Loopback for BGP connectivity

Evan Ray · ‎01-18-2015

I'm learning BGP at the moment, and from my understanding you should establish neighborship with the loopback interface rather than the physical interface because if the physical interface goes down, the neighborship would be lost, while a loopback interfacd never goes down. What!?!? If the router goes down, it's down. The loopback is just to test connectivity, which I still just can't grasp that concept. I've Googled and read in my books, and I get it, but I don't. Very frustrating.

I'm just trying to make sense of this concept. I guess I just don't understand how the loopback will help if it's down. You have two routers, connected via serial 0/0/0 on one end, and serial 0/0/0 on the other. One of the routers goes down, therefore serial 0/0/0 can no longer communite with the other router, so how will the loopback help here? It can't get to that loopback interface IP. I must be missing something here.

Boyan Sotirov · ‎01-18-2015

What if... both routers are connected over parallel interfaces :)

The situation you describe (only one interface connecting the routers) indeed does not benefit from using the Loopback interface. When you have multiple connections between the BGP peers, and the Loopback interface network is either advertised dynamically or with static routes - than establishing the conection on the Loopback has the benefit that the connection "never goes down". Assuming off course that at least one of the connections between the BGP peers is still active :)

Evan Ray · ‎01-18-2015

Good point. I guess when I think of the router being down, I think of it completely being down ...as in it died :) When they are saying that it goes down, they are referring to the interface. I actually need to think bigger, as in a large router with multiple line cards for example, not some simple 2 interface router. Your response actually clarified some fuzziness. Thank you!!

Boyan Sotirov · ‎01-18-2015

You're very welcome :)

And you're right... whenever BGP is involved, you have to think big! :)

Enjoy!