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The BGP Restart Session After Max-Prefix Limit feature enhances the capabilities of the neighbor maximum-prefix command with the introduction of the restart keyword. This enhancement allows the network operator to configure the time interval at which a peering session is reestablished by a router when the number of prefixes that have been received from a peer has exceeded the maximum prefix limit. The restart keyword has a configurable timer argument that is specified in minutes. The time range of the timer argument is from 1 to 65535.
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This document assumes that BGP is configured in your network and that peering has been established.
This feature attempts to reestablish a disabled peering 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many prefixes can cause instability in the network, where an excessive number of prefixes are rapidly advertised and withdrawn.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 can be configured to disable the restart capability, while the network operator corrects the underlying problem.
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Note 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. The use of this command should be necessary only when troubleshooting or tuning a router that is sending an excessive number of prefixes. |
This section contains the following procedures:
There is a configurable limit on the maximum number of prefixes that a router that is running BGP can receive from a peer router. This limit is configured with the neighbor maximum-prefix command. When the router receives too many prefixes from a peer router 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. Entering the clear ip bgp command clears stored prefixes.
The BGP Restart Session After Maximum-Prefix Limit feature enhances the capabilities of the neighbor maximum-prefix command with the introduction of the restart keyword. This enhancement allows the network operator to configure a router to automatically reestablish a peering session when one has been disabled or brought down. There is configurable time interval at which peering can be reestablished automatically. The configurable timer argument for the restart keyword is specified in minutes. The time range is from 1 to 65,535 minutes.
This section contains the following procedures:
The network operator can configure a router that is running BGP to automatically reestablish a peering 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.
This feature attempts to reestablish a disabled peering 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 can be configured to disable the restart capability, while the network operator corrects the underlying problem.
5. neighbor {ip-address | peer-group-name} {maximum-prefix maximum [threshold]}
The commands in the following table can be useful for trouble shooting issues related to configuring this feature:
Display of the following error messages can indicate an underlying problem that is causing the peering session to become disabled. The network operator should check the values that are configured for the maximum-prefix limit and the configuration of any peers that are sending an excessive number of prefixes. The following sample error messages below are similar to the error messages that may be displayed:
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. The use of this command should be necessary only when troubleshooting or tuning a router that is sending an excessive number of prefixes.
1. show ip bgp neighbors ip-address
The following example sets the maximum number of prefixes allowed from the neighbor at 192.168.6.6 to 2000 and configures the router to reestablish a peering session after 30 minutes if one has been disabled:
neighbor 192.168.6.6 maximum-prefix 2000 restart 30
To verify that a router has been configured to automatically reestablish disabled peering sessions, use the show ip bgp neighbors command. The output of this command will display the status and configured restart timer value for the BGP Restart Session After Maximum-Prefix Limit feature. The following 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%
For additional information related to BGP Restart Session After Max-Prefix Limit feature, refer to the following references:
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This section documents modified commands. All other commands used with this feature are documented in the Cisco IOS command reference publications.
To control how many prefixes can be received from a neighbor, use the neighbor maximum-prefix command in router configuration mode. To disable this function, use the no form of this command.
Syntax Description
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Defaults
This command is disabled by default. If the restart-interval is not configured, a disabled session will stay down by default after the maximum-prefix limit is exceeded. There is no default limit on the number of prefixes that can be configured with this command. Limitations on the number of prefixes that can be configured are determined by the amount of available system resources and are configured by the network operator. Peering sessions will be disabled (by default) when the configured maximum number of prefixes has been exceeded.
Command Modes
Command History
Usage Guidelines
This command allows you to configure a maximum number of prefixes that a BGP router is allowed to receive from a peer. It adds another mechanism (in addition to distribute lists, filter lists, and route maps) to control prefixes received from a peer.
When the number of received prefixes exceeds the maximum number configured, the router disables the peering session (by default).
If the restart keyword is configured, the router will automatically reestablish the peering session at the configured time interval.
If the warning-only keyword is configured, the router instead only sends a log message, but continues peering with the sender. If the peer is terminated, the peer stays down until the clear ip bgp command is issued.
Examples
The following example sets the maximum number of prefixes allowed from the neighbor at 192.168.6.6 to 1000:
The following example sets the maximum number of prefixes allowed from the neighbor at 192.168.6.6 to 5000 and configures the router to display warning messages when the router reaches 2500 prefixes or 50 percent of the maximum-prefix limit:
neighbor 192.168.6.6 maximum-prefix 5000 50
The following example sets the maximum number of prefixes allowed from the neighbor at 192.168.6.6 to 2000 and configures the router to reestablish a peering session after 30 minutes if one has been disabled:
The following example sets the maximum number of prefixes allowed from the neighbor at 192.168.6.6 to 500 and configures a warning to be displayed when the maximum-prefix limit has been exceeded:
Related Commands
To display information about the TCP and Border Gateway Protocol (BGP) connections to neighbors, use the show ip bgp neighbors command in EXEC mode.
Syntax Description
Command Modes
Command History
Examples
The following is sample output from the show ip bgp neighbors command in privileged EXEC mode:
Table 1 describes the significant fields shown in the display.
The following is sample output from the show ip bgp neighbors command with the advertised-routes keyword in privileged EXEC mode:
The following is sample output from the show ip bgp neighbors command with the routes keyword in privileged EXEC mode:
Table 2 describes the significant fields shown in the displays.
The following is sample output from the show ip bgp neighbors command with the paths keyword in privileged EXEC mode:
Table 3 describes the significant fields shown in the display.
Posted: Thu Aug 21 13:55:46 PDT 2003
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