Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 5.x

Hello Packet

OSPFv2 routers periodically send Hello packets on every OSPF-enabled interface. The hello interval determines how frequently the router sends these Hello packets and is configured per interface. OSPFv2 uses Hello packets for the following tasks:

• Neighbor discovery
• Keepalives
• Bidirectional communications
• Designated router election (see the “Designated Routers” section)

The Hello packet contains information about the originating OSPFv2 interface and router, including the assigned OSPFv2 cost of the link, the hello interval, and optional capabilities of the originating router. An OSPFv2 interface that receives these Hello packets determines if the settings are compatible with the receiving interface settings. Compatible interfaces are considered neighbors and are added to the neighbor table (see the “Neighbors” section).

Hello packets also include a list of router IDs for the routers that the originating interface has communicated with. If the receiving interface sees its own router ID in this list, then bidirectional communication has been established between the two interfaces.

OSPFv2 uses Hello packets as a keepalive message to determine if a neighbor is still communicating. If a router does not receive a Hello packet by the configured dead interval (usually a multiple of the hello interval), then the neighbor is removed from the local neighbor table.

Neighbors

An OSPFv2 interface must have a compatible configuration with a remote interface before the two can be considered neighbors. The two OSPFv2 interfaces must match the following criteria:

• Hello interval
• Dead interval
• Area ID (see the “Areas” section)
• Authentication
• Optional capabilities

If there is a match, the following information is entered into the neighbor table:

• Neighbor ID—The router ID of the neighbor.
• Priority—Priority of the neighbor. The priority is used for designated router election (see the “Designated Routers” section).
• State—Indication of whether the neighbor has just been heard from, is in the process of setting up bidirectional communications, is sharing the link-state information, or has achieved full adjacency.
• Dead time—Indication of the time since the last Hello packet was received from this neighbor.
• IP Address—The IP address of the neighbor.
• Designated Router—Indication of whether the neighbor has been declared as the designated router or as the backup designated router (see the “Designated Routers” section).
• Local interface—The local interface that received the Hello packet for this neighbor.

Adjacency

Not all neighbors establish adjacency. Depending on the network type and designated router establishment, some neighbors become fully adjacent and share LSAs with all their neighbors, while other neighbors do not. For more information, see the “Designated Routers” section.

Adjacency is established using Database Description packets, Link State Request packets, and Link State Update packets in OSPF. The Database Description packet includes just the LSA headers from the link-state database of the neighbor (see the “Link-State Database” section). The local router compares these headers with its own link-state database and determines which LSAs are new or updated. The local router sends a Link State Request packet for each LSA that it needs new or updated information on. The neighbor responds with a Link State Update packet. This exchange continues until both routers have the same link-state information.

Designated Routers

Networks with multiple routers present a unique situation for OSPF. If every router floods the network with LSAs, the same link-state information is sent from multiple sources. Depending on the type of network, OSPFv2 might use a single router, the designated router ( DR), to control the LSA floods and represent the network to the rest of the OSPFv2 area (see the “Areas” section). If the DR fails, OSPFv2 selects a backup designated router (BDR). If the DR fails, OSPFv2 uses the BDR.

Network types are as follows:

• Point-to-point—A network that exists only between two routers. All neighbors on a point-to-point network establish adjacency and there is no DR.
• Broadcast—A network with multiple routers that can communicate over a shared medium that allows broadcast traffic, such as Ethernet. OSPFv2 routers establish a DR and BDR that controls LSA flooding on the network. OSPFv2 uses the well-known IPv4 multicast addresses 224.0.0.5 and a MAC address of 0100.5300.0005 to communicate with neighbors.

The DR and BDR are selected based on the information in the Hello packet. When an interface sends a Hello packet, it sets the priority field and the DR and BDR field if it knows who the DR and BDR are. The routers follow an election procedure based on which routers declare themselves in the DR and BDR fields and the priority field in the Hello packet. As a final tie breaker, OSPFv2 chooses the highest router IDs as the DR and BDR.

All other routers establish adjacency with the DR and the BDR and use the IPv4 multicast address 224.0.0.6 to send LSA updates to the DR and BDR. Figure 6-1 shows this adjacency relationship between all routers and the DR.

DRs are based on a router interface. A router might be the DR for one network and not for another network on a different interface.

Figure 6-1 DR in Multi-Access Network

Areas

Figure 6-2 OSPFv2 Areas

The ABR has a separate link-state database for each area to which it connects. The ABR sends Network Summary (type 3) LSAs (see the “Route Summarization” section) from one connected area to the backbone area. The backbone area sends summarized information about one area to another area. In Figure 6-2, Area 0 sends summarized information about Area 5 to Area 3.

OSPFv2 defines one other router type: the autonomous system boundary router (ASBR). This router connects an OSPFv2 area to another autonomous system. An autonomous system is a network controlled by a single technical administration entity. OSPFv2 can redistribute its routing information into another autonomous system or receive redistributed routes from another autonomous system. For more information, see the “Advanced Features” section.)

