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.

Using auto-summary has advantages and disadvantages. One major advantage is that the receiving router receives fewer prefixes. This helps with routers with CPU and/or memory constraints. The disadvantage is that you have a loss of detailed routing information. It’s possible that you will receive traffic for prefixes that you don’t have, but that match the advertised summary. Nowadays, we have powerful hardware so it’s best to avoid auto-summary. If you use it, you might want to consider using a route-map so that you only advertise to to specific routers.


Let me give you an example to explain what I’m talking about. I’ll use these two routers:

BGP R1 R2 AS1 AS2 Topology

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

R1#show run | section bgp
router bgp 1
 bgp log-neighbor-changes
 neighbor remote-as 2
R2#show run | section bgp
router bgp 2
 bgp log-neighbor-changes
 neighbor 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

R1(config)#router bgp 1

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
% 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

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

We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You’ve Ever Spent on Your Cisco Career!
  • Full Access to our 792 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)

1490 Sign Ups in the last 30 days

100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!

Forum Replies

  1. Hi Rene,

    So I am assuming [no auto-summary] is the default on BGP? Also, is there a command to see whether auto-sum is enabled or not. When I run [show run | section bgp] there is no mention of auto-sum.


  2. Hi Mario,

    No auto-summary is the default for BGP. It’s one of those commands that doesn’t show up in the running-config because it’s default.

    I think you can see it with “show run all” though.


  3. Hi Rene,

    I have followed the same config you have explained but it’s not working at all.
    Refer to the below outputs.

    IOU1#show run int lo0
    Building configuration...
    Current configuration : 61 bytes
    interface Loopback0
     ip address
    IOU1#show run | s bgp
    router bgp 1
     bgp log-neighbor-changes
     neighbor remote-as 2
    IOU1#show ip bgp summ
    BGP router identifier, local AS number 1
    BGP table version is 1, main routing table version 1
    Neighbor        V           AS MsgRcvd MsgSent   T
    ... Continue reading in our forum

  4. Hello Krishendu

    At first glance, i don’t see anything wrong with your configuration, but remember, there are two sides to the link. Check to see what configuration setup exists on the other BGP router. Also, try using some debugging commands such as debug ip bgp to see what is being exchanged between the routers.

    I hope this has been helpful!


  5. Hi,

    If we can advertise exact network and subnet mask so whats is the need of auto-summary command?

    What would be the reason to advertise network address without subnet mask? can you please give example of the network where have to advertise a network without subnet mask and then use auto-summary command?

15 more replies! Ask a question or join the discussion by visiting our Community Forum