OSPF LinkID for the Loopback Interface Configured as Point-to-Point on a Cisco Router

Question

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I agree with the comments from Ryan and the answer provided by RahulKrishnan R A that we have to declare the interface as point-to-point in order to carry the mask for the loopback interface that's is configured with a mask other than /32.

My question is: in this case, what will be the LinkID for that link? Since for point-to-point the LinkID is supposed to be the 'Neighbor Router ID', what will it's value be for the loopback - which actually is a stub and has no neighbor.

May be that's one of the reasons a Juniper router doesn't allow a loopback address other than /32 for ipv4.

Answer 1

You actually found and posted the relevant section from the RFC. So I believe you already answered your own question! :)

Routers have two choices when setting the LinkID for an OSPF point-to-point link:

A)If the router knows the neighbor's IP address then that becomes the LinkID.

B) If the router does not know the neighbor's IP adress, the LinkID is set to the subnet address for the stub link. So in this case it would be 2.2.2.0 and the Link Data will be the original /24 subnet mask.

The real question about why this happens is due to how OSPF treats loopback addresses WITHOUT the point-to-point command. A loopback is a special type of network in OSPF and is always advertised as a host route UNLESS the point-to-point command is configured.