OSPF Forwarding Address

[rakeshvelagala]

Dear All,

Please advise on the below.

In OSPF, route will not be installed in the routing table if the forwarding address is not learnt via OSPF. So if I have created a static route for the forwarding address on the router, the route will be missing though it will be in database.

I understand it is RFC specification. But what is the reason for designing that way?

Thanks

[Rolf Fischer · Accepted solution]

Hi,

the purpose of a non-zero forwarding-address is to avoid unnecessary hops if a third-party next-hop exists for an external route.

Several checks to prevent routing-loops and ensure reachability must be passed before an ASBR sets the forwarding-address to an non-zero IP-address:

The third-party next-hop has to be directly connected on an ASBR's interface, which is

• (non-passive) OSPF enabled
• no OSPF point-to-point or point-to-multipoint interface

When this conditions are met, the corresponding network must be learned as an OSPF internal route. Before another router installs the external route in its routing-table, it just checks if this is the case; otherwise something went obviously wrong and it doesn't install the route.

Cisco Document 13682: Common Routing Problem with OSPF Forwarding Address

HTH

Rolf

P.S.: I focussed on standard area types, the usage of the FA in NSSAs is slightly different.