BGP Prefix Limitation

If a large amount of route information (prefix) is sent due to a neighbor’s misconfiguration, for example, the load on the router may increase and have an adverse effect on the router. Therefore, the neighbor maximum-prefix command is used to limit the number of route information received from the neighbor. This command prevents the router from being overloaded by limiting the number of route information received from the neighbor.

The command format for neighbor maximum-prefix is as follows.

BGP Prefix Limitation command syntax

Router(config)#router bgp <AS>
Router(config-router)#neighbor <ip-address> maximum-prefix <maximum> [<threshold>] [restart <restart-interval>] [warning-only]

<AS>:AS number
<ip-address>:neighbor IP address
<maximum>:The maximum number of route information to be received. By default, if the maximum number is exceeded, the neighbor is taken down.
<threshold>:Threshold to display warnings. Specified in %. Default 75%.
restart:Option to automatically re-establish the neighbor
<restart-interval>:The time it takes to automatically re-establish the neighbor. Minutes.
warning-only:If the maximum number of route information to be received is exceeded, it will only display warnings without taking down the neighbor.

BGP Prefix Limitation Configuration Example

The following is a simple example of configuring the BGP prefix limitation.

Four routes are sent from ISP1 to R1. set the maximum number of incoming routes to 8 at R1 as follows.

R1 maxmum-prefix

router bgp 100
 neighbor 172.16.1.11 maximum-prefix 8

Now, ISP1 sends more 100.1.4.0/24 and 100.1.5.0/24 and 100.1.6.0/24 to R1. R1 will then display the following warning message.

R1 maxmum-prefix warning

*Mar  1 01:31:02.293: %BGP-4-MAXPFX: No. of prefix received from 172.16.1.11 (afi 0) reaches 7, max 8

By default, the threshold is 75%. A warning is displayed because more routes were received than 6, which is 75% of the maximum number 8. In addition, ISP1 sends route information for 100.1.7.0/24 and 100.1.8.0/24 to R1.

R1 Exceeding the maximum prefix

R1#
*Mar  1 01:36:02.369: %BGP-4-MAXPFX: No. of prefix received from 172.16.1.11 (afi 0) reaches 8, max 8
R1#
*Mar  1 01:37:02.392: %BGP-3-MAXPFXEXCEED: No. of prefix received from 172.16.1.11 (afi 0):
9 exceed limit 8
*Mar  1 01:37:02.392: %BGP-5-ADJCHANGE: neighbor 172.16.1.11 Down BGP Notification sent
*Mar  1 01:37:02.392: %BGP-3-NOTIFICATION: sent to neighbor 172.16.1.11 3/1 (update malformed) 0 bytes
R1# FFFF FFFF FFFF FFFF FFFF FFFF FFFF FFFF 0034 0200 0000 1940 0101 0040 0204 0201 0001
4003 04AC 1001 0B80 0404 0000 0000 1864 0108
R1#

Since R1 has received a route that exceeds the maximum number of prefixes set by ISP1 of 8, we can see that a NOTIFICATION message has been sent to bring down the neighbor with ISP1. Also, if you look at the status of the neighbor in the show ip bgp summary, you can see that it is in a state where the number of prefixes is exceeded.

R1 show ip bgp summary

R1#show ip bgp summary
BGP router identifier 100.100.0.1, local AS number 100
BGP table version is 31, main routing table version 31
Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
172.16.1.11     4     1      88      98        0    0    0 00:02:30 Idle (PfxCt)

Then add a warning-only option to the maximum-prefix setting in R1.

R1 maximum-prefix warning-only

router bgp 100
 neighbor 172.16.1.11 maximum-prefix 8 warning-only

Then, even if more than eight routes are received, as shown below, you will only see a warning message and the neighbor will not go down.

R1 warning-only

*Mar  1 01:44:19.023: %BGP-5-ADJCHANGE: neighbor 172.16.1.11 Up
*Mar  1 01:44:19.128: %BGP-4-MAXPFX: No. of prefix received from 172.16.1.11 (afi 0) reaches 7, max 8
R1#
*Mar  1 01:44:19.132: %BGP-3-MAXPFXEXCEED: No. of prefix received from 172.16.1.11 (afi 0):
9 exceed limit 8

How the BGP works