Link-State Advertisements

OSPFv2 uses link-state advertisements (LSAs) to build its routing table.

This section includes the following topics:

• LSA Types
• Link Cost
• Flooding and LSA Group Pacing
• Link-State Database
• Opaque LSAs

LSA Types

Table 6-1 shows the LSA types supported by Cisco NX-OS.

Link Cost

Each OSPFv2 interface is assigned a link cost. The cost is an arbitrary number. By default, Cisco NX-OS assigns a cost that is the configured reference bandwidth divided by the interface bandwidth. By default, the reference bandwidth is 40 Gb/s. The link cost is carried in the LSA updates for each link.

Flooding and LSA Group Pacing

When an OSPFv2 router receives an LSA, it forwards that LSA out every OSPF-enabled interface, flooding the OSPFv2 area with this information. This LSA flooding guarantees that all routers in the network have identical routing information. LSA flooding depends on the OSPFv2 area configuration (see the “Areas” section). The LSAs are flooded based on the link-state refresh time (every 30 minutes by default). Each LSA has its own link-state refresh time.

You can control the flooding rate of LSA updates in your network by using the LSA group pacing feature. LSA group pacing can reduce high CPU or buffer usage. This feature groups LSAs with similar link-state refresh times to allow OSPFv2 to pack multiple LSAs into an OSPFv2 Update message.

By default, LSAs with link-state refresh times within 10 seconds of each other are grouped together. You should lower this value for large link-state databases or raise it for smaller databases to optimize the OSPFv2 load on your network.

Link-State Database

Each router maintains a link-state database for the OSPFv2 network. This database contains all the collected LSAs, and includes information on all the routes through the network. OSPFv2 uses this information to calculate the bast path to each destination and populates the routing table with these best paths.

LSAs are removed from the link-state database if no LSA update has been received within a set interval, called the MaxAge. Routers flood a repeat of the LSA every 30 minutes to prevent accurate link-state information from being aged out. Cisco NX-OS supports the LSA grouping feature to prevent all LSAs from refreshing at the same time. For more information, see the “Flooding and LSA Group Pacing” section.

Opaque LSAs

Opaque LSAs allow you to extend OSPF functionality. Opaque LSAs consist of a standard LSA header followed by application-specific information. This information might be used by OSPFv2 or by other applications. OSPFv2 uses Opaque LSAs to support OSPFv2 Graceful Restart capability (see the “High Availability and Graceful Restart” section). Three Opaque LSA types are defined as follows:

• LSA type 9—Flooded to the local network.
• LSA type 10—Flooded to the local area.
• LSA type 11—Flooded to the local autonomous system.

OSPFv2 and the Unicast RIB

OSPFv2 runs the Dijkstra shortest path first algorithm on the link-state database. This algorithm selects the best path to each destination based on the sum of all the link costs for each link in the path. The resultant shortest path for each destination is then put in the OSPFv2 route table. When the OSPFv2 network is converged, this route table feeds into the unicast RIB. OSPFv2 communicates with the unicast RIB to do the following:

• Add or remove routes
• Handle route redistribution from other protocols
• Provide convergence updates to remove stale OSPFv2 routes and for stub router advertisements (see the “OSPFv2 Stub Router Advertisements” section)

OSPFv2 also runs a modified Dijkstra algorithm for fast recalculation for summary and external (type 3, 4, 5, and 7) LSA changes.

Authentication

You can configure authentication on OSPFv2 messages to prevent unauthorized or invalid routing updates in your network. Cisco NX-OS supports two authentication methods:

• Simple password authentication
• MD5 authentication digest

You can configure the OSPFv2 authentication for an OSPFv2 area or per interface.

Simple Password Authentication

Simple password authentication uses a simple clear-text password that is sent as part of the OSPFv2 message. The receiving OSPFv2 router must be configured with the same clear-text password to accept the OSPFv2 message as a valid route update. Because the password is in clear text, anyone who can watch traffic on the network can learn the password.

MD5 Authentication

You should use MD5 authentication to authenticate OSPFv2 messages. You configure a password that is shared at the local router and all remote OSPFv2 neighbors. For each OSPFv2 message, Cisco NX-OS creates an MD5 one-way message digest based on the message itself and the encrypted password. The interface sends this digest with the OSPFv2 message. The receiving OSPFv2 neighbor validates the digest using the same encrypted password. If the message has not changed, the digest calculation is identical and the OSPFv2 message is considered valid.

MD5 authentication includes a sequence number with each OSPFv2 message to ensure that no message is replayed in the network.

