Aggregating Routes on BGP

Posted in NETWORKING/IP ROUTING by edeguzman on October 3, 2007

This configuration shows the different ways of advertsing an aggregate routes to a peer. This configuration is placed in a border gateway of a AS. Below is the basic configuration on the applying of the aggregate route.

router eigrp 100
!
network 192.168.199.0
!
router bgp 100
network 192.168.192.0 mask 255.255.248.0
neighbor 192.168.1.253 remote-as 200
!
ip route 192.168.192.0 255.255.248.0 Null0

The network 192.168.192.0 mask 255.255.248.0 simply advertise the aggregate route to its peer. There is also a static entry in the configuration which its pointing to a Null0, this will static entry simply serve as black hole for the border gateway. Whenever a route matches the aggregate address 192.168.192.0 (take note that this 192.168.192.0 is the more-specific route in the border gateway’s routing table, so this will be the basis for summarization or aggregation of every route passing to the border gateway) it will be forwarded to the border gateway and the border gateway will check to its routing table the more-specific address that will match the route. If there is a match it will forward the packet to the destination but if there’s no match for the route then the route will be match to the static entry and this will go to the Null0 to be drop.

Below is another way of doing the aggregation.

router eigrp 100
passive-interface Serial0/0
network 192.168.199.0
no auto-summary
!
router bgp 100
bgp log-neighbor-changes
aggregate-address 192.168.192.0 255.255.248.0 summary-only
redistribute eigrp 100
neighbor 192.168.1.253 remote-as 200
!

aggregate-address 192.168.192.0 255.255.248.0 summary-only this command will advertise a aggregate address to its peer. The summary-only command makes the other routes suppressed, only the aggregate address is being advertised to its peer.

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