OSPF Default information originate

[arsalan.jawaidstu1]

Hi experts,

I was wondering how does a router R2 in an area know about a default route which was originated from an ASBR R1 in the same area i.e. area 0.

I know that all communication in ospf is done through LSA. And internally only LSA 1 and 2 are passed with each other depending on the network. I checked the LSA, there was no mention of the 0.0.0.0 or default route. Is there any flag bit in the ospf packet? I am curious on how this happens.

Thank you

Arsalan

[Richard Burts]

I am not sure what you are saying when you mention that internally only LSA 1 and 2 are passed with each other. It sounds like you are suggesting that R1 will send only type 1 and 2 LSA to R2. But that is not the case If R1 is configured with default-information originate then it becomes an ASBR and will generate type 5 LSA to advertise the default route.

For additional information you might want to look at this very excellent article which discusses OSPF default route and how it is advertised depends on the type of area.

http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13692-21.html

HTH

Rick

[arsalan.jawaidstu1]

Hi Richard,

Thank you for answering

Yes that is what I'm suggesting. So ASBR is in area 0 and it generates a type 5 LSA. R2 gets to know the external network through that LSA. But why doesn't the OSPF linkstate database contain any reference to that LSA when we see the database in R2?

[Richard Burts]

Arsalan

I would expect to see the LSA in the database. Could you post the output of show ip ospf database external so that we can see what is there?

HTH

Rick

[arsalan.jawaidstu1]

Hi Richard,

Thank you for your response

I was confused whether the ABR knew this network through LSA 1 and 2 or not. It seems I have the answer now, it does have an LSA 5 in its database. But earlier I thought the 0.0.0.0 network would be advertised by LSA 1 or 2 because everyone said in an area only LSA 1 or 2 is used. So yea now I know LSA 5 is also used, which propagates throughout the network as the same LSA 5.

Thanks

Arsalan

[Richard Burts]

The discussions that you have seen that focus on LSA type 1 and 2 are discussions using a very simple OSPF topology with all routers in a single area and with no ASBR. This is a good approach when the focus is on the fundamentals of OSPF. Once the fundamentals of OSPF are clear then the discussion could cover more complex aspects of OSPF including the use of LSA type 5 for default routes.

HTH

Rick

[Prashant Sheshasayee]

Default-Information originate

It creates type 5 LSA for default route.It tells the ASBRS to flood a default route into ospf but only if the ASBR itself has a default route in its IP routing table.

[paul driver]

Hello

I agree , you will should a type 5 external LSA

on R2 < sh ip ospf database external
on R1 < sh ip ospf database external self-originate

res

Paul