Contents

Introduction

Background Information

OSPF versus RIP

What Do We Mean by Link-States?

Shortest Path First Algorithm

OSPF Cost

Shortest Path Tree

Areas and Border Routers

Link-State Packets

Enable OSPF on the Router

OSPF Authentication

Simple Password Authentication

Message Digest Authentication

The Backbone and Area 0

Virtual Links

Areas Not Physically Connected to Area 0

the Backbone

Neighbors

Adjacencies

DR Election

Build the Adjacency

Adjacencies on Point-to-Point Interfaces

Adjacencies on Non-Broadcast Multi-Access (NBMA) Networks

Avoid DRs and neighbor Command on NBMA

Point-to-Point Subinterfaces

Select Interface Network Types

OSPF and Route Summarization

Inter-Area Route Summarization

External Route Summarization

Stub Areas

Redistribute Routes into OSPF

E1 vs. E2 External Routes

Redistribute OSPF into Other Protocols

Use of a Valid Metric

VLSM

Mutual Redistribution

Inject Defaults into OSPF

OSPF Design Tips

Number of Routers per Area

Number of Neighbors

Number of Areas per ABR

Full Mesh vs. Partial Mesh

Memory Issues

Summary

Appendix A: Link-State Database Synchronization

Link-State Advertisements

OSPF Database Example

Appendix B: OSPF and IP Multicast Address

Appendix C: Variable Length Subnet Masks (VLSM)

Related Information

Introduction

This document describes how OSPF works and how it can be used to design and build large and complicated networks.

Background Information

The Open Shortest Path First (OSPF) protocol, defined in RFC 2328 , is an Interior Gateway Protocol used to distribute routing information within a single Autonomous System.

OSPF protocol was developed due to a need in the internet community to introduce a high functionality non-proprietary Internal Gateway Protocol (IGP) for the TCP/IP protocol family.

The discussion of the creation of a common interoperable IGP for the Internet started in 1988 and did not get formalized until 1991.

At that time, the OSPF Working Group requested that OSPF be considered for advancement to Draft Internet Standard.

The OSPF protocol is based on link-state technology, which is a departure from the Bellman-Ford vector based algorithms used in traditional Internet routing protocols such as RIP.

OSPF has introduced new concepts such as authentication of routing updates, Variable Length Subnet Masks (VLSM), route summarization, and so forth.

These chapters discuss the OSPF terminology, algorithm and the benefits and nuances of the protocol in the design of the large, complicated networks of today.

OSPF versus RIP

The rapid growth and expansion of modern networks has pushed Routing Information Protocol (RIP) to its limits. RIP has certain limitations that can cause problems in large networks:

- RIP has a limit of 15 hops. A network that spans more than 15 hops (15 routers) is considered unreachable.

- RIP cannot handle Variable Length Subnet Masks (VLSM). Given the shortage of IP addresses and the flexibility VLSM gives in the efficient assignment of IP addresses, this is considered a major flaw.

Periodic broadcasts of the full routing table consume a large amount of bandwidth. This is a major problem with large networks especially on slow links and WAN clouds.

- RIP converge is slower than OSPF. In large networks convergence gets to be in the order of minutes.

- RIP routers go through a period of a hold-down and garbage collection and slowly time-out information that has not been received recently. This is inappropriate in large environments and could cause routing inconsistencies.

- RIP has no concept of network delays and link costs. Routing decisions are based on hop counts. The path with the lowest hop count to the destination is always preferred even if the longer path has a better aggregate link bandwidth and less delays.

- RIP networks are flat networks. There is no concept of areas or boundaries. With the introduction of classless routing and the intelligent use of aggregation and summarization, RIP networks have fallen behind.

Enhancements were introduced in a new version of RIP called RIP2. RIP2 addresses the issues of VLSM, authentication, and multicast routing updates.

RIP2 is not a big improvement over RIP (now called RIP1) because it still has the limitations of hop counts and slow convergence which are essential in large networks.

OSPF, on the other hand, addresses most of the issues previously presented:

- With OSPF, there is no limitation on the hop count.

- The intelligent use of VLSM is very useful in IP address allocation.

- OSPF uses IP multicast to send link-state updates. This ensures less process resource consumption on routers that do not listen to OSPF packets. Updates are only sent in case routing changes occur instead of periodically. This ensures efficient bandwidth.

- OSPF has better convergence than RIP. This is because routing changes are propagated instantaneously and not periodically.

- OSPF allows for better load balancing.

- OSPF allows for a logical definition of networks where routers can be divided into areas. This limits the explosion of link state updates over the whole network. This also provides a mechanism to aggregate routes and decrease the unnecessary propagation of subnet information.

- OSPF allows for routing authentication through different methods of password authentication.

- OSPF allows for the transfer and tagging of external routes injected into an Autonomous System. This keeps track of external routes injected by exterior protocols such as BGP.

This leads to more complexity in the configuration and troubleshooting of OSPF networks.

Administrators that are used to the simplicity of RIP are challenged with the amount of new information they have to learn in order to keep up with OSPF networks.

This introduces more overhead in memory allocation and CPU utilization. Some of the routers which run RIP have to be upgraded in order to handle the overhead caused by OSPF.

What Do We Mean by Link-States?

OSPF is a link-state protocol. Think of a link as an interface on the router. The state of the link is a description of that interface and of its relationship to its neighbor routers.

A description of the interface would include, for example, the IP address of the interface, the mask, the type of network it is connected to, the routers connected to that network and so on.

The collection of all these link-states would form a link-state database.

Shortest Path First Algorithm

OSPF uses a shortest path first algorithm to build and calculate the shortest path to all destinations. The shortest path is calculated with the Dijkstra algorithm.

The algorithm by itself is complicated. This is a high level look at the various steps of the algorithm:

1. Upon initialization or due to any change in routing information, a router generates a link-state advertisement. This advertisement represents the collection of all link-states on that router.

2. All routers exchange link-states through floods. Each router that receives a link-state update must store a copy in its link-state database and then propagate the update to other routers.

3. After the database of each router is completed, the router calculates a Shortest Path Tree to all destinations. The router uses the Dijkstra algorithm in order to calculate the shortest path tree, destinations, associated cost, and next hop to reach those destinations form the IP routing table.

4. In case changes in the OSPF network do not occur, such as cost of a link or a network which is added or deleted, OSPF remains very quiet. Changes are communicated through link-state packets, and the Dijkstra algorithm is recalculated to find the shortest path.

The algorithm places each router at the root of a tree and calculates the shortest path to each destination based on the cumulative cost required to reach that destination.

Each router has its own view of the topology even though all the routers build a shortest path tree which uses the same link-state database. These sections indicate what is involved in the creation of a shortest path tree.

OSPF Cost

The cost (also called metric) of an interface in OSPF is an indication of the overhead required to send packets across a certain interface.

The cost of an interface is inversely proportional to the bandwidth of that interface. A higher bandwidth indicates a lower cost

There is more overhead (higher cost) and time delays involved through a 56k serial line than through a 10M ethernet line.

The formula used to calculate the cost is:

- cost= 10000 0000/bandwidth in bps

For example, it costs 10 EXP8/10 EXP7 = 10 to cross a 10M Ethernet line and 10 EXP8/1544000 = 64 to cross a T1 line.

By default, the cost of an interface is calculated based on the bandwidth; you can force the cost of an interface with the ip ospf cost <value>  interface subconfiguration mode command.

Shortest Path Tree

Refer to this network diagram with the indicated interface costs. In order to build the shortest path tree for RTA, we would have to make RTA the root of the tree and calculate the smallest cost for each destination.

This is the view of the network as seen from RTA. Note the direction of the arrows in the cost calculation.

The cost of RTB interface to network 198.51.100.1 is not relevant when the cost is calculated to 192.168.0.1.

RTA can reach 192.168.0.1 via RTB with a cost of 15 (10+5).

RTA can also reach 203.0.113.1 via RTC with a cost of 20 (10+10) or via RTB with a cost of 20 (10+5+5).

In case equal cost paths exist to the same destination, the implementation of OSPF keeps track of up to six (6) next hops to the same destination.

After the router builds the shortest path tree, it builds the routing table. Directly connected networks are reached via a metric (cost) of 0 and other networks are reached in accord with the cost calculated in the tree.

Areas and Border Routers

As previously mentioned, OSPF uses floods to exchange link-state updates between routers. Any change in routing information is flooded to all routers in the network.

Areas are introduced to put a boundary on the explosion of link-state updates. Floods and calculation of the Dijkstra algorithm on a router is limited to changes within an area.

All routers within an area have the exact link-state database. Routers that belong to multiple areas, and connect these areas to the backbone area are called area border routers (ABR).

ABRs must therefore maintain information that describes the backbone areas and other attached areas.

An area is interface specific. A router that has all of its interfaces within the same area is called an internal router (IR).

A router that has interfaces in multiple areas is called an area border router (ABR).

Routers that act as gateways (redistribution) between OSPF and other routing protocols (IGRP, EIGRP, IS-IS, RIP, BGP, Static) or other instances of the OSPF routing process are called autonomous system boundary router (ASBR). Any router can be an ABR or an ASBR.

Link-State Packets

There are different types of Link State Packets, those are what you normally see in an OSPF database (Appendix A and illustrated here).

The router links are an indication of the state of the interfaces on a router in a certain designated area. Each router generates a router link for all of its interfaces.

Summary links are generated by ABRs; this is how network reachability information is disseminated between areas.

Normally, all information is injected into the backbone (area 0) and in turn the backbone passes it on to other areas.

ABRs also propagate the reachability of the ASBR. This is how routers know how to get to external routes in other ASs.

Network Links are generated by a Designated Router (DR) on a segment (DRs are discussed later).

This information is an indication of all routers connected to a particular multi-access segment such as Ethernet, Token Ring and FDDI (NBMA also).

External Links are an indication of networks outside of the AS. These networks are injected into OSPF via redistribution. The ASBR injects these routes into an autonomous system.

Enable OSPF on the Router

OSPF enable on the router involves two steps in config mode:

1. Enable an OSPF process with the router ospf <process-id>  command.

2. Area assignment to the interfaces with the network <network or IP address> <mask> <area-id>  command.

The OSPF process-id is a numeric value local to the router. It does not have to match process-ids on other routers.

It is possible to run multiple OSPF processes on the same router, but is not recommended as it creates multiple database instances that add extra overhead to the router.

The network command is an assignment method of an interface to a certain area. The mask is used as a shortcut and it puts a list of interfaces in the same area with one line configuration line.

The mask contains wild card bits where 0 is a match and 1 is a "do not care" bit, for example, 0.0.255.255 indicates a match in the first two bytes of the network number.

The area-id is the area number we want the interface to be in. The area-id can be an integer between 0 and 4294967295 or can take a form similar to an IP address A.B.C.D.

Here is an example:

RTA#

interface Ethernet0

ip address 192.168.0.2 255.255.255.0

interface Ethernet1

ip address 192.168.0.5 255.255.255.0

interface Ethernet2

ip address 192.168.0.3 255.255.255.0

router ospf 100

network 192.168.0.4  0.0.255.255 area 0.0.0.0

network 192.168.0.3 0.0.0.0 area 23

The first network statement puts both E0 and E1 in the same area 0.0.0.0, and the second network statement puts E2 in area 23. Note the mask of 0.0.0.0, which indicates a full match on the IP address.

This is an easy way to put an interface in a certain area if you are are unable to resolve a mask.

OSPF Authentication

It is possible to authenticate the OSPF packets such that routers can participate in routing domains based on predefined passwords.

By default, a router uses a Null authentication which means that routing exchanges over a network are not authenticated. Two other authentication methods exist: Simple password authentication and Message Digest authentication (MD-5).

Simple Password Authentication

Simple password authentication allows a password (key) to be configured per area. Routers in the same area that want to participate in the routing domain has to be configured with the same key.

The drawback of this method is that it is vulnerable to passive attacks. Anybody with a link analyzer could easily get the password off the wire.

To enable password authentication, use these commands:

- ip ospf authentication-key key  (this goes under the specific interface)

- area area-id authentication (this goes under router ospf <process-id>)

Here is an example:

interface Ethernet0

ip address 10.0.0.1 255.255.255.0

ip ospf authentication-key mypassword

router ospf 10

network 10.0.0.0 0.0.255.255 area 0

area 0 authentication

Message Digest Authentication

Message Digest authentication is a cryptographic authentication. A key (password) and key-id are configured on each router.

