CISCO EXPERT – CCIE#23373

Ricardo Martins

Cisco Expert – BGP load-balacing between 2 AS’s and Ebgp-multihop

Posted by Ricardo Martins on June 27, 2008

Let’s start with something simple BGP hehe, just kidding.

I always remember how this peace of technology works because I have spent quite sometime trying to solve the mystery why 2 EBGP neighboors wouldn’t peer even when everything seemed to be well configured.

I really believe that 99% of the problems you run into when trying to configure some technology is caused by not fully understanding the technology itself.

In this example, we are going to configure a peer between AS1 and AS2 using load-balacing between 2 links. Now, we all now that for 2 routers to peer via EBGP, they need to be directly connected which they are, so we should not have any problem with it, or do we??

Let’s take a closer look into the diagram and configurations:

Important part of the configuration:

R1:

ip route 200.2.2.2 255.255.255.255 10.10.10.2
ip route 200.2.2.2 255.255.255.255 10.10.20.2
!
router bgp 1
neighboor 200.2.2.2 remote-as 2
neighboor 200.2.2.2 update-source loopback 0
neighboor 200.2.2.2 ebgp-multihop

R2:

ip route 100.1.1.1 255.255.255.255 10.10.10.1
ip route 100.1.1.1 255.255.255.255 10.10.20.1
!
router bgp 2
neighboor 100.1.1.1 remote-as 1
neighboor 100.1.1.1 update-source loopback 0
neighboor 100.1.1.1 ebgp-multihop

First of all, we need to use 2 static routes in both sides pointing to the loopbacks with a next-hop address. There is nothing special about this so far.

What I believe is trick here, is that for 2 EBGP neighboors to peer they need either be directly connected or using the command ebgp multihop if not directly connected.
In this particulary case both routers are directly connected but we still need to use the command ebgp-multihop because we are peering with the loopbacks networks which they are 2 hop’s away.

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