Advanced Features

Cisco NX-OS supports advanced OSPFv2 features that enhance the usability and scalability of OSPFv2 in the network. This section includes the following topics:

• Stub Area
• Not-So-Stubby Area
• Virtual Links
• Route Redistribution
• Route Summarization
• High Availability and Graceful Restart
• OSPFv2 Stub Router Advertisements
• Multiple OSPFv2 Instances
• SPF Optimization
• BFD
• Virtualization Support

Stub Area

You can limit the amount of external routing information that floods an area by making it a stub area. A stub area is an area that does not allow AS External (type 5) LSAs (see the “Link-State Advertisements” section). These LSAs are usually flooded throughout the local autonomous system to propagate external route information. Stub areas have the following requirements:

• All routers in the stub area are stub routers. See the “Stub Routing” section.
• No ASBR routers exist in the stub area.
• You cannot configure virtual links in the stub area.

Figure 6-3 shows an example of an OSPFv2 autonomous system where all routers in area 0.0.0.10 have to go through the ABR to reach external autonomous systems. Area 0.0.0.10 can be configured as a stub area.

Figure 6-3 Stub Area

Stub areas use a default route for all traffic that must go through the backbone area to the external autonomous system. The default route is 0.0.0.0 for IPv4.

Not-So-Stubby Area

A Not-so-Stubby Area ( NSSA) is similar to a stub area, except that an NSSA allows you to import autonomous system external routes within an NSSA using redistribution. The NSSA ASBR redistributes these routes and generates NSSA External (type 7) LSAs that it floods throughout the NSSA. You can optionally configure the ABR that connects the NSSA to other areas to translate this NSSA External LSA to AS External (type 5) LSAs. The ABR then floods these AS External LSAs throughout the OSPFv2 autonomous system. Summarization and filtering are supported during the translation. See the “Link-State Advertisements” section for information about NSSA External LSAs.

You can, for example, use NSSA to simplify administration if you are connecting a central site using OSPFv2 to a remote site that is using a different routing protocol. Before NSSA, the connection between the corporate site border router and a remote router could not be run as an OSPFv2 stub area because routes for the remote site could not be redistributed into a stub area. With NSSA, you can extend OSPFv2 to cover the remote connection by defining the area between the corporate router and remote router as an NSSA (see the “Configuring NSSA” section).

The backbone Area 0 cannot be an NSSA.

Virtual Links

Virtual links allow you to connect an OSPFv2 area ABR to a backbone area ABR when a direct physical connection is not available. Figure 6-4 shows a virtual link that connects Area 3 to the backbone area through Area 5.

Figure 6-4 Virtual Links

You can also use virtual links to temporarily recover from a partitioned area, which occurs when a link within the area fails, isolating part of the area from reaching the designated ABR to the backbone area.

Route Redistribution

OSPFv2 can learn routes from other routing protocols by using route redistribution. See the “Route Redistribution” section. You configure OSPFv2 to assign a link cost for these redistributed routes or a default link cost for all redistributed routes.

Route redistribution uses route maps to control which external routes are redistributed. You must configure a route map with the redistribution to control which routes are passed into OSPFv2. A route map allows you to filter routes based on attributes such as the destination, origination protocol, route type, route tag, and so on. You can use route maps to modify parameters in the AS External (type 5) and NSSA External (type 7) LSAs before these external routes are advertised in the local OSPFv2 autonomous system. See “Configuring Route Policy Manager,” for information about configuring route maps.

Route Summarization

Because OSPFv2 shares all learned routes with every OSPF-enabled router, you might want to use route summarization to reduce the number of unique routes that are flooded to every OSPF-enabled router. Route summarization simplifies route tables by replacing more-specific addresses with an address that represents all the specific addresses. For example, you can replace 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24 with one summary address, 10.1.0.0/16.

Typically, you would summarize at the boundaries of area border routers (ABRs). Although you could configure summarization between any two areas, it is better to summarize in the direction of the backbone so that the backbone receives all the aggregate addresses and injects them, already summarized, into other areas. The two types of summarization are as follows:

• Inter-area route summarization
• External route summarization

You configure inter-area route summarization on ABRs, summarizing routes between areas in the autonomous system. To take advantage of summarization, you should assign network numbers in areas in a contiguous way to be able to lump these addresses into one range.

External route summarization is specific to external routes that are injected into OSPFv2 using route redistribution. You should make sure that external ranges that are being summarized are contiguous. Summarizing overlapping ranges from two different routers could cause packets to be sent to the wrong destination. Configure external route summarization on ASBRs that are redistributing routes into OSPF.

When you configure a summary address, Cisco NX-OS automatically configures a discard route for the summary address to prevent routing black holes and route loops.

High Availability and Graceful Restart

Cisco NX-OS provides a multilevel high-availability architecture. OSPFv2 supports stateful restart, which is also referred to as non-stop routing (NSR). If OSPFv2 experiences problems, it attempts to restart from its previous run-time state. The neighbors do not register any neighbor event in this case. If the first restart is not successful and another problem occurs, OSPFv2 attempts a graceful restart.

