BGP Support for Next-Hop Address Tracking

The BGP Support for Next-Hop Address Tracking feature is enabled by default when a supporting Cisco software image is installed. BGP next-hop address tracking is event driven. BGP prefixes are automatically tracked as peering sessions are established. Next-hop changes are rapidly reported to the BGP routing process as they are updated in the RIB. This optimization improves overall BGP convergence by reducing the response time to next-hop changes for routes installed in the RIB. When a bestpath calculation is run in between BGP scanner cycles, only next-hop changes are tracked and processed.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and 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 table at the end of this module.

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.

Information About BGP Support for Next-Hop Address Tracking

BGP Next-Hop Address Tracking

The BGP next-hop address tracking feature is enabled by default when a supporting Cisco software image is installed. BGP next-hop address tracking is event driven. BGP prefixes are automatically tracked as peering sessions are established. Next-hop changes are rapidly reported to the BGP routing process as they are updated in the RIB. This optimization improves overall BGP convergence by reducing the response time to next-hop changes for routes installed in the RIB. When a best-path calculation is run in between BGP scanner cycles, only next-hop changes are tracked and processed.

BGP Next-Hop Dampening Penalties

If the penalty threshold value is higher than 950, then the delay is calculated as the reuse time using the dampening calculations. The dampening calculations use the following parameters:

  • Penalty

  • Half-life time

  • Reuse time

  • max-suppress-time

The values for the dampening parameters used are a max-suppress-time of 60 seconds, the half-life of 8 seconds, and the reuse-limit of 100.

For example, if the original penalty of 1600 is added, then after 16 seconds it becomes 800, and after 40 seconds, the penalty becomes 100. Hence, for the route update penalty of 1600, a delay of 40 seconds is used to schedule the BGP scanner.

These parameters (penalty threshold and any of the dampening parameters) cannot be modified.

Default BGP Scanner Behavior

BGP monitors the next hop of installed routes to verify next-hop reachability and to select, install, and validate the BGP best path. By default, the BGP scanner is used to poll the RIB for this information every 60 seconds. During the 60 second time period between scan cycles, Interior Gateway Protocol (IGP) instability or other network failures can cause black holes and routing loops to temporarily form.

BGP Next_Hop Attribute

The Next_Hop attribute identifies the next-hop IP address to be used as the BGP next hop to the destination. The device makes a recursive lookup to find the BGP next hop in the routing table. In external BGP (eBGP), the next hop is the IP address of the peer that sent the update. Internal BGP (iBGP) sets the next-hop address to the IP address of the peer that advertised the prefix for routes that originate internally. When any routes to iBGP that are learned from eBGP are advertised, the Next_Hop attribute is unchanged.

A BGP next-hop IP address must be reachable in order for the device to use a BGP route. Reachability information is usually provided by the IGP, and changes in the IGP can influence the forwarding of the next-hop address over a network backbone.

Selective BGP Next-Hop Route Filtering

BGP selective next-hop route filtering was implemented as part of the BGP Selective Address Tracking feature to support BGP next-hop address tracking. Selective next-hop route filtering uses a route map to selectively define routes to help resolve the BGP next hop.

The ability to use a route map with the bgp nexthop command allows the configuration of the length of a prefix that applies to the BGP Next_Hop attribute. The route map is used during the BGP bestpath calculation and is applied to the route in the routing table that covers the next-hop attribute for BGP prefixes. If the next-hop route fails the route map evaluation, the next-hop route is marked as unreachable. This command is per address family, so different route maps can be applied for next-hop routes in different address families.


Note

Use route map on ASR series devices to set the next hop as BGP peer for the route and apply that route map in outbound direction towards the peer.


Note

Only match ip address and match source-protocol commands are supported in the route map. No set commands or other match commands are supported.

BGP Support for Fast Peering Session Deactivation

BGP Hold Timer

By default, the BGP hold timer is set to run every 180 seconds in Cisco software. This timer value is set as the default to protect the BGP routing process from instability that can be caused by peering sessions with other routing protocols. BGP devices typically carry large routing tables, so frequent session resets are not desirable.

BGP Fast Peering Session Deactivation

BGP fast peering session deactivation improves BGP convergence and response time to adjacency changes with BGP neighbors. This feature is event driven and configured on a per-neighbor basis. When this feature is enabled, BGP will monitor the peering session with the specified neighbor. Adjacency changes are detected and terminated peering sessions are deactivated in between the default or configured BGP scanning interval.

Selective Address Tracking for BGP Fast Session Deactivation

In Cisco IOS XE Release 2.1 and later releases, the BGP Selective Address Tracking feature introduced the use of a route map with BGP fast session deactivation. The route-map keyword and map-name argument are used with the neighbor fall-over BGP neighbor session command to determine if a peering session with a BGP neighbor should be reset when a route to the BGP peer changes. The route map is evaluated against the new route, and if a deny statement is returned, the peer session is reset. The route map is not used for session establishment.


Note

Only match ip address and match source-protocol commands are supported in the route map. No set commands or other match commands are supported.

How to Configure BGP Support for Next-Hop Address Tracking

Configuring BGP Next-Hop Address Tracking

The tasks in this section show how configure BGP next-hop address tracking. BGP next-hop address tracking significantly improves the response time of BGP to next-hop changes in the RIB. However, unstable Interior Gateway Protocol (IGP) peers can introduce instability to BGP neighbor sessions. We recommend that you aggressively dampen unstable IGP peering sessions to reduce the possible impact to BGP. For more details about configuring route dampening, see “Configuring BGP Route Dampening.”

Configuring BGP Selective Next-Hop Route Filtering

Perform this task to configure selective next-hop route filtering using a route map to filter potential next-hop routes. This task uses prefix lists and route maps to match IP addresses or source protocols and can be used to avoid aggregate addresses and BGP prefixes being considered as next-hop routes. Only match ip address and match source-protocol commands are supported in the route map. No set commands or other match commands are supported.

For more examples of how to use the bgp nexthop command, see the “Examples: Configuring BGP Selective Next-Hop Route Filtering” section in this module.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    router bgp autonomous-system-number

    4.    address-family ipv4 [unicast | multicast| vrf vrf-name]

    5.    bgp nexthop route-map map-name

    6.    exit

    7.    exit

    8.    ip prefix-list list-name [seq seq-value] {deny network / length | permit network/length} [ge ge-value] [le le-value]

    9.    route-map map-name [permit | deny] [sequence-number]

    10.    match ip address prefix-list prefix-list-name [prefix-list-name...]

    11.    exit

    12.    route-map map-name [permit | deny] [sequence-number]

    13.    end

    14.    show ip bgp [network] [network-mask]


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Device> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 configure terminal


    Example:
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3 router bgp autonomous-system-number


    Example:
    Device(config)# router bgp 45000
     

    Enters router configuration mode and creates a BGP routing process.

     
    Step 4 address-family ipv4 [unicast | multicast| vrf vrf-name]


    Example:
    Device(config-router)# address-family ipv4 unicast
     

    Specifies the IPv4 address family and enters address family configuration mode.

    • The unicast keyword specifies the IPv4 unicast address family. By default, the router is placed in address family configuration mode for the IPv4 unicast address family if the unicast keyword is not specified with the address-family ipv4 command.

    • The multicast keyword specifies IPv4 multicast address prefixes.

    • The vrf keyword and vrf-name argument specify the name of the VRF instance to associate with subsequent IPv4 address family configuration mode commands.

     
    Step 5 bgp nexthop route-map map-name


    Example:
    Device(config-router-af)# bgp nexthop route-map CHECK-NEXTHOP
     

    Permits a route map to selectively define routes to help resolve the BGP next hop.

    • In this example the route map named CHECK-NEXTHOP is created.

     
    Step 6 exit


    Example:
    Device(config-router-af)# exit
     

    Exits address family configuration mode and enters router configuration mode.

     
    Step 7 exit


    Example:
    Device(config-router)# exit
     

    Exits router configuration mode and enters global configuration mode.

     
    Step 8 ip prefix-list list-name [seq seq-value] {deny network / length | permit network/length} [ge ge-value] [le le-value]


    Example:
    Device(config)# ip prefix-list FILTER25 seq 5 permit 0.0.0.0/0 le 25
     

    Creates a prefix list for BGP next-hop route filtering.

    • Selective next-hop route filtering supports prefix length matching or source protocol matching on a per address-family basis.

    • The example creates a prefix list named FILTER25 that permits routes only if the mask length is more than 25; this will avoid aggregate routes being considered as the next-hop route.

     
    Step 9 route-map map-name [permit | deny] [sequence-number]


    Example:
    Device(config)# route-map CHECK-NEXTHOP deny 10
     

    Configures a route map and enters route map configuration mode.

    • In this example, a route map named CHECK-NEXTHOP is created. If there is an IP address match in the following match command, the IP address will be denied.

     
    Step 10 match ip address prefix-list prefix-list-name [prefix-list-name...]


    Example:
    Device(config-route-map)# match ip address prefix-list FILTER25
     

    Matches the IP addresses in the specified prefix list.

    • Use the prefix-list-name argument to specify the name of a prefix list. The ellipsis means that more than one prefix list can be specified.

    Note   

    Only the syntax applicable to this task is used in this example. For more details, see the Cisco IOS IP Routing: BGP Command Reference.

     
    Step 11 exit


    Example:
    Device(config-route-map)# exit
     

    Exits route map configuration mode and enters global configuration mode.

     
    Step 12 route-map map-name [permit | deny] [sequence-number]


    Example:
    Device(config)# route-map CHECK-NEXTHOP permit 20
     

    Configures a route map and enters route map configuration mode.

    • In this example, all other IP addresses are permitted by route map CHECK-NEXTHOP.

     
    Step 13 end


    Example:
    Device(config-route-map)# end
     

    Exits route map configuration mode and enters privileged EXEC mode.

     
    Step 14 show ip bgp [network] [network-mask]


    Example:
    Device# show ip bgp
     

    Displays the entries in the BGP routing table.

    • Enter this command to view the next-hop addresses for each route.

    Note   

    Only the syntax applicable to this task is used in this example. For more details, see the Cisco IOS IP Routing: BGP Command Reference.

     

    Example

    The following example from the show ip bgp command shows the next-hop addresses for each route: 

    BGP table version is 7, local router ID is 172.17.1.99
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
   Network          Next Hop            Metric LocPrf Weight Path
*  10.1.1.0/24      192.168.1.2              0             0 40000 i
*  10.2.2.0/24      192.168.3.2              0             0 50000 i
*> 172.16.1.0/24    0.0.0.0                  0         32768 i
*> 172.17.1.0/24    0.0.0.0                  0         32768

    Adjusting the Delay Interval for BGP Next-Hop Address Tracking

    Perform this task to adjust the delay interval between routing table walks for BGP next-hop address tracking.

    You can increase the performance of this feature by tuning the delay interval between full routing table walks to match the tuning parameters for the Interior Gateway protocol (IGP). The default delay interval is 5 seconds. This value is optimal for a fast-tuned IGP. In the case of an IGP that converges more slowly, you can change the delay interval to 20 seconds or more, depending on the IGP convergence time.

    BGP next-hop address tracking significantly improves the response time of BGP to next-hop changes in the RIB. However, unstable Interior Gateway Protocol (IGP) peers can introduce instability to BGP neighbor sessions. We recommend that you aggressively dampen unstable IGP peering sessions to reduce the possible impact to BGP.

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    router bgp autonomous-system-number

      4.    address-family ipv4 [[mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name] | vpnv4 [unicast]]

      5.    bgp nexthop trigger delay delay-timer

      6.    end


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example: 
      Device> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.

       
      Step 2 configure terminal


      Example: 
      Device# configure terminal
       

      Enters global configuration mode.

       
      Step 3 router bgp autonomous-system-number


      Example: 
      Device(config)# router bgp 64512
       

      Enters router configuration mode to create or configure a BGP routing process.

       
      Step 4 address-family ipv4 [[mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name] | vpnv4 [unicast]]


      Example: 
      Device(config-router)# address-family ipv4 unicast
       

      Enter address family configuration mode to configure BGP peers to accept address family-specific configurations.

      • The example creates an IPv4 unicast address family session.

       
      Step 5 bgp nexthop trigger delay delay-timer


      Example: 
      Device(config-router-af)# bgp nexthop trigger delay 20
       

      Configures the delay interval between routing table walks for next-hop address tracking.

      • The time period determines how long BGP will wait before starting a full routing table walk after notification is received.

      • The value for the delay-timer argument is a number from 1 to 100 seconds. The default value is 5 seconds.

      • The example configures a delay interval of 20 seconds.

       
      Step 6 end


      Example: 
      Device(config-router-af)# end
       

      Exits address-family configuration mode, and enters privileged EXEC mode.

       

      Disabling BGP Next-Hop Address Tracking

      Perform this task to disable BGP next-hop address tracking. BGP next-hop address tracking is enabled by default under the IPv4 and VPNv4 address families. Beginning with Cisco IOS Release 12.2(33)SB6, BGP next-hop address tracking is also enabled by default under the VPNv6 address family whenever the next hop is an IPv4 address mapped to an IPv6 next-hop address.