The router uses an algorithm based on the OSPF packet, the key, and the key-id to generate a "message digest" that gets appended to the packet.

Unlike the simple authentication, the key is not exchanged over the wire. A non-decreasing sequence number is also included in each OSPF packet to protect against replay attacks.

This method also allows for uninterrupted transitions between keys. This is helpful for administrators who wish to change the OSPF password without communication disruption.

If an interface is configured with a new key, the router sends multiple copies of the same packet, each authenticated by different keys.

The router does not send duplicate packets when it detects that all of its neighbors have adopted the new key.

These are the commands used for message digest authentication:

- ip ospf message-digest-key keyid md5 key  (used under the interface)

- area area-id authentication message-digest (used under router ospf <process-id>)

Here is an example:

interface Ethernet0

ip address 10.0.0.1 255.255.255.0

ip ospf message-digest-key 10 md5 mypassword

router ospf 10

network 10.0.0.0 0.0.255.255 area 0

area 0 authentication message-digest

The Backbone and Area 0

OSPF has special restrictions when multiple areas are involved. If more than one area is configured, one of these areas has be to be area 0. This is called the backbone.

It is good network design practice to start with area 0 and then expand into other areas later on.

The backbone has to be at the center of all other areas, that is, all areas have to be physically connected to the backbone.

The reason is that OSPF expects all areas to inject routing information into the backbone and in turn the backbone disseminates that information into other areas.

This diagram illustrates the flow of information in an OSPF network:

In this diagram, all areas are directly connected to the backbone. In the rare situations where a new area is introduced that cannot have a direct physical access to the backbone, a virtual link has to be configured.

Virtual links are discussed in the next section. Note the different types of routing information. Routes that are generated from within an area (the destination belongs to the area) are called intra-area routes.

These routes are normally represented by the letter O in the IP routing table. Routes that originate from other areas are called inter-area or Summary routes.

The notation for these routes is O IA in the IP routing table. Routes that originate from other routing protocols (or different OSPF processes) and that are injected into OSPF via redistribution are called external routes.

These routes are represented by O E2 or O E1 in the IP routing table. Multiple routes to the same destination are preferred in this order:  intra-area, inter-area, external E1, external E2. External types E1 and E2 are explained later.

Virtual Links

Virtual links are used for two purposes:

- To an area that does not have a physical connection to the backbone

- To patch the backbone in case discontinuity of area 0 occurs.

Areas Not Physically Connected to Area 0

As mentioned earlier, area 0 has to be at the center of all other areas. In some rare case where it is impossible to have an area physically connected to the backbone, a virtual link is used.

The virtual link provides the disconnected area a logical path to the backbone. The virtual link has to be established between two ABRs that have a common area, with one ABR connected to the backbone.

In this example, area 1 does not have a direct physical connection into area 0. A virtual link has to be configured between RTA and RTB. Area 2 is to be used as a transit area and RTB is the entry point into area 0.

In this way RTA and area 1 has a logical connection to the backbone. In order to configure a virtual link, use the area <area-id> virtual-link <RID> router OSPF sub-command on both RTA and RTB, where area-id is the transit area.

In the diagram, this is area 2. The RID is the router-id. The OSPF router-id is usually the highest IP address on the box, or the highest loopback address if one exists.

The router-id is only calculated at boot time. To find the router-id, use the show ip ospf interface command.

Consider that 10.0.0.11 and 10.0.0.22 are the respective RIDs of RTA and RTB, the OSPF configuration for both routers would be:

RTA#

router ospf 10

area 2 virtual-link 10.0.0.22

RTB#

router ospf 10

area 2 virtual-link 10.0.0.11

the Backbone

OSPF allows for discontinuous parts of the backbone to link through a virtual link. In some cases, different area 0s need to be linked together.

This can occur if, for example, a company tries to merge two separate OSPF networks into one network with a common area 0. In other instances, virtual-links are added for redundancy in case some router failure causes the backbone to be split into two.

A virtual link can be configured between separate ABRs that touch area 0 from each side and share a common area (illustrated here).

In this diagram, two area 0s are linked together via a virtual link. In case a common area does not exist, an additional area, such as area 3, could be created to become the transit area.

In case any area which is different than the backbone becomes partitioned, the backbone takes care of the partition effort without the use of any virtual links.

One part of the partioned area is known to the other part via inter-area routes rather than intra-area routes.

Neighbors

Routers that share a common segment become neighbors on that segment. Neighbors are elected via the Hello protocol. Hello packets are sent periodically out of each interface through IP multicast (Appendix B).

Routers become neighbors as soon as they see themselves listed in the neighbor Hello packet. This way, a two way communication is guaranteed. Neighbor negotiation applies to the primary address only.

Secondary addresses can be configured on an interface with a restriction that they have to belong to the same area as the primary address.

Two routers do not become neighbors unless they agree with this criteria.

- Area-id: Two routers which have a common segment; their interfaces have to belong to the same area on that segment. The interfaces must belong to the same subnet and have a similar mask.

- Authentication: OSPF allows for the configuration of a password for a specific area. Routers that want to become neighbors have to exchange the same password on a particular segment.

- Hello and Dead Intervals: OSPF exchanges Hello  packets on each segment. This is a form of keepalive used by routers in order to acknowledge their existence on a segment and in order to elect a designated router (DR) on multiaccess segments.

The Hello  interval specifies the length of time, in seconds, between the Hello  packets that a router sends on an OSPF interface.

The dead interval is the number of seconds that a router Hello  packets have not been seen before its neighbors declare the OSPF router down.

- OSPF requires these intervals to be exactly the same between two neighbors. If any of these intervals are different, these routers do not become neighbors on a particular segment. The router interface commands used to set these timers are: ip ospf hello-interval seconds and ip ospf dead-interval seconds.

- Stub area flag:  Two routers have to also agree on the stub area flag in the Hello packets in order to become neighbors. Stub areas are discussed in a later section. Consider that definition of stub areas affect the neighbor election process.

Adjacencies

Adjacency is the next step after the neighbor process. Adjacent routers are routers that go beyond the simple Hello exchange and proceed into the database exchange process.

In order to minimize the amount of information exchange on a particular segment, OSPF elects one router to be a designated router (DR), and one router to be a backup designated router (BDR), on each multi-access segment.

The BDR is elected as a backup mechanism in case the DR goes down. The idea behind this is that routers have a central point of contact for information exchange.

Rather than exchange updates with every other router on the segment, every router exchanges information with the DR and BDR.

The DR and BDR relay the information to everybo creationdy else. In mathematical terms, this cuts the information exchange from O(n*n) to O(n) where n is the number of routers on a multi-access segment.

This router model illustrates the DR and BDR:

In this diagram, all routers share a common multi-access segment. Due to the exchange of Hello packets, one router is elected DR and another is elected BDR.

Each router on the segment (which already became a neighbor) tries to establish an adjacency with the DR and BDR.

DR Election

DR and BDR election is done via the Hello protocol. Hello packets are exchanged via IP multicast packets (Appendix B) on each segment.

The router with the highest OSPF priority on a segment becomes the DR for that segment. The same process is repeated for the BDR. In case of a tie, the router with the highest RID prevails.

The default for the interface OSPF priority is one. Remember that the DR and BDR concepts are per multiaccess segment. The OSPF priority value on an interface is done with the ip ospf priority <value>  interface command.

A priority value of zero indicates an interface which is not to be elected as DR or BDR. The state of the interface with priority zero is DROTHER. This illustrates the DR election:

In this diagram, RTA and RTB have the same interface priority but RTB has a higher RID. RTB would be DR on that segment. RTC has a higher priority than RTB. RTC is DR on that segment.

Build the Adjacency

The adjacency build process takes effect after multiple stages have been fulfilled. Routers that become adjacent have the exact link-state database.

Here is a summary of states which an interface passes through before it becomes adjacent to another router:

- Down: No information has been received from anybody on the segment.

- Attempt: On non-broadcast multi-access clouds such as Frame Relay and X.25, this state indicates that no recent information has been received from the neighbor. To contact the neighbor, send Hello packets at the reduced rate Poll Interval .

- Init: The interface has detected a Hello packet from a neighbor but bi-directional communication has not yet been established.

- Two-way: There is bi-directional communication with a neighbor. The router has seen itself in the Hello packets from a neighbor. At the end of this stage the DR and BDR election would have been done. At the end of the 2-way stage, routers decides whether to proceed in an adjacency build. The decision is based on whether one of the routers is a DR or BDR or the link is a point-to-point or a virtual link.

- Exstart: Routers try to establish the initial sequence number to be used in the information exchange packets. The sequence number insures that routers always get the most recent information. One router becomes the primary and the other becomes secondary. The primary router polls the secondary for information.

- Exchange: Routers describe their entire link-state database through sent database description packets. At this state, packets could be flooded to other interfaces on the router.

- Load: At this state, routers finalize the information exchange. Routers have built a link-state request list and a link-state retransmission list. Any information that looks incomplete or outdated are put on the request list. Updates are put on the retransmission list until acknowledged.

- Full: At this state, the adjacency is complete. The neighbor routers are fully adjacent. Adjacent routers have a similar link-state database.

Here is an example:

RTA, RTB, RTD, and RTF share a common segment (E0) in area 0.0.0.0. These are the configurations of RTA and RTF. RTB and RTD must have a similar configuration to RTF and are not included.

RTA#

hostname RTA

interface Loopback0

ip address 203.0.113.41 255.255.255.0

interface Ethernet0

ip address 203.0.113.141 255.255.255.0

router ospf 10

network 203.0.113.41 0.0.0.0 area 1

network 203.0.113.100 0.0.255.255 area 0.0.0.0

RTF#

hostname RTF

interface Ethernet0

ip address 203.0.113.142 255.255.255.0

router ospf 10

network 203.0.113.100 0.0.255.255 area 0.0.0.0

This is a simple example that demonstrates a couple of commands that are very useful in debugging OSPF networks.

- show ip ospf interface  <interface>

This command is a quick check to determine if all of the interfaces belong to the areas they are supposed to be in. The sequence in which the OSPF network commands are listed is very important.

In RTA configuration, if the "network 203.0.113.100 0.0.255.255 area 0.0.0.0" statement was put before the "network 203.0.113.41 0.0.0.0 area 1" statement, all of the interfaces would be in area 0, which is incorrect because the loopback is in area 1.

Here is the command output on RTA, RTF, RTB, and RTD:

RTA#show ip ospf interface e0

Ethernet0 is up, line protocol is up

Internet Address 203.0.113.141 255.255.255.0, Area 0.0.0.0

Process ID 10, Router ID 203.0.113.41, Network Type BROADCAST, Cost:

10

Transmit Delay is 1 sec, State BDR, Priority 1

Designated Router (ID) 203.0.113.151, Interface address 203.0.113.142

Backup Designated router (ID) 203.0.113.41, Interface address

203.0.113.141

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Hello due in 0:00:02

Neighbor Count is 3, Adjacent neighbor count is 3

Adjacent with neighbor 203.0.113.151  (Designated Router)

Loopback0 is up, line protocol is up

Internet Address 203.0.113.41 255.255.255.255, Area 1

Process ID 10, Router ID 203.0.113.41, Network Type LOOPBACK, Cost: 1

Loopback interface is treated as a stub Host

RTF#show ip ospf interface e0

Ethernet0 is up, line protocol is up

Internet Address 203.0.113.142 255.255.255.0, Area 0.0.0.0

Process ID 10, Router ID 203.0.113.151, Network Type BROADCAST, Cost: 10

Transmit Delay is 1 sec, State DR, Priority 1

Designated Router (ID) 203.0.113.151, Interface address 203.0.113.142

Backup Designated router (ID) 203.0.113.41, Interface address

203.0.113.141

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Hello due in 0:00:08

Neighbor Count is 3, Adjacent neighbor count is 3

Adjacent with neighbor 203.0.113.41  (Backup Designated Router)

RTD#show ip ospf interface e0

Ethernet0 is up, line protocol is up

Internet Address 203.0.113.144 255.255.255.0, Area 0.0.0.0

Process ID 10, Router ID 192.0.2.174, Network Type BROADCAST, Cost:

10

Transmit Delay is 1 sec, State DROTHER, Priority 1

Designated Router (ID) 203.0.113.151, Interface address 203.0.113.142

Backup Designated router (ID) 203.0.113.41, Interface address

203.0.113.141

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Hello due in 0:00:03

Neighbor Count is 3, Adjacent neighbor count is 2

Adjacent with neighbor 203.0.113.151  (Designated Router)

Adjacent with neighbor 203.0.113.41  (Backup Designated Router)

RTB#show ip ospf interface e0

Ethernet0 is up, line protocol is up

Internet Address 203.0.113.143 255.255.255.0, Area 0.0.0.0

Process ID 10, Router ID 203.0.113.121, Network Type BROADCAST, Cost: 10

Transmit Delay is 1 sec, State DROTHER, Priority 1

Designated Router (ID) 203.0.113.151, Interface address 203.0.113.142

Backup Designated router (ID) 203.0.113.41, Interface address

203.0.113.141

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Hello due in 0:00:03

Neighbor Count is 3, Adjacent neighbor count is 2

Adjacent with neighbor 203.0.113.151  (Designated Router)

Adjacent with neighbor 203.0.113.41  (Backup Designated Router)

This output shows very important information. At RTA output, Ethernet0 is in area 0.0.0.0. The process ID is 10 (router ospf 10) and the router ID is 203.0.113.41.

