[VIDEO] CCIE Enterprise Infrastructure - OSPF MTU Mismatch

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This video explores the OSPF MTU check and mismatch scenario in detail. 

The MTU is strictly speaking not included in the parameter checks but it is tightly related to the OSPF adjacency establishment process. 

First of all, the MTU or Maximum Transmission Unit is the size in bytes of the largest IP datagram that can be sent out the associated interface, without fragmentation.

RFC 2328 - 10.6.  Receiving Database Description Packets

If the Interface MTU field in the Database Description packet indicates an IP datagram size that is larger than the router can accept on the receiving interface without fragmentation, the Database Description packet is rejected.

The MTU must be equal for the DD packets to be successfully processed between neighbors. If the MTU values are not equal, the routers will not go beyond the EXSTART state and the adjacency will fail.

The routers exchange their MTU value in DBD packets when the OSPF adjacency is initialized. If the MTU field in the Database Description packet indicates an IP datagram size that is larger than the router can accept on the receiving interface without fragmentation, the Database Description packet is rejected.

This entire process is controlled by a single check to verify if the MTU values match. This check can be disabled. This is done with the ‘ip ospf mtu-ignore’ command. This command disables the MTU mismatch detection on received DBD packets.

ip ospf mtu-ignore

To disable Open Shortest Path First (OSPF) maximum transmission unit (MTU) mismatch detection on receiving database descriptor (DBD) packets, use the ip ospf mtu-ignore command in interface configuration mode. To enable OSPF mismatch detection, use the no form of this command.

Usage Guidelines

OSPF checks whether neighbors are using the same MTU on a common interface. This check is performed when neighbors exchange DBD packets. If the receiving MTU in the DBD packet is higher than the IP MTU configured on the incoming interface, OSPF adjacency will not be established.

In summary, here are the most important take-aways from this video:

  1. OSPF performs a check to verify if the MTU values match between neighbors on a common interface.

  2. This check is performed when the neighbors exchange DBD packets. If the received MTU in the DBD packet is larger than the MTU configured on the incoming interface, OSPF adjacency will not be established.

  3. This check can be disabled with the ‘ip ospf mtu-ignore’ command.

  4. The proper solution is to configure the correct MTU on the interfaces

Thank you for watching!

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