      Disabling next hop address tracking may be useful if you the network has unstable IGP peers and route dampening is not resolving the stability issues. To reenable BGP next-hop address tracking, use the bgp nexthopcommand with the trigger and enable keywords.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    router bgp autonomous-system-number

        4.    address-family ipv4 [[mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name] | vpnv4 [unicast] | vpnv6 [unicast]]

        5.    no bgp nexthop trigger enable

        6.    end


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 enable


        Example: 
        Device> enable
         

        Enables privileged EXEC mode.

        • Enter your password if prompted.

         
        Step 2 configure terminal


        Example: 
        Device# configure terminal
         

        Enters global configuration mode.

         
        Step 3 router bgp autonomous-system-number


        Example: 
        Device(config)# router bgp 64512
         

        Enters router configuration mod to create or configure a BGP routing process.

         
        Step 4 address-family ipv4 [[mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name] | vpnv4 [unicast] | vpnv6 [unicast]]


        Example: 
        Device(config-router)# address-family ipv4 unicast
         

        Enter address family configuration mode to configure BGP peers to accept address family-specific configurations.

        • The example creates an IPv4 unicast address family session.

         
        Step 5 no bgp nexthop trigger enable


        Example: 
        Device(config-router-af)# no bgp nexthop trigger enable
         

        Disables BGP next-hop address tracking.

        • Next-hop address tracking is enabled by default for IPv4 and VPNv4 address family sessions.

        • The example disables next-hop address tracking.

         
        Step 6 end


        Example: 
        Device(config-router-af)# end
         

        Exits address-family configuration mode, and enters Privileged EXEC mode.

         

        Configuring Fast Session Deactivation

        The tasks in this section show how to configure BGP next-hop address tracking. BGP next-hop address tracking significantly improves the response time of BGP to next-hop changes in the RIB. However, unstable Interior Gateway Protocol (IGP) peers can introduce instability to BGP neighbor sessions. We recommend that you aggressively dampen unstable IGP peering sessions to reduce the possible impact to BGP. For more details about route dampening, see the "Configuring Internal BGP Features" module.

        Configuring Fast Session Deactivation for a BGP Neighbor

        Perform this task to establish a peering session with a BGP neighbor and then configure the peering session for fast session deactivation to improve the network convergence time if the peering session is deactivated.

        Enabling fast session deactivation for a BGP neighbor can significantly improve BGP convergence time. However, unstable IGP peers can still introduce instability to BGP neighbor sessions. We recommend that you aggressively dampen unstable IGP peering sessions to reduce the possible impact to BGP.

        SUMMARY STEPS

          1.    enable

          2.    configure terminal

          3.    router bgp autonomous-system-number

          4.    address-family ipv4 [mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name]

          5.    neighbor ip-address remote-as autonomous-system-number

          6.    neighbor ip-address fall-over

          7.    end


        DETAILED STEPS
           Command or ActionPurpose
          Step 1 enable


          Example: 
          Device> enable
           

          Enables privileged EXEC mode.

          • Enter your password if prompted.

           
          Step 2 configure terminal


          Example: 
          Device# configure terminal
           

          Enters global configuration mode.

           
          Step 3 router bgp autonomous-system-number


          Example: 
          Device(config)# router bgp 50000
           

          Enters router configuration mode to create or configure a BGP routing process.

           
          Step 4 address-family ipv4 [mdt | multicast | tunnel | unicast [vrf vrf-name] | vrf vrf-name]


          Example: 
          Device(config-router)# address-family ipv4 unicast
           

          Enters address family configuration mode to configure BGP peers to accept address family-specific configurations.

          • The example creates an IPv4 unicast address family session.

           
          Step 5 neighbor ip-address remote-as autonomous-system-number


          Example: 
          Device(config-router-af)# neighbor 10.0.0.1 remote-as 50000
           

          Establishes a peering session with a BGP neighbor.

           
          Step 6 neighbor ip-address fall-over


          Example: 
          Device(config-router-af)# neighbor 10.0.0.1 fall-over
           

          Configures the BGP peering to use fast session deactivation.

          • BGP will remove all routes learned through this peer if the session is deactivated.

           
          Step 7 end


          Example: 
          Device(config-router-af)# end
           

          Exits configuration mode and returns to privileged EXEC mode.

