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