Loopback interfaces and routing updates

Hello there.
Routing protocols often use loopback interfaces to exchange updates, with the reasoning that these interfacers never go down.

If R1 & R2 have a point-to-point connection, and exchange updates via their loopbacks, does this mean that the routers can still exchange updates via their loopbacks, even though the link between them might be down? Do routers need layer2 connectivity to exchange updates via their loopbacks?

Hello Sheldon.

Internal gateway routing protocols (IGRPs) such as EIGRP, OSPF and RIP use loopback interfaces not to exchange routing information, but to define the router ID. This occurs in case the router ID is not manually configured. The exchange of information occurs on interfaces that have been configured to participate in the exchange of routing information (usually using the networkcommand). If a point to point connection between two routers that exchange routing information is down, information cannot be exchanged.

In the case of BGP, which is an exterior gateway routing protocol (EGRP), routing information can in fact be exchanged between loopback interfaces if it is configured in this manner. (This is considered best practice by the way.) Even if the loopback interfaces are up, however, in order for information to be successfully exchanged, a path must exist between the two routers exchanging information. If there is no link, information cannot be exchanged.

I hope this has been helpful!


Laz, thank you for clearing this up for me. Absolutely it has been helpful.