BGP loopback source IP shouldn't be advertised in BGP?

[Adnan Shabbir]

I heard that if you are using a loopback source IP for BGP neighborship, while it is advertised in IGP for reachability, it shouldn't be adertised in BGP. What is the reason behind it ? 

[pmckenzie]

Hi Adnan

               you use the Loopback in BGP to ensure that the interface between two IBGP peers is stable.

A user doesn't need to go to a Loopback network.  Users want to go to a LAN.

A loopback does not appear in the BGP routing table but it is still in the neighbor table.

[Giuseppe Larosa]

Hello @Adnan Shabbir ,

if we speak of iBGP deployment with an IGP like OSPF or IS-IS used to advertise loopbacks you want to keep separate the IGP infrastructure subnets used for building up the network including the iBGP sessions from service or customer oriented prefixes that are usually published in BGP only.

Advertising a loopback interface in iBGP is not forbidden but it does not solve the issues of making that loopback "reachable" by other devices.

The reason is that in iBGP the next-hop is not changed but preserved so a remote router iBGP speaker would see something like

B [200/0] 10.1.1.1/32 via 10.1.1.1

if it is not able to find an indipendent way to resolve the next-hop 10.1.1.1 it is a dead lock.

On the other way if 10.1.1.1/32 is published in IGP there is no real reason to publish it also in iBGP as that route will never be used for its higher admin distance. It would be used if the IGP route disappears but if this happen you would see the loopback address in BGP with next-hop the same IP address and this would not be acceptable by other routers.

So it is better to avoid to advertise in BGP the loopback used as source address for iBGP sessions.

Hope to help

Giuseppe