IP Routing: BGP Configuration Guide, Cisco IOS XE Release 3S (Cisco ASR 1000)
BGP Restart Neighbor Session After Max-Prefix Limit Reached
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BGP Restart Neighbor Session After Max-Prefix Limit Reached

The BGP Restart Session After Max-Prefix Limit Reached feature adds the restart keyword to the neighbor maximum-prefix command. This allows a network operator to configure the time interval at which a peering session is reestablished by a device when the number of prefixes that have been received from a peer has exceeded the maximum prefix limit.

Finding Feature Information

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Information About BGP Neighbor Session Restart After Max-Prefix Limit Reached

Prefix Limits and BGP Peering Sessions

Use the neighbor maximum-prefix command to limit the maximum number of prefixes that a device running BGP can receive from a peer. When the device receives too many prefixes from a peer and the maximum-prefix limit is exceeded, the peering session is disabled or brought down. The session stays down until the network operator manually brings the session back up by entering the clear ip bgp command, which clears stored prefixes.

BGP Neighbor Session Restart with the Maximum Prefix Limit

The restart keyword was added to the neighbor maximum-prefix command so that a network operator can configure a device to automatically reestablish a BGP neighbor peering session when the peering session has been disabled or brought down. The time interval at which peering can be reestablished automatically is configurable. The restart-interval for the restart keyword is specified in minutes; range is from 1 to 65,535 minutes.

How to Configure a Device to Reestablish a Neighbor Session After the Maximum Prefix Limit Has Been Exceeded

Configuring a Router to Reestablish a Neighbor Session After the Maximum Prefix Limit Has Been Exceeded

Perform this task to configure the time interval at which a BGP neighbor session is reestablished by a device when the number of prefixes that have been received from a BGP peer has exceeded the maximum prefix limit.

The network operator can configure a device running BGP to automatically reestablish a neighbor session that has been brought down because the configured maximum-prefix limit has been exceeded. No intervention from the network operator is required when this feature is enabled.


Note


This task attempts to reestablish a disabled BGP neighbor session at the configured time interval that is specified by the network operator. However, the configuration of the restart timer alone cannot change or correct a peer that is sending an excessive number of prefixes. The network operator will need to reconfigure the maximum-prefix limit or reduce the number of prefixes that are sent from the peer. A peer that is configured to send too many prefixes can cause instability in the network, where an excessive number of prefixes are rapidly advertised and withdrawn. In this case, the warning-only keyword of the neighbor maximum-prefix command can be configured to disable the restart capability while the network operator corrects the underlying problem.


SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    router bgp autonomous-system-number

    4.    neighbor {ip-address | peer-group-name} maximum-prefix maximum [threshold] [restart minutes] [warning-only]

    5.    end

    6.    show ip bgp neighbors ip-address


DETAILED STEPS
      Command or Action Purpose
    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:
    Router(config)# router bgp 101
     

    Enters router configuration mode and creates a BGP routing process.

     
    Step 4 neighbor {ip-address | peer-group-name} maximum-prefix maximum [threshold] [restart minutes] [warning-only]


    Example:
    Device(config-router)# neighbor 10.4.9.5 maximum-prefix 1000 90 restart 60
     

    Configures the maximum-prefix limit on a router that is running BGP.

    • Use the restart keyword and minutes argument to configure the router to automatically reestablish a neighbor session that has been disabled because the maximum-prefix limit has been exceeded. The configurable range of minutes is from 1 to 65535 minutes.
    • Use the warning-only keyword to configure the device to disable the restart capability to allow you to adjust a peer that is sending too many prefixes.
    Note   

    If the minutes argument is not configured, the disabled session will stay down after the maximum-prefix limit is exceeded. This is the default behavior.

     
    Step 5 end


    Example:
    Device(config-router)# end
     

    Exits router configuration mode and enters privileged EXEC mode.

     
    Step 6 show ip bgp neighbors ip-address


    Example:
    Device# show ip bgp neighbors 10.4.9.5
     

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

    • In this example, the output from this command will display the maximum prefix limit for the specified neighbor and the configured restart timer value.
    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 sample output from the show ip bgp neighbors command verifies that a device has been configured to automatically reestablish disabled neighbor sessions. The output shows that the maximum prefix limit for neighbor 10.4.9.5 is set to 1000 prefixes, the restart threshold is set to 90 percent, and the restart interval is set at 60 minutes.

