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Why is the Interface MTU showing 0 in OSPF DB Description packet ?

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TitleWhy is the Interface MTU showing 0 in OSPF DB Description packet ?
Question
This article explains anissue related to OSPF interface MTUsize which is exchanged between neighboring routers during OSPF adjacency formation.
Environment
Answer
The Interface MTUisshowing '0' in OSPF DB Description packet even thoughthe IP MTU is configured for 9000 or anyothersizeas we do not send out the MTU size during the database exchange. This is the default and expectedbehavior and isworking per design.

Interface IPMTU configuration.

!
interface ethernet 1/1
enable
ip address 192.168.254.2/24
ip mtu 9000
!


This can be seen in thewireshark output of the DB Description packet as shown below.

- Open Shortest Path First
+ OSPF Header
- OSPF DB Description
Interface MTU: 0
+ Options: 0x42 (0, E)
+ DB Description: 0x07 (I, M, MS)
DD Sequence: 11798327


However this can cause ