BGP Summarization

[Nick Currie]

Hi there, hoping someone can help with a question i have regarding BGP network statements.

If I have a remote site with the following networks:

192.168.120.0/24

192.168.121.0/24

192.168.122.0/24

192.168.123.0/24

192.168.124.0/24

192.168.125.0/24

192.168.126.0/24

192.168.127.0/24

This can be summarized for static routing as 192.168.120.0/21

Is it possible to reduce the number of network statements in the BGP configuration to something similar, or do I have to include each actual class C network in the BGP configuration.

ie 

router bgp 5501
 no synchronization
 bgp log-neighbor-changes
 network 192.168.120.0
 network 192.168.121.0

etc

Thanks for any help with this! :-)

[Rolf Fischer · Accepted solution]

Hello Nick,

the following example shows a conditional aggregation (at least one more specific route within the specified range has to be in the BGP table):

router bgp 5501
 network 192.168.120.0
 [network 192.168.121.0]
 [network ...]
 aggregate-address 192.168.120.0 255.255.248.0 summary-only

Another (unconditional) method would be a static discard-route for the summary and one matching network- or redistribution statement under the BGP process:

ip route 192.168.120.0 255.255.248.0 null 0
router bgp 5501
 network 192.168.120.0 mask 255.255.248.0

(Note that the masks of static route and network statement need to match!)

HTH

Rolf

[Basically what Joseph has already stated while I was typing ...]

[Nick Currie]

Thanks guys, that's great. I have now successfully implemented both examples in my GNS3 environment, Rolf thanks for going the extra mile with config examples im very new to BGP and this is invaluable. Much appreciated!

[jmcgrady1]

regarding the use of 'aggregate-address'; does this summarize the advertisements TO BGP peers? Or does it summarize the advertisements received FROM peers?

Also, is it possible to summarize for advertisements to specific BGP neighbors?

[Joseph W. Doherty]

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Posting

Yes, it's possible to summarize within BGP.  You do not need to include a network statement for each /24.

You also can inject the summary into BGP alone, or you can pick up each active /24 within the summary range.

Instead of actually picking of the prefixes dynamically, often a null route will be used for the whole address block and a network statement used for that.