Thanks again

Thanks again

I do like your analysis methodology. As Wael stated we can cover these tiny details in separate posts, comments from people like your self energizes us to proceed.

Back to the last part of your question, I like to address it from another prospective, the loopback trick is fine when we are talking about a single router connected to multiple areas having only a single router per area connected to it, but in reality you might have a situation with multiple routers connected to multiple areas (most probably when repairing a partitioned backbone area), and thus you need a mean of moving the Inter-area information to the other router(s) in order for the other areas on the other router(s) to be able to learn them (since Inter-area information can never go from an area to another directly – at least with Cisco’s implementation), and here comes the virtual-link, it would provide a virtual backbone connection between the different routers in order to be able to exchange the different Inter-area information that each knows (notice that with Inter-area things tend to distance-vector behavior).

Well as Wael said, these details require separate posts, and again comments as yours energizes us to go on with such posts.

BR,
Mohammed Mahmoud.

Good question; it needs a separate post to cover the issues you have touched in the comment.

But we can try to briefly clarify it here

- The rule that Inter-area routing must traverse area zero is only a recommendation in the RFC and was made as an obligation by CISCO implementation only.

- Having routes moving from area to area without traversing a common backbone can introduce routing loops.

- The trick you have done works in simple networks (between two areas only). Try to add an area zero adjacency to one of the other routers in different areas you will see that LSAs will not be installed in routing tables of other ABR routers.

This is only a simple fast clarification for your case and I promise to make a detailed post.

Good lucks with your LABs

Regarding to the Area concepts of the OSPF, every area should be attached to Area 0 via ABR to be able to get the latest LSAs that roams in the Area 0. Ok, that’s cool, but why?

When tested via dynamips, I reached that, the router which for example, gather Area 1 and Area 2 isn’t marked as ABR when we do “sh ip ospf”, so the router consider itself as non-ABR and shouldn’t flood LSAs from Area 1 to Area 2.

That’s also cool, and logical.

Now, I’ve configured the non-ABR which attach Area 1 with Area 2 with a loopback interface and put this interface in Area 0 at the interface level, so the non-ABR router became an ABR and it’s now flood the LSAs learnt from the other Areas into the isolated Area!

So, I guess it worked fine, I can receive the interarea and external routes perfectly! Why then we need to creat a virtual link when we can do a “fake Area 0″ at the loopback of the non-ABR we want to make it as ABR?

Thanks for your effort.

http://www.networkworld.com/community/node/19293