BGP Aggregate experiment

By | January 2, 2019

The BGP aggregate-address can be used to summarise a set of networks into a single prefix. For this post, I just wanted to show the difference between aggregate-address and aggregate-address with summary only.

We have below topology. I’m going to summarise prefixes in R1.

R1 config

hostname R1
!
interface GigabitEthernet0/0
 ip address 10.10.10.1 255.255.255.252
!
router bgp 10
 bgp log-neighbor-changes
 network 192.168.1.0
 network 192.168.2.0
 network 192.168.3.0
 neighbor 10.10.10.2 remote-as 20
!
ip route 192.168.1.0 255.255.255.0 Null0
ip route 192.168.2.0 255.255.255.0 Null0
ip route 192.168.3.0 255.255.255.0 Null0
!

R2 config

hostname R2
!
interface GigabitEthernet0/0
 ip address 10.10.10.2 255.255.255.252
!
router bgp 20
 bgp log-neighbor-changes
 neighbor 10.10.10.1 remote-as 10
!

Case 1: without aggregate-address

R2#sh ip bgp
     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.1.0      10.10.10.1               0             0 10 i
 *>  192.168.2.0      10.10.10.1               0             0 10 i
 *>  192.168.3.0      10.10.10.1               0             0 10 i

Case 2: with aggregate-address
R1 config

router bgp 10
 bgp log-neighbor-changes
 network 192.168.1.0
 network 192.168.2.0
 network 192.168.3.0
 aggregate-address 192.168.0.0 255.255.252.0
Router#sh ip bgp
     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.0.0/22   10.10.10.1               0             0 10 i
 *>  192.168.1.0      10.10.10.1               0             0 10 i
 *>  192.168.2.0      10.10.10.1               0             0 10 i
 *>  192.168.3.0      10.10.10.1               0             0 10 i

Note that we will be having the original /24 routes (longer prefix) and summarised /22 route.

Case 3: aggregate-address with summary only
R1 config

router bgp 10
 bgp log-neighbor-changes
 network 192.168.1.0
 network 192.168.2.0
 network 192.168.3.0
 aggregate-address 192.168.0.0 255.255.252.0 summary-only
R2#sh ip bgp
     Network          Next Hop            Metric LocPrf Weight Path
 *>  192.168.0.0/22   10.10.10.1               0             0 10 i

All the longer-prefixes inside of the aggregate address are suppressed.