0 BGP AND LOOPBACK INTERFACESUSING A LOOPBACK INTERFACE TO DEFINE NE...

4.0 BGP and Loopback interfaces

Using a loopback interface to define neighbors is commonly used with IBGP

rather than EBGP. Normally the loopback interface is used to make sure

that the IP address of the neighbor stays up and is independent of an

interface that might be flaky. In the case of EBGP, most of the time the

peer routers are directly connected and loopback does not apply.

If the IP address of a loopback interface is used in the neighbor com-

mand, some extra configuration needs to be done on the neighbor router.

The neighbor router needs to tell BGP that it is using a loopback

interface rather than a physical interface to initiate the BGP neighbor

TCP connection. The command used to indicate a loopback interface is:

neighbor

ip-address update-source interface

The following example should illustrate the use of this command.