Sunday, October 19, 2014

OSPF Route Preference: You Sure You Know It ? ... RFC 3101...Part 2 (Final)

It's sunday, got some time today. Let's wrap up this series quickly.

The physical and logical topology is gonna remain same. The only thing I changed in this part is ... IOS Version :)

R3 is Still the Translator:

Let's review the routing table of R2 quickly again and see how things are:

So now the question is why results are different with different IOS ? ( IOS used in part 1 was 12.4 (20) T ).

The previous version was actually following RFC 1587 in which the route preference is same as mentioned in post 1 i.e. E2 is preferred over N2.

But the later IOS followed the RFC3101 implementation. Which states that LSA with P-bit set is preferred over the one which has P bit as Zero. The P bit is set to 1 by ASBR while redistributing the routes into NSSA except the situation where ASBR is also ABR and P bit remains unset (0).

But as an Engineer if you would like you can still go back to older IOS behavior as following:

The reason in Part 1 , the traffic (Data Plane) took the direct path because of same cost to Forwarding Address.

Further Readings:

Deepak Arora

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