           

          Configuring Selective Address Tracking for Fast Session Deactivation

          Perform this task to configure selective address tracking for fast session deactivation. The optional route-map keyword and map-name argument of the neighbor fall-over command are used to determine if a peering session with a BGP neighbor should be deactivated (reset) when a route to the BGP peer changes. The route map is evaluated against the new route, and if a deny statement is returned, the peer session is reset.


          Note

          Only match ip address and match source-protocol commands are supported in the route map. No set commands or other match commands are supported.

          SUMMARY STEPS

            1.    enable

            2.    configure terminal

            3.    router bgp autonomous-system-number

            4.    neighbor {ip-address| peer-group-name} remote-as autonomous-system-number

            5.    neighbor ip-address fall-over [route-map map-name]

            6.    exit

            7.    ip prefix-list list-name [seq seq-value]{deny network / length | permit network / length}[ge ge-value] [le le-value]

            8.    route-map map-name [permit | deny][sequence-number]

            9.    match ip address prefix-list prefix-list-name [prefix-list-name...]

            10.    end


          DETAILED STEPS
             Command or ActionPurpose
            Step 1 enable


            Example: 
            Device> enable
             

            Enables privileged EXEC mode.

            • Enter your password if prompted.

             
            Step 2 configure terminal


            Example: 
            Device# configure terminal
             

            Enters global configuration mode.

             
            Step 3 router bgp autonomous-system-number


            Example: 
            Device(config)# router bgp 45000
             

            Enters router configuration mode for the specified routing process.

             
            Step 4 neighbor {ip-address| peer-group-name} remote-as autonomous-system-number


            Example: 
            Device(config-router)# neighbor 192.168.1.2 remote-as 40000
             

            Adds the IP address or peer group name of the neighbor in the specified autonomous system to the IPv4 multiprotocol BGP neighbor table of the local router.

             
            Step 5 neighbor ip-address fall-over [route-map map-name]


            Example: 
            Device(config-router)# neighbor 192.168.1.2 fall-over route-map CHECK-NBR
             

            Applies a route map when a route to the BGP changes.

            • In this example, the route map named CHECK-NBR is applied when the route to neighbor 192.168.1.2 changes.

             
            Step 6 exit


            Example: 
            Device(config-router)# exit
             

            Exits router configuration mode and enters global configuration mode.

             
            Step 7 ip prefix-list list-name [seq seq-value]{deny network / length | permit network / length}[ge ge-value] [le le-value]


            Example: 
            Device(config)# ip prefix-list FILTER28 seq 5 permit 0.0.0.0/0 ge 28
             

            Creates a prefix list for BGP next-hop route filtering.

            • Selective next-hop route filtering supports prefix length matching or source protocol matching on a per-address-family basis.

            • The example creates a prefix list named FILTER28 that permits routes only if the mask length is greater than or equal to 28.

             
            Step 8 route-map map-name [permit | deny][sequence-number]


            Example: 
            Device(config)# route-map CHECK-NBR permit 10
             

            Configures a route map and enters route-map configuration mode.

            • In this example, a route map named CHECK-NBR is created. If there is an IP address match in the following match command, the IP address will be permitted.

             
            Step 9 match ip address prefix-list prefix-list-name [prefix-list-name...]


            Example: 
            Device(config-route-map)# match ip address prefix-list FILTER28
             

            Matches the IP addresses in the specified prefix list.

            • Use the prefix-list-name argument to specify the name of a prefix list. The ellipsis means that more than one prefix list can be specified.

            Note   

            Only the syntax applicable to this task is used in this example. For more details, see the Cisco IOS IP Routing: BGP Command Reference.

             
            Step 10 end


            Example: 
            Device(config-route-map)# end
             

            Exits configuration mode and returns to privileged EXEC mode.

