IP Routing: BGP Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850 Switches)
BGP Soft Reset
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BGP Soft Reset

BGP Soft Reset

BGP Soft Reset feature provides automatic support for dynamic soft reset of inbound BGP routing table updates that is not dependent upon stored routing table update information. The new method requires no preconfiguration (as with the neighbor soft-reconfiguration command) and requires much less memory than the previous soft reset method for inbound routing table updates.

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.

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 Soft Reset

BGP Session Reset

Whenever the routing policy changes due to a configuration change, BGP peering sessions must be reset by using the clear ip bgp command. Cisco software supports the following three mechanisms to reset BGP peering sessions:

  • Hard reset—A hard reset tears down the specified peering sessions including the TCP connection and deletes routes coming from the specified peer.

  • Soft reset—A soft reset uses stored prefix information to reconfigure and activate BGP routing tables without tearing down existing peering sessions. Soft reconfiguration uses stored update information, at the cost of additional memory for storing the updates, to allow you to apply new BGP policy without disrupting the network. Soft reconfiguration can be configured for inbound or outbound sessions.

  • Dynamic inbound soft reset—The route refresh capability, as defined in RFC 2918, allows the local device to reset inbound routing tables dynamically by exchanging route refresh requests to supporting peers. The route refresh capability does not store update information locally for nondisruptive policy changes. It instead relies on dynamic exchange with supporting peers. Route refresh must first be advertised through BGP capability negotiation between peers. All BGP devices must support the route refresh capability. To determine if a BGP device supports this capability, use the show ip bgp neighbors command. The following message is displayed in the output when the device supports the route refresh capability:

         Received route refresh capability from peer.

The bgp soft-reconfig-backup command was introduced to configure BGP to perform inbound soft reconfiguration for peers that do not support the route refresh capability. The configuration of this command allows you to configure BGP to store updates (soft reconfiguration) only as necessary. Peers that support the route refresh capability are unaffected by the configuration of this command.

Routing Policy Change Management

Routing policies for a peer include all the configurations for elements such as a route map, distribute list, prefix list, and filter list that may impact inbound or outbound routing table updates. Whenever there is a change in the routing policy, the BGP session must be soft-cleared, or soft-reset, for the new policy to take effect. Performing inbound reset enables the new inbound policy configured on the device to take effect. Performing outbound reset causes the new local outbound policy configured on the device to take effect without resetting the BGP session. As a new set of updates is sent during outbound policy reset, a new inbound policy of the neighbor can also take effect. This means that after changing inbound policy, you must do an inbound reset on the local device or an outbound reset on the peer device. Outbound policy changes require an outbound reset on the local device or an inbound reset on the peer device.

There are two types of reset: hard reset and soft reset. The table below lists their advantages and disadvantages.

Table 1 Advantages and Disadvantages of Hard and Soft Resets

Type of Reset

Advantages

Disadvantages

Hard reset

No memory overhead.

The prefixes in the BGP, IP, and Forwarding Information Base (FIB) tables provided by the neighbor are lost. A hard reset is not recommended.

Outbound soft reset

No configuration, and no storing of routing table updates.

Does not reset inbound routing table updates.

Dynamic inbound soft reset

Does not clear the BGP session and cache.

Does not require storing of routing table updates, and has no memory overhead.

Both BGP devices must support the route refresh capability.

Note   

Does not reset outbound routing table updates.

Configured inbound soft reset (uses the neighbor soft-reconfiguration router configuration command)

Can be used when both BGP devices do not support the automatic route refresh capability.

The bgp soft-reconfig-backup command was introduced to configure inbound soft reconfiguration for peers that do not support the route refresh capability.

Requires preconfiguration.

Stores all received (inbound) routing policy updates without modification; is memory-intensive.

Recommended only when absolutely necessary, such as when both BGP devices do not support the automatic route refresh capability.

Note   

Does not reset outbound routing table updates.

Once you have defined two devices to be BGP neighbors, they will form a BGP connection and exchange routing information. If you subsequently change a BGP filter, weight, distance, version, or timer, or if you make a similar configuration change, you must reset BGP connections in order for the configuration change to take effect.

