To configure a not-so-stubby area ( NSSA), use the area nssa command in router address family topology or router configuration mode. To remove the NSSA distinction from the area, use the no form of this command.

Syntax Description

Command Default

No NSSA area is defined.

Command Modes

Router address family topology configuration (config-router-af-topology) Router configuration (config-router)

Command History

Usage Guidelines

To remove the specified area from the software configuration, use the no area area-idcommand (with no other keywords). That is, the no area area-id command removes all area options, including area authentication, area default-cost, area nssa, area range, area stub, and area virtual-link.

Release 12.2(33)SRB

If you plan to configure the Multi-Topology Routing (MTR) feature, you need to enter the area nssacommand in router address family topology configuration mode in order for this OSPF router configuration command to become topology-aware.

Related Commands

To configure a not-so-stubby area ( NSSA) and to configure the OSPF Forwarding Address Suppression in Translated Type-5 LSAs feature, use the area nssa translatecommand in router address family topology or router configuration mode. To remove the NSSA distinction from the area, use the no form of this command.

Syntax Description

Command Default

The ABRs connecting an NSSA and the backbone areas elect one of them to translate LSAs, which means that a router might be elected as translator.

Command Modes

Router address family topology configuration (config-router-af-topology) Router configuration (config-router)

Usage Guidelines

To configure the OSPF Forwarding Address Suppression in Translated Type-5 LSAs feature, configure the translate type7 suppress-fa keywords. Consider the following caution.

Caution

Configuring the OSPF Forwarding Address Suppression in Translated Type-5 LSAs feature causes the router to be noncompliant with RFC 1587. Also, suboptimal routing might result because there might be better paths to reach the destination’s forwarding address. This feature should not be configured without careful consideration and not until the network topology is understood.

If the the no-redistribution or default-information-originate keywords are used, two separate lines for the area nssa command appear in the configuration file for ease of readability. For example, if the area 6 nssa translate type7 suppress-fa no-redistributioncommand is configured, the following lines would appear in the configuration file:

router ospf 1
 area 6 nssa no-redistribution
 area 6 nssa translate type7 suppress-fa

Cisco IOS Release 15.1(2)S and later releases support RFC 3101 and include the always keyword, which allows you to configure an NSSA ABR router as a forced NSSA LSA translator. This means that the NSSA ABR router will unconditionally assume the role of LSA translator, preempting the default behavior, which would only include it among the candidates to be elected as translator.

Note	Even a forced translator might not translate all LSAs; translation depends on the contents of each LSA.

You can configure the always keyword only in router configuration mode, not in router address family topology configuration mode.

To remove the specified area from the software configuration, use the no area area-idcommand (with no other keywords). That is, the no area area-id command removes all area options, such as area authentication, area default-cost, area nssa, area range, area stub, and area virtual-link.

Release 12.2(33)SRB

If you plan to configure the Multi-Topology Routing (MTR) feature with this command, you you must do so in router address family topology configuration mode in order for this OSPF router configuration command to become topology-aware.

Related Commands

To define an Open Shortest Path First (OSPF) virtual link, use the area virtual-link command in router address family topology, router configuration, or address family configuration mode. To remove a virtual link, use the no form of this command.

Syntax Description

Command Default

No OSPF virtual link is defined.

Command Modes

Router address family topology configuration (config-router-af-topology)

Router configuration (config-router)

Address family configuration (config-router-af)

Command History

Usage Guidelines

In OSPF, all areas must be connected to a backbone area. A lost connection to the backbone can be repaired by establishing a virtual link.

The shorter the hello interval, the faster topological changes will be detected, but more routing traffic will ensue. The setting of the retransmit interval should be conservative, or needless retransmissions will result. The value should be larger for serial lines and virtual links.

You should choose a transmit delay value that considers the transmission and propagation delays for the interface.

To configure a virtual link in OSPF for IPv6, you must use a router ID instead of an address. In OSPF for IPv6, the virtual link takes the router ID rather than the IPv6 prefix of the remote router.

Use the ttl-security hops hop-count keywords and argument to enable checking of TTL values on OSPF packets from neighbors or to set TTL values sent to neighbors. This feature adds an extra layer of protection to OSPF.

Note	In order for a virtual link to be properly configured, each virtual link neighbor must include the transit area ID and the corresponding virtual link neighbor router ID. To display the router ID, use the show ip ospf or the show ipv6 ospf command in privileged EXEC mode.

Note	To remove the specified area from the software configuration, use the no area area-id command (with no other keywords). That is, the no area area-id command removes all area options, such as area default-cost, area nssa, area range, area stub, and area virtual-link.

Release 12.2(33)SRB

If you plan to configure the Multitopology Routing (MTR) feature, you need to enter the area virtual-link command in router address family topology configuration mode in order for this OSPF router configuration command to become topology-aware.

Related Commands

To suppress the provider edge (PE) specific checks on a router when the Open Shortest Path First (OSPF) process is associated with the VPN routing and forwarding instance (VRF), use the capabilityvrf-lite command in router configuration mode. To restore the checks, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

Disabled. PE specific checks are performed if the process is associated with VRF command modes.

Command Modes

Router configuration

Command History

Usage Guidelines

This command works only if the OSPF process is associated with the VRF.

When the OSPF process is associated with the VRF, several checks are performed when link-state advertisements (LSAs) are received. PE checks are needed to prevent loops when the PE is performing a mutual redistribution between OSPF and Border Gateway Protocol (BGP) interfaces.

The table below describes the PE checks performed when Type-3, Type-5, and Type-7 LSAs are received.

In some situations, performing PE checks might not be desirable. The concept of VRFs can be used on a router that is not a PE router (that is, a router that is not running BGP). With the capabilityvrf-lite command, the checks can be turned off to allow correct population of the VRF routing table with routes to IP prefixes.

To suppress the provider edge (PE)-specific checks on a router when the Open Shortest Path First version 3 (OSPFv3) process is associated with the VPN routing and forwarding (VRF) instance, use the capability vrf-lite command in address family configuration mode. To restore the checks, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

Disabled. PE-specific checks are performed if the process is associated with VRF command modes.

Command Modes

Address family configuration (config-router-af)#

Command History

Usage Guidelines

This command works only if the OSPFv3 process is associated with the VRF.

When the OSPFv3 process is associated with the VRF, several checks are performed when link-state advertisements (LSAs) are received. PE checks are needed to prevent loops when the PE is performing a mutual redistribution between OSPF and Border Gateway Protocol (BGP) interfaces.

The table below describes the PE checks performed when inter-area-prefix LSAs, AS-External LSAs, or not-so-stubby area (NSSA) LSAs are received.

Related Commands

To clear redistribution based on the Open Shortest Path First (OSPF) routing process ID, use the clear ip ospf command in privileged EXEC mode.

Syntax Description

Command Modes

Privileged EXEC

Command History

Usage Guidelines

Use the pidargument to clear only one OSPFprocess. If the pid argumentis not specified,all OSPF processesare cleared.

To replace RFC 3101 compatibility with RFC 1587 compatibility for route selection in not-so-stubby area (NSSA) Area Border Routers (ABRs), use the compatible rfc1587command in router configuration mode or address family configuration mode. To restore RFC 3101 compatibility, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

Route selection is compatible with RFC 3101.

Command Modes

Router configuration (config-router)
Address family configuration (config-router-af)

Command History

Usage Guidelines

In Cisco IOS Release 15.1(2)S and later releases, RFC 3101 replaces RFC 1587, and RFC 3101 behavior is automatically enabled. You can choose the route selection behavior by configuring a router to run as RFC 3101 or RFC 1587 compatible.

See Appendix F of RFC3101 The OSPF Not-So-Stubby Area (NSSA) Option for a detailed list of differences between RFC1587 and RFC3101.

To generate a default external route into an Open Shortest Path First (OSPF) routing domain, use the default-information originate command in router configuration or router address family topology configuration mode. To disable this feature, use the no form of this command.

Syntax Description

Command Default

This command is disabled by default. No default external route is generated into the OSPF routing domain.

Command Modes

Router configuration (config-router) Router address family topology configuration (config-router-af-topology)

Command History

Usage Guidelines

Whenever you use the redistribute or the default-information router configuration command to redistribute routes into an OSPF routing domain, the Cisco IOS software automatically becomes an Autonomous System Boundary Router (ASBR). However, an ASBR does not, by default, generate a default route into the OSPF routing domain. The software must still have a default route for itself before it generates one, except when you have specified the always keyword.

When a route map is used, the origination of the default route by OSPF is not bound to the existence of a default route in the routing table.

Release 12.2(33)SRB

If you plan to configure the Multi-Topology Routing (MTR) feature, you need to enter the default-information originatecommand in router address family topology configuration mode in order for this OSPF router configuration command to become topology-aware.

Related Commands

To set default metric values for the Open Shortest Path First (OSPF) routing protocol, use the default-metric command in router address family topology or router configuration mode. To r eturn to the default state, use the no form of this command.

Syntax Description

Command Default

Built-in, automatic metric translations, as appropriate for each routing protocol. The metric of redistributed connected and static routes is set to 0.

Command Modes

Router address family topology configuration (config-router-af-topology) Router configuration (config-router)

Command History

Usage Guidelines

T he default-metric command is used in conjunction with the redistribute router configuration command to cause the current routing protocol to use the same metric value for all redistributed routes. A default metric helps solve the problem of redistributing routes with incompatible metrics. Whenever metrics do not convert, using a default metric provides a reasonable substitute and enables the redistribution to proceed.

Note	When enabled, the default-metric command applies a metric value of 0 to redistributed connected routes. The default-metric command does not override metric values that are applied with the redistribute command.

Release 12.2(33)SRB

If you plan to configure the Multi-Topology Routing (MTR) feature, you need to enter the default-metric command in router address family topology configuration mode in order for this OSPF router configuration command to become topology-aware.

Related Commands

To define Open Shortest Path First (OSPF) route administrative distances based on route type, use the distance ospf command in router address family topology or router configuration mode. To restore the default value, use the no form of this command.

Syntax Description

Command Default

dist1 : 110

dist2 : 110

dist3 : 110

Command Modes

Router address family topology configuration (config-router-af-topology) Router configuration (config-router)

Command History

Usage Guidelines

You must specify at least one of the keywords.

This command performs the same function as the distance command used with an access list. However, the distance ospfcommand allows you to set a distance for an entire group of routes, rather than a specific route that passes an access list.

A common reason to use the distance ospf command is when you have multiple OSPF processes with mutual redistribution, and you want to prefer internal routes from one over external routes from the other.

Release 12.2(33)SRB

If you plan to configure the Multi-Topology Routing (MTR) feature, you need to enter the distance ospfcommand in router address family topology configuration mode in order for this OSPF router configuration command to become topology-aware.

Related Commands

To configure the BGP/MPLS VPN domain ID, use the domain-id command in address-family configuration mode. To restore the default value, use the no form of this command.

Syntax Description

Command Default

The default value for the domain-id command is NULL.

Command Modes

address-family configuration

Command History

Usage Guidelines

The value of the BGP extended community used to carry the domain-id can be one of 0005, 0105, 0205, or 8005. In OSPFv2, a default non-NULL domain-id is provided by using the process-id of the router instance. In OSPFv3, the default value is NULL.

To suppress the sending of syslog messages when the router receives link-state advertisement (LSA) Type 6 Multicast OSPF ( MOSPF) packets, which are unsupported, use the ignore lsa mospfcommand in router configuration mode. To restore the sending of syslog messages, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

This command is disabled by default. Each MOSPF packet causes the router to send a syslog message.

Command Modes

Router configuration

Command History

Usage Guidelines

Cisco routers do not support LSA Type 6 MOSPF packets, and they generate syslog messages if they receive such packets. If the router is receiving many MOSPF packets, you might want to configure the router to ignore the packets and thus prevent a large number of syslog messages.

To enable Open Shortest Path First version 2 (OSPFv2) on an interface, use the ip ospf area command in interface configuration mode. To disable OSPFv2 on the interface, use the no form of this command.

Syntax Description

Command Default

If the secondaries nonekeywords are entered in the no form of this command, the secondary IP addresses will be advertised. If the secondaries nonekeywords are not present, OSPFv2 will be disabled.

Command Modes

Interface configuration (config-if) Virtual network interface (config-if-vnet)

Command History

Usage Guidelines

OSPF is enabled on an interface when the network address for the interface matches the range of addresses that is specified by the network area command that is entered in router configuration mode. You can enable OSPFv2 explicitly on an interface with the ip ospf area command that is entered in interface configuration mode. This capability simplifies the configuration of unnumbered interfaces with different areas.

The ip ospf areacommand that is entered in interface configuration mode will supersede the effects of the network areacommand. Therefore, an interface that is configured with the ip ospf area command in interface configuration mode will not be affected by the network areacommand.

Note	If you later disable the ip ospf area command, the interface will still run OSPFv2 as long as its network address matches the range of addresses that is specified by the network areacommand.

Related Commands

To filter outgoing link-state advertisements (LSAs) to an Open Shortest Path First (OSPF) interface, use the ip ospf database-filter all out command in interface or virtual network interface configuration modes. To restore the forwarding of LSAs to the interface, use the no form of this command.

Syntax Description

Command Default

This command is disabled by default. All outgoing LSAs are flooded to the interface.

Command Modes

Interface configuration (config-if)

Virtual network interface (config-if-vnet)

Command History

Usage Guidelines

This command performs the same function that the neighbor database-filter command performs on a neighbor basis.

If the ip ospf database-filter all out command is enabled for a virtual network and you want to disable it, use the disable keyword in virtual network interface configuration mode.

Related Commands

To configure Open Shortest Path First (OSPF) to treat the interface as an OSPF demand circuit, use the ip ospf demand-circuit command in interface configuration mode or virtual network interface configuration mode. To remove the OSPF demand circuit functionality from the interface, use the no form of this command.

Syntax Description

Command Default

The circuit is not an OSPF demand circuit.

Command Modes

Interface configuration (config-if)

Virtual network interface (config-if-vnet)

Command History

Usage Guidelines

On point-to-point interfaces, only one end of the demand circuit must be configured with the ip ospf demand-circuit command. Periodic hello messages are suppressed and periodic refreshes of link-state advertisements (LSAs) do not flood the demand circuit. This command allows the underlying data-link layer to be closed when the topology is stable. In point-to-multipoint topology, only the multipoint end must be configured with this command.

If the ip ospf demand-circuit command is enabled for a virtual network and you want to disable it, use the disable keyword in virtual network interface configuration mode.

Related Commands

To suppress the unnecessary flooding of link-state advertisements (LSAs) in stable topologies, use the ip ospf flood-reduction command in interface configuration mode. To disable this feature, use the no form of this command.

Syntax Description

Command Default

This command is disabled by default.

Command Modes

Interface configuration (config-if)

Virtual network interface (config-if-vnet)

Command History

Usage Guidelines

All routers supporting the Open Shortest Path First (OSPF) demand circuit are compatible and can interact with routers supporting flooding reduction.

If the ip ospf flood-reduction command is enabled for a virtual network and you want to disable it, use the disable keyword in virtual network interface configuration mode.

Related Commands

To configure Open Shortest Path First (OSPF) to look up Domain Name System (DNS) names for use in all OSPF show EXEC command displays, use the ip ospf name-lookup command in global configuration mode. To disable this function, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

This command is disabled by default.

Command Modes

Global configuration

Command History

Usage Guidelines

This command makes it easier to identify a router because the router is displayed by name rather than by its router ID or neighbor ID.

To configure the Open Shortest Path First (OSPF) network type to a type other than the default for a given medium, use the ip ospf network command in interface configuration mode. To return to the default value, use the no form of this command.

Syntax Description

Command Default

Depends on the network type.

Command Modes

Interface configuration (config-if) Virtual network interface (config-if-vnet)

Command History

Usage Guidelines

Using this feature, you can configure broadcast networks as NBMA networks when, for example, routers in your network do not support multicast addressing. You can also configure nonbroadcast multiaccess networks (such as X.25, Frame Relay, and Switched Multimegabit Data Service (SMDS)) as broadcast networks. This feature saves you from needing to configure neighbors.

Configuring NBMA networks as either broadcast or nonbroadcast assumes that there are virtual circuits from every router to every router or fully meshed networks. However, there are other configurations where this assumption is not true. For example, a partially meshed network. In these cases, you can configure the OSPF network type as a point-to-multipoint network. Routing between two routers that are not directly connected will go through the router that has virtual circuits to both routers. You need not configure neighbors when using this feature.

If this command is issued on an interface that does not allow it, this command will be ignored.

OSPF has two features related to point-to-multipoint networks. One feature applies to broadcast networks; the other feature applies to nonbroadcast networks:

• On point-to-multipoint, broadcast networks, you can use the neighbor command, and you must specify a cost to that neighbor.
• On point-to-multipoint, nonbroadcast networks, you must use the neighbor command to identify neighbors. Assigning a cost to a neighbor is optional.

