Monday, March 1, 2010

Troubleshooting OSPF Adjacency - MTU Mismatch Issue

Today I am gonna focus little more on OSPF Adjacency Troubleshooting. One of the criteria which should match to form Neighborship is Interface MTU Size. In case two ospf neighbors trying to form neighborship are getting failed and you get the following message on console "Too many retransmissions". Chances are pretty high that it's a common ospf MTU size mismatch issue. I have seen many books talking about OSPF so far which didn't mention that MTU size of neighbors should also match in order to form adjacency. So in this short post I am going to tell you how to fix it by putting the command under the interface " ip ospf mtu-ignore". After entering this command OSPF basically doesn't consider MTU size to form neighborship and ignores it. Although this command can be entered on one side but it's better to enter it on both routers. Other way of course is to configure MTU size manually on both sides to match.

Here is another quick command to add more fun into ospf, I'll discuss this command in detail soon in my next post. Have you ever seen something like this earlier under OSPF config :) are other details that you may wanna look at. In my topology I just connected R1 to R2 with serial interfaces (S0/0) on both sides.

Deepak Arora

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