Remember that the RID is the highest IP address on the box or the loopback interface, calculated at boot time or whenever the OSPF process is restarted.

The state of the interface is BDR. Since all routers have the same OSPF priority on Ethernet 0 (default is 1), the RTF interface was elected as DR because of the higher RID.

In the same way, RTA was elected as BDR. RTD and RTB are neither a DR or BDR and their state is DROTHER.

Take notice of the neighbor count and the adjacent count. RTD has three neighbors and is adjacent to two of them, the DR and the BDR. RTF has three neighbors and is adjacent to all of them because it is the DR.

The information about the network type is important and determines the state of the interface. On broadcast networks such as Ethernet, the election of the DR and BDR are irrelevant to the end user.

It does not matter who the DR or BDR are. In other cases, such as NBMA media such as Frame Relay and X.25, this becomes very important for OSPF to function correctly.

With the introduction of point-to-point and point-to-multipoint subinterfaces, DR election is no longer an issue. OSPF over NBMA is discussed in the next section.

Another command we need to look at is:

- show ip ospf neighbor

Let us look at the RTD output:

RTD#show ip ospf neighbor

Neighbor ID    Pri State         Dead Time  Address      Interface

203.0.113.121    1  2WAY/DROTHER  0:00:37   203.0.113.143  Ethernet0

203.0.113.151    1  FULL/DR       0:00:36   203.0.113.142  Ethernet0

203.0.113.41   1  FULL/BDR      0:00:34   203.0.113.141  Ethernet0

The show ip ospf neighbor command shows the state of all the neighbors on a particular segment. Do not be alarmed if the Neighbor ID does not belong to the segment that you look at.

In our case 203.0.113.121 and 203.0.113.151 are not on Ethernet0. The Neighbor ID is actually the RID which could be any IP address on the box.

RTD and RTB are just neighbors, that is why the state is 2WAY/DROTHER. RTD is adjacent to RTA and RTF and the state is FULL/DR and FULL/BDR.

Adjacencies on Point-to-Point Interfaces

OSPF always forms an adjacency with the neighbor on the other side of a point-to-point interface such as point-to-point serial lines. There is no concept of DR or BDR. The state of the serial interfaces is point to point.

Adjacencies on Non-Broadcast Multi-Access (NBMA) Networks

Special care must be taken with configuration of OSPF over multi-access non-broadcast medias such as Frame Relay, X.25, ATM. The protocol considers these media like any other broadcast media such as Ethernet.

NBMA clouds are usually built in a hub and spoke topology. PVCs or SVCs are laid out in a partial mesh and the physical topology does not provide the multi access that OSPF can detect.

The selection of the DR becomes an issue because the DR and BDR need to have full physical connectivity with all routers that exist on the cloud.

Because of the lack of broadcast capabilities, the DR and BDR need to have a static list of all other routers attached to the cloud.

This is achieved with the neighbor ip-address [priority number] [poll-interval seconds] command, where the "ip-address" and "priority" are the IP address and the OSPF priority given to the neighbor.

A neighbor with priority 0 is considered ineligible for DR election. The "poll-interval" is the amount of time an NBMA interface waits before the poll (a sent Hello) to a presumably dead neighbor.

The neighbor command applies to routers with DR- or BDR-potential (interface priority not equal to 0). This shows a network diagram where DR selection is very important:

In this diagram, it is essential for the RTA interface to the cloud to be elected DR. This is because RTA is the only router that has full connectivity to other routers.

The election of the DR could be influenced by the ospf priority parameter on the interfaces. Routers that do not need to become DRs or BDRs have a priority of 0 other routers could have a lower priority.

The  neighbor command is not covered in depth in this document and becomes obsolete through new interface Network Type irrespective of the underlying physical media. This is explained in the next section.

Avoid DRs and neighbor Command on NBMA

Different methods can be used to avoid the complications of static neighbor configuration and specific routers which become DRs or BDRs on the non-broadcast cloud.

To specify which method to use is influenced by whether we are start the network from the start or if we rectify a design which already exists.

Point-to-Point Subinterfaces

A subinterface is a logical way to define an interface. The same physical interface can be split into multiple logical interfaces, with each subinterface defined as point-to-point.

This was originally created in order to better handle issues caused by split horizon over NBMA and vector based routing protocols.

A point-to-point subinterface has the properties of any physical point-to-point interface. As far as OSPF is concerned, an adjacency is always formed over a point-to-point subinterface with no DR or BDR election.

This is an illustration of point-to-point subinterfaces:

In this diagram, on RTA, we can split Serial 0 into two point-to-point subinterfaces, S0.1 and S0.2. This way, OSPF considers the cloud as a set of point-to-point links rather than one multi-access network.

The only drawback for the point-to-point is that each segment belongs to a different subnet. This is unacceptable because some administrators have already assigned one IP subnet for the whole cloud.

Another workaround is to use IP unnumbered interfaces on the cloud. This is also a problem for administrators who manage the WAN based on IP addresses of the serial lines. This is a typical configuration for RTA and RTB:

RTA#

interface Serial 0

no ip address

encapsulation frame-relay

interface Serial0.1 point-to-point

ip address 198.51.100.36 255.255.252.0

frame-relay interface-dlci 20

interface Serial0.2 point-to-point

ip address 198.51.100.46 255.255.252.0

frame-relay interface-dlci 30

router ospf 10

network 198.51.100.1 0.0.255.255 area 1

RTB#

interface Serial 0

no ip address

encapsulation frame-relay

interface Serial0.1 point-to-point

ip address 198.51.100.35 255.255.252.0

frame-relay interface-dlci 40

interface Serial1

ip address 198.51.100.11 255.255.255.0

router ospf 10

network 198.51.100.1 0.0.255.255 area 1

network 198.51.100.10 0.0.255.255 area 0

Select Interface Network Types

The command used to set the network type of an OSPF interface is:

ip ospf network {broadcast | non-broadcast | point-to-multipoint}

Point-to-Multipoint Interfaces

An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface with one or more neighbors. This concept takes the previously discussed point-to-point concept one step further.

Administrators do not have to worry about multiple subnets for each point-to-point link. The cloud is configured as one subnet.

This works well for those who migrate into the point-to-point concept with no change in IP address on the cloud. Also, they can disregard DRs and neighbor statements.

OSPF point-to-multipoint works through the exchange of additional link-state updates that contain a number of information elements that describe connectivity to the neighbor routers.

RTA#

interface Loopback0

ip address 203.0.113.101 255.255.255.0

interface Serial0

ip address 198.51.100.101 255.255.255.0

encapsulation frame-relay

ip ospf network point-to-multipoint

router ospf 10

network 198.51.100.1 0.0.255.255 area 1

RTB#

interface Serial0

ip address 198.51.100.102 255.255.255.0

encapsulation frame-relay

ip ospf network point-to-multipoint

interface Serial1

ip address 198.51.100.11 255.255.255.0

router ospf 10

network 198.51.100.1 0.0.255.255 area 1

network 198.51.100.10 0.0.255.255 area 0

Note that no static frame relay map statements were configured; this is because Inverse ARP takes care of the DLCI to IP address mapping. Let us look at some of show ip ospf interface and show ip ospf route outputs:

RTA#show ip ospf interface s0

Serial0 is up, line protocol is up

Internet Address 198.51.100.101 255.255.255.0, Area 0

Process ID 10, Router ID 203.0.113.101, Network Type

POINT_TO_MULTIPOINT, Cost: 64

Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT,

Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5

Hello due in 0:00:04

Neighbor Count is 2, Adjacent neighbor count is 2

Adjacent with neighbor 198.51.100.174

Adjacent with neighbor 198.51.100.130

RTA#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

198.51.100.103      1   FULL/  -        0:01:35     198.51.100.103    Serial0

198.51.100.102      1   FULL/  -        0:01:44     198.51.100.102    Serial0

RTB#show ip ospf interface s0

Serial0 is up, line protocol is up

Internet Address 198.51.100.102 255.255.255.0, Area 0

Process ID 10, Router ID 198.51.100.102, Network Type

POINT_TO_MULTIPOINT, Cost: 64

Transmit Delay is 1 sec, State POINT_TO_MULTIPOINT,

Timer intervals configured, Hello 30, Dead 120, Wait 120, Retransmit 5

Hello due in 0:00:14

Neighbor Count is 1, Adjacent neighbor count is 1

Adjacent with neighbor 203.0.113.101

RTB#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

203.0.113.101      1   FULL/  -        0:01:52     198.51.100.101    Serial0

The only drawback for point-to-multipoint is that it generates multiple Hosts routes (routes with mask 255.255.255.255) for all the neighbors. Note the Host routes in the IP routing table for RTB:

RTB#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

203.0.113.210 255.255.255.255 is subnetted, 1 subnets

O       203.0.113.101 [110/65] via 198.51.100.101,  Serial0

198.51.100.1 is variably subnetted, 3 subnets, 2 masks

O    198.51.100.103 255.255.255.255

[110/128] via 198.51.100.101, 00:00:00, Serial0

O    198.51.100.101 255.255.255.255

[110/64] via 198.51.100.101, 00:00:00, Serial0

C       198.51.100.100 255.255.255.0 is directly connected, Serial0

172.16.0.0 255.255.255.0 is subnetted, 1 subnets

C       172.16.0.1 is directly connected, Serial1

RTC#show ip route

203.0.113.210 255.255.255.255 is subnetted, 1 subnets

O       203.0.113.101 [110/65] via 198.51.100.101, Serial1

198.51.100.1 is variably subnetted, 4 subnets, 2 masks

O       198.51.100.102 255.255.255.255 [110/128] via 198.51.100.101,Serial1

O       198.51.100.101 255.255.255.255 [110/64] via 198.51.100.101, Serial1

C       198.51.100.100 255.255.255.0 is directly connected, Serial1

172.16.0.0 255.255.255.0 is subnetted, 1 subnets

O       172.16.0.1 [110/192] via 198.51.100.101, 00:14:29, Serial1

Note that in the RTC IP routing table, network 172.16.0.1 is reachable via next hop 198.51.100.101 and not via 198.51.100.102, normally seen over Frame Relay clouds which share the same subnet.

This is one advantage of the point-to-multipoint configuration because you do not need static mapping on RTC to reach next hop 198.51.100.102.

Broadcast Interfaces

This approach is a workaround for the neighbor command which statically lists all current neighbors. The interface is logically set to broadcast and behaves as if the router were connected to a LAN.

DR and BDR election are performed so assure that either a full mesh topology or a static selection of the DR based on the interface priority. The command that sets the interface to broadcast is:

ip ospf network broadcast

OSPF and Route Summarization

To summarize is to consolidate multiple routes into one single advertisement. This is normally done at the boundaries of Area Border Routers (ABRs).

Although summarization is configured between any two areas, it is better to summarize in the direction of the backbone. This way the backbone receives all the aggregate addresses and in turn injects them, already summarized, into other areas.

There are two types of summarization:

- Inter-area route summarization

- External route summarization

Inter-Area Route Summarization

Inter-area route summarization is done on ABRs and it applies to routes from within the AS. It does not apply to external routes injected into OSPF via redistribution.

In order to take advantage of summarization, network numbers in areas must be assigned in a contiguous way to lump these addresses into one range.