Related Commands

To modify the number of retransmissions of database exchange and update packets for both demand and non-demand circuits, use the limit retransmissions command in router configuration mode. To reset the maximum number of retransmissions back to the default value of 24, use the no form of this command.

Syntax Description

Command Default

Maximum number of retransmissions is 24.

Command Modes

Router configuration (config-router)#

Address family configuration (config-router-af)#

Command History

Usage Guidelines

There is a limit to the number of retransmissions of database exchange and update packets for both demand and nondemand circuits. The retransmission of these packets stops once this retry limit is reached, thus preventing unnecessary use of the link in continual retransmission of the packets if, for some reason, a neighbor is not responding during adjacency forming. The limit for both demand circuit and nondemand circuit retransmissions is 24.

The limit-retransmissions command allows you to either remove (disable) the limit or change the maximum number of retransmissions to be a number from 1 to 255. The configuration of this command provides for backward compatibility for previous or other releases of Cisco IOS Software or other routers that do not have this feature.

The limit to the number of retransmissions does not apply for update packets on nonbroadcast multiaccess (NBMA) point-to-multipoint direct circuits. In this situation, the dead timer is used to end communication with nonresponding neighbors and thus stop the retransmissions.

Note	This command can be used in the router configuration mode and address-family mode. The command is also applicable for both OSPF and OSPFv3 protocols.

Related Commands

To configure the router to send a syslog message when an Open Shortest Path First (OSPF) neighbor goes up or down, use the log-adjacency-changes command in router configuration mode. To turn off this function, use the no form of this command.

Syntax Description

Command Default

Enabled

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

This command allows you to know about OSPF neighbors going up or down without turning on the debug ip ospf packetcommand or the debug ipv6 ospf adjacency command. The log-adjacency-changes command provides a higher level view of those changes of the peer relationship with less output than the debug command provides. The log-adjacency-changes command is on by default but only up/down (full/down) events are reported, unless the detail keyword is also used.

Related Commands

To limit the number of nonself-generated link-state advertisements (LSAs) that an Open Shortest Path First (OSPF) routing process can keep in the OSPF link-state database (LSDB), use the max-lsa command in router configuration mode. To remove the limit of non self-generated LSAs that an OSPF routing process can keep in the OSPF LSDB, use the no form of this command.

Syntax Description

Command Default

The number of nonself-generated LSAs that an OSPF routing process can keep in the OSPF LSDB is not limited.

threshold-percentage : 75 percentwarning-only warning message: disabledignore-time minutes: 5 minutesignore-count count-number:5 timesreset-time minutes: 10 minutes

Command Modes

Router configuration

Command History

Usage Guidelines

To prevent the OSPF process from endlessly changing from the normal state of operation to the ignore state as a result of the LSA count exceeding the maximum configured number immediately after it returns from the ignore state to the normal state of operation, the OSPF process keeps a counter on how many times the process went into the ignore state. This counter is called the ignore count. If the ignore count exceeds the maximum number of LSAs that is specified by the ignore-count keyword and counter-number argument, the OSPF process remains in the ignore state permanently. To return the OSPF process to the state of normal operation, enter the clear ip ospf command.

If the router is placed into a permanent ignore state, we recommend that you identify and correct the cause of the problem involving the router that is generating the LSAs, or, if possible, increase the limit that has been configured by the max-lsa command before you try to bring the router back into normal operation.

If the router that has generated large numbers of LSAs is not reachable, these LSAs cannot be removed from the OSPF area and domain. As a result, any other router leaving the ignore state and returning to normal operation may reach the ignore state again. We recommend that you take one of the following actions in order to bring the router back into the network:

• Temporarily increase the LSA limit to account for the stale LSAs.
• Wait until the stale LSAs are removed as a result of reaching their maximum age.
• Make sure that the router that has generated the large number of LSAs is connected to the network and is no longer generating large numbers of LSAs.

When the warning-only keyword is used, the OSPF process never enters the ignore state. When the LSA count exceeds the maximum limit that is specified by the maximum-number argument, only an error message is logged and the OSPF process continues in its normal operation.

When the max-lsa command is entered for the first time or when any of the parameters of the command are changed, the OSPF process undergoes a soft-reset procedure.

Related Commands

To configure a router that is running the Open Shortest Path First (OSPF) protocol to advertise a maximum metric so that other routers do not prefer the router as an intermediate hop in their shortest path first (SPF) calculations, use the max-metric router-lsacommand in router address family topology or router configuration mode. To disable the advertisement of a maximum metric, use the no form of this command.

