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!


1 Like


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!


Hello, with auto summary we advertised the network I understand that with auto summary we don’t have to specify a subnet mask. Network can now be seen on R2. But how does R2 now know where the Network-Part (Network-ID) starts at without the knowing of the subnetmask?

thank you in advance

Hello Roberto

As stated in the lesson, when you don’t add the mask parameters, BGP will automatically advertise the classful network, or a subnet of that classful network if it exists in the routing table.

I hope this has been helpful!


I think when Auto summary is enabled only classful boundaries are advertised, you have /24 subnet for Class A which could be the problem here.

Hello Sampath

I just tried labbing this up and used the same parameters and I was able to get the network to appear in the routing table. It is true that auto-summary enables classful boundaries, but it is successfully applied whenever there is an interface that is configured with an IP address that is within that specific network boundary. In both the lesson, where we have and @bapikrishnendu’s example where we have, both of these subnets fall within the network that is used in the network command under the BGP configuration.

So the problem seems to be elsewhere. It’s a good idea to check the BGP configuration as I stated in my previous response to this post.

I hope this has been helpful!