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OSPF MTU MISTMATCH ISSUE AND FIX

  • While maintaining and creating neighbor adjacency OSPF sends IP MTU in its DBD packets.
  • MTU should match on both ends for forming and even to maintain neighbor adjacency.
  • In case there is MTU mismatch, the OSPF router gets stuck in the Exstart state.
  • Let’s check the issue and how to overcome it.
  • R1 is configure manually with MTU: 1400 bytes, while R2 and R3 are set to default: 1500 bytes.

 

We have enabled, debug for neighbor adjacency, as shown below, we see that we are receiving higher MTU value from 3.3.3.3 our neighbor while MTU configured on R1 interface is 1400.

R1(config)#int gig
R1(config)#int gigabitEthernet 0/0

R1(config-if)#ip mtu 1400

This indicates issue with MTU size, same is show in the below packet captures as well.

How to fix this issue and bring neighbor adjacency from Exstart to FULL.

Solutions:

  1. Change the IP MTU to MTU size of the neighbor.

In this case we can set IP MTU to default (1500) on R1 and it will be fixed as show below:

2. Using IP MTU IGNORE.

In case, there is multi vendors, and we are not sure of the MTU configured at the remote end and we still want to bring the neighborship in Full state, then we can use the below command to ignore the MTU and bring OSPF neighbor adjacency up.

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