OSPF Forwarding Address

Unanswered Question
Dec 10th, 2007

Hi All...

What is the main use of Forwarding address in OSPF... Is the traffic destined for Type 5 routes are routed to Forwarding address ?

( i think forwarding address is only for Type 5 routes.. correct me if im wrong)

Thanks in advance

I have this problem too.
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nordick26 Mon, 12/10/2007 - 06:38

Hi,

Type 5 LSA are originated by AS boundary routers and is flooded throughout the AS. Normaly, these routes are redistributed in this ASBR router. Type 5 LSA are not propagete to stub, totally-stubby and not-so-stubby areas in OSPF.

I guess, by forwarding address you mean LSA type 4. It is sent from the ABR to your ASBR. The LSA contains the metric from the ABR to the ASBR. This is known as a Type 4 LSA.

Do you mean something else?

regards

[rate any helpful posts]

mohammedmahmoud Mon, 12/10/2007 - 06:44

Hi,

The concept of the OSPF forwarding address was introduced to avoid extra hops when traffic is routed to an external autonomous system. The value of the forwarding address specified by the ASBR can be either 0.0.0.0 or non-zero. The 0.0.0.0 address indicates that the originating router (the ASBR) is the next hop. Please do check this documents for more details:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009405a.shtml

And yes, obviously its only for external routes Type 5 and 7 routes, for Type 7 there is a special case:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122newft/122t/122t15/ftoadsup.htm

HTH,

Mohammed Mahmoud.

Harold Ritter Mon, 12/10/2007 - 06:50

The forwarding address is used to provide optimal forwarding to an external destination.

For example, if RTRa, RTRb and RTRc share a common link, RTRa and RTRb run OSPF between themselves, RTRb and RTRc run another routing protocol between themselves (BGP for instance) and RTRb redistributes the routes received from RTRc into OSPF. RTRb sets the forwarding address to RTRc IP address so RTRa can forward packets directly to RTRc.

The concept of forwarding address also applies to LSA type 7.

Regards,

arun kumar Mon, 12/10/2007 - 07:17

Hi Mahmud & Harold,

Thanks for your valubale reply.. Im clear now in Forwarding address..

royalblues Mon, 12/10/2007 - 08:01

Hey Mohammed

Good to see you back... Hope you got ur number :-)

Narayan

arun kumar Wed, 12/12/2007 - 02:45

Hi All,

One more point on Forwarding Address... Forwarding address should be either Type 1, TYpe 2 or Type 3 LSA. If a router has forwarding address to be other than this type of LSA then the router will reject the route ?

We faced an issue in our network on Multilink (Due to the characteristic of PPP, directly connected interface will get injected as host route into the routing protocol domain).

Need some clarification on the above...

Thanks in advance..

Kevin Dorrell Wed, 12/12/2007 - 03:47

So, do I understand this correctly? That the "forwarding address" is a little bit like the "next-hop" address in BGP: it tells the other routers in the AS "if you know a good way to get to this address, then that is the way to go".

So, by extension, doing an area n nssa translate type7 suppress-fa is a bit like doing next-hop-self in BGP.

Am I getting my parallels right here?

Kevin Dorrell

Luxembourg

mohammedmahmoud Wed, 12/12/2007 - 05:01

Hi Kevin,

Yes, they are pretty analogous, but not exactly, i prefer to think of its concept like the redirects on Ethernet networks to assure optimal routing, one thing to take care of, this keyword takes effect only on an NSSA ABR or an NSSA ASBR.

And by the way have you confirmed your recertification issue ? I hope you are safe.

BR,

Mohammed Mahmoud.

Kevin Dorrell Wed, 12/12/2007 - 05:32

Thanks Mohammed, that is also a useful analogy.

As for re-certification, I am safe for the moment. As I thought, the written deadline gets extended by 12 months with each failed lab attempt, up to 3 years:

http://www.cisco.com/web/learning/le3/ccie/policies/index.html#16

It didn't really answer the question about whether I get stripped of the right to do the lab if I fail the written before the three years are up. I think I shall concentrate on getting the lab for now, and only re-sit the written if I fail the lab again in May.

Thanks again.

Kevin Dorrell

Luxembourg

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