Hi,

But in RFC2328 section 12.4.4.1,they have mentioned "if two routers, both reachable from one another, originate functionally equivalent AS-external-LSAs (i.e., same destination, cost and non-zero forwarding address), then the LSA originated by the router having the highest OSPF Router ID is used. The router having the lower OSPF Router ID can then flush its LSA. "

In my secnario, I am advertising 100.0.0.0/8 with same metric from R1 and R3, but the forwarding address is different. In that case, both LSAs should be in the database, rt ?

Thanks,

Vijaybabu

Vijaybabu,

That is correct. RFC2328 clearly states that the destination, cost and non-zero forwarding address have to be the same for this rule to apply. So in your case the two type 5 LSAs should be present in the LSDB as the FAs are different.

Regards,

Good explaination. But, I have a doubt. What is the significance of forwarding address in type 5 LSA?To reach all external networks, every router (in the topology given by Vijay) has to reach ASBR only. Then, what is there to do with forwarding adress? Does it play any key role?

Thanks,

Balajee

Balajee,

The forwarding address does play a key role indeed. If it is set in the type 5 LSA then the reachibility of the external route is based on the reachibility of this forwarding address via an OSPF internal route.

Hope this helps,

Hi ritter,

As per you explaination, I got this info ie OSPF routers use Forwarding address only to reach external networks, not ASBRs. right!

Thanks for the valuable info.

Thanks,

Balajee

Hi ritter,

Why forward address is 0.0.0.0 in type LSA when the routes are originated from non-OSPF networks? Why it shouldnt be ASBR address itself?

http://cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080124c7d.shtml

Balajee,

RFC2328 states the following:

"If the forwarding address is set to 0.0.0.0, packets should be sent to the ASBR itself."

Regards,

Correct.