A graceful restart, or nonstop forwarding (NSF), allows OSPFv2 to remain in the data forwarding path through a process restart. When OSPFv2 needs to perform a graceful restart, it sends a link-local opaque (type 9) LSA, called a grace LSA (see the “Opaque LSAs” section). This restarting OSPFv2 platform is called NSF capable.

The grace LSA includes a grace period, which is a specified time that the neighbor OSPFv2 interfaces hold onto the LSAs from the restarting OSPFv2 interface. (Typically, OSPFv2 tears down the adjacency and discards all LSAs from a down or restarting OSPFv2 interface.) The participating neighbors, which are called NSF helpers, keep all LSAs that originate from the restarting OSPFv2 interface as if the interface was still adjacent.

When the restarting OSPFv2 interface is operational again, it rediscovers its neighbors, establishes adjacency, and starts sending its LSA updates again. At this point, the NSF helpers recognize that the graceful restart has finished.

Stateful restart is used in the following scenarios:

• First recovery attempt after the process experiences problems
• ISSU
• User-initiated switchover using the system switchover command

Graceful restart is used in the following scenarios:

• Second recovery attempt after the process experiences problems within a 4-minute interval
• Manual restart of the process using the restart ospf command
• Active supervisor removal
• Active supervisor reload using the reload module active-sup command

OSPFv2 Stub Router Advertisements

You can configure an OSPFv2 interface to act as a stub router using the OSPFv2 Stub Router Advertisements feature. Use this feature when you want to limit the OSPFv2 traffic through this router, such as when you want to introduce a new router to the network in a controlled manner or limit the load on a router that is already overloaded. You might also want to use this feature for various administrative or traffic engineering reasons.

OSPFv2 stub router advertisements do not remove the OSPFv2 router from the network topology, but they do prevent other OSPFv2 routers from using this router to route traffic to other parts of the network. Only the traffic that is destined for this router or directly connected to this router is sent.

OSPFv2 stub router advertisements mark all stub links (directly connected to the local router) to the cost of the local OSPFv2 interface. All remote links are marked with the maximum cost (0xFFFF).

Multiple OSPFv2 Instances

Cisco NX-OS supports multiple instances of the OSPFv2 protocol that run on the same node. You cannot configure multiple instances over the same interface. By default, every instance uses the same system router ID. You must manually configure the router ID for each instance if the instances are in the same OSPFv2 autonomous system.

SPF Optimization

Cisco NX-OS optimizes the SPF algorithm in the following ways:

• Partial SPF for Network (type 2) LSAs, Network Summary (type 3) LSAs, and AS External (type 5) LSAs—When there is a change on any of these LSAs, Cisco NX-OS performs a faster partial calculation rather than running the whole SPF calculation.
• SPF timers—You can configure different timers for controlling SPF calculations. These timers include exponential backoff for subsequent SPF calculations. The exponential backoff limits the CPU load of multiple SPF calculations.

BFD

This feature supports bidirectional forwarding detection (BFD). BFD is a detection protocol that provides fast forwarding-path failure detection times. BFD provides subsecond failure detection between two adjacent devices and can be less CPU-intensive than protocol hello messages because some of the BFD load can be distributed onto the data plane on supported modules. See the Cisco Nexus 7000 Series NX-OS Interfaces Configuration Guide, Release 5.x, for more information.

Virtualization Support

OSPFv2 supports virtual routing and forwarding (VRF) instances. VRFs exist within virtual device contexts (VDCs). By default, Cisco NX-OS places you in the default VDC and default VRF unless you specifically configure another VDC and VRF. You can have up to four instances of OSPFv2 in a VDC.

Each OSPFv2 instance can support multiple VRFs, up to the system limit. For more information, see the Cisco Nexus 7000 Series NX-OS Virtual Device Context Configuration Guide, Release 5.x, and see Chapter14, “Configuring Layer 3 Virtualization”

Enabling OSPFv2

You must enable the OSPFv2 feature before you can configure OSPFv2.

BEFORE YOU BEGIN

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. feature ospf

3. (Optional) show feature

4. (Optional) copy running-config startup-config

DETAILED STEPS

To disable the OSPFv2 feature and remove all associated configuration, use the no feature ospf command in configuration mode:

Creating an OSPFv2 Instance

The first step in configuring OSPFv2 is to create an OSPFv2 instance. You assign a unique instance tag for this OSPFv2 instance. The instance tag can be any string.

For more information about OSPFv2 instance parameters, see the “Configuring Advanced OSPFv2” section.

BEFORE YOU BEGIN

Ensure that you have enabled the OSPF feature (see the “Enabling OSPFv2” section).

Use the show ip ospf instance-tag command to verify that the instance tag is not in use.

OSPFv2 must be able to obtain a router identifier (for example, a configured loopback address) or you must configure the router ID option.