A soft reset updates the routing table for inbound and outbound routing updates. Cisco software supports soft reset without any prior configuration. This soft reset allows the dynamic exchange of route refresh requests and routing information between BGP devices, and allows the subsequent readvertisement of the respective outbound routing table. There are two types of soft reset:

  • When soft reset is used to generate inbound updates from a neighbor, it is called dynamic inbound soft reset.

  • When soft reset is used to send a new set of updates to a neighbor, it is called outbound soft reset.

To use soft reset without preconfiguration, both BGP peers must support the soft route refresh capability, which is advertised in the OPEN message sent when the peers establish a TCP session.

How to Configure BGP Soft Reset

Performing BGP Dynamic Inbound Soft Reset

SUMMARY STEPS

    1.    enable

    2.    clear ip bgp {* | autonomous-system-number | neighbor-address} soft in


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Device> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.

     
    Step 2 clear ip bgp {* | autonomous-system-number | neighbor-address} soft in


    Example:
    Device# clear ip bgp 192.168.1.2 soft in
     

    Performs a dynamic soft reset on the connection specified.

     

    Performing BGP Outbound Soft Reset

    SUMMARY STEPS

      1.    enable

      2.    clear ip bgp {* | autonomous-system-number | neighbor-address} soft out


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example:
      Device> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.

       
      Step 2 clear ip bgp {* | autonomous-system-number | neighbor-address} soft out


      Example:
      Device# clear ip bgp 192.168.1.2 soft out
       

      Performs an outbound soft reset on the connection specified.

       

      Configuring Inbound Soft Reconfiguration When Route Refresh Capability Is Missing

      Perform this task to configure inbound soft reconfiguration using the bgp soft-reconfig-backup command for BGP peers that do not support the route refresh capability. BGP peers that support the route refresh capability are unaffected by the configuration of this command. Note that the memory requirements for storing the inbound update information can become quite large.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    router bgp autonomous-system-number

        4.    bgp log-neighbor-changes

        5.    bgp soft-reconfig-backup

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

        7.    neighbor {ip-address | peer-group-name} soft-reconfiguration [inbound]

        8.    neighbor {ip-address | peer-group-name} route-map map-name {in | out}

        9.    Repeat Steps 6 through 8 for every peer that is to be configured with inbound soft reconfiguration.

        10.    exit

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

        12.    set ip next-hop ip-address

        13.    end

        14.    show ip bgp neighbors [neighbor-address]

        15.    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 for the specified routing process.

         
        Step 4 bgp log-neighbor-changes


        Example:
        Device(config-router)# bgp log-neighbor-changes
         

        Enables logging of BGP neighbor resets.

         
        Step 5 bgp soft-reconfig-backup


        Example:
        Device(config-router)# bgp soft-reconfig-backup
         

        Configures a BGP speaker to perform inbound soft reconfiguration for peers that do not support the route refresh capability.

        • This command is used to configure BGP to perform inbound soft reconfiguration for peers that do not support the route refresh capability. The configuration of this command allows you to configure BGP to store updates (soft reconfiguration) only as necessary. Peers that support the route refresh capability are unaffected by the configuration of this command.

         
        Step 6 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 of the neighbor in the specified autonomous system to the IPv4 multiprotocol BGP neighbor table of the local device.

         
        Step 7 neighbor {ip-address | peer-group-name} soft-reconfiguration [inbound]


        Example:
        Device(config-router)# neighbor 192.168.1.2 soft-reconfiguration inbound
         

        Configures the Cisco software to start storing updates.

        • All the updates received from this neighbor will be stored unmodified, regardless of the inbound policy. When inbound soft reconfiguration is done later, the stored information will be used to generate a new set of inbound updates.

         
        Step 8 neighbor {ip-address | peer-group-name} route-map map-name {in | out}


        Example:
        Device(config-router)# neighbor 192.168.1.2 route-map LOCAL in
         

        Applies a route map to incoming or outgoing routes.

        • In this example, the route map named LOCAL will be applied to incoming routes.

         
        Step 9 Repeat Steps 6 through 8 for every peer that is to be configured with inbound soft reconfiguration.  

         
        Step 10 exit


        Example:
        Device(config-router)# exit
         

        Exits router configuration mode and enters global configuration mode.

         
        Step 11 route-map map-name [permit | deny] [sequence-number]


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

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

        • In this example, a route map named LOCAL is created.