Syntax Description

Command Default

Router link-state advertisements (LSAs) are originated with normal link metrics.

Command Modes

Router address family topology configuration (config-router-af-topology) Router configuration (config-router)

Command History

Usage Guidelines

Enabling the max-metric router-lsa command will cause a router to originate LSAs with a maximum metric (LSInfinity: 0xFFFF) through all nonstub links, which allows BGP routing tables to converge without attracting transit traffic (if there are not alternate lower cost paths around the router). The router will advertise accurate (normal) metrics after the configured or default timers expire or after BGP sends a notification that routing tables have converged.

Note	Directly connected links in a stub network are not affected by the configuration of a maximum or infinite metric because the cost of a stub link is always set to the output interface cost.

The max-metric router-lsa command is useful in the following situations:

• Reloading a router. After a router is reloaded, Interior Gateway Protocols (IGPs) converge very quickly, and other routers may try to forward traffic through the newly reloaded router. If the router is still building BGP routing tables, packets destined for other networks that the router has not learned through BGP may be dropped. In the case of an Internet backbone router, a large number of packets may be dropped.
• Introducing a router into a network without routing traffic through it. You may want to connect a router to an OSPF network but not want real traffic flowing through the router if there are better alternate paths. If there are no alternate paths, this router would still accept transit traffic as before.
• Gracefully removing a router from a network. This feature allows you to gracefully remove a router from the network by advertising a maximum metric through all links, which allows other routers to select alternate paths for transit traffic to follow before the router is shut down.

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

Note	In older OSPF implementations (RFC 1247 and earlier implementations), the router link costs in received LSAs with a metric of LSInfinity are not used during SPF calculations, which means that no transit traffic will be sent to the routers that originate these LSAs.

Release 12.2(33)SRB

If you plan to configure the Multi-Topology Routing (MTR) feature, you need to enter the max-metric router-lsacommand in router address family topology configuration mode in order for this OSPF router configuration command to become topology-aware.

Related Commands

To configure Open Shortest Path First (OSPF) routers interconnecting to nonbroadcast networks, use the neighbor command in router address family topology or router configuration mode. To remove a configuration, use the no form of this command.

Syntax Description

Command Default

This command is disabled by default. No configuration is specified.

Command Modes

Router address family topology configuration (config-router-af-topology) Router configuration (config-router)

Command History

Usage Guidelines

X.25 and Frame Relay provide an optional broadcast capability that can be configured in the map to allow OSPF to run as a broadcast network. At the OSPF level you can configure the router as a broadcast network. Refer to the x25 map and frame-relay map commands in the “X.25 Commands” and “Frame Relay Commands” chapters, respectively, in the Cisco IOSWide-Area Networking Command Referencefor more detail.

One neighbor entry must be included in the Cisco IOS software configuration for each known nonbroadcast network neighbor. The neighbor address must be on the primary address of the interface.

If a neighboring router has become inactive (hello packets have not been received for the Router Dead Interval period), it may still be necessary to send hello packets to the dead neighbor. These hello packets will be sent at a reduced rate called Poll Interval.

When the router first starts up, it sends only hello packets to those routers with nonzero priority, that is, routers that are eligible to become designated routers (DRs) and backup designated routers (BDRs). After the DR and BDR are selected, DR and BDR will then start sending hello packets to all neighbors in order to form adjacencies.

Note	You cannot use the neighbor (OSPF) command to specify an Open Shortest Path First (OSPF) neighbor on non-broadcast networks within an OSPF Virtual Private Network (VPN) routing instance.

Prior to Cisco IOS Release 12.0, the neighbor command applied to NBMA networks only. With Release 12.0, the neighbor command applies to NBMA networks and point-to-multipoint networks. On NBMA networks, the cost keyword is not accepted.

Release 12.2(33)SRB

If you plan to configure the Multi-Topology Routing (MTR) feature, you need to enter the neighborcommand in router address family topology configuration mode in order for this OSPF router configuration command to become topology-aware.

Related Commands

To define the interfaces on which Open Shortest Path First (OSPF) runs and to define the area ID for those interfaces, use the network area command in router configuration mode. To disable OSPF routing for interfaces defined with the ip-address wildcard-maskpair, use the no form of this command.