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. router-id ip-address

4. (Optional) show ip ospf instance-tag

5. (Optional) copy running-config startup-config

DETAILED STEPS

To remove the OSPFv2 instance and all associated configuration, use the no router ospf command in configuration mode.

Note This command does not remove the OSPF configuration in interface mode. You must manually remove any OSPFv2 commands configured in interface mode.

Configuring Optional Parameters on an OSPFv2 Instance

You can configure optional parameters for OSPF.

For more information about OSPFv2 instance parameters, see the “Configuring Advanced OSPFv2” section.

BEFORE YOU BEGIN

Ensure that you have enabled the OSPF feature (see the “Enabling OSPFv2” section).

OSPFv2 must be able to obtain a router identifier (for example, a configured loopback address) or you must configure the router ID option.

Ensure that you are in the correct VDC (or use the switchto vdc command).

DETAILED STEPS

You can configure the following optional parameters for OSPFv2 in router configuration mode:

This example shows how to create an OSPFv2 instance:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# copy running-config startup-config

Configuring Networks in OSPFv2

You can configure a network to OSPFv2 by associating it through the interface that the router uses to connect to that network (see the “Neighbors” section). You can add all networks to the default backbone area (Area 0), or you can create new areas using any decimal number or an IP address.

Note All areas must connect to the backbone area either directly or through a virtual link.

Note OSPF is not enabled on an interface until you configure a valid IP address for that interface.

BEFORE YOU BEGIN

Ensure that you have enabled the OSPF feature (see the “Enabling OSPFv2” section).

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. interface interface-type slot/port

3. ip address ip-prefix/length

4. ip router ospf instance-tag area area-id [ secondaries none ]

5. (Optional) show ip ospf instance-tag interface interface-type slot/por t

6. (Optional) copy running-config startup-config

DETAILED STEPS

You can configure the following optional parameters for OSPFv2 in interface configuration mode:

This example shows how to add a network area 0.0.0.10 in OSPFv2 instance 201:

switch# configure terminal

switch(config)# interface ethernet 1/2

switch(config-if)# ip address 192.0.2.1/16

switch(config-if)# ip router ospf 201 area 0.0.0.10

switch(config-if)# copy running-config startup-config

Use the show ip ospf interface command to verify the interface configuration. Use the show ip ospf neighbor command to see the neighbors for this interface.

Configuring Authentication for an Area

You can configure authentication for all networks in an area or for individual interfaces in the area. Interface authentication configuration overrides area authentication.

BEFORE YOU BEGIN

Ensure that you have enabled the OSPF feature (see the “Enabling OSPFv2” section).

Ensure that all neighbors on an interface share the same authentication configuration, including the shared authentication key.

Create the key chain for this authentication configuration. See the Cisco Nexus 7000 Series NX-OS Security Configuration Guide, Release 5.x.

Note For OSPFv2, the key identifier in the key key-id command supports values from 0 to 255 only.

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. area area-id authentication [ message-digest ]

4. interface interface-type slot/port

5. (Optional) ip ospf authentication-key [ 0 | 3 ] key

or

ip ospf message-digest-key key-id md5 [ 0 | 3 ] key

6. (Optional) show ip ospf instance-tag interface interface-type slot/por t

7. (Optional) copy running-config startup-config

DETAILED STEPS

Configuring Authentication for an Interface

You can configure authentication for individual interfaces in the area. Interface authentication configuration overrides area authentication.

BEFORE YOU BEGIN

Ensure that you have enabled the OSPF feature (see the “Enabling OSPFv2” section).

Ensure that all neighbors on an interface share the same authentication configuration, including the shared authentication key.

Create the key chain for this authentication configuration. See the Cisco Nexus 7000 Series NX-OS Security Configuration Guide, Release 5.x.

Note For OSPFv2, the key identifier in the key key-id command supports values from 0 to 255 only.

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. interface interface-type slot/port

3. ip ospf authentication [ message-diges t]

4. (Optional) ip ospf authentication key-chain key-id

5. (Optional) ip ospf authentication-key [ 0 | 3 | 7] key

6. (Optional) ip ospf message-digest-key key-id md5 [ 0 | 3 | 7 ] key

7. (Optional) show ip ospf instance-tag interface interface-type slot/por t

8. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to set an interface for simple, unencrypted passwords and set the password for Ethernet interface 1/2:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# exit

switch(config)# interface ethernet 1/2

switch(config-if)# ip router ospf 201 area 0.0.0.10

switch(config-if)# ip ospf authentication

switch(config-if)# ip ospf authentication-key 0 mypass

switch(config-if)# copy running-config startup-config

Configuring Filter Lists for Border Routers

You can separate your OSPFv2 domain into a series of areas that contain related networks. All areas must connect to the backbone area through an area border router (ABR). OSPFv2 domains can connect to external domains through an autonomous system border router (ASBR). See the “Areas” section.

