BGP Peering with Loopback Interfaces

[JUANNN]

Hello,

I been using BGP peerings with loopbacks for a while, but I am trying to understand the reason for it. I've searched in other Community Discussions but I could not find any answers:

A lot of answers use the following scenario, where is clear that using the loopbacks from both routers to peer in BGP will keep the BGP session UP even if one of the links goes down, (as long as there is a route to the loopback), without configuring BGP sessions between the physical interfaces.

But this would be the same as BGP peering from both links individually (2 BGP sessions, 1 per link). BGP will receive the same routes from both links and will choose the path according to the attributes, and then when a link goes down, the other link with the other BGP session will replace it. I labbed it and it works perfectly fine, no outages.

So why or when should I use the loopback interfaces to peer in BGP then? Is it because is not practical to configure a single BGP session for each redundant link? Or because if we have multiple paths to our BGP peer, several hops away, then not always I am going to be able to know each physical interface IP address from the distant peer?

Thanks,

Juan

[Richard Burts · Accepted solution]

Juan

There are several reasons not to have BGP session on both physical links. Part of the rationale is that a single logical session between peers  can utilize the redundancy of the physical links and avoid the overhead of two active peer relationships. 

[MHM Cisco World · Accepted solution]

Where in mp-bgp mpls, where cisco recommended using LO to establish ibgp between two peers.

Vxlan also use bgp and use LO to establish bgp between leaf and spine. 

MHM

[JUANNN]

Rick and MHM

Thanks, both answers make sense. 

Juan

[Richard Burts]

Juan

You are quite welcome. Yes there are several reasons why BGP peering with loopback interfaces is recommended. 

It is certainly possible to do the BGP peering using the physical interfaces. But that requires keeping 2 active BGP sessions to the same neighbor and that requires more resources than a single peering using loopback interfaces.

[Joseph W. Doherty]

Another possible reason would be to avoid Cisco's BGP default of not performing ECMP.

Also, for a pair of Ethernet links between devices, using Etherchannel might be an option.