Cisco IOS IP Routing Protocols Configuration Guide, Release 12.4T - OSPF Stub Router Advertisement [Cisco IOS Software Releases 12.4 T]

Table Of Contents

OSPF Stub Router Advertisement

Feature Overview

Allowing Routing Tables to Converge

Configuring a Graceful Shutdown

Benefits

Related Features and Technologies

Supported Platforms

Supported Standards, MIBs, and RFCs

Configuration Tasks

Configuring Advertisement on Startup

Configuring Advertisement Until Routing Tables Converge

Configuring Advertisement for a Graceful Shutdown

Verifying the Advertisement of a Maximum Metric

Monitoring and Maintaining OSPF Stub Router Advertisement

Configuration Examples

Advertisement on Startup Example

Advertisement Until Routing Tables Converge Example

Graceful Shutdown Example

Command Reference

OSPF Stub Router Advertisement

Feature History

Release

Modification

12.1(8)E

This feature was introduced.

12.0(15)S

This feature was integrated into Cisco IOS Release 12.0(15)S.

12.0(15)SC

This feature was integrated into Cisco IOS Release 12.0(15)SC.

12.0(16)ST

This feature was integrated into Cisco IOS Release 12.0(16)ST.

12.2(4)T

This feature was integrated into Cisco IOS Release 12.2(4)T.

12.2(4)T3

Support for the Cisco 7500 series was added in Cisco IOS Release 12.2(4)T3.

12.2(14)S

This feature was integrated into Cisco IOS Release 12.2(14)S.

This document describes the OSPF Stub Router Advertisement feature. It includes the following sections:

•Feature Overview

•Benefits

•Related Features and Technologies

•Supported Platforms

•Supported Standards, MIBs, and RFCs

•Configuration Tasks

•Monitoring and Maintaining OSPF Stub Router Advertisement

•Configuration Examples

•Command Reference

Feature Overview

The OSPF Stub Router Advertisement feature allows you to bring a new router into a network without immediately routing traffic through the new router and allows you to gracefully shut down or reload a router without dropping packets that are destined for other networks. This feature introduces three configuration options that allow you to configure a router that is running the Open Shortest Path First (OSPF) protocol to advertise a maximum or infinite metric to all neighbors.

When any of these three configuration options are enabled on a router, the router will originate link-state advertisements (LSAs) with a maximum metric (LSInfinity: 0xFFFF) through all nonstub links. The advertisement of a maximum metric causes other routers to assign a cost to the new router that is higher than the cost of using an alternate path. Because of the high cost assigned to paths that pass through the new router, other routers will not use a path through the new router as a transit path to forward traffic that is destined for other networks, which allows switching and routing functions to be up and running and routing tables to converge before transit traffic is routed through this router.

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.

Allowing Routing Tables to Converge

Two configuration options introduced by the OSPF Stub Router Advertisement feature allow you to bring a new router into a network without immediately routing traffic through the new router. These configuration options are useful because Interior Gateway Protocols (IGPs) converge very quickly upon a router during startup or after a reload, often before Border Gateway Protocol (BGP) routing tables have completely converged. If neighbor routers forward traffic through a router while that router is building BGP routing tables, packets that have been received for other destinations may be dropped. Advertising a maximum metric during startup will allow routing tables to converge before traffic that is destined for other networks is sent through the router. The following two configuration options enable a router to advertise a maximum metric at startup:

•You can configure a timer to advertise a maximum metric when the router is started or reloaded. When this option is configured, the router will advertise a maximum metric, which forces neighbor routers to select alternate paths until the timer expires. When the timer expires, the router will advertise accurate (normal) metrics, and other routers will send traffic to this router depending on the cost. The configurable range of the timer is from 5 to 86,400 seconds.

•You can configure a router to advertise a maximum metric at startup until BGP routing tables converge or until the default timer expires (600 seconds). Once BGP routing tables converge or the default timer expires, the router will advertise accurate (normal) metrics and other routers will send traffic to this router, depending on the cost.