To specify an address range, perform this task in router configuration mode:

area area-id range address mask

Where the area-id is the area which contains networks to be summarized. The "address" and "mask" specifies the range of addresses to be summarized in one range. This is an example of summarization:

In this diagram, RTB isummarizes the range of subnets from 172.16.0.64 to 172.16.0.95 into one range: 172.16.0.64 255.255.224.0. To achieve this, mask the first three left most bits of 64 with a mask of 255.255.224.0.

In the same way, RTC generates the summary address 172.16.0.96 255.255.224.0 into the backbone. Note that this summarization was successful because we have two distinct ranges of subnets, 64-95 and 96-127.

It is difficult to summarize if the subnets between area 1 and area 2 overlapped. The backbone area would receive summary ranges that overlap and routers in the middle would not know where to send the traffic based on the summary address.

This is the relative configuration of RTB:

RTB#

router ospf 100

area 1 range 172.16.0.64 255.255.224.0

Prior to Cisco IOS® Software Release 12.1(6), it was recommended to manually configure, on the ABR, a discard static route for the summary address to prevent possible routing loops. For the summary route shown, use this command:

ip route 172.16.0.64 255.255.224.0 null0

In Cisco IOS® 12.1(6) and higher, the discard route is automatically generated by default. To discard route, configure commands under router ospf:

- Either [no] discard-route internal

- Or [no] discard-route external

Note about summary address metric calculation: RFC 1583  called to calculate the metric for summary routes based on the minimum metric of the component paths available.

RFC 2178  (now obsoleted by RFC 2328 ) changed the specified method to calculate metrics for summary routes so the component of the summary with the maximum (or largest) cost would determine the cost of the summary.

Prior to Cisco IOS® 12.0, Cisco was compliant with the then-current RFC 1583 . As of Cisco IOS® 12.0, Cisco changed the behavior of OSPF to be compliant with the new standard, RFC 2328 .

This situation created the possibility of sub-optimal routing if all of the ABRs in an area were not upgraded to the new code at the same time.

In order to address this potential problem, a command has been added to the OSPF configuration of Cisco IOS® that allows you to selectively disable compatibility with RFC 2328 .

The new configuration command is under router ospf, and has the syntax:

[no] compatible rfc1583

The default parameter is compatible with RFC 1583 . This command is available in these versions of Cisco IOS®:

- 12.1(03)DC

- 12.1(03)DB

- 12.001(001.003) - 12.1 Mainline

- 12.1(01.03)T - 12.1 T-Train

- 12.000(010.004) - 12.0 Mainline

- 12.1(01.03)E - 12.1 E-Train

- 12.1(01.03)EC

- 12.0(10.05)W05(18.00.10)

- 12.0(10.05)SC

External Route Summarization

External route summarization is specific to external routes that are injected into OSPF via redistribution. Also, make sure that external ranges that are summarized are contiguous.

Summarization of overlapped ranges from two different routers could cause packets to be sent to the wrong destination. Summarization is done via the router ospf subcommand:

summary-address ip-address mask

This command is effective only on ASBR redistribution into OSPF.

In this diagram, RTA and RTD inject external routes into OSPF by redistribution. RTA injects subnets in the range 128.213.64-95 and RTD injects subnets in the range 128.213.96-127. To summarize the subnets into one range on each router:

RTA#

router ospf 100

summary-address 172.16.0.64 255.255.224.0

redistribute bgp 50 metric 1000 subnets

RTD#

router ospf 100

summary-address 172.16.0.96 255.255.224.0

redistribute bgp 20 metric 1000 subnets

This causes RTA to generate one external route 172.16.0.64 255.255.224.0 and causes RTD to generate 172.16.0.96 255.255.224.0.

Note that the summary-address command has no effect if used on RTB because RTB does not perform the redistribution into OSPF.

Stub Areas

OSPF allows certain areas to be configured as stub areas. External networks, such as those redistributed from other protocols into OSPF, are not allowed to be flooded into a stub area.

Routing from these areas to the outside world is based on a default route. Stub area configuration reduces the topological database size inside an area and reduces the memory requirements of routers inside that area.

An area could be qualified a stub when there is a single exit point from that area or if routing to outside of the area does not have to take an optimal path.

The latter description is an indication that a stub area that has multiple exit points also has one or more area border routers which inject a default into that area.

Routing to the outside world could take a sub-optimal path to the destination out of the area via an exit point which is farther to the destination than other exit points.

Other stub area restrictions are that a stub area cannot be used as a transit area for virtual links. Also, an ASBR cannot be internal to a stub area.

These restrictions are made because a stub area is mainly configured not to carry external routes and any of these situations cause external links to be injected in that area. The backbone cannot be configured as stub.

All OSPF routers inside a stub area have to be configured as stub routers. When an area is configured as stub, all interfaces that belong to that area exchange Hello packets with a flag that indicates that the interface is stub.

Actually this is just a bit in the Hello packet (E bit) that gets set to 0. All routers that have a common segment have to agree on that flag. Otherwise, then they do not become neighbors and routing does not take effect.

An extension to stub areas is called totally stubby areas. Cisco indicates this with the addition of a no-summary keyword to the stub area configuration.

A totally stubby area is one that blocks external routes and summary routes (inter-area routes) from entrance into the area.

This way, intra-area routes and the default of 0.0.0.0 are the only routes injected into that area.

- The command that configures an area as stub is:  area <area-id> stub [no-summary]

- The command that configures a default-cost into an area is: area area-id default-cost cost

If the cost is not set with that command, a cost of 1 is advertised by the ABR.

Assume that area 2 is to be configured as a stub area. This example shows the routing table of RTE before and after area 2 stub configuration.

RTC#

interface Ethernet 0

ip address 203.0.113.141 255.255.255.0

interface Serial1

ip address 203.0.113.151 255.255.255.252

router ospf 10

network 203.0.113.150 0.0.0.255 area 2

network 203.0.113.140 0.0.0.255 area 0

RTE#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

203.0.113.150 255.255.255.252 is subnetted, 1 subnets

C       203.0.113.150 is directly connected, Serial0

O IA 203.0.113.140 [110/74] via 203.0.113.151, 00:06:31, Serial0

198.51.100.1 is variably subnetted, 2 subnets, 2 masks

O E2    172.16.0.64 255.255.192.0

[110/10] via 203.0.113.151, 00:00:29, Serial0

O IA    172.16.0.63 255.255.255.252

[110/84] via 203.0.113.151, 00:03:57, Serial0

172.16.0.108 255.255.255.240 is subnetted, 1 subnets

O       172.16.0.208 [110/74] via 203.0.113.151, 00:00:10, Serial0

RTE has learned the inter-area routes (O IA) 203.0.113.140 and 172.16.0.63 and it has learned the intra-area route (O) 172.16.0.208 and the external route (O E2) 172.16.0.64.

To configure area 2 as stub:

RTC#

interface Ethernet 0

ip address 203.0.113.141 255.255.255.0

interface Serial1

ip address 203.0.113.151 255.255.255.252

router ospf 10

network 203.0.113.150 0.0.0.255 area 2

network 203.0.113.140 0.0.0.255 area 0

area 2 stub

RTE#

interface Serial1

ip address 203.0.113.152 255.255.255.252

router ospf 10

network 203.0.113.150 0.0.0.255 area 2

area 2 stub

Note that the stub command is configured on RTE also, otherwise RTE never becomes a neighbor to RTC. The default cost was not set, so RTC advertises 0.0.0.0 to RTE with a metric of 1.

RTE#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is 203.0.113.151 to network 0.0.0.0

203.0.113.150 255.255.255.252 is subnetted, 1 subnets

C       203.0.113.150 is directly connected, Serial0

O IA 203.0.113.140 [110/74] via 203.0.113.151, 00:26:58, Serial0

198.51.100.1 255.255.255.252 is subnetted, 1 subnets

O IA    172.16.0.63 [110/84] via 203.0.113.151, 00:26:59, Serial0

172.16.0.108 255.255.255.240 is subnetted, 1 subnets

O       172.16.0.208 [110/74] via 203.0.113.151, 00:26:59, Serial0

O*IA 0.0.0.0 0.0.0.0 [110/65] via 203.0.113.151, 00:26:59, Serial0

Note that all the routes show up except the external routes which were replaced by a default route of 0.0.0.0. The cost of the route happened to be 65 (64 for a T1 line + 1 advertised by RTC).

We now configure area 2 to be totally stubby, and change the default cost of 0.0.0.0 to 10.

RTC#

interface Ethernet 0

ip address 203.0.113.141 255.255.255.0

interface Serial1

ip address 203.0.113.151 255.255.255.252

router ospf 10

network 203.0.113.150 0.0.0.255 area 2

network 203.0.113.140 0.0.0.255 area 0

area 2 stub no-summary

area 2 default cost 10

RTE#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

203.0.113.150 255.255.255.252 is subnetted, 1 subnets

C       203.0.113.150 is directly connected, Serial0

172.16.0.108 255.255.255.240 is subnetted, 1 subnets

O       172.16.0.208 [110/74] via 203.0.113.151, 00:31:27, Serial0

O*IA 0.0.0.0 0.0.0.0 [110/74] via 203.0.113.151, 00:00:00, Serial0

Note that the only routes that show up are the intra-area routes (O) and the default-route 0.0.0.0. The external and inter-area routes have been blocked.

The cost of the default route is now 74 (64 for a T1 line + 10 advertised by RTC). No configuration is needed on RTE in this case.

The area is already stub, and the no-summary command does not affect the Hello packet at all as the stub command does.

Redistribute Routes into OSPF

Redistribute routes into OSPF from other routing protocols or from static causes these routes to become OSPF external routes. To redistribute routes into OSPF, use this command in router configuration mode:

redistribute protocol [process-id] [metric value] [metric-type value] [route-map map-tag] [subnets]

Note: This command must be on one line.

The protocol and process-id are the protocol that we inject into OSPF and its process-id if it exits. The metric is the cost which we are assign to the external route.

If no metric is specified, OSPF puts a default value of 20 when routes are redistributed from all protocols except BGP routes, which get a metric of 1. The metric-type is discussed in the next paragraph.

The route-map is a method used to control the redistribution of routes between routing domains. The format of a route map is:

route-map map-tag [[permit | deny] | [sequence-number]]

With route redistribution into OSPF, only routes that are not subnetted are redistributed if the subnets keyword is not specified.

E1 vs. E2 External Routes

External routes fall under two categories, external type 1 and external type 2. The difference between the two is in the way the cost (metric) of the route is calculated.

The cost of a type 2 route is always the external cost, irrespective of the interior cost to reach that route.

A type 1 cost is the addition of the external cost and the internal cost used to reach that route.

A type 1 route is always preferred over a type 2 route for the same destination.

As the this diagram shows, RTA redistributes two external routes into OSPF. N1 and N2 both have an external cost of x. The only difference is that N1 is redistributed into OSPF with a metric-type 1 and N2 is redistributed with a metric-type 2.

If we track the routes as they flow from Area 1 to Area 0, the cost to reach N2 as seen from RTB or RTC is always  x. The internal cost along the way is not considered. On the other hand, the cost to reach N1 is incremented by the internal cost. The cost is x+y as seen from RTB and x+y+z as seen from RTC.

If the external routes are both type 2 routes and the external costs to the destination network are equal, then the path with the lowest cost to the ASBR is selected as the best path.

Unless otherwise specified, the default external type given to external routes is type 2.

Suppose we added two static routes which point to E0 on RTC: 10.0.0.16 255.255.255.0 (the /24 notation indicates a 24 bit mask which starts from the far left) and 198.51.100.1 255.255.0.0.

