IOS XE BGP maximum-prefix feature imposes a maximum limit on the number of prefixes that are received from a neighbor for a given address family. Whenever the number of prefixes received exceeds the maximum number configured, the BGP session is terminated, which is the default behavior, after sending a cease notification to the neighbor. The session is down until a manual clear is performed by the user. The session can be resumed by using the clear bgp command. It is possible to configure a period after which the session can be automatically brought up by using the maximum prefix command with the restart keyword. The maximum prefix limit can be configured by the user.

Note	Maximum-Prefix feature on dynamic neighbors is only supported when the Persistent Dynamic Neighbors feature is configured. For more information refer Persistent Dynamic Neighbor

In earlier versions of IOS-XE, the logging of maximum-prefix warnings was limited to one warning per log type within a 60-second time window, regardless of the specific neighbor triggering the warning. This means that if multiple neighbors exceeded the maximum-prefix limit within a short time frame, only the first warning was logged, and subsequent warnings were considered time-limited and not logged individually. From Cisco IOS XE 17.13.1a, the enhancement ensures the logging of maximum-prefix warnings are now time-limited per-neighbor within a 60-second time window.

An option to discard extra is added to the maximum-prefix configuration. Configuring the discard extra option drops all excess prefixes received from the neighbor when the prefixes exceed the configured maximum value. This drop does not, however, result in session flap.

On the same lines, the following describes the actions when the maximum prefix value is changed:

• If the maximum value alone is changed, a route-refresh message is sourced, if applicable.

• If the new maximum value is greater than the current prefix count state, the new prefix states are saved.

• If the new maximum value is less than the current prefix count state, then some existing prefixes are deleted to match the new configured state value.

• When the router drops prefixes, it is inconsistent with the rest of the network, resulting in possible routing loops.

• If prefixes are dropped, the standby and active BGP sessions may drop different prefixes. Consequently, an NSR switchover results in inconsistent BGP tables.

• The discard extra configuration cannot co-exist with the soft reconfig configuration.

• There is currently no way to control which prefixes are deleted.

• A peer may withdraw prefixes after some prefixes have been discarded. This may result in having discarded prefixes and still be below the prefix limit. To recover discarded prefixes up to the prefix limit, users may perform a soft clear on the neighbor.

• All maximum-prefix sub-options are mutually exclusive, only one can be configured at a time for a given neighbor.

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.

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.

Border Gateway Protocol (BGP) imposes maximum limits on the maximum number of prefixes that are accepted from a peer for a given address family. This limitation safeguards the device from resource depletion caused by misconfiguration, either locally or on the remote neighbor. To prevent a peer from flooding BGP with advertisements, a limit is placed on the number of prefixes that are accepted from a peer for each supported address family. The default limits can be overridden through configuration of the maximum-prefix limit command for the peer for the appropriate address family.

A cease notification message is sent to the neighbor and the peering with the neighbor is terminated when the number of prefixes received from the peer for a given address family exceeds the maximum limit (either set by default or configured by the user) for that address family. It is possible that the maximum number of prefixes for a neighbor for a given address family has been configured after the peering with the neighbor has been established and a certain number of prefixes have already been received from the neighbor for that address family. A cease notification message is sent to the neighbor and peering with the neighbor is terminated immediately after the configuration if the configured maximum number of prefixes is fewer than the number of prefixes that have already been received from the neighbor for the address family.