IP Routing: BGP Configuration Guide, Cisco IOS XE Fuji 16.9.x
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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.
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module.
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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.
Subcodes for BGP Cease
Notification
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.
The following
subcodes are supported for the BGP cease notification message:
Maximum number
of prefixes reached
Administrative
shutdown
Peer
de-configured
Administrative
reset
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.
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 Reached
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.
Configures a
BGP neighbor to member of a peer group.
% keyword is the IPv6 link-local
address identifier. This keyword needs to be added whenever a link-local IPv6
address is used outside the context of its interface.
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 9
end
Example:
Device(config-router)# end
Exits router
configuration mode and enters privileged EXEC mode.
Step 10
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.
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: Configuring a Router
to Reestablish a Neighbor Session After the Maximum Prefix Limit
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:
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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
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.
The following commands were modified:
neighbor
maximum-prefix and
show ip bgp
neighbors .
BGP—Subcodes
for BGP Cease Notification
Support for
subcodes for BGP cease notification has been added.
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