This shows the different behaviors when different parameters are used in the redistribute command on RTC:

RTC#

interface Ethernet0

ip address 203.0.113.142 255.255.255.0

interface Serial1

ip address 203.0.113.151 255.255.255.252

router ospf 10

redistribute static

network 203.0.113.150 0.0.0.255 area 2

network 203.0.113.140 0.0.0.255 area 0

ip route 10.0.0.16 255.255.255.0 Ethernet0

ip route 198.51.100.1 255.255.0.0 Ethernet0

RTE#

interface Serial0

ip address 203.0.113.152 255.255.255.252

router ospf 10

network 203.0.113.150 0.0.0.255 area 2

The output of show ip route on RTE:

RTE#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

203.0.113.150 255.255.255.252 is subnetted, 1 subnets

C       203.0.113.150 is directly connected, Serial0

O IA 203.0.113.140 [110/74] via 203.0.113.151, 00:02:31, Serial0

O E2 198.51.100.1 [110/20] via 203.0.113.151, 00:02:32, Serial0

Note that the only external route that has appeared is 198.51.100.1, because we did not use the subnet keyword. Remember that if the subnet keyword is not used, only routes that are not subnetted are redistributed. In our case 10.0.0.16 is a class A route that is subnetted and it did not get redistributed. Since the metric keyword was not used (or a default-metric statement under router OSPF), the cost allocated to the external route is 20 (the default is 1 for BGP).

redistribute static metric 50 subnets

RTE#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M

- mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

10.0.0.16 255.255.255.0 is subnetted, 1 subnets

O E2    10.0.0.16 [110/50] via 203.0.113.151, 00:00:02, Serial0

203.0.113.150 255.255.255.252 is subnetted, 1 subnets

C       203.0.113.150 is directly connected, Serial0

O IA 203.0.113.140 [110/74] via 203.0.113.151, 00:00:02, Serial0

O E2 198.51.100.1 [110/50] via 203.0.113.151, 00:00:02, Serial0

Note that 10.0.0.16 has shown up now and the cost to external routes is 50. Since the external routes are of type 2 (E2), the internal cost has not been added. Suppose now, we change the type to E1:

redistribute static metric 50 metric-type 1 subnets

RTE#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

10.0.0.16 255.255.255.0 is subnetted, 1 subnets

O E1    10.0.0.16 [110/114] via 203.0.113.151, 00:04:20, Serial0

203.0.113.150 255.255.255.252 is subnetted, 1 subnets

C       203.0.113.150 is directly connected, Serial0

O IA 203.0.113.140 [110/74] via 203.0.113.151, 00:09:41, Serial0

O E1 198.51.100.1 [110/114] via 203.0.113.151, 00:04:21, Serial0

Note that the type has changed to E1 and the cost has been incremented by the internal cost of S0 which is 64, the total cost is 64+50=114.

Assume that we add a route map to the RTC configuration:

RTC#

interface Ethernet0

ip address 203.0.113.142 255.255.255.0

interface Serial1

ip address 203.0.113.151 255.255.255.252

router ospf 10

redistribute static metric 50 metric-type 1 subnets route-map STOPUPDATE

network 203.0.113.150 0.0.0.255 area 2

network 203.0.113.140 0.0.0.255 area 0

ip route 10.0.0.16 255.255.255.0 Ethernet0

ip route 198.51.100.1 255.255.0.0 Ethernet0

access-list 1 permit 198.51.100.1 0.0.255.255

route-map STOPUPDATE permit 10

match ip address 1

The route map only permits 198.51.100.1 to be redistributed into OSPF and denies the rest. This is why 10.0.0.16 does not show up in the RTE routing table anymore.

RTE#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

203.0.113.150 255.255.255.252 is subnetted, 1 subnets

C       203.0.113.150 is directly connected, Serial0

O IA 203.0.113.140 [110/74] via 203.0.113.151, 00:00:04, Serial0

O E1 198.51.100.1 [110/114] via 203.0.113.151, 00:00:05, Serial0

Redistribute OSPF into Other Protocols

Use of a Valid Metric

Whenever you redistribute OSPF into other protocols, you have to respect the rules of those protocols. In particular, the metric applied must match the metric used by that protocol.

For example, the RIP metric is a hop count between 1 and 16, where 1 indicates that a network is one hop away and 16 indicates that the network is unreachable. On the other hand IGRP and EIGRP require a metric of the form:

default-metric bandwidth delay reliability loading mtu

VLSM

Another issue to consider is VLSM (Variable Length Subnet Guide)(Appendix C). OSPF can carry multiple subnet information for the same major net, but other protocols such as RIP and IGRP (EIGRP is OK with VLSM) cannot.

If the same major net crosses the boundaries of an OSPF and RIP domain, VLSM information redistributed into RIP or IGRP is lost and static routes have to be configured in the RIP or IGRP domains. This example illustrates this problem.

In this diagram, RTE runsOSPF and RTA runs RIP. RTC does the the redistribution between the two protocols. The problem is that the class C network 203.0.113.150 is variably subnetted, it has two different masks 255.255.255.252 and 255.255.255.192.

Here is the configuration and the routing tables of RTE and RTA:

RTA#

interface Ethernet0

ip address 203.0.113.68 255.255.255.192

router rip

network 203.0.113.150

RTC#

interface Ethernet0

ip address 203.0.113.67 255.255.255.192

interface Serial1

ip address 203.0.113.151 255.255.255.252

router ospf 10

redistribute rip metric 10 subnets

network 203.0.113.150 0.0.0.255 area 0

router rip

redistribute ospf 10 metric 2

network 203.0.113.150

RTE#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

203.0.113.150 is variably subnetted, 2 subnets, 2 masks

C       203.0.113.150 255.255.255.252 is directly connected, Serial0

O       203.0.113.64 255.255.255.192

[110/74] via 203.0.113.151, 00:15:55, Serial0

RTA#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

203.0.113.150 255.255.255.192 is subnetted, 1 subnets

C       203.0.113.64 is directly connected, Ethernet0

Note that RTE has recognized that 203.0.113.150 has two subnets while RTA thinks that it has only one subnet (the one configured on the interface).

Information about subnet 203.0.113.150 255.255.255.252 is lost in the RIP domain. In order to reach that subnet, a static route needs to be configured on RTA:

RTA#

interface Ethernet0

ip address 203.0.113.68 255.255.255.192

router rip

network 203.0.113.150

ip route 203.0.113.150 255.255.255.0 203.0.113.67

This way RTA is able to reach the other subnets.

Mutual Redistribution

Mutual redistribution between protocols must be done very carefully and in a controlled manner. Incorrect configuration could lead to potential looping of routing information.

A rule of thumb for mutual redistribution is not to allow information learned from a protocol to be injected back into the same protocol.

Passive interfaces and distribute lists must be applied on the redistribution routers. To filter information with link-state protocols such as OSPF is a difficult.

Distribute-list out works on the ASBR to filter redistributed routes into other protocols. Distribute-list in works on any router to prevent routes from the routing table, but it does not prevent link-state packets from propagation; downstream routers would still have the routes.

It is better to avoid any OSPF filter as much as possible if filters can be applied on the other protocols to prevent loops.

To illustrate, suppose RTA, RTC, and RTE run RIP. RTC and RTA also run OSPF. Both RTC and RTA do redistribution between RIP and OSPF.

If you do not want the RIP from RTE to be injected into the OSPF domain, put a passive interface for RIP on E0 of RTC. However, you have allowed the RIP from RTA to be injected into OSPF. Here is the outcome:

Note: Do not use this configuration.

RTE#

interface Ethernet0

ip address 203.0.113.151 255.255.255.192

interface Serial0

ip address 203.0.113.152 255.255.255.192

router rip

network 203.0.113.150

RTC#

interface Ethernet0

ip address 203.0.113.67 255.255.255.192

interface Serial1

ip address 203.0.113.151 255.255.255.192

router ospf 10

redistribute rip metric 10 subnets

network 203.0.113.150 0.0.0.255 area 0

router rip

redistribute ospf 10 metric 2

passive-interface Ethernet0

network 203.0.113.150

RTA#

interface Ethernet0

ip address 203.0.113.68 255.255.255.192

router ospf 10

redistribute rip metric 10 subnets

network 203.0.113.150 0.0.0.255 area 0

router rip

redistribute ospf 10 metric 1

network 203.0.113.150

RTC#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

203.0.113.150 255.255.255.192 is subnetted, 4 subnets

C       203.0.113.150 is directly connected, Serial1

C       203.0.113.64 is directly connected, Ethernet0

R       203.0.113.151 [120/1] via 203.0.113.68, 00:01:08, Ethernet0

[120/1] via 203.0.113.152, 00:00:11, Serial1

O       203.0.113.151 [110/20] via 203.0.113.68, 00:21:41, Ethernet0

Note that RTC has two paths to reach 203.0.113.151 subnet: Serial 1 and Ethernet 0 (E0 is obviously the wrong path). This happened because RTC gave that entry to RTA via OSPF and RTA gave it back via RIP because RTA did not learn it via RIP.

This example is a very small scale of loops that can occur because of an incorrect configuration. In large networks this situation gets even more aggravated.

In order to fix the situation in our example, do not send RIP on RTA Ethernet 0 via a passive interface. This is not suitable in case some routers on the Ethernet are RIP only routers.

In this case, you could allow RTC to send RIP on the Ethernet; this way RTA does not send it back on the wire because of split horizon (this does not work on NBMA media if split horizon is off).

Split horizon does not allow updates to be sent back on the same interface they were learned from (via the same protocol).

Another good method is to apply distribute-lists on RTA to deny subnets learned via OSPF from the return to RIP on the Ethernet. The latter is used:

RTA#

interface Ethernet0

ip address 203.0.113.68 255.255.255.192

router ospf 10

redistribute rip metric 10 subnets

network 203.0.113.150 0.0.0.255 area 0

router rip

redistribute ospf 10 metric 1

network 203.0.113.150

distribute-list 1 out ospf 10

And the output of the RTC routing table would be:

RTF#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set

203.0.113.150 255.255.255.192 is subnetted, 4 subnets

C       203.0.113.150 is directly connected, Serial1

C       203.0.113.64 is directly connected, Ethernet0

R       203.0.113.151 [120/1] via 203.0.113.152, 00:00:19, Serial1

O       203.0.113.151 [110/20] via 203.0.113.68, 00:21:41, Ethernet0

Inject Defaults into OSPF

An autonomous system boundary router (ASBR) can be forced to generate a default route into the OSPF domain. A router becomes an ASBR whenever routes are redistributed into an OSPF domain.

However, an ASBR does not, by default, generate a default route into the OSPF routing domain.

To have OSPF generate a default route use:

default-information originate [always] [metric metric-value] [metric-type type-value] [route-map map-name]

Note: This command must be on one line.

There are two ways to generate a default. The first is to advertise 0.0.0.0 inside the domain, but only if the ASBR itself already has a default route. The second is to advertise 0.0.0.0 regardless whether the ASBR has a default route. The latter can be set with the keyword always.

Use caution when the always keyword is used. If your router advertises a default (0.0.0.0) inside the domain and does not have a default itself or a path to reach the destinations, routing is broken.

The metric and metric type are the cost and type (E1 or E2) assigned to the default route. The route map specifies the set of conditions that need to be satisfied in order for the default to be generated.

Assume that RTE injects a default-route 0.0.0.0 into RIP. RTC has a gateway of last resort of 203.0.113.152. RTC does not propagate the default to RTA until we configure RTC with a default-information originate command.

RTC#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is 203.0.113.152 to network 0.0.0.0

203.0.113.150 255.255.255.192 is subnetted, 4 subnets

C       203.0.113.150 is directly connected, Serial1

C       203.0.113.64 is directly connected, Ethernet0

R       203.0.113.151 [120/1] via 203.0.113.152, 00:00:17, Serial1

O       203.0.113.151 [110/20] via 203.0.113.68, 2d23, Ethernet0

R*   0.0.0.0 0.0.0.0 [120/1] via 203.0.113.152, 00:00:17, Serial1

[120/1] via 203.0.113.68, 00:00:32, Ethernet0

RTC#

interface Ethernet0

ip address 203.0.113.67 255.255.255.192

interface Serial1

ip address 203.0.113.151 255.255.255.192

router ospf 10

redistribute rip metric 10 subnets

network 203.0.113.150 0.0.0.255 area 0

default-information originate metric 10

router rip

redistribute ospf 10 metric 2

passive-interface Ethernet0

network 203.0.113.150

RTA#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is 203.0.113.67 to network 0.0.0.0

203.0.113.150 255.255.255.192 is subnetted, 4 subnets

O       203.0.113.150 [110/74] via 203.0.113.67, 2d23, Ethernet0

C       203.0.113.64 is directly connected, Ethernet0

O E2    203.0.113.151 [110/10] via 203.0.113.67, 2d23, Ethernet0

C       203.0.113.151 is directly connected, Ethernet1

O*E2 0.0.0.0 0.0.0.0 [110/10] via 203.0.113.67, 00:00:17, Ethernet0

Note that RTA has learned 0.0.0.0 as an external route with metric 10. The gateway of last resort is set to 203.0.113.67 as expected.

OSPF Design Tips

The OSPF RFC (1583) did not specify any guidelines for the number of routers in an area or number the of neighbors per segment or what is the best way to architect a network.

