ospf forwarding address for external routes

[Kashish_Patel]

I read this in a document:

"When OSPF learns an external LSA, it makes sure that the forwarding address is known

through an OSPF intra-area or interarea route before it installs it into the routing table. If the

forwarding address is not known through an intra-area or interarea route, OSPF will not install

the route in the routing table."

I am trying to understand from where this requirement comes from? Why OSPF will not install the route in the routing table if the forwarding address is known via ospf external route?

[Rolf Fischer]

Hi,

the originating router sets the FA to a non-zero IP-address only when certain conditions are met:

The interface pointing to the next-hop IP of the external route

• has to be OSPF-enabled (non-passive)

AND

• has to be a multiacces-interface (network type broadcast or non-broadcast)

So if there is no OSPF interal route to reach the FA on other routers, someting went wrong and it's safer not to install the route.

Link: Common Routing Problem with OSPF Forwarding Address

HTH

Rolf