ABRs have the following optional configuration parameters:

• Area range—Configures route summarization between areas. See the “Configuring Route Summarization” section.
• Filter list—Filters the Network Summary (type 3) LSAs that are allowed in from an external area.

ASBRs also support filter lists.

BEFORE YOU BEGIN

Ensure that you have enabled the OSPF feature (see the “Enabling OSPFv2” section).

Create the route map that the filter list uses to filter IP prefixes in incoming or outgoing Network Summary (type 3) LSAs. See Chapter16, “Configuring Route Policy Manager”

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. area area-id filter-list route-map map-name {in | out}

4. (Optional) show ip ospf policy statistics area id filter-list {in | out}

5. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to configure a filter list in area 0.0.0.10:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# area 0.0.0.10 filter-list route-map FilterLSAs in

switch(config-router)# copy running-config startup-config

Configuring Stub Areas

You can configure a stub area for part of an OSPFv2 domain where external traffic is not necessary. Stub areas block AS External (type 5) LSAs and limit unnecessary routing to and from selected networks. See the “Stub Area” section. You can optionally block all summary routes from going into the stub area.

BEFORE YOU BEGIN

Ensure that you have enabled the OSPF feature (see the “Enabling OSPFv2” section).

Ensure that there are no virtual links or ASBRs in the proposed stub area.

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. area area-id stub

4. (Optional) area area-id default-cost cost

5. (Optional) show ip ospf instance-tag

6. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to create a stub area:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# area 0.0.0.10 stub

switch(config-router)# copy running-config startup-config

Configuring a Totally Stubby Area

You can create a totally stubby area and prevent all summary route updates from going into the stub area.

To create a totally stubby area, use the following command in router configuration mode:

Configuring NSSA

You can configure an NSSA for part of an OSPFv2 domain where limited external traffic is required. For information about NSSAs, see the “Not-So-Stubby Area” section. You can optionally translate this external traffic to an AS External (type 5) LSA and flood the OSPFv2 domain with this routing information. An NSSA can be configured with the following optional parameters:

• No redistribution— Redistributed routes bypass the NSSA and are redistributed to other areas in the OSPFv2 autonomous system. Use this option when the NSSA ASBR is also an ABR.
• Default information originate—Generates an NSSA External (type 7) LSA for a default route to the external autonomous system. Use this option on an NSSA ASBR if the ASBR contains the default route in the routing table. This option can be used on an NSSA ABR whether or not the ABR contains the default route in the routing table.
• Route map—Filters the external routes so that only those routes that you want are flooded throughout the NSSA and other areas.
• Translate—Translates NSSA External LSAs to AS External LSAs for areas outside the NSSA. Use this command on an NSSA ABR to flood the redistributed routes throughout the OSPFv2 autonomous system. You can optionally suppress the forwarding address in these AS External LSAs. If you choose this option, the forwarding address is set to 0.0.0.0.
• No summary—Blocks all summary routes from flooding the NSSA. Use this option on the NSSA ABR.

BEFORE YOU BEGIN

Ensure that you have enabled the OSPF feature (see the “Enabling OSPFv2” section).

Ensure that there are no virtual links in the proposed NSSA and that it is not the backbone area.

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. area area-id nssa [ no-redistribution ] [ default-information-originate [ route-map map-name ]] [ no-summary ] [ translate type7 { always | never } [ suppress-fa ]]

4. (Optional) area area-id default-cost cost

5. (Optional) show ip ospf instance-tag

6. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to create an NSSA that blocks all summary route updates:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# area 0.0.0.10 nssa no-summary

switch(config-router)# copy running-config startup-config

This example shows how to create an NSSA that generates a default route:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# area 0.0.0.10 nssa default-info-originate

switch(config-router)# copy running-config startup-config

This example shows how to create an NSSA that filters external routes and blocks all summary route updates:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# area 0.0.0.10 nssa route-map ExternalFilter no-summary

switch(config-router)# copy running-config startup-config

This example shows how to create an NSSA that always translates NSSA External (type 5) LSAs to AS External (type 7) LSAs:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# area 0.0.0.10 nssa translate type 7 always

switch(config-router)# copy running-config startup-config

Configuring Virtual Links

A virtual link connects an isolated area to the backbone area through an intermediate area. See the “Virtual Links” section. You can configure the following optional parameters for a virtual link:

• Authentication—Sets a simple password or MD5 message digest authentication and associated keys.
• Dead interval—Sets the time that a neighbor waits for a Hello packet before declaring the local router as dead and tearing down adjacencies.
• Hello interval—Sets the time between successive Hello packets.
• Retransmit interval—Sets the estimated time between successive LSAs.
• Transmit delay—Sets the estimated time to transmit an LSA to a neighbor.

Note You must configure the virtual link on both routers involved before the link becomes active.

You cannot add a virtual link to a stub area.

BEFORE YOU BEGIN

Ensure that you have enabled OSPF (see the “Enabling OSPFv2” section).