Configuring a Graceful Shutdown

The third configuration option introduced by the OSPF Stub Router Advertisement 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. There are many situations where you may need to remove a router from the network. If a router is removed from a network and neighbor routers cannot detect that the physical interface is down, neighbors will need to wait for dead timers to expire before the neighbors will remove the adjacency and routing tables will reconverge. This situation may occur when there is a switch between other routers and the router that is shut down. Packets may be dropped while the neighbor routing tables reconverge.

When this third option is configured, the router advertises a maximum metric, which allows neighbor routers to select alternate paths before the router is shut down. This configuration option could also be used to remove a router that is in a critical condition from the network without affecting traffic that is destined for other networks.

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.

Benefits

Improved Stability and Availability

Advertising a maximum metric through all links at startup or during a reload will prevent neighbor routers from using a path through the router as a transit path, thereby reducing the number of packets that are dropped and improving the stability and availability of the network.

Graceful Removal from the Network

Advertising a maximum metric before shutdown allows other routers to select alternate paths before the transit path through a router becomes inaccessible.

Related Features and Technologies

The OSPF Stub Router Advertisement feature is an extension of the OSPF routing protocol. For more information about configuring OSPF and BGP, refer to the Release 12.2 Cisco IOS IP Routing Configuration Guide and the Cisco IOS IP Command Reference, Volume 2 of 3: Routing Protocols.

Supported Platforms

The OSPF Stub Router Advertisement feature is supported by the following platforms in Cisco IOS Release 12.2(14)S that support OSPF:

•Cisco 7200 series

•Cisco 7400 series

•Cisco 7500 series

Determining Platform Support Through Cisco Feature Navigator

Cisco IOS software is packaged in feature sets that support specific platforms. To get updated information regarding platform support for this feature, access Cisco Feature Navigator. Cisco Feature Navigator dynamically updates the list of supported platforms as new platform support is added for the feature.

Cisco Feature Navigator is a web-based tool that enables you to determine which Cisco IOS software images support a specific set of features and which features are supported in a specific Cisco IOS image. You can search by feature or release. Under the release section, you can compare releases side by side to display both the features unique to each software release and the features in common.

To access Cisco Feature Navigator, you must have an account on Cisco.com. If you have forgotten or lost your account information, send a blank e-mail to cco-locksmith@cisco.com. An automatic check will verify that your e-mail address is registered with Cisco.com. If the check is successful, account details with a new random password will be e-mailed to you. Qualified users can establish an account on Cisco.com by following the directions at http://www.cisco.com/register.

Cisco Feature Navigator is updated regularly when major Cisco IOS software releases and technology releases occur. For the most current information, go to the Cisco Feature Navigator home page at the following URL:

http://www.cisco.com/go/fn

Availability of Cisco IOS Software Images

Platform support for particular Cisco IOS software releases is dependent on the availability of the software images for those platforms. Software images for some platforms may be deferred, delayed, or changed without prior notice. For updated information about platform support and availability of software images for each Cisco IOS software release, refer to the online release notes or, if supported, Cisco Feature Navigator.

Supported Standards, MIBs, and RFCs

Standards

No new or modified standards are supported by this feature.

MIBs

No new or modified MIBs are supported by this feature.

To obtain lists of supported MIBs by platform and Cisco IOS release, and to download MIB modules, go to the Cisco MIB website on Cisco.com at the following URL:

http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

RFCs

•RFC 3137 OSPF Stub Router Advertisement

Configuration Tasks

See the following sections for configuration tasks to configure OSPF to advertise a maximum metric. This feature has three different configuration options. All tasks are optional and should be individually configured.

•Configuring Advertisement on Startup (optional)

•Configuring Advertisement Until Routing Tables Converge (optional)

•Configuring Advertisement for a Graceful Shutdown (optional)

•Verifying the Advertisement of a Maximum Metric (optional)

Configuring Advertisement on Startup

To configure a router that is running OSPF to advertise a maximum metric during startup, use the following commands beginning in global configuration mode:

Command

Purpose

Step 1

Router(config)# router ospf process-id

Places the router in router configuration mode and enables an OSPF routing process.

Step 2

Router(config-router)# max-metric router-lsa on-startup announce-time

Configures OSPF to advertise a maximum metric during startup for a configured period of time. The announce-time argument is a configurable timer that must follow the on-startup keyword to be configured. There is no default timer value. The configurable time range is from 5 to 86,400 seconds.

Configuring Advertisement Until Routing Tables Converge

To configure a router that is running OSPF to advertise a maximum metric until BGP routing tables converge, use the following commands beginning in global configuration mode:

Command

Purpose

Step 1

Router(config)# router ospf process-id

Places the router in router configuration mode and enables an OSPF routing process.

Step 2

Router(config-router)# max-metric router-lsa on-startup wait-for-bgp

Configures OSPF to advertise a maximum metric until BGP routing tables have converged or until the default timer has expired. The wait-for-bgp keyword must follow the on-startup keyword to be configured. The default timer value is 600 seconds.

Configuring Advertisement for a Graceful Shutdown

To configure a router that is running OSPF to advertise a maximum metric for a graceful shutdown or removal from the network, use the following commands beginning in global configuration mode:

Command

Purpose

Step 1

Router(config)# router ospf process-id

Places the router in router configuration mode and enables an OSPF routing process.

Step 2

Router(config-router)# max-metric router-lsa

Configures OSPF to advertise a maximum metric until the router is shut down.

Step 3

Router(config-router)# exit

Exits router configuration mode.

Step 4

Router(config)# exit

Exits configuration mode and places the router in privileged EXEC mode.

Step 5

Router# show ip ospf

Displays general information about OSPF routing processes. The show ip ospf command is entered in order to verify that the max-metric router-lsa command has been enabled before the router is shut down or reloaded.

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.

Verifying the Advertisement of a Maximum Metric

To verify that the advertisement of a maximum metric has been configured correctly, use the show ip ospf or show ip ospf database command.

The output of the show ip ospf command will display the condition, state, and remaining time delay of the advertisement of a maximum metric, depending on which options were configured with the max-metric router-lsa command.

The following sample output is similar to the output that will be displayed when the on-startup keyword and announce-time argument are configured with the max-metric router-lsa command:
Router# show ip ospf
Routing Process "ospf 1998" with ID 10.18.134.155
     Supports only single TOS(TOS0) routes
     Supports opaque LSA
     It is an area border and autonomous system boundary router
     Redistributing External Routes from,
        static, includes subnets in redistribution
     Originating router-LSAs with maximum metric, Time remaining: 00:01:18
        Condition: on startup for 300 seconds, State: active
     SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
     Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
     Number of external LSA 7. Checksum Sum 0x47261 
     Number of opaque AS LSA 0. Checksum Sum 0x0     
     Number of DCbitless external and opaque AS LSA 0
     Number of DoNotAge external and opaque AS LSA 0
     Number of areas in this router is 2. 1 normal 0 stub 1 nssa
     External flood list length 0
        Area BACKBONE(0)
           Number of interfaces in this area is 1
           Area has no authentication
           SPF algorithm executed 3 times
           Area ranges are
           Number of LSA 8. Checksum Sum 0x474AE 
           Number of opaque link LSA 0. Checksum Sum 0x0 
The following sample output is similar to the output that will be displayed when the on-startup and wait-for-bgp keywords are configured with the max-metric router-lsa command:
Router# show ip ospf
Routing Process "ospf 1998" with ID 10.18.134.155
     Supports only single TOS(TOS0) routes
     Supports opaque LSA
     It is an area border and autonomous system boundary router
     Redistributing External Routes from,
        static, includes subnets in redistribution
     Originating router-LSAs with maximum metric, Time remaining: 00:01:18
        Condition: on startup while BGP is converging, State: active
     SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
     Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
     Number of external LSA 7. Checksum Sum 0x47261 
     Number of opaque AS LSA 0. Checksum Sum 0x0     
     Number of DCbitless external and opaque AS LSA 0
     Number of DoNotAge external and opaque AS LSA 0
     Number of areas in this router is 2. 1 normal 0 stub 1 nssa
     External flood list length 0
        Area BACKBONE(0)
           Number of interfaces in this area is 1
           Area has no authentication
           SPF algorithm executed 3 times
           Area ranges are
           Number of LSA 8. Checksum Sum 0x474AE 
           Number of opaque link LSA 0. Checksum Sum 0x0 
The following sample output is similar to the output that will be displayed when the max-metric router-lsa command is configured without any keywords or arguments:
Router# show ip ospf
Routing Process "ospf 1998" with ID 10.18.134.155
     Supports only single TOS(TOS0) routes
     Supports opaque LSA
     It is an area border and autonomous system boundary router
     Redistributing External Routes from,
        static, includes subnets in redistribution
     Originating router-LSAs with maximum metric
        Condition: always, State: active
     SPF schedule delay 5 secs, Hold time between two SPFs 10 secs
     Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs
     Number of external LSA 7. Checksum Sum 0x47261 
     Number of opaque AS LSA 0. Checksum Sum 0x0     
     Number of DCbitless external and opaque AS LSA 0
     Number of DoNotAge external and opaque AS LSA 0
     Number of areas in this router is 2. 1 normal 0 stub 1 nssa
     External flood list length 0
        Area BACKBONE(0)
           Number of interfaces in this area is 1
           Area has no authentication
           SPF algorithm executed 3 times
           Area ranges are
           Number of LSA 8. Checksum Sum 0x474AE 
           Number of opaque link LSA 0. Checksum Sum 0x0 
The output of the show ip ospf database command will display information about OSPF LSAs and indicate if the router is announcing maximum cost links. The following sample output is similar to the output that will be displayed when any form of the max-metric router-lsa command is configured:
Router# show ip ospf database
      Exception Flag: Announcing maximum link costs
      LS age: 68
      Options: (No TOS-capability, DC)
      LS Type: Router Links
      Link State ID: 172.18.134.155
      Advertising Router: 172.18.134.155
      LS Seq Number: 80000002
      Checksum: 0x175D
      Length: 60
      Area Border Router
      AS Boundary Router
      Number of Links: 3
        Link connected to: a Transit Network
        (Link ID) Designated Router address: 192.168.1.11
        (Link Data) Router Interface address: 192.168.1.14
         Number of TOS metrics: 0
          TOS 0 Metrics: 65535 (metric used for local calculation: 10)
        Link connected to: a Transit Network
        (Link ID) Designated Router address: 10.1.145.11
        (Link Data) Router Interface address: 10.1.145.14
         Number of TOS metrics: 0
          TOS 0 Metrics: 65535 (metric used for local calculation: 10)
        Link connected to: a Stub Network
        (Link ID) Network/subnet number: 10.11.12.0
        (Link Data) Network Mask: 255.255.255.0
         Number of TOS metrics: 0
          TOS 0 Metrics: 1 
Monitoring and Maintaining OSPF Stub Router Advertisement

To monitor and maintain the advertisement of a maximum metric, use the following EXEC commands:

Command

Purpose

Router# show ip ospf

Displays general information about OSPF routing processes and provides information about the configuration settings and status of the OSPF Stub Router Advertisement feature.

Router# show ip ospf database router

Displays information about router LSAs, and indicates if a router is announcing maximum link costs.

Configuration Examples

This section provides the following configuration examples:

•Advertisement on Startup Example

•Advertisement Until Routing Tables Converge Example

•Graceful Shutdown Example

Advertisement on Startup Example

In the following example, a router that is running OSPF is configured to advertise a maximum metric at startup for 300 seconds:
Router(config)# router ospf 100
 Router(config-router)# max-metric router-lsa on-startup 300
Advertisement Until Routing Tables Converge Example

In the following example, a router that is running OSPF is configured to advertise a maximum metric until BGP routing tables converge or until the default timer expires (600 seconds):
Router(config)# router ospf 100
 Router(config-router)# max-metric router-lsa on-startup wait-for-bgp
Graceful Shutdown Example

In the following example, a router that is running OSPF is configured to advertise a maximum metric until the router is shut down:
Router(config)# router ospf 100
 Router(config-router)# max-metric router-lsa
 Router(config-router)# exit
 Router(config)# exit
 Router# show ip ospf
Command Reference

The following commands are introduced or modified in the feature or features documented in this module. For information about these commands, see the Cisco IOS IP Routing Protocols Command Reference at http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_book.html. For information about all Cisco IOS commands, go to the Command Lookup Tool at http://tools.cisco.com/Support/CLILookup or to the Cisco IOS Master Commands List.

•max-metric router-lsa

•show ip ospf

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