BGP With Two AS

Unanswered Question
Jan 6th, 2008
User Badges:


I have a scenario where there are Three Routers Router A , B & C. Router A is configured in BGP AS 100 & Router B & C are configured in BGP AS 200. Router B & C are connected to Router A Directly. On the Router A I am getting the Routes of Router B & C but on Router B I am not getting the route for Router C ( WAN & LAN Network) & Vice Versa.

There is no direct connectivity between Router B & C.

Any thing further I have to configure?

Please check the Diagram attached for more concept & BGP Configurations.


  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4 (1 ratings)
haarisalibaig Sun, 01/06/2008 - 10:16
User Badges:

Hi Micheal,

Please find attached the running configurations of all the three sites & as well the output of Sh ip bgp / sh ip bgp summary / sh ip bgp neig / sh ip route.

*** Instead of BGP AS 100 65000 is used & instead of BGP AS 200 64522 is used.


douhanm Sun, 01/06/2008 - 10:23
User Badges:

WHat you are trying to do is impossible, you cannot have two routers without direct connectivty using the same AS number, as soon as the router sees its own AS in the PATH advertised from A it will disregard the route, for sites that are not interconnected you must use different AS numbers ( or get really complicated with AS override etc )

Jon Marshall Sun, 01/06/2008 - 10:25
User Badges:
  • Super Blue, 32500 points or more
  • Hall of Fame,

    Founding Member

  • Cisco Designated VIP,

    2017 LAN, WAN


Your problem is that an EBGP router will not by default accept a route advertisement with it's own AS in the AS PATH. So when B sends advert to A it adds it's AS number to the path. Then A adds it's own and sends to C. C rejects the packet because it sees it's own AS in it.

To get around this on B & C under the BGP config

neighbor "neighbor IP" allowas-in 1

This will allow B and C to accept routes that have 1 occurence of their own AS in it.



haarisalibaig Sun, 01/06/2008 - 21:35
User Badges:

Hi Jon,

thanks alot for your reply...

Is there a way that I dont do any further configurations on Router B & C. Instead should do something on the Router A to make this scenario work. Actually in our city we have a service provider who is doing the same & we are simulating that. The configuratios on B & C they provide are the same, but we are not aware of what they are configuring on Router A.


bvsnarayana03 Sun, 01/06/2008 - 22:04
User Badges:
  • Silver, 250 points or more

Dont see how you can find B & C's routes in each other without configuring IBGP between these 2.

haarisalibaig Sun, 01/06/2008 - 22:47
User Badges:


If I have to configure IBGP it is simlpe without any efforts we can do it in minutes. As I mentioned this scenario is already working with our Service Provider.I am sure this will work, but definetly require high level technical expertise & understanding.


s.arunkumar Tue, 01/08/2008 - 05:26
User Badges:
  • Bronze, 100 points or more

i think ur router A config might be configured with as-override option.

ie,at router A config would

neighbor as-override.

neigbor as-override

this will override the 200 in as path with value 100 as per ur example..check ur as path by sh ip bgp command.

If it show 100 100,then this option might be there..

haarisalibaig Sat, 01/12/2008 - 21:28
User Badges:

Hi Arun,

First of all thanks for your reply.

The command you have given is not there (as-ovveride)

Router_A(config-router)#neighbor a?

activate advertise-map advertisement-interval allowas-in

The above is the output from my Router_A.


Danilo Dy Tue, 01/08/2008 - 06:57
User Badges:
  • Blue, 1500 points or more


Try creating IP GRE Tunnel between Router B & C through Router A. IP GRE Tunnel source and destination address can be achieved by static routing.

Once IP GRE Tunnel is up. Try running iBGP between Router B & C thru IP GRE Tunnel.




This Discussion