There are different approaches to OSPF network design. The important thing to remember is that any protocol can fail under pressure.

The idea is not to challenge the protocol but rather to work with it in order to get the best behavior.

Number of Routers per Area

The maximum number of routers per area depends on several factors:

- What kind of area do you have?

- What kind of CPU power do you have in that area?

- What kind of media?

- Does OSPF run in NBMA mode?

- Is your NBMA network meshed?

- Do you have a lot of external LSAs in the network?

- Are other areas well summarized?

For this reason, it is difficult to specify a maximum number of routers per area. Consult your local sales or system engineer for specific network design help.

Number of Neighbors

The number of routers connected to the same LAN is also important. Each LAN has a DR and BDR that build adjacencies with all other routers.

The fewer neighbors that exist on the LAN, the smaller the number of adjacencies a DR or BDR have to build. That depends on how much power your router has.

You could always change the OSPF priority to select your DR. Avoid the same router as DR on more than one segment.

If DR selection is based on the highest RID, then one router could accidently become a DR over all segments it is connected to. This router requires extra effort while other routers are idle.

Number of Areas per ABR

ABRs keep a copy of the database for all areas they service. If a router is connected to five areas for example, it has to keep a list of five different databases.

The number of areas per ABR is a number that is dependent on many factors, which includetype of area (normal, stub, NSSA), ABR CPU power, number of routes per area, and number of external routes per area.

For this reason, a specific number of areas per ABR cannot be recommended. It is not preferable to overload an ABR when you can always spread the areas over other routers.

This diagram shows the difference between one ABR which holds five different databases (which includes area 0) and two ABRs which hold three databases each.

These are just guidelines. More areas configured per ABR result in lower performance. In some cases, the lower performance can be tolerated.

Full Mesh vs. Partial Mesh

The combination of low bandwidth and too many link-states (associated with Non Broadcast Multi-Access (NBMA) clouds such as Frame Relay or X.25) are always a challenge

A partial mesh topology has proven to behave much better than a full mesh. A carefully laid out point-to-point or point-to-multipoint network works much better than multipoint networks that have to deal with DR issues.

Memory Issues

It is not easy to figure out the memory needed for a particular OSPF configuration. Memory issues usually come up when too many external routes are injected in the OSPF domain.

A backbone area with 40 routers and a default route to the outside world would have less memory issues compared with a backbone area with 4 routers and 33,000 external routes injected into OSPF.

Memory is also conserved through good OSPF design. Summarization at the area border routers and use of stub areas could further minimize the number of routes exchanged.

The total memory used by OSPF is the sum of the memory used in the routing table (show ip route summary) and the memory used in the link-state database.

The numbers are a rule of thumb estimate. Each entry in the routing table consumes between approximately 200 and 280 bytes plus 44 bytes per extra path.

Each LSA consumes a 100 byte overhead plus the size of the actual link state advertisement, possibly another 60 to 100 bytes (for router links, this depends on the number of interfaces on the router).

This must be added to memory used by other processes and by the Cisco IOS® itself. To know the exact number, run show memory with and without OSPF turned on.

The difference in the processor memory used would be the answer (keep a backup copy of the configs).

Normally, a routing table with less than 500K bytes could be accommodated with 2 to 4 MB RAM; Large networks with greater than 500K need 8 to 16 MB, or 32 to 64 MB if full routes are injected from the Internet.

Summary

The OSPF protocol defined in RFC 1583, provides a high functionality open protocol that allows multiple vendor networks to communicate with the TCP/IP protocol family.

Some of the benefits of OSPF are, fast convergence, VLSM, authentication, hierarchical segmentation, route summarization, and aggregation which are needed to handle large and complicated networks.

Appendix A: Link-State Database Synchronization

In this diagram, routers on the same segment go through a series of states before they form a successful adjacency. The neighbor and DR election are done via the Hello protocol.

When a router sees itself in its neighbor Hello packet, the state transitions to "2-Way". At that point DR and BDR election is performed on multi-access segments.

A router continues to form an adjacency with a neighbor if either of the two routers is a DR or BDR or they are connected via a point-to-point or virtual link.

In the Exstart state,the two neighbors form a primary/secondary relationship where they agree on a initial sequence number. The sequence number is used to detect old or duplicate Link-State Advertisements (LSA).

In the Exchange state, Database Description Packets (DD)  gets exchanged. These are abbreviated link-state advertisements in the form of link-state headers. The header supplies enough information to identify a link.

The primary node sends DD packets which are acknowledged with DD packets from the secondary node. All adjacencies in exchange state or greater are used by the flood procedure.

These adjacencies are fully capable of transmission and reception of all types of OSPF routing protocol packets.

In the Load state, link-state request packets are sent to neighbors, to ask for more recent advertisements that have been discovered but not yet received. Each router builds a list of required LSAs to bring its adjacency up to date.

A Retransmission List is maintained to make sure that every LSA is acknowledged. To specify the number of seconds between link-state advertisement retransmissions for the adjacency you can use:

ip ospf retransmit-interval seconds

Link-state update packets are sent in response to request packets. The link-state update packets are flooded over all adjacencies.

In the Full state, the neighbor routers are fully adjacent. The databases for a common area are an exact match between adjacent routers.

Each LSA has an age field that gets periodically incremented while it is contained in the database or as it gets flooded throughout the area. When an LSA reaches a Maxage it gets flushed from the database if that LSA is not on any neighbors retransmission list.

Link-State Advertisements

Link-state advertisements are broken into five types. Router Links (RL) are generated by all routers. These links describe the state of the router interfaces inside a particular area.

These links are only flooded inside the router area. Network Links (NL) are generated by a DR of a particular segment; these are an indication of the routers connected to that segment.

Summary Links (SL) are the inter-area links (type 3); these links list the networks inside other areas but still belong to the autonomous system.

Summary links are injected by the ABR from the backbone into other areas and from other areas into the backbone. These links are used for aggregation between areas.

Other types of summary links are the asbr-summary links. These are type 4 links that point to the ASBR. This is to make sure that all routers know the way to exit the autonomous system.

The last type is type 5, External Links (EL), these are injected by the ASBR into the domain.

The previous diagram illustrates the different link types. RTA generates a router link (RL) into area 1, and it also generates a network link (NL) since it happens the be the DR on that particular segment.

RTB is an ABR, and it generates RL into area 1 and area 0. RTB also generates summary links into area 1 and area 0. These links are the list of networks that are interchanged between the two areas.

An ASBR summary link is also injected by RTB into area 1. This is an indication of the existence of RTD, the autonomous system boundary router (ASBR).

Similarly RTC, which is another ABR, generates RL for area 0 and area 2, and a SL (3) into area 2 (since it does not announce any ASBR), and a SL (3,4) into area 0 to announce RTD.

RTD generates a RL for area 2 and generates an EL for external routes learned via BGP. The external routers are flooded all over the domain.

This table is a summary of the link state advertisements.

LS Type

Advertisement Description

1

Router Link advertisements. Generated by each router for each area it belongs to. They describe the states of the router link to the area. These are only flooded within a particular area.

2

Network Link advertisements. Generated by Designated Routers. They describe the set of routers attached to a particular network. Flooded in the area that contains the network.

3 or 4

Summary Link advertisements. Generated by Area Border routers. They describe inter-area (between areas) routes. Type 3 describes routes to networks, also used to aggregate routes. Type 4 describes routes to ASBR.

5

AS external link advertisements. Originated by ASBR. They describe routes to destinations external to the AS. Flooded all over except stub areas.

If you look at the OSPF database in detail, with show ip ospf database detail, there are different keywords such as Link-Data, Link-ID, and Link-state ID. These terms become inconsistent as the value of each depends on the link state type and the link-type.

We review this terminology and provide a detailed example on the OSPF database as seen from the router.

The Link-State ID basically defines the identity of the link-state depending on the LS type.

Router Links are identified by the router ID (RID) of the router that originated the advertisement.

Network Links are identified by the relative IP address of the DR. This makes sense because Network Links are originated by the Designated Router.

Summary Links (type 3) are identified by the IP network numbers of the destinations to which they point.

ASBR Summary Links (Summary Links type 4) are identified by the RID of the ASBR.

External Links are identified by the IP network numbers of the external destinations to which they point. This table summarizes this information:

LS Type

Link State ID (In the high level view of the database when a router is referenced, this is called Link ID)

1

The origin Router ID (RID).

2

The IP interface address of the network Designated Router.

3

The destination network number.

4

The router ID of the described AS boundary router.

5

The external network number.

The different links available:

Stub network links: This term has nothing to do with stub areas. A stub segment is a segment that has one router only attached to it.

An Ethernet or Token Ring segment that has one attached router is considered a link to a stub network. A loopback interface is also considered a link to stub network with a 255.255.255.255 mask (Host route).

Point-to-point links: These could be physical or logical (subinterfaces) point-to-point serial link connections. These links could be numbered (an IP address is configured on the link) or unnumbered.

Transit links: These are interfaces connected to networks that have more than one router attached, hence the name transit.

Virtual links: These are logical links that connect areas that do not have physical connections to the backbone. Virtual links are treated as numbered point-to-point links.

The link-ID is an identification of the link itself. This is different for each link type.

A transit link is identified by the IP address of the DR on that link.

A numbered point-to-point link is identified by the RID of the neighbor router on the point-to-point link.

Virtual links are identical to point-to-point links.

Stub network links are identified by the IP address of the interface to the stub network. This table summarizes this information:

Link Type

Link ID (This applies to individual Links)

Point-to-Point

Neighbor Router ID

Link to transit network

Interface address of DR

Link to stub network (In case of loopback mask is 255.255.255.255)

Network/subnet number

Virtual Link

Neighbor Router ID

The Link Data is the IP address of the link, except for stub network where the link data is the network mask.

Link Type

Link Data

Stub network

Network Mask

Other networks (applies to router links only)

Router - associated IP interface address

Finally, an Advertising Router is the RID of the router that has sent the LSA.

OSPF Database Example

Given this network diagram, the configurations, and the IP route tables, here are different ways to understand the OSPF database.

RTA#

interface Loopback0

ip address 203.0.113.41 255.255.255.255

interface Ethernet0

ip address 203.0.113.68 255.255.255.192

interface Ethernet1

ip address 203.0.113.151 255.255.255.192

router ospf 10

network 203.0.113.100 0.0.255.255 area 0

RTA#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is 203.0.113.67 to network 0.0.0.0

203.0.113.128 255.255.255.192 is subnetted, 1 subnets

O E2    203.0.113.1288 [110/10] via 203.0.113.67, 00:00:50, Ethernet0

203.0.113.30 255.255.255.255 is subnetted, 1 subnets

C       203.0.113.41 is directly connected, Loopback0

203.0.113.150 255.255.255.192 is subnetted, 3 subnets

O IA    203.0.113.150 [110/74] via 203.0.113.67, 00:00:50, Ethernet0

C       203.0.113.64 is directly connected, Ethernet0

C       203.0.113.151 is directly connected, Ethernet1

O*E2 0.0.0.0 0.0.0.0 [110/10] via 203.0.113.67, 00:00:50, Ethernet0

RTE#

ip subnet-zero

interface Ethernet0

ip address 203.0.113.16 255.255.255.192

interface Serial0

ip address 203.0.113.152 255.255.255.192

router ospf 10

redistribute rip metric 10 subnets

network 203.0.113.150 0.0.0.63 area 1

default-information originate metric 10

router rip

network 203.0.113.128

ip route 0.0.0.0 0.0.0.0 Ethernet0

RTE#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is 0.0.0.0 to network 0.0.0.0

203.0.113.128 255.255.255.192 is subnetted, 1 subnets

C       203.0.113.1288 is directly connected, Ethernet0

203.0.113.30 is variably subnetted, 2 subnets, 2 masks

O IA    203.0.113.41 255.255.255.255

[110/75] via 203.0.113.151, 00:16:31, Serial0

203.0.113.150 255.255.255.192 is subnetted, 3 subnets

C       203.0.113.150 is directly connected, Serial0

O IA    203.0.113.64 [110/74] via 203.0.113.151, 00:16:31, Serial0

O IA    203.0.113.151 [110/84] via 203.0.113.151, 00:16:31, Serial0

S*   0.0.0.0 0.0.0.0 is directly connected, Ethernet0

RTC#

ip subnet-zero

interface Ethernet0

ip address 203.0.113.67 255.255.255.192

interface Serial1

ip address 203.0.113.151 255.255.255.192

router ospf 10

network 203.0.113.64 0.0.0.63 area 0

network 203.0.113.150 0.0.0.63 area 1

RTF#show ip route

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP

i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is 203.0.113.152 to network 0.0.0.0