         
        Step 12 set ip next-hop ip-address


        Example:
        Device(config-route-map)# set ip next-hop 192.168.1.144
         

        Specifies where output packets that pass a match clause of a route map for policy routing.

        • In this example, the ip address is set to 192.168.1.144.
         
        Step 13 end


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

        Exits route-map configuration mode and enters privileged EXEC mode.

         
        Step 14 show ip bgp neighbors [neighbor-address]


        Example:
        Device# show ip bgp neighbors 192.168.1.2
         

        (Optional) Displays information about the TCP and BGP connections to neighbors.

        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 15 show ip bgp [network] [network-mask]


        Example:
        Device# show ip bgp 
         

        (Optional) Displays the entries in the BGP routing table.

        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.

         

        Examples

        The following partial output from the show ip bgp neighbors command shows information about the TCP and BGP connections to the BGP neighbor 192.168.2.1. This peer supports route refresh.

        BGP neighbor is 192.168.1.2,  remote AS 40000, external link
         Neighbor capabilities:
           Route refresh: advertised and received(new)
        

        The following partial output from the show ip bgp neighbors command shows information about the TCP and BGP connections to the BGP neighbor 192.168.3.2. This peer does not support route refresh so the soft-reconfig inbound paths for BGP peer 192.168.3.2 will be stored because there is no other way to update any inbound policy updates.

        BGP neighbor is 192.168.3.2,  remote AS 50000, external link
         Neighbor capabilities:
           Route refresh: advertised
        

        The following sample output from the show ip bgp command shows the entry for the network 172.17.1.0. Both BGP peers are advertising 172.17.1.0/24, but only the received-only path is stored for 192.168.3.2.

        BGP routing table entry for 172.17.1.0/24, version 11
        Paths: (3 available, best #3, table Default-IP-Routing-Table, RIB-failure(4))
        Flag: 0x820
         Advertised to update-groups:
            1         
         50000
           192.168.3.2 from 192.168.3.2 (172.17.1.0)
             Origin incomplete, metric 0, localpref 200, valid, external
         50000, (received-only)
           192.168.3.2 from 192.168.3.2 (172.17.1.0)
             Origin incomplete, metric 0, localpref 100, valid, external
         40000
           192.168.1.2 from 192.168.1.2 (172.16.1.0)
             Origin incomplete, metric 0, localpref 200, valid, external, best

        Configuration Examples for BGP Soft Reset

        Examples: BGP Soft Reset

        The following examples show two ways to reset the connection for BGP peer 192.168.1.1.

        Example: Dynamic Inbound Soft Reset

        The following example shows the command used to initiate a dynamic soft reconfiguration in the BGP peer 192.168.1.1. This command requires that the peer support the route refresh capability.

        clear ip bgp 192.168.1.1 soft in

        Example: Inbound Soft Reset Using Stored Information

        The following example shows how to enable inbound soft reconfiguration for the neighbor 192.168.1.1. All the updates received from this neighbor will be stored unmodified, regardless of the inbound policy. When inbound soft reconfiguration is performed later, the stored information will be used to generate a new set of inbound updates.

        router bgp 100
         neighbor 192.168.1.1 remote-as 200
         neighbor 192.168.1.1 soft-reconfiguration inbound 
        

        The following example clears the session with the neighbor 192.168.1.1:

        clear ip bgp 192.168.1.1 soft in

        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

        Standards and RFCs

        Standard/RFC

        Title

        RFC 2918

        Route Refresh Capability for BGP-4

        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 Soft Reset

        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 . An account on Cisco.com is not required.
        Table 2 Feature Information for BGP Soft Reset

        Feature Name

        Releases

        Feature Information

        BGP Soft Reset

        Cisco IOS XE Release 3.2SE

        Cisco IOS XE Release 3.3SE

        BGP Soft Reset feature provides automatic support for dynamic soft reset of inbound BGP routing table updates that is not dependent upon stored routing table update information. The new method requires no preconfiguration (as with the neighbor soft-reconfiguration command) and requires much less memory than the previous soft reset method for inbound routing table updates.

        In Cisco IOS XE Release 3.3SE, support was added for the Cisco Catalyst 3650 Series Switches and Cisco Catalyst 3850 Series Switches.