OSPF: Best path between inter-area routes

[CSCO11598534]

Hello all,

I will keep it simple with only two routers. Please see the diagram.

R1 will prefer to reach Loopback0 of R2 over the f0/0 (Area0).

Even making higher the cost in f0/0, selection does not change!

I thought that it may something have to do with capability transit command.

Although enabled in both routers, nothing changed.

Anyone can explain please?? Is there a rule that says that you must transit Area0 regardless the cost??

[Joseph W. Doherty]

This is expected behavior.

As R2's loopback is in an area not defined on R1, destination is only logically reachable via area 0 from R1.

If R2's loopback was in area 1, then R1 would send via area 1 if traffic also sourced in area 1 otherwise it would use area 0 again.

[CSCO11598534]

Thanks for answering. I quess what you are saying is that traffic from non-transit area cannot go directly to another non-transit area, but has to go via transit area 0.

Correct? If that so, can you please provide a link (even better from an RFC) that states that clearly?

[Joseph W. Doherty]

Correct.

The only exception would be if the router already had the destination area on it (and destination network in the same route table).

As to a documentation reference, sorry, don't have one at hand.

[Georg Pauwen]

Hello,

in addition to Joseph's remarks, the RFC you are looking for is 2328. Here is an excerpt (page 27 in he link provided):

"The topology of the backbone dictates the backbone paths used
between areas. The topology of the backbone can be enhanced by
adding virtual links. This gives the system administrator some
control over the routes taken by inter-area traffic."

RFC 2328

https://www.ietf.org/rfc/rfc2328.txt

[CSCO11598534 · Accepted solution]

Thank you both for your response.

Indeed the mentioned RFC states it clearly:

"When routing a packet between two non-backbone areas the
        backbone is used.  The path that the packet will travel can be
        broken up into three contiguous pieces: an intra-area path from
        the source to an area border router, a backbone path between the
        source and destination areas, and then another intra-area path
        to the destination.  The algorithm finds the set of such paths
        that have the smallest cost."

Thank you for your support

[Joseph W. Doherty]

Yes, but again, if you had, for example, area 1 and area 2, but not area zero, on the same router (and sharing a route table), and packet arrived on router in area 1, and destination network was in area 2, router would forward packet to area 2, bypassing the need to use area zero for transit. I.e. the router will ignore the OSPF rule.

I re-mention this, because its results, for such a example, would appear different from what the RFC states.

[CSCO11598534]

Joseph you got me a bit confused there.

I get what you are saying but even more:

I would expect even in the case that the router had attached areas 0,1,2 and traffic had to go from area 1 to 2, it would go directly and not via area 0.

Am i wrong?

[Joseph W. Doherty]

Yes, if router had all three areas, traffic should flow between areas 1 and 2 without using area 0, but also true if router had just area 1 and 2, although the latter violates the OSPF RFC.

[CSCO11598534]

In my mind, RFC is technically violated in both cases, because Area0 is ignored.

Can you please clarify why you make that distinction?

[Joseph W. Doherty]

I made the distinction, because as you note, it appears to be a technical violation of the RFC, but you could argue it's not, as OSPF is being ignored.

Just mentioned so if you also bumped into it, again, you wouldn't think its unexpected behavior.

[CSCO11598534]

Thank you.

As a general rule I believe we can conclude that:

For traffic between non-backbone areas the packets should traverse Area 0.

The only exception is when both source and destination areas are attached to the same router.

[Joseph W. Doherty]

"The only exception is when both source and destination areas are attached to the same router."

If they share a route table.