Loopback interface

Daniel333

I guess I am a little confused as the purpose of the loopback address. I understand why I have a loopback on my PC, and I assumed that is why they were on routers as well. But then I ran into lab 8 from the Boston sim (I am at work, so I have so use the sims here)

They had me apply RIP to it, and bam. It was routed. I could ping a loopback from other Routers and workstations. Why would you want this? What level of functionality or purpose does this have?

thanks as always,

Webmaster

Besides being useful for testing purposes, some routing protocols use the highest loopback IP address as an ID (like the router ID in OSPF, which otherwise would use the highest IP address of a regular interface, unless configured with the router-id command). It can also be used to identify a router by a single IP address, making it the source for traffic originating from that server, making it easier to recognize and filter in other areas of the network. (i.e. a DNS A and PTR record pointing to that loopback interface). It's usually configured with a 255.255.255.255 mask and propagated as a 'host route', allowing you to, for example, use a single class C to address 254 routers in an internetwork. It can also be used to ID to router for logging, i.e. by using the logging source-interface loopback0.

malweth

It's also an interface on the router that will never go down. This is another reason it's useful for router management (via SNMP especially).