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. area area-id virtual-link router-id

4. (Optional) show ip ospf virtual-link [ brief ]

5. (Optional) copy running-config startup-config

DETAILED STEPS

You can configure the following optional commands in virtual link configuration mode:

This example shows how to create a simple virtual link between two ABRs.

The configuration for ABR 1 (router ID 27.0.0.55) is as follows:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# area 0.0.0.10 virtual-link 10.1.2.3

switch(config-router)# copy running-config startup-config

The configuration for ABR 2 (Router ID 10.1.2.3) is as follows:

switch# configure terminal

switch(config)# router ospf 101

switch(config-router)# area 0.0.0.10 virtual-link 27.0.0.55

switch(config-router)# copy running-config startup-config

Configuring Redistribution

You can redistribute routes learned from other routing protocols into an OSPFv2 autonomous system through the ASBR.

You can configure the following optional parameters for route redistribution in OSPF:

• Default information originate—Generates an AS External (type 5) LSA for a default route to the external autonomous system.

Note Default information originate ignores match statements in the optional route map.

• Default metric—Sets all redistributed routes to the same cost metric.

Note If you redistribute static routes, Cisco NX-OS also redistributes the default static route.

BEFORE YOU BEGIN

Ensure that you have enabled OSPF (see the “Enabling OSPFv2” section).

Create the necessary route maps used for redistribution.

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. redistribute { bgp id | direct | eigrp id | isis id | ospf id | rip id | static } route-map map-name

4. default-information originate [ always ] [ route-map map-name ]

5. default-metric cost

6. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to redistribute the Border Gateway Protocol (BGP) into OSPF:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# redistribute bgp route-map FilterExternalBGP

switch(config-router)# copy running-config startup-config

Limiting the Number of Redistributed Routes

Route redistribution can add many routes to the OSPFv2 route table. You can configure a maximum limit to the number of routes accepted from external protocols. OSPFv2 provides the following options to configure redistributed route limits:

• Fixed limit—Logs a message when OSPFv2 reaches the configured maximum. OSPFv2 does not accept any more redistributed routes. You can optionally configure a threshold percentage of the maximum where OSPFv2 logs a warning when that threshold is passed.
• Warning only—Logs a warning only when OSPFv2 reaches the maximum. OSPFv2 continues to accept redistributed routes.
• Withdraw—Starts the timeout period when OSPFv2 reaches the maximum. After the timeout period, OSPFv2 requests all redistributed routes if the current number of redistributed routes is less than the maximum limit. If the current number of redistributed routes is at the maximum limit, OSPFv2 withdraws all redistributed routes. You must clear this condition before OSPFv2 accepts more redistributed routes.
• You can optionally configure the timeout period.

BEFORE YOU BEGIN

Ensure that you have enabled OSPF (see the “Enabling OSPFv2” section).

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. redistribute { bgp id | direct | eigrp id | isis id | ospf id | rip id | static } route-map map-name

4. redistribute maximum-prefix max [ threshold ] [ warning-only | withdraw [ num-retries timeout ]]

5. (Optional) show running-config ospf

6. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to limit the number of redistributed routes into OSPF:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# redistribute bgp route-map FilterExternalBGP

switch(config-router)# redistribute maximum-prefix 1000 75

Configuring Route Summarization

You can configure route summarization for inter-area routes by configuring an address range that is summarized. You can also configure route summarization for external, redistributed routes by configuring a summary address for those routes on an ASBR. For more information, see the “Route Summarization” section.

BEFORE YOU BEGIN

Ensure that you have enabled OSPF (see the “Enabling OSPFv2” section).

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. area area-id range ip-prefix/length [ no-advertise ] [ cost cost ]

or

4. summary-address ip-prefix/length [ no-advertise | tag tag-id ]

5. (Optional) show ip ospf summary-address

6. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to create summary addresses between areas on an ABR:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# area 0.0.0.10 range 10.3.0.0/16

switch(config-router)# copy running-config startup-config

This example shows how to create summary addresses on an ASBR:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# summary-address 10.5.0.0/16

switch(config-router)# copy running-config startup-config

Configuring Stub Route Advertisements

Use stub route advertisements when you want to limit the OSPFv2 traffic through this router for a short time. For more information, see the “OSPFv2 Stub Router Advertisements” section.

Stub route advertisements can be configured with the following optional parameters:

• On startup—Sends stub route advertisements for the specified announce time.
• Wait for BGP—Sends stub router advertisements until BGP converges.

Note You should not save the running configuration of a router when it is configured for a graceful shutdown because the router continues to advertise a maximum metric after it is reloaded.

BEFORE YOU BEGIN

Ensure that you have enabled OSPF (see the “Enabling OSPFv2” section).

