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OSPF LSA5 forwarding address

Did you know that OSPF can redistribute routes with their original next-hop addresses?

In the topology above, the leftmost router (lets call it router 1) injects network 172.16.1.0/24 into EIGRP. ASBR redistributes to OSPF. ASBR will as we now create a type5 LSA for the external type network. But did you know that it can preserve the original next-hop address from EIGRP?  The feature is called “OSPF Type 5 LSA forwarding address feature” and lets us preserve the next-hop address in some topologies in order to spare one unnecessary hop.

Lets look at a LSU for this advertisement, we see that the Forwarding Address is set to 1.23.1, which is the ip address of the originating router, originally advertised with EIGRP!

 

This is a really cool feature which lets us spare one hop for multiaccess networks where a router redistribute routes into ospf. But it has some restrictions, all of them quite obvious though.

  • ASBR must have OSPF enabled on the next-hop interface
  • OSPF network type must not be point-to-point or point-to-multipoint.

What if we change ospf network type then?

We see that after a change to p-mp, the LSU contains a LSA5 with the next-hop set to 0.0.0.0. Which wouldn’t come as a surprise.

When to look out for this in the lab?

The reason I write this blog is still because I am after my CCIE numbers, and this feature can appear on the lab exam. But how?
For any multiaccess networks, where an OSPF ASBR redistributes like this case, a task can be something like “make sure that packets that are destined to network 172.16.1.0/24 is forwarded from R2 to R1.” In a case like that we would want to look out for how the next-hop is advertise and remember that we can preserve the next-hop address from the EIGRP router.

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