203.0.113.128 255.255.255.192 is subnetted, 1 subnets

O E2    203.0.113.1288 [110/10] via 203.0.113.152, 04:49:05, Serial1

203.0.113.30 255.255.255.255 is subnetted, 1 subnets

O       203.0.113.41 [110/11] via 203.0.113.68, 04:49:06, Ethernet0

203.0.113.150 255.255.255.192 is subnetted, 3 subnets

C       203.0.113.150 is directly connected, Serial1

C       203.0.113.64 is directly connected, Ethernet0

O       203.0.113.151 [110/20] via 203.0.113.68, 04:49:06, Ethernet0

O*E2 0.0.0.0 0.0.0.0 [110/10] via 203.0.113.152, 04:49:06, Serial1

General View of the Database

RTC#show ip ospf database

OSPF Router with ID (203.0.113.67) (Process ID 10)

Router Link States (Area 1)

Link ID         ADV Router      Age    Seq#       Checksum Link count

203.0.113.67   203.0.113.67   48     0x80000008 0xB112   2

203.0.113.16  203.0.113.16  212    0x80000006 0x3F44   2

Summary Net Link States (Area 1)

Link ID         ADV Router      Age    Seq#       Checksum

203.0.113.41   203.0.113.67   602    0x80000002 0x90AA

203.0.113.64   203.0.113.67   620    0x800000E9 0x3E3C

203.0.113.151  203.0.113.67   638    0x800000E5 0xA54E

Router Link States (Area 0)

Link ID         ADV Router      Age    Seq#       Checksum Link count

203.0.113.41   203.0.113.41   179    0x80000029 0x9ADA   3

203.0.113.67   203.0.113.67   675    0x800001E2 0xDD23   1

Net Link States (Area 0)

Link ID         ADV Router      Age    Seq#       Checksum

203.0.113.68   203.0.113.41   334    0x80000001 0xB6B5

Summary Net Link States (Area 0)

Link ID         ADV Router      Age    Seq#       Checksum

203.0.113.150    203.0.113.67   792    0x80000002 0xAEBD

Summary ASB Link States (Area 0)

Link ID         ADV Router      Age    Seq#       Checksum

203.0.113.16  203.0.113.67   579    0x80000001 0xF9AF

AS External Link States

Link ID         ADV Router      Age    Seq#       Checksum Tag

0.0.0.0         203.0.113.16  1787   0x80000001 0x98CE   10

203.0.113.1288  203.0.113.16  5      0x80000002 0x93C4   0

This is a general look at the whole OSPF database. The database is listed in accord with the areas. In this case, we look at the RTC database which is an ABR. Both area 1 and area 0's databases are listed.

Area 1 is composed of router links and summary links. No network links exist because no DR exists on any of the segments in area 1. No Summary ASBR links exist in area 1 because the only ASBR happens to be in area 0.

External links do not belong to any particular area as they are flooded all over. Note that all the links are the cumulative links collected from all routers in an area.

Concentrate on the database in area 0. The Link-ID indicated here is actually the Link-State ID. This is a representation of the whole router, not a particular link. This seems ambiguous.

Remember that this high level Link-ID (actually Link-State ID) represents the whole router and not just a link.

Router Links

Router Link States (Area 0)

Link ID         ADV Router      Age    Seq#       Checksum Link count

203.0.113.41   203.0.113.41   179    0x80000029 0x9ADA   3

203.0.113.67   203.0.113.67   675    0x800001E2 0xDD23   1

Start with the router links. There are two entries listed for 203.0.113.41 and 203.0.113.67, these are the RIDs of the two routers in area 0. The number of links in area 0 for each router is also indicated. RTA has three links to area 0 and RTC has one link. A detailed view of the RTC router links:

RTC#show ip ospf database router 203.0.113.67

OSPF Router with ID (203.0.113.67) (Process ID 10)

Router Link States (Area 1)

LS age: 1169

Options: (No TOS-capability)

LS Type: Router Links

Link State ID: 203.0.113.67

Advertising Router: 203.0.113.67

LS Seq Number: 80000008

Checksum: 0xB112

Length: 48

Area Border Router

Number of Links: 2

Link connected to: another Router (point-to-point)

(Link ID) Neighbor Router ID: 203.0.113.16

(Link Data) Router Interface address: 203.0.113.151

Number of TOS metrics: 0

TOS 0 Metrics: 64

Link connected to: a Stub Network

(Link ID) Network/subnet number: 203.0.113.150

(Link Data) Network Mask: 255.255.255.192

Number of TOS metrics: 0

TOS 0 Metrics: 64

One thing to note here is that OSPF generates an extra stub link for each point-to-point interface. Do not get confused if you see the link count larger than the number of physical interfaces.

Router Link States (Area 0)

LS age: 1227

Options: (No TOS-capability)

LS Type: Router Links

Link State ID: 203.0.113.67

Advertising Router: 203.0.113.67

LS Seq Number: 80000003

Checksum: 0xA041

Length: 36

Area Border Router

Number of Links: 1

Link connected to: a Transit Network

(Link ID) Designated Router address: 203.0.113.68

(Link Data) Router Interface address: 203.0.113.67

Number of TOS metrics: 0

TOS 0 Metrics: 10

Note that the Link ID is equal to the IP address (not the RID) of the attached DR; in this case it is 203.0.113.68. The Link Data is the RTC IP address.

Network Links

Net Link States (Area 0)

Link ID         ADV Router      Age    Seq#       Checksum

203.0.113.68   203.0.113.41   334    0x80000001 0xB6B5

One network link is listed, indicated by the interface IP address (not the RID) of the DR, in this case 203.0.113.68. A detailed view of this entry:

RTC#show ip ospf database network

OSPF Router with ID (203.0.113.67) (Process ID 10)

Net Link States (Area 0)

Routing Bit Set on this LSA

LS age: 1549

Options: (No TOS-capability)

LS Type: Network Links

Link State ID: 203.0.113.68 (address of Designated Router)

Advertising Router: 203.0.113.41

LS Seq Number: 80000002

Checksum: 0xB4B6

Length: 32

Network Mask: 255.255.255.192

Attached Router: 203.0.113.41

Attached Router: 203.0.113.67

Note that the network link lists the RIDs of the routers attached to the transit network; in this case the RIDs of RTA and RTC are listed.

Summary Links

Summary Net Link States (Area 0)

Link ID         ADV Router      Age    Seq#       Checksum

203.0.113.150    203.0.113.67   792    0x80000002 0xAEBD

Area 0 has one summary link represented by the IP network address of the

link 203.0.113.150. This link was injected by the ABR RTC from area 1 into

area 0. A detailed view of this summary link, summary links for

area 1 are not listed here:

RTC#show ip ospf database summary (area 1 is not listed)

Summary Net Link States (Area 0)

LS age: 615

Options: (No TOS-capability)

LS Type: Summary Links(Network)

Link State ID: 203.0.113.150 (summary Network Number)

Advertising Router: 203.0.113.67

LS Seq Number: 80000003

Checksum: 0xACBE

Length: 28

Network Mask: 255.255.255.192 TOS: 0  Metric: 64

Summary ASBR Links

Summary ASB Link States (Area 0)

Link ID         ADV Router      Age    Seq#       Checksum

203.0.113.16  203.0.113.67   579    0x80000001 0xF9AF

This is an indication of who the ASBR is. In this case the ASBR is RTE represented by its RID 203.0.113.16. The advertising router for this entry into area 0 is RTC with RID 203.0.113.67. A detailed view of the summary ASBR entry:

RTC#show ip ospf database asbr-summary

OSPF Router with ID (203.0.113.67) (Process ID 10)

Summary ASB Link States (Area 0)

LS age: 802

Options: (No TOS-capability)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 203.0.113.16 (AS Boundary Router address)

Advertising Router: 203.0.113.67

LS Seq Number: 80000003

Checksum: 0xF5B1

Length: 28

Network Mask: 0.0.0.0 TOS: 0  Metric: 64

External Links

AS External Link States

Link ID         ADV Router      Age    Seq#       Checksum Tag

0.0.0.0         203.0.113.16  1787   0x80000001 0x98CE   10

203.0.113.1288  203.0.113.16  5      0x80000002 0x93C4   0

We have two external Links, the first one is the 0.0.0.0 injected into OSPF via the default-information originate command.

The other entry is network 203.0.113.128 8 which is injected into OSPF by redistribution.

The router advertising these networks is 203.0.113.16, the RID of RTE.

This is the detailed view of the external routes:

RTC#show ip ospf database external

OSPF Router with ID (203.0.113.67) (Process ID 10)

AS External Link States

Routing Bit Set on this LSA

LS age: 208

Options: (No TOS-capability)

LS Type: AS External Link

Link State ID: 0.0.0.0 (External Network Number )

Advertising Router: 203.0.113.16

LS Seq Number: 80000002

Checksum: 0x96CF

Length: 36

Network Mask: 0.0.0.0

Metric Type: 2 (Larger than any link state path)

TOS: 0

Metric: 10

Forward Address: 0.0.0.0

External Route Tag: 10

Routing Bit Set on this LSA

LS age: 226

Options: (No TOS-capability)

LS Type: AS External Link

Link State ID: 203.0.113.1288 (External Network Number)

Advertising Router: 203.0.113.16

LS Seq Number: 80000002

Checksum: 0x93C4

Length: 36

Network Mask: 255.255.255.192

Metric Type: 2 (Larger than any link state path)

TOS: 0

Metric: 10

Forward Address: 0.0.0.0

External Route Tag: 0

Note the forward address. Whenever this address is 0.0.0.0 it indicates that the external routes are reachable via the advertising router, in this case 203. 250.16.130.

This is why the identity of the ASBR is injected by ABRs into other areas which use ASBR summary links.

This forward address is not always 0.0.0.0. In some cases, it could be the IP address of another router on the same segment. This diagram illustrates this situation:

In this situation RTB, runs BGP with RTA, and OSPF with the rest of the domain. RTA does not run OSPF. RTB redistributes BGP routes into OSPF.

In accord with OSPF, RTB is an ASBR advertising external routes. The forwarding address in this case is set to 172.16.0.11 and not to the advertising router (0.0.0.0) RT B.

There is no need to make the extra hop. Routers inside the OSPF domain must reach the forwarding address via OSPF in order for the external routes to be put in the IP routing table.

If the forwarding address is reached via some other protocol or no t accessible, the external entries would be in the database but not in the IP routing table.

Another situation would arise if both RTB and RTC are ASBRs (RTC runs BGP with RTA). In this situation, in order to eliminate the duplication of the effort, one of the two routers do not advertise (flushes) the external routes. The router with the higher RID prevails.

