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.