Syntax Description

Command Default

This command is disabled by default.

Command Modes

Router configuration

Command History

Usage Guidelines

The ip-address and wildcard-mask arguments together allow you to define one or multiple interfaces to be associated with a specific OSPF area using a single command. Using the wildcard-mask argument allows you to define one or multiple interfaces to be associated with a specific OSPF area using a single command. If you intend to associate areas with IP subnets, you can specify a subnet address as the value of the area-idargument.

For OSPF to operate on the interface, the primary address of the interface must be covered by the network areacommand. If the network areacommand covers only the secondary address, it will not enable OSPF over that interface.

The Cisco IOS software sequentially evaluates the ip-address wildcard-mask pair for each interface as follows:

1. The wildcard-maskargument is logically ORed with the interface IP address.
2. The wildcard-maskargument is logically ORed with the ip-addressargument in the network command.
3. The software compares the two resulting values. If they match, OSPF is enabled on the associated interface and this interface is attached to the OSPF area specified.

There is no limit to the number of network area commands you can use on the router.

Note

Any individual interface can only be attached to a single area. If the address ranges specified for different areas overlap, the software will adopt the first area in the network command list and ignore the subsequent overlapping portions. In general, we recommend that you configure address ranges that do not overlap in order to avoid inadvertent conflicts.

When a more specific OSPF network range is removed, interfaces belonging to that network range will be retained and remain active if and only if a less specific network range exists.

For example, consider the following configuration:

router ospf 1
 network 192.168.129.16 0.0.0.3 area 20
 network 192.168.129.40 0.0.0.3 area 20
 network 192.168.129.44 0.0.0.3 area 20
 network 192.168.129.96 0.0.0.3 area 20
 network 192.168.128.0  0.0.127.255 area 20
!

Enter the following:

no network 192.168.129.40 0.0.0.3 area 20 

Interfaces falling into the network range 192.168.129.40/0.0.0.3 will still remain active because the superset, 192.168.128.0/0.0.127.255, exists for area 20. A more specific network statement will cause interfaces belonging to that range to be removed from a different area only if a less specific network statement (superset) exists.

Consider a configuration such as the following:

!
router ospf 1
 network 192.168.128.0 0.0.127.255 area 20
!

If the following network statement is entered:

network 192.168.129.96 0.0.0.3 area 40 

then interfaces belonging to range 192.168.129.96/0.0.0.3, if any, are removed from area 20 and moved to area 40. Network statements with identical ranges but with different area IDs are considered as area changes. For example, the following network statements will cause interfaces belonging to network range 192.168.129.40/0.0.0.3 to move from area 20 to area 40:

network 192.168.129.40 0.0.0.3 area 20
network 192.168.129.40 0.0.0.3 area 40

Related Commands

To disable Cisco nonstop forwarding (NSF) helper mode on a Cisco router that is running Open Shortest Path First (OSPF), use the nsf cisco helper disablecommand in router configuration mode. To reenable Cisco NSF helper mode, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

This command is enabled by default; therefore, NSF helper mode is disabled on a Cisco router that is running OSPF.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

When a router in an OSPF process has NSF enabled, the router is said to be NSF-capable and will operate in graceful restart mode--the OSPF router process performs nonstop forwarding recovery due to a Route Processor (RP) switchover. By default, the neighboring routers of the NSF-capable router will be NSF-aware and will operate in NSF helper mode. When the NSF-capable router is performing graceful restart, the helper routers assist in the nonstop forwarding recovery process. If you do not want the router to help the restarting neighbor with nonstop forwarding recovery, enter the nsf cisco helper disable command.

Related Commands

To disable Internet Engineering Task Force (IETF) nonstop forwarding (NSF) helper mode on a router that is running Open Shortest Path First (OSPF), use the nsf ietf helper disablecommand in router configuration mode. To reenable IETF NSF helper mode, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

This command is disabled by default; therefore, IETF NSF helper mode is enabled on a router that is running OSPF.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

When a router in an OSPF process has NSF enabled, the router is said to be NSF-capable and will operate in graceful restart mode--the OSPF router process performs nonstop forwarding recovery due to a Route Processor (RP) switchover. By default, the neighboring routers of the NSF-capable router will be NSF-aware and will operate in NSF helper mode. When the NSF-capable router is performing graceful restart, the helper routers assist in the nonstop forwarding recovery process. If you do not want the router to help the restarting neighbor with nonstop forwarding recovery, enter the nsf ietf helper disable command.

