CCIE Enterprise Infrastructure - OSPF MTU Mismatch

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

0:00 Introduction
0:40 Explaining the MTU
1:29 Verifying OSPF configuration
2:04 Changing the MTU on R1
4:52 Restarting the OSPF process
5:57 Analyzing the output
7:57 Explaining the OSPF MTU check
10:11 Debugging OSPF MTU mismatch
12:48 Recap
13:24 ip ospf mtu-ignore
14:31 Problem with ignoring the MTU
15:57 Using extended ping to send oversized packets
18:09 Explaining LSA exchange failure and proposing the correct solution
19:38 Summary
20:33 Bonus: Wireshark packet capture
21:46 Bonus: IP MTU vs. OSPF for IPv6
28:27 Ending

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.

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.

INE blog "OSPF and MTU Mismatch": https://blog.ine.com/2011/03/30/ospf-...

OSPF packet capture (OSPF_broadcast_adjacencies.cap): https://packetlife.net/media/captures...

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