BGP Routing

[Harmeet Singh]

Hello,

I create a topology (See Attachment) in GNS 3 with BGP. I advertise all directly connected network through BGP on every router in this topology. Create external and internal neighbors through neighbor command. When i run SHOW IP ROUTE on R3, it shows network 3.0.0.0 via only 30.0.0.2. Why it doesn't show 3.0.0.0 via 10.0.0.1. And when i Shut down port 30.0.0.1 then route 3.0.0.0 doesn't show in my routing table.

[John Blakley]

Where is 3.0.0.0 being advertised from? Can you post "show ip bgp summary" and "show ip bgp"?

[Vasileios Bouloukos MSc, ITIL, CCIE]

Hi Harmeet,

According to your design and the subnetting I assume the 3.0.0.0 subnet is the subnet between R5-R6.

The reason that R3 does not receive this route by R2 is due to the BGP AS loop prevention mechanismwhich does not allow to advertise routes learned from as X to AS X again.

So the 3.0.0.0 subnet is advertised to R2 by R5 with AS 20. Then, R2 is not permited to advertise this prefix to R3 due to the fact that R3 is in AS 20 again,

You can overcome this problem if you configure the neigbor x.x.x.x as-overide command to R2 for the peering with R3.

Alternatively, you can confgure the allowas-in command to the R3 router.

Hope that helps!

Vasilis