When does Recursive or Reverse Routing occurs!

Unanswered Question
Jan 22nd, 2008

Hi,

I am facing reverse routing issue , where in traffic on oneside is going to one route and return in different route. This is happening on one of the Core Router of our network.

Apprciate your ideas on this with possible Route Cause description..

~Akber.

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Richard Burts Wed, 01/23/2008 - 04:34

Akber

I understand the concept of recursive routing, which is what happens when you have an entry in the routing table which has a next hop identified, and then you have to look in the routing table again to find how to get to the next hop. An example might be like this - if you have in the routing table:

ip route 192.168.100.0/24 with next hop 172.16.31.23

but that next hop is not a connected subnet on your router, then you would need to look in the routing table again to find the next hop address

ip route 172.16.31.0/24 with next hop of 10.10.10.16

which does resolve to a connected subnet.

Recursive routing is not related to the symptom that you describe.

What you describe with traffic taking one path outbound and a different path inbound is assymetric routing. There are several different things that can cause this and without knowing more about your environment it is difficult to know which would apply to you:

- you could be routing to some external destination and their choice of return path (perhaps using BGP) does not match your choice.

- you could be routing to some destination in a remote part of your network which is using a different routing protocol and their choice of return path does not match your choice.

- you could be routing to some destination in your network and its metrics are not configured the same way that yours are.

- you could be routing to some destination and they are using Policy Based Routing to select a different return path.

If you can tell us more about your environment we might be able to help identify the cause of this behavior.

HTH

Rick

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