BGP Auto-Summary

This topic is to discuss the following lesson:

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.


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.


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 TblVer InQ OutQ Up/Down State/PfxRcd 4 2 24 24 1 0 0 00:18:17 0
IOU1#show ip bgp
IOU1#show ip bgp
% Network not in table

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!



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?

Hello Card

BGP Auto-Summary is useful when you have many different subnets that you want to advertise. For example, let’s say you have 250 subnets of the form:



These are all discontiguous networks and if you want to advertise them all, you have to explicitly state each one with its subnet mask. That’s 250 network commands. You can use the auto-summary command to simplify configuration and reduce the number of network commands to the classful networks that contain all of the networks you want to advertise.

I hope this has been helpful!