Thursday, November 12, 2015

Masking MTU Miss Match In OSPF

MTU is a common OSPF adjacency attribute. Which means that the MTU from both sides (interfaces connected) must be same. Anyhow if it is different from one side a unique problem of forming adjacency occurs.

So the neighbors will flap between in Exstart / Down states. (click on the images to view in full screen)

If the output and errors are like this, most probably the issue is with MTUs. Enter debug ip ospf adj on both routers to gather more information about the issue since this is a adjacency problem.

By reading the output we can easily say that the router with ID (R1) has a lower MTU of 1000 Bytes & the (R2) has the default MTU of 1500 Bytes.

MTU can be manually changed by entering ip mtu <size> command in interface level. But because MTU is a function of physical link you may not be able to change it for most cases. Even if so, you can mask the problem by entering the command ip ospf mtu-ignore in interface level on the lower MTU side so that the adjacencies will happen and the routers will work fine. But it is not a recommended solution because when the router with the larger MTU goes for a flooding event, the router with the lower MTU will not be able to handle it.

So even though you can technically issue the above command to solve the issue it will be never a valid design because it will probably call for more troubles in the future.


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