The Full Database

This is a list of the whole database as an exercise. You are now able to review and explain each entry:

RTC#show ip ospf database router

OSPF Router with ID (203.0.113.67) (Process ID 10)

Router Link States (Area 1)

LS age: 926

Options: (No TOS-capability)

LS Type: Router Links

Link State ID: 203.0.113.67

Advertising Router: 203.0.113.67

LS Seq Number: 80000035

Checksum: 0x573F

Length: 48

Area Border Router

Number of Links: 2

Link connected to: another Router (point-to-point)

(Link ID) Neighbor Router ID: 203.0.113.16

(Link Data) Router Interface address: 203.0.113.151

Number of TOS metrics: 0

TOS 0 Metrics: 64

Link connected to: a Stub Network

(Link ID) Network/subnet number: 203.0.113.150

(Link Data) Network Mask: 255.255.255.192

Number of TOS metrics: 0

TOS 0 Metrics: 64

Routing Bit Set on this LSA

LS age: 958

Options: (No TOS-capability)

LS Type: Router Links

Link State ID: 203.0.113.16

Advertising Router: 203.0.113.16

LS Seq Number: 80000038

Checksum: 0xDA76

Length: 48

AS Boundary Router

Number of Links: 2

Link connected to: another Router (point-to-point)

(Link ID) Neighbor Router ID: 203.0.113.67

(Link Data) Router Interface address: 203.0.113.152

Number of TOS metrics: 0

TOS 0 Metrics: 64

Link connected to: a Stub Network

(Link ID) Network/subnet number: 203.0.113.150

(Link Data) Network Mask: 255.255.255.192

Number of TOS metrics: 0

TOS 0 Metrics: 64

Router Link States (Area 0)

Routing Bit Set on this LSA

LS age: 1107

Options: (No TOS-capability)

LS Type: Router Links

Link State ID: 203.0.113.41

Advertising Router: 203.0.113.41

LS Seq Number: 8000002A

Checksum: 0xC0B0

Length: 60

AS Boundary Router

Number of Links: 3

Link connected to: a Stub Network

(Link ID) Network/subnet number: 203.0.113.41

(Link Data) Network Mask: 255.255.255.255

Number of TOS metrics: 0

TOS 0 Metrics: 1

Link connected to: a Stub Network

(Link ID) Network/subnet number: 203.0.113.151

(Link Data) Network Mask: 255.255.255.192

Number of TOS metrics: 0

TOS 0 Metrics: 10

Link connected to: a Transit Network

(Link ID) Designated Router address: 203.0.113.68

(Link Data) Router Interface address: 203.0.113.68

Number of TOS metrics: 0

TOS 0 Metrics: 10

LS age: 1575

Options: (No TOS-capability)

LS Type: Router Links

Link State ID: 203.0.113.67

Advertising Router: 203.0.113.67

LS Seq Number: 80000028

Checksum: 0x5666

Length: 36

Area Border Router

Number of Links: 1

Link connected to: a Transit Network

(Link ID) Designated Router address: 203.0.113.68

(Link Data) Router Interface address: 203.0.113.67

Number of TOS metrics: 0

TOS 0 Metrics: 10

RTC#show ip ospf database network

OSPF Router with ID (203.0.113.67) (Process ID 10)

Net Link States (Area 0)

Routing Bit Set on this LSA

LS age: 1725

Options: (No TOS-capability)

LS Type: Network Links

Link State ID: 203.0.113.68 (address of Designated Router)

Advertising Router: 203.0.113.41

LS Seq Number: 80000026

Checksum: 0x6CDA

Length: 32

Network Mask: 255.255.255.192

Attached Router: 203.0.113.41

Attached Router: 203.0.113.67

RTC#show ip ospf database summary

OSPF Router with ID (203.0.113.67) (Process ID 10)

Summary Net Link States (Area 1)

LS age: 8

Options: (No TOS-capability)

LS Type: Summary Links(Network)

Link State ID: 203.0.113.41 (summary Network Number)

Advertising Router: 203.0.113.67

LS Seq Number: 80000029

Checksum: 0x42D1

Length: 28

Network Mask: 255.255.255.255 TOS: 0  Metric: 11

LS age: 26

Options: (No TOS-capability)

LS Type: Summary Links(Network)

Link State ID: 203.0.113.64 (summary Network Number)

Advertising Router: 203.0.113.67

LS Seq Number: 80000030

Checksum: 0xB182

Length: 28

Network Mask: 255.255.255.192 TOS: 0  Metric: 10

LS age: 47

Options: (No TOS-capability)

LS Type: Summary Links(Network)

Link State ID: 203.0.113.151 (summary Network Number)

Advertising Router: 203.0.113.67

LS Seq Number: 80000029

Checksum: 0x1F91

Length: 28

Network Mask: 255.255.255.192 TOS: 0  Metric: 20

Summary Net Link States (Area 0)

LS age: 66

Options: (No TOS-capability)

LS Type: Summary Links(Network)

Link State ID: 203.0.113.150 (summary Network Number)

Advertising Router: 203.0.113.67

LS Seq Number: 80000025

Checksum: 0x68E0

Length: 28

Network Mask: 255.255.255.192 TOS: 0  Metric: 64

RTC#show ip ospf asbr-summary

OSPF Router with ID (203.0.113.67) (Process ID 10)

Summary ASB Link States (Area 0)

LS age: 576

Options: (No TOS-capability)

LS Type: Summary Links(AS Boundary Router)

Link State ID: 203.0.113.16 (AS Boundary Router address)

Advertising Router: 203.0.113.67

LS Seq Number: 80000024

Checksum: 0xB3D2

Length: 28

Network Mask: 0.0.0.0 TOS: 0  Metric: 64

RTC#show ip ospf database external

OSPF Router with ID (203.0.113.67) (Process ID 10)

AS External Link States

Routing Bit Set on this LSA

LS age: 305

Options: (No TOS-capability)

LS Type: AS External Link

Link State ID: 0.0.0.0 (External Network Number)

Advertising Router: 203.0.113.16

LS Seq Number: 80000001

Checksum: 0x98CE

Length: 36

Network Mask: 0.0.0.0

Metric Type: 2 (Larger than any link state path)

TOS: 0

Metric: 10

Forward Address: 0.0.0.0

External Route Tag: 10

Routing Bit Set on this LSA

LS age: 653

Options: (No TOS-capability)

LS Type: AS External Link

Link State ID: 203.0.113.1288 (External Network Number)

Advertising Router: 203.0.113.16

LS Seq Number: 80000024

Checksum: 0x4FE6

Length: 36

Network Mask: 255.255.255.192

Metric Type: 2 (Larger than any link state path)

TOS: 0

Metric: 10

Forward Address: 0.0.0.0

External Route Tag: 0

Appendix B: OSPF and IP Multicast Address

OSPF used IP multicast to exchange Hello packets and Link State Updates. An IP multicast address is implemented with class D addresses. A class D address ranges from 224.0.0.0 to 239.255.255.255.

Some special IP multicast addresses are reserved for OSPF:

- 224.0.0.5: All OSPF routers must be able to transmit and listen to this address.

- 224.0.0.6: All DR and BDR routers must be able to transmit and listen to this address.

The mapping between IP multicast addresses and MAC addresses has the rule:

For multiaccess networks that support multicast, the low order 23 bits of the IP address are used as the low order bits of the MAC multicast address 01-005E-00-00- 00. For example:

- 224.0.0.5 would be mapped to 01-00-5E-00-00-05

- 224.0.0.6 would be mapped to 01-00-5E-00-00-06

OSPF uses broadcast on Token Ring networks.

Appendix C: Variable Length Subnet Masks (VLSM)

This is a binary/decimal conversion chart:

0000

0001

0010

0011

0100

0101

0110

0111

0

0000

16

0000

32

0000

48

0000

64

0000

80

0000

96

0000

112

0000

1

0001

17

0001

33

0001

49

0001

65

0001

81

0001

97

0001

113

0001

2

0010

18

0010

34

0010

50

0010

66

0010

82

0010

98

0010

114

0010

3

0011

19

0011

35

0011

51

0011

67

0011

83

0011

99

0011

115

0011

4

0100

20

0100

36

0100

52

0100

68

0100

84

0100

100

0100

116

0100

5

0101

21

0101

37

0101

53

0101

69

0101

85

0101

101

0101

117

0101

6

0110

22

0110

38

0110

54

0110

70

0110

86

0110

102

0110

118

0110

7

0111

23

0111

39

0111

55

0111

71

0111

87

0111

103

0111

119

0111

8

1000

24

1000

40

1000

56

1000

72

1000

88

1000

104

1000

120

1000

9

1001

25

1001

41

1001

57

1001

73

1001

89

1001

105

1001

121

1001

10

1010

26

1010

42

1010

58

1010

74

1010

90

1010

106

1010

122

1010

11

1011

27

1011

43

1011

59

1011

75

1011

91

1011

107

1011

123

1011

12

1100

28

1100

44

1100

60

1100

76

1100

92

1100

108

1100

124

1100

13

1101

29

1101

45

1101

61

1101

77

1101

93

1101

109

1101

125

1101

14

1110

30

1110

46

1110

62

1110

78

1110

94

1110

110

1110

126

1110

15

1111

31

1111

47

1111

63

1111

79

1111

95

1111

111

1111

127

1111

1000

1001

1010

1011

1100

1101

1110

1111

128

0000

144

0000

160

0000

176

0000

192

0000

208

0000

224

0000

240

0000

129

0001

145

0001

161

0001

177

0001

193

0001

209

0001

225

0001

241

0001

130

0010

146

0010

162

0010

178

0010

194

0010

210

0010

226

0010

242

0010

131

0011

147

0011

163

0011

179

0011

195

0011

211

0011

227

0011

243

0011

132

0100

148

0100

164

0100

180

0100

196

0100

212

0100

228

0100

244

0100

133

0101

149

0101

165

0101

181

0101

197

0101

213

0101

229

0101

245

0101

134

0110

150

0110

166

0110

182

0110

198

0110

214

0110

230

0110

246

0110

135

0111

151

0111

167

0111

183

0111

199

0111

215

0111

231

0111

247

0111

136

1000

152

1000

168

1000

184

1000

200

1000

216

1000

232

1000

248

1000

137

1001

153

1001

169

1001

185

1001

201

1001

217

1001

233

1001

249

1001

138

1010

154

1010

170

1010

186

1010

202

1010

218

1010

234

1010

250

1010

139

1011

155

1011

171

1011

187

1011

203

1011

219

1011

235

1011

251

1011

140

1100

156

1100

172

1100

188

1100

204

1100

220

1100

236

1100

252

1100

141

1101

157

1101

173

1101

189

1101

205

1101

221

1101

237

1101

253

1101

142

1110

158

1110

174

1110

190

1110

206

1110

222

1110

238

1110

254

1110

143

1111

159

1111

175

1111

191

1111

207

1111

223

1111

239

1111

255

1111

The idea behind variable length subnet masks is to offer more flexibility to divide a major net into multiple subnets and remain able to maintain an adequate number of hosts in each subnet.

Without VLSM, one subnet mask only can be applied to a major network. This restricts the number of hosts given the number of subnets required.

If you pick the mask such that you have enough subnets, you are not able to allocate enough hosts in each subnet. The same is true for the hosts; a mask that allows enough hosts does not provide enough subnet space.

For example, suppose you were assigned a class C network 192.168.0.0 and you need to divide that network into three subnets with 100 hosts in one subnet and 50 hosts for the remainder of the subnets.

Ignore the two end limits 0 and 255, and you have theoretically available to you 256 addresses (192.168.0.0 - 192.168.0.255). This cannot be done without VLSM.

There are a handful of subnet masks that can be used; note that a mask must have a contiguous number of ones that start from the left and the rest of the bits are all 0s.

-252 (1111 1100) The address space is divided into 64.

-248 (1111 1000) The address space is divided into 32.

-240 (1111 0000) The address space is divided into 16.

-224 (1110 0000) The address space is divided into 8.

-192 (1100 0000) The address space is divided into 4.

-128 (1000 0000) The address space is divided into 2.

Without VLSM you have the choice to use mask 255.255.255.128 and divide the addresses into 2 subnets with 128 hosts each or use 255.255.255.192 and divide the space into 4 subnets with 64 hosts each.

This does not meet the requirement. If you use multiple masks, you can use mask 128 and further subnet the second chunk of addresses with mask 192.

This table shows how you have divided the address space:

Use caution in the allocation of IP addresses to each mask. Once you assign an IP address to the router or to a host, you have used up the whole subnet for that segment.

For example, if you assign 192.168.0.10 255.255.255.128 to E2, the whole range of addresses between 192.168.0.0 and 192.168.0.127 is consumed by E2.

In the same way if you assign 192.168.0.160 255.255.255.128 to E2, the whole range of addresses between 192.168.0.128 and 192.168.0.255 is consumed by the E2 segment.

This is an illustration of how the router interprets these addresses. Remember that any time you use a mask different than the natural mask, for instance to create a subnet, the router complains if the combination IP address and mask result in a subnet zero.

Use the ip subnet-zero command on the router in order to resolve this issue.

RTA#

ip subnet-zero

interface Ethernet2

ip address 192.168.0.10 255.255.255.128

interface Ethernet3

ip address 192.168.0.160 255.255.255.192

interface Ethernet4

ip address 192.168.0.226 255.255.255.192

RTA#show ip route connected

192.168.0.0  is variably subnetted, 3 subnets, 2 masks

C       192.168.0.0  255.255.255.128 is directly connected, Ethernet2

C       192.168.0.128 255.255.255.192 is directly connected, Ethernet3

C       192.168.0.192 255.255.255.192 is directly connected, Ethernet4

Related Information

- Cisco IOS® IP Routing: OSPF Command Reference

- OSPF and MTU

- OSPF neighbors are stuck in exstart and exchange state due to MTU mismatch

- OSPF Support Page

- OSPF: Frequently Asked Questions

- Technical Support & Documentation - Cisco Systems