OSPF Stub Router Advertisement

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.

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Information About 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

SUMMARY STEPS

    1.    Router(config)# router ospf process-id

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


DETAILED STEPS
     Command or ActionPurpose
    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

    SUMMARY STEPS

      1.    Router(config)# router ospf process-id

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


    DETAILED STEPS
       Command or ActionPurpose
      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

      SUMMARY STEPS

        1.    Router(config)# router ospfprocess-id

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

        3.    Router(config-router)# end

        4.    Router# show ip ospf


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 Router(config)# router ospfprocess-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.

         
        What to Do Next


        Note

        Do 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 ospfor show ip ospf databasecommand.

        The output of the show ip ospfcommand 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-startupkeyword 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-startupand 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 databasecommand 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

        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

        Example Advertisement on Startup

        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

        Example Advertisement Until Routing Tables Converge

        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

        Example Graceful Shutdown

        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

        Related Documents

        Related Topic

        Document Title

        Cisco IOS commands

        Cisco IOS Master Commands List, All Releases

        OSPF commands: complete command syntax, command mode, defaults, command history, usage guidelines, and examples

        Cisco IOS IP Routing: OSPF Command Reference

        Configuring OSPF

        “Configuring OSPF” in the IP Routing: OSPF Configuration Guide.

        OSPFv2 loop-free alternate fast reroute

        “OSPFv2 Loop-Free Alternate Fast Reroute” in the IP Routing: OSPF Configuration Guide

        Standards and RFCs

        Standard/RFC

        Title

        RFC 5286

        Basic Specification for IP Fast Reroute: Loop-Free Alternates

        Technical Assistance

        Description

        Link

        The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

        http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

        Feature Information for OSPF Stub Router Advertisement

        The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

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