Question about OSPF neighbor relationships and interfaces.

/usr

How does a router choose which interface it uses when it forms a neighbor relationship?

I have a router that is correctly advertising its routes, but I'm curious as to why I see I see one of it's subinterfaces when I do a show ip ospf neighbor.

R1

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R2

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R3

On R3, 10.1.3.100 is the interface that's actually connected to R2. 10.1.30.1 and 10.1.40.1 are subinterfaces off of the Fa0/0 interface.

When I do a show ip ospf neighbor on R2 and R3, I see 10.1.40.1 and it's the DR.

Any idea why this interface is showing up rather than the connected Fa0/0 ethernet port and the 10.1.3.100 network?

shednik

unless you statically assign the router id in the ospf configuration it will choose the highest loopback address if present and if not it will choose the highest ip address as the Router ID.

So 10.1.4.1 is higher then 10.1.3.100 and that makes 10.1.4.1 is the winner.

kalebksp

Are you talking about the Neighbor ID in the output of show ip ospf nei? The router id is the numerically highest IP address on the router, unless there are loopback addresses, in which case it's the numerically highest loopback IP address. There should be an Address field in the output that shows the IP of the interface it's connected through.

EDIT: shednik beat me...

/usr

Thanks a lot, just needed to clear that up!