    Device# show ip bgp neighbors 10.4.9.5
     
    BGP neighbor is 10.4.9.5,  remote AS 101, internal link
      BGP version 4, remote router ID 10.4.9.5
      BGP state = Established, up for 2w2d
      Last read 00:00:14, hold time is 180, keepalive interval is 60 seconds
      Neighbor capabilities:
        Route refresh: advertised and received(new)
        Address family IPv4 Unicast: advertised and received
      Message statistics:
        InQ depth is 0
        OutQ depth is 0
                             Sent       Rcvd
        Opens:                  1          1
        Notifications:          0          0
        Updates:                0          0
        Keepalives:         23095      23095
        Route Refresh:          0          0
        Total:              23096      23096
      Default minimum time between advertisement runs is 5 seconds
     For address family: IPv4 Unicast
      BGP table version 1, neighbor versions 1/0 1/0
      Output queue sizes : 0 self, 0 replicated
      Index 2, Offset 0, Mask 0x4
      Member of update-group 2
                                     Sent       Rcvd
      Prefix activity:               ----       ----
        Prefixes Current:               0          0
        Prefixes Total:                 0          0
        Implicit Withdraw:              0          0
        Explicit Withdraw:              0          0
        Used as bestpath:             n/a          0
        Used as multipath:            n/a          0
                                       Outbound    Inbound
      Local Policy Denied Prefixes:    --------    -------
        Total:                                0          0
    !Configured maximum number of prefixes and restart interval information!
      Maximum prefixes allowed 1000
      Threshold for warning message 90%, restart interval 60 min
      Number of NLRIs in the update sent: max 0, min 0
      Connections established 1; dropped 0
      Last reset never
    Connection state is ESTAB, I/O status: 1, unread input bytes: 0
    Local host: 10.4.9.21, Local port: 179
    Foreign host: 10.4.9.5, Foreign port: 11871
    Enqueued packets for retransmit: 0, input: 0  mis-ordered: 0 (0 bytes)
    Event Timers (current time is 0x5296BD2C):
    Timer          Starts    Wakeups            Next
    Retrans         23098          0             0x0
    TimeWait            0          0             0x0
    AckHold         23096      22692             0x0
    SendWnd             0          0             0x0
    KeepAlive           0          0             0x0
    GiveUp              0          0             0x0
    PmtuAger            0          0             0x0
    DeadWait            0          0             0x0
    iss: 1900546793  snduna: 1900985663  sndnxt: 1900985663     sndwnd:  14959
    irs: 2894590641  rcvnxt: 2895029492  rcvwnd:      14978  delrcvwnd:   1406
    SRTT: 300 ms, RTTO: 607 ms, RTV: 3 ms, KRTT: 0 ms
    minRTT: 0 ms, maxRTT: 316 ms, ACK hold: 200 ms
    Flags: passive open, nagle, gen tcbs
    Datagrams (max data segment is 1460 bytes):
    Rcvd: 46021 (out of order: 0), with data: 23096, total data bytes: 438850
    Sent: 46095 (retransmit: 0, fastretransmit: 0), with data: 23097, total data by9

    Troubleshooting Tips

    Use the clear ip bgp command to reset a BGP connection using BGP soft reconfiguration. This command can be used to clear stored prefixes to prevent a device that is running BGP from exceeding the maximum-prefix limit.

    Display of the following error messages can indicate an underlying problem that is causing the neighbor session to become disabled. You should check the values configured for the neighbor maximum-prefix command and the configuration of any peers that are sending an excessive number of prefixes. The following sample error messages are similar to the error messages that may be displayed:

    00:01:14:%BGP-5-ADJCHANGE:neighbor 10.10.10.2 Up
    00:01:14:%BGP-4-MAXPFX:No. of unicast prefix received from 10.10.10.2 reaches 5, max 6
    00:01:14:%BGP-3-MAXPFXEXCEED:No.of unicast prefix received from 10.10.10.2:7 exceed limit6
    00:01:14:%BGP-5-ADJCHANGE:neighbor 10.10.10.2 Down - BGP Notification sent
    00:01:14:%BGP-3-NOTIFICATION:sent to neighbor 10.10.10.2 3/1 (update malformed) 0 byte
    

    The bgp dampening command can be used to configure the dampening of a flapping route or interface when a peer is sending too many prefixes and causing network instability. Use this command only when troubleshooting or tuning a device that is sending an excessive number of prefixes. For more details about BGP route dampening, see the “Configuring Advanced BGP Features” module.

    Configuration Example for BGP Restart Neighbor Session After Max-Prefix Limit Reached

    Example: Restart Session After Maximum Number of Prefixes from Neighbor Reached

    The following example sets the maximum number of prefixes allowed from the neighbor at 192.168.6.6 to 2000 and configures the device to reestablish a peering session after 30 minutes if one has been disabled:

    router bgp 101
     network 172.16.0.0
     neighbor 192.168.6.6 maximum-prefix 2000 restart 30

    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 Restart Neighbor Session after Max-Prefix Limit

    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 Restart Session After Max-Prefix Limit

    Feature Name

    Releases

    Feature Information

    BGP Restart Session After Max-Prefix Limit

    12.0(22)S

    12.2(15)T

    12.2(18)S

    15.0(1)S

    Cisco IOS XE Release 2.1

    Cisco IOS XE Release 3.2SE

    The BGP Restart Session After Max-Prefix Limit Reached feature adds the restart keyword to the neighbor maximum-prefix command. This allows a network operator to configure the time interval at which a peering session is reestablished by a device when the number of prefixes that have been received from a peer has exceeded the maximum prefix limit.

    This feature was introduced on the Cisco ASR 1000 Series Aggregation Services Routers.

    The following commands were modified: neighbor maximum-prefix and show ip bgp neighbors.