OSPF and loopback addresses

SpencerClark

I have been read about OSPF areas, and routers that use OSPF can set up loopback interfaces if required. OSPF uses areas to divide up networks. My question is where to loopback addresses fit in the topology of the area map. Do they get assigned to an area or do they just exist outside of the areas. I was just curious if any one knew more about the relationship between them.

Thanks!

SteveO86

SpencerClark wrote: »

I have been read about OSPF areas, and routers that use OSPF can set up loopback interfaces if required. OSPF uses areas to divide up networks. My question is where to loopback addresses fit in the topology of the area map. Do they get assigned to an area or do they just exist outside of the areas. I was just curious if any one knew more about the relationship between them.

Thanks!

The loopbacks will be typicaly be used as the router-id, so you can easily identify routers in an OSPF area, so OSPF updates would come from the loopback address and OSPF neighbor addresses would be the loopback address. The loopback address would be used more-so for management and not routing. Think of it as the router's OSPF name.

DragonNOA1

Also, the loopbacks can be any subnet and can be advertised so that you can get to them.

pogue

Let's be clear on this. Loopback interfaces are not solely for OSPF.

They can be used with any routing protocol, or even without a routing protocol.

They are simply a "virtual" interface that always stays in an up/up status. There are many different ways that a loopback interface can be used, only one of which is being used as an OSPF router-id.

Since the network set up on a loopback interface is routable,, it can be set up as the destination of a tunnel. If you are tunneling from router A to router B's loopback interface, then as long as any route in between router A and B stays up, the tunnel will be successful.

There are numerous other examples.

Just wanted to throw this in there lest you think loopback interfaces are some sort of OSPF feature, which they are not. They are simply a tool that can be used by numerous protocol within the Cisco framework.

Russ

ColbyG

SpencerClark wrote: »

Do they get assigned to an area or do they just exist outside of the areas.

Along with what everyone else said, I want to address this question. All routes within an OSPF domain will belong to some area, this includes loopbacks. So if the loopback is advertised into OSPF (which isn't required for it to be a RID), then it will have to be in an area.

SpencerClark

Thanks everyone! I appreciate the clarification! These were the comments that I was exactly looking for!