IP Routing: BGP Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850 Switches)
BGP Restart Neighbor Session After Max-Prefix Limit Reached
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BGP Restart Neighbor Session After Max-Prefix Limit Reached

Last Updated: November 21, 2012

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

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 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 Reached

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 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:

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

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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 1Feature 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.

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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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