Saturday, October 18, 2014

OSPF Route Preference: You Sure You Know It ? ... RFC 3101...Part 1

Well I wanted to write about this for quite a while but seem to lost in time. We all know that OSPF is fairly a complex protocol to understand and work on though It makes me wonder why People usually come with OSPF as preferred choice if I ask them to choose IGP for New Enterprise Network Design. Well let's keep that story for my Network Design Series.

Now if you know me, I am one of those people who find reading books cover to cover quite hard. I prefer other methods like Video Trainings, Labing Up the topic I am trying to understand, Cisco Documentation and of course above all the Blog Posts at different forums.

While reading one of those exiting blog posts long back I came across this post from Ethan Banks on an interesting OSPF issue which he discovered during practicing one of Net Master Class CCIE R&S Lab from their workbook.

So I decided to test the scenario by my own thinking I can re-produce the issue and see get it working the way Ethan suggested.

So I created this quick topology (Of course Replaced Cat 1 with R4).

I configured everything like shown in Diagram but to my surprise the Issue didn't occur.

Now the questions was why so ?

I mean from theory standpoint the issue should have occurred. Let's review the topology from OSPF LSA Standpoint and see:

In my topology R3 became the translator.

Now by theory R2 should have seen LSA-5 (Translated by R3 & Forwarded By R1) as well as LSA-7 received directly from R4. In which case R2 should prefer LSA-5 over LSA-7 based on OSPF Route Selection Mechanism:

Regardless of a route’s metric or administrative distance, OSPF will choose routes in the following order:

Intra-Area (O)
Inter-Area (O IA)
External Type 1 (E1)
External Type 2 (E2)
NSSA Type 1 (N1)
NSSA Type 2 (N2)

Now let's review what routing table of R2 tells us first:

Well it seems to be following same logic. Let's take a closer look at Database Table now:

Now control plane seems to be in Sync with what Ethan suggested. But what about Data plane ?. Will R2 send traffic towards R3 to get to network ?

So the behavior seems to be changed in recent IOS version as we didn't had to suppress the forwarding address manually or doing anything fancy.

The behavior is described under RFC 3101 which I figured out through a friend.

Well per RFC it's all about fun with P-bit. 

We will continue the discussion in next post. In the mean while go through recommended readings list:

Recommended Readings:

Deepak Arora

Full Configuration:

1 comment:

Anonymous said...

Hopefully you will Complete this series