BGP Auto-Summary

In a previous lesson I explained how the BGP network command works. When we enable auto-summary for BGP, the way the network command works changes slightly.

Normally when you advertise a network in BGP you have to type in the exact network and subnet mask that you want to advertise or it won’t be placed in the BGP table.

With auto-summary enabled, you can advertise a classful network and you don’t have to add the mask parameter. BGP will automatically advertise the classful network if you have the classful network or a subnet of this network in your routing table. Let me give you an example to explain what I’m talking about. I’ll use these two routers:

These routers are configured for eBGP, there’s a loopback interface on R1 with network 1.1.1.1 /32. Here’s the configuration:

R1#show run | section bgp
router bgp 1
 bgp log-neighbor-changes
 neighbor 192.168.12.2 remote-as 2

R2#show run | section bgp
router bgp 2
 bgp log-neighbor-changes
 neighbor 192.168.12.1 remote-as 1

The configuration is straight-forward, we only configured eBGP, no networks have been advertised and auto-summary is disabled. Let’s see if we can advertise classful network 1.0.0.0/8:

R1(config)#router bgp 1
R1(config-router)#network 1.0.0.0

Note that I didn’t specify a subnet mask with the mask parameter. Take a look at the BGP table now:

R1#show ip bgp 1.0.0.0
% Network not in table

As expected there is nothing in the BGP table since we require the exact network and subnet mask. Let’s enable auto-summary now so you can see the difference:

R1(config)#router bgp 1
R1(config-router)#auto-summary

After enabling auto-summary things will change. Take a look at the BGP table of R1:

R1#show ip bgp 1.0.0.0
BGP routing table entry for 1.0.0.0/8, version 2
Paths: (1 available, best #1, table default)
  Advertised to update-groups:
     1
  Local
    0.0.0.0 from 0.0.0.0 (192.168.12.1)
      Origin IGP, metric 0, localpref 100, weight 32768, valid, sourced, local, best

R1 now has an entry for classful network 1.0.0.0/8. It was able to install this in its BGP table because auto-summary is enabled and we have 1.1.1.1/32 in our routing table. This network will also be advertised to R2:

R2#show ip bgp
BGP table version is 2, local router ID is 192.168.12.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale, m multipath, b backup-path, x best-external, f RT-Filter
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 1.0.0.0          192.168.12.1             0             0 1 i

That’s all there is to it.

hostname R1
!
interface Loopback 0
 ip address 1.1.1.1 255.255.255.255
!
interface fastEthernet0/0
 ip address 192.168.12.1 255.255.255.0
!
router bgp 1
 neighbor 192.168.12.2 remote-as 2
 network 1.0.0.0
 auto-summary
!
end

hostname R2
!
interface fastEthernet0/0
 ip address 192.168.12.2 255.255.255.0
!
router bgp 2
 neighbor 192.168.12.1 remote-as 1
!
end

I hope this example has been useful, if you have any questions feel free to leave a comment!