             

            Configuration Examples for BGP Support for Next-Hop Address Tracking

            Example: Enabling and Disabling BGP Next-Hop Address Tracking

            In the following example, next-hop address tracking is disabled under the IPv4 address family session: 

            router bgp 50000 
 address-family ipv4 unicast 
 no bgp nexthop trigger enable

            Example: Adjusting the Delay Interval for BGP Next-Hop Address Tracking

            In the following example, the delay interval for next-hop tracking is configured to occur every 20 seconds under the IPv4 address family session: 

            router bgp 50000 
 address-family ipv4 unicast 
 bgp nexthop trigger delay 20

            Examples: Configuring BGP Selective Next-Hop Route Filtering

            The following example shows how to configure BGP selective next-hop route filtering to avoid using a BGP prefix as the next-hop route. If the most specific route that covers the next hop is a BGP route, then the BGP route will be marked as unreachable. The next hop must be an IGP or static route. 

            router bgp 45000 
 address-family ipv4 unicast
 bgp nexthop route-map CHECK-BGP
 exit
 exit
route-map CHECK-BGP deny 10
 match source-protocol bgp 1
 exit
route-map CHECK-BGP permit 20
 end

            The following example shows how to configure BGP selective next-hop route filtering to avoid using a BGP prefix as the next-hop route and to ensure that the prefix is more specific than /25. 

            router bgp 45000 
 address-family ipv4 unicast
 bgp nexthop route-map CHECK-BGP25
 exit
 exit
ip prefix-list FILTER25 seq 5 permit 0.0.0.0/0 le 25
route-map CHECK-BGP25 deny 10
 match ip address prefix-list FILTER25
 exit
route-map CHECK-BGP25 deny 20
 match source-protocol bgp 1
 exit
route-map CHECK-BGP25 permit 30
 end

            Example: Configuring Fast Session Deactivation for a BGP Neighbor

            In the following example, the BGP routing process is configured on device A and device B to monitor and use fast peering session deactivation for the neighbor session between the two devices. Although fast peering session deactivation is not required at both devices in the neighbor session, it will help the BGP networks in both autonomous systems to converge faster if the neighbor session is deactivated.

            Device A

            router bgp 40000 
 neighbor 192.168.1.1 remote-as 45000 
 neighbor 192.168.1.1 fall-over 
 end

            Device B

            router bgp 45000 
 neighbor 192.168.1.2 remote-as 40000 
 neighbor 192.168.1.2 fall-over 
 end

            Example: Configuring Selective Address Tracking for Fast Session Deactivation

            The following example shows how to configure the BGP peering session to be reset if a route with a prefix of /28 or a more specific route to a peer destination is no longer available: 

            router bgp 45000 
 neighbor 192.168.1.2 remote-as 40000 
 neighbor 192.168.1.2 fall-over route-map CHECK-NBR
 exit
ip prefix-list FILTER28 seq 5 permit 0.0.0.0/0 ge 28
route-map CHECK-NBR permit 10
 match ip address prefix-list FILTER28
 end

            Additional References

            Related Documents

            Related Topic

            Document Title

            Cisco IOS commands

            Cisco IOS Master Command List, All Releases

            BGP commands

            Cisco IOS IP Routing: BGP Command Reference

            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 BGP Support for Next-Hop Address Tracking

            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 BGP Support for Next-Hop Address Tracking

            Feature Name

            Releases

            Feature Information

            BGP Support for Next-Hop Address Tracking

            12.0(29)S

            15.0(1)S

            The BGP Support for Next-Hop Address Tracking feature is enabled by default when a supporting Cisco IOS software image is installed. BGP next-hop address tracking is event driven. BGP prefixes are automatically tracked as peering sessions are established. Next-hop changes are rapidly reported to the BGP routing process as they are updated in the RIB. This optimization improves overall BGP convergence by reducing the response time to next-hop changes for routes installed in the RIB. When a bestpath calculation is run in between BGP scanner cycles, only next-hop changes are tracked and processed.

            The following command was introduced in this feature: bgp nexthop.

            BGP Selective Address Tracking

            The BGP Selective Address Tracking feature introduces the use of a route map for next-hop route filtering and fast session deactivation. Selective next-hop filtering uses a route map to selectively define routes to help resolve the BGP next hop, or a route map can be used to determine if a peering session with a BGP neighbor should be reset when a route to the BGP peer changes.

            The following commands were modified by this feature: bgp nexthop, neighbor fall-over.

            BGP Support for Fast Peering Session Deactivation

            12.0(29)S

            15.0(1)S

            The BGP Support for Fast Peering Session Deactivation feature introduced an event-driven notification system that allows a Border Gateway Protocol (BGP) process to monitor BGP peering sessions on a per-neighbor basis. This feature improves the response time of BGP to adjacency changes by allowing BGP to detect an adjacency change and deactivate the terminated session in between standard BGP scanning intervals. Enabling this feature improves overall BGP convergence.

            The following command was modified by this feature: neighbor fall-over.