Related Commands

To enable nonstop routing (NSR) operations on a router that is running Open Shortest Path First (OSPF), use the nsrcommand in router configuration mode. To disable NSR and return to the default, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

NSR is disabled.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

This command enables NSR on an OSPF router. With NSR enabled, a router with redundant Route Processors (RPs) is allowed to maintain its OSPF state and adjacencies across planned and RP switchovers. It does this by checkpointing state information from OSPF on the active RP to the standby RP. Later, following a switchover to the standby RP, OSPF can use this checkpointed information to continue operation without interruption.

This command is present only in images for platforms that have a hardware or software redundancy capability.

Related Commands

To limit the number of prefixes redistributed into Open Shortest Path First (OSPF) or to generate a warning when the number of prefixes redistributed into OSPF reaches a maximum, use the redistribute maximum-prefixcommand in router configuration mode. To remove the values, use the no form of this command.

Syntax Description

Command Default

The percentage default is 75.

Command Modes

Router configuration (config-router)

Command History

Usage Guidelines

If someone mistakenly injects a large number of IP prefixes into IS-IS, perhaps by redistributing Border Gateway Protocol (BGP) into IS-IS, the network can be severely flooded. Limiting the number of redistributed prefixes prevents this potential problem.

When the redistribute maximum-prefix command is configured and the number of redistributed prefixes reaches the maximum value configured, no more prefixes will be redistributed (unless the warning-only keyword was configured).

The redistribution limit applies only to external IP prefixes. Default prefixes and summarized prefixes are not limited.

The limit is tracked separately for each not-so-stubby-area (NSSA) because redistribution to NSSAs is done independently for each NSSA and independently of all other regular areas.

Select a maximum value based on your knowledge of how many prefixes are redistributed on the router to the OSPF process.

To configure an Open Shortest Path First (OSPF) routing process, use the router ospf command in global configuration mode. To terminate an OSPF routing process, use the noform of this command.

Syntax Description

Command Default

No OSPF routing process is defined.

Command Modes

Global configuration

Command History

Usage Guidelines

You can specify multiple OSPF routing processes in each router.

After you enter the router ospf command, you can enter the maximum number of paths. There can be from 1 to 32 paths.

Related Commands

To use a fixed router ID, use the router-id command in router configuration mode. To force Open Shortest Path First (OSPF) to use the previous OSPF router ID behavior, use the no form of this command.

Syntax Description

Command Default

No OSPF routing process is defined.

Command Modes

Router configuration

Command History

Usage Guidelines

You can configure an arbitrary value in the IP address format for each router. However, each router ID must be unique.

If this command is used on an OSPF router process which is already active (has neighbors), the new router-ID is used at the next reload or at a manual OSPF process restart. To manually restart the OSPF process, use the clear ip ospf command.

Related Commands

To display general information about Open Shortest Path First (OSPF) routing processes, use the showipospf command in user EXEC or privileged EXEC mode.

Syntax Description

Command Modes

User EXEC Privileged EXEC

Command History

To display the internal Open Shortest Path First (OSPF) routing table entries to an Area Border Router (ABR) and Autonomous System Boundary Router (ASBR), use the showipospfborder-routers command in privileged EXEC mode.

Syntax Description

This command has no arguments or keywords.

Command Modes

Privileged EXEC

Command History

To display lists of information related to the Open Shortest Path First (OSPF) database for a specific router, use the showipospfdatabase command in EXEC mode.

Syntax Description

Command Modes

EXEC

Command History

Usage Guidelines

The various forms of this command deliver information about different OSPF link state advertisements.

To display a list of Open Shortest Path First (OSPF) link-state advertisements (LSAs) waiting to be flooded over an interface, use the showipospfflood-listcommand in EXEC mode.

Syntax Description

Command Modes

EXEC

Command History

Usage Guidelines

Use this command to observe OSPF packet pacing.

To display interface information related to Open Shortest Path First (OSPF), use the show ip ospf interface command in user EXEC or privileged EXEC mode.

Syntax Description

Command Modes

Command History

To display Open Shortest Path First (OSPF) neighbor information on a per-interface basis, use the showipospfneighbor command in privileged EXEC mode.

Syntax Description

Command Modes

Privileged EXEC (#)

Command History