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. max-metric router-lsa [ on-startup [ announce-time ] [ wait-for bgp tag ]]

4. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to enable the stub router advertisements on startup for the default 600 seconds:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# max-metric router-lsa on-startup

switch(config-router)# copy running-config startup-config

Modifying the Default Timers

OSPFv2 includes a number of timers that control the behavior of protocol messages and shortest path first (SPF) calculations. OSPFv2 includes the following optional timer parameters:

• LSA arrival time—Sets the minimum interval allowed between LSAs that arrive from a neighbor. LSAs that arrive faster than this time are dropped.
• Pacing LSAs—Sets the interval at which LSAs are collected into a group and refreshed, checksummed, or aged. This timer controls how frequently LSA updates occur and optimizes how many are sent in an LSA update message (see the “Flooding and LSA Group Pacing” section).
• Throttle LSAs—Sets the rate limits for generating LSAs. This timer controls how frequently LSAs are generated after a topology change occurs.
• Throttle SPF calculation—Controls how frequently the SPF calculation is run.

At the interface level, you can also control the following timers:

• Retransmit interval—Sets the estimated time between successive LSAs.
• Transmit delay—Sets the estimated time to transmit an LSA to a neighbor.

See the “Configuring Networks in OSPFv2” section for information about the hello interval and dead timer.

BEFORE YOU BEGIN

Ensure that you have enabled OSPF (see the “Enabling OSPFv2” section).

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. timers lsa-arrival msec

4. timers lsa-group-pacing seconds

5. timers throttle lsa start-time hold-interval max-time

6. timers throttle spf delay-time hold-time

7. interface type slot/port

8. ip ospf hello-interval seconds

9. ip ospf dead-interval seconds

10. ip ospf retransmit-interval seconds

11. ip ospf transmit-delay seconds

12. (Optional) show ip ospf

13. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to control LSA flooding with the lsa-group-pacing option:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# timers lsa-group-pacing 300

switch(config-router)# copy running-config startup-config

Configuring Graceful Restart

Graceful restart is enabled by default. You can configure the following optional parameters for graceful restart in an OSPFv2 instance:

• Grace period—Configures how long neighbors should wait after a graceful restart has started before tearing down adjacencies.
• Helper mode disabled—Disables helper mode on the local OSPFv2 instance. OSPFv2 does not participate in the graceful restart of a neighbor.
• Planned graceful restart only—Configures OSPFv2 to support graceful restart only in the event of a planned restart.

BEFORE YOU BEGIN

Ensure that you have enabled OSPF (see the “Enabling OSPFv2” section).

Ensure that all neighbors are configured for graceful restart with matching optional parameters set.

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. router ospf instance-tag

3. graceful-restart

4. (Optional) graceful-restart grace-period seconds

5. (Optional) graceful-restart helper-disable

6. (Optional) graceful-restart planned-only

7. (Optional) show ip ospf instance-tag

8. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to enable a graceful restart if it has been disabled and set the grace period to 120 seconds:

switch# configure terminal

switch(config)# router ospf 201

switch(config-router)# graceful-restart

switch(config-router)# graceful-restart grace-period 120

switch(config-router)# copy running-config startup-config

Restarting an OSPFv2 Instance

You can restart an OSPv2 instance. This action clears all neighbors for the instance.

To restart an OSPFv2 instance and remove all associated neighbors, use the following command:

Configuring OSPFv2 with Virtualization

You can configure multiple OSPFv2 instances in each VDC. You can also create multiple VRFs within each VDC and use the same or multiple OSPFv2 instances in each VRF. You assign an OSPFv2 interface to a VRF.

Note Configure all other parameters for an interface after you configure the VRF for an interface. Configuring a VRF for an interface deletes all the configuration for that interface.

BEFORE YOU BEGIN

Create the VDCs.

Ensure that you have enabled OSPF (see the “Enabling OSPFv2” section).

Ensure that you are in the correct VDC (or use the switchto vdc command).

SUMMARY STEPS

1. configure terminal

2. vrf context vrf_name

3. router ospf instance-tag

4. vrf vrf-name

5. (Optional) maximum-paths paths

6. interface interface-type slot/port

7. vrf member vrf-name

8. ip-address ip-prefix/length

9. router ospf instance-tag area area-id

10. (Optional) copy running-config startup-config

DETAILED STEPS

This example shows how to create a VRF and add an interface to the VRF:

switch# configure terminal

switch(config)# vrf context NewVRF

switch(config)# router ospf 201

switch(config)# interface ethernet 1/2

switch(config-if)# vrf member NewVRF

switch(config-if)# ip address 192.0.2.1/16

switch(config-if)# ip router ospf 201 area 0

switch(config)# copy running-config startup-config

OSPF RFC Compatibility Mode Example

The following example shows how to configure OSPF to be compatible with routers that comply with RFC 1583:

Note You must configure RFC 1583 compatibility on any VRF that connects to routers running only RFC1583 compatible OSPF.

Related Documents

MIBs