Cisco IOS XE IP Routing Protocols Configuration Guide, Release 2
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Table Of Contents

OSPF Stub Router Advertisement

Finding Feature Information

Contents

Information About OSPF Stub Router Advertisement

OSPF Stub Router Advertisement Functionality

Maximum Metric Allows Routing Tables to Converge

Maximum Metric Allows Graceful Shutdown of a Router

Benefits of OSPF Stub Router Advertisement

How to Configure OSPF Stub Router Advertisement

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 of OSPF Stub Router Advertisement

Advertisement on Startup: Example

Advertisement Until Routing Tables Converge: Example

Graceful Shutdown: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for OSPF Stub Router Advertisement


OSPF Stub Router Advertisement

Last Updated: May 4, 2009

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.

Finding Feature Information

For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for OSPF Stub Router Advertisement" section.

Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Information About OSPF Stub Router Advertisement

•How to Configure OSPF Stub Router Advertisement

•Configuration Examples of OSPF Stub Router Advertisement

•Additional References

•Feature Information for OSPF Stub Router Advertisement

Information About OSPF Stub Router Advertisement

Before you configure the OSPF Stub Router Advertisement feature, you should understand the following concepts:

•OSPF Stub Router Advertisement Functionality

•Maximum Metric Allows Routing Tables to Converge

•Maximum Metric Allows Graceful Shutdown of a Router

•Benefits of OSPF Stub Router Advertisement

OSPF Stub Router Advertisement Functionality

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.

Maximum Metric Allows 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.

Maximum Metric Allows Graceful Shutdown of a Router

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 of OSPF Stub Router Advertisement

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.

How to Configure OSPF Stub Router Advertisement

The following tasks 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)

•Monitoring and Maintaining OSPF Stub Router Advertisement (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)# end

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

Step 4 

Router# show ip ospf

Displays general information about OSPF routing processes.

•Use the show ip ospf command 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 of OSPF Stub Router Advertisement

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)# end

 Router# show ip ospf

Additional References

The following sections provide references related to OSPF Stub Router Advertisement.

Related Documents

Related Topic
Document Title

Configuring OSPF

"Configuring OSPF" chapter in the Cisco IOS XE IP Routing Protocols Configuration Guide, Release 2

OSPF commands

Cisco IOS IP Routing Protocols Command Reference

Cisco IOS master command list, all releases

Cisco IOS Master Command List, All Releases


Standards

Standard
Title

None

—


MIBs

MIB
MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco IOS XE software releases , and feature sets, use Cisco MIB Locator found at the following URL:

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


RFCs

RFC
Title

RFC 3137

OSPF Stub Router Advertisement


Technical Assistance

Description
Link

The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies.

To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds.

Access to most tools on the Cisco Support website requires a Cisco.com user ID and password.

http://www.cisco.com/techsupport


Feature Information for OSPF Stub Router Advertisement

Table 1 lists the release history for this feature.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 1 lists only the Cisco IOS XE software release that introduced support for a given feature in a given Cisco IOS XE software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE software release train also support that feature.

Table 1 Feature Information for OSPF Stub Router Advertisement

Feature Name
Releases
Feature Information

OSPF Stub Router Advertisement

Cisco IOS XE
Release 2.1

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.

The following commands are introduced or modified in the feature documented in this module:

•max-metric router-lsa

•show ip ospf


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