ospf path selection

[sumeet_bpathak]

Hi,

Just wanted to understand in below scenario (attached) is it possiable that Area1 can directly communicate Area 2 through 50 mb link ? or is it necessary to go trough area 0.

[Atif Awan]

No it is not possible. Area 1 will not have any 'direct' routing information for Area 2. Any inter-area information that Area 1 will have will come from it's ABRs which makes it mandatory for inter-area traffic to pass through the backbone.

Atif

[dijohn]

I would say YES. You will need some virtual links though.

OSPF Area Transit Capability

In short, this is a special property of a non-backbone area that allows  this area to transport traffic for other areas (either zero or  non-zero). Per the OSPF definition, a transit area is the area that has a  virtual-link connecting two or more ABRs attached to this area. Thus,  having a virtual-link provisioned across the area is the necessary thing  to make the area transit. In fact, it’s just an alternate definition of  a transit area.

could you please review the following

http://blog.ine.com/2009/09/14/understanding-ospf-transit-capability/

http://www.cisco.com/en/US/docs/ios/12_0s/feature/guide/ospfatc.html#wp1054427

/divin

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[Peter Paluch · Accepted solution]

Hello Divin,

I would say both you and Atif are right in your own ways. By definition, OSPF disallows direct inter-area communication and requires that inter-area traffic must flow through backbone, or to be more precise, it must at least come to an ABR (which is both in a non-backbone area and in area 0) and then it may again leave towards other area, if the topology permits such a route. From this viewpoint, Atif is absolutely correct.

The virtual link is actually a cheat on this principle - it is a fictive vertex interconnecting the true ABR (which is already in area 0) and another want-to-be ABR, and the vertex itself is put into area 0. This way, the second router is made an ABR by making it virtually connected to the backbone although physically, it does not have any interfaces in area 0. So, yes, with a virtual link, even a non-backbone area may become a transit area and thus allow inter-area communication - but it is done by actually by attaching that area (albeit virtually) to backbone, therefore meeting the requirement Atif stated earlier.

Best regards,

Peter

[cadet alain]

Hi Peter,

So, yes, with a virtual link, even a non-backbone area may become a transit area and thus allow inter-area communication

But this non-backbone area must be a standard area, if it is a stub area then it can't participate in a virtual-link but you can use workaround by using a tunnel passing through this area linking the backbone router and the router in the discontiguous stub area.

Regards.

Alain.

[Peter Paluch · Accepted solution]

Alain,

Yes, absolutely, an area through which a virtual link is configured must be a regular area (i.e. no stubby, totally stubby, NSSA or NSSA-TS) because it must be able to accept all LSA-3, 4 and 5 to provide full routing capabilities for the inter-area traffic. And yes, you can cheat this by creating a tunnel Thank you for pointing out that fact!

Best regards,

Peter

[Kishore Chennupati]

Hi Divin,

Thanks for that INE blog link. It was awesome. some good stuff in that article

Regards,

Kishore