BGP Advertisements through an eBGP Peer not occurring between Two Peers in the same AS

Issue

BGP routes from a device is not advertised to another device in its own AS through an external BGP peer.

Consider the scenario in the diagram below:

As shown in the diagram, R1 in AS # 10 is advertising its routes to R2 in the same AS via an eBGP peer (Firewall) AS # 20. The routes advertised by R1 reach the firewall, however the firewall is not advertising it out to R2.

Resolution

R1 is advertising its routes through the eBGP to the firewall. Here is the bgp loc-rib and rib-out table from R1

Once the firewall receives the routes, it will install it in its routing table. However, the routes do not show up in the rib-out table

R2 will not learn routes advertised by R1 through an external peer since R2 expects to know routes from R1 via iBGP since both the devices are in the same AS. In this case, the firewall will not advertise the routes to R2 since the routes were learnt from R1 which in the same AS as R2. Here is the bgp table from R2 where there are no routes received in the loc-rib:

So routes cannot be learnt and sent to the same AS. Devices within the same AS must have full mesh connectivity to make sure all the routes are learnt through the iBGP.

In this scenario, to make the firewall advertise R1 routes to R2, the firewall must also be made part of the same AS as R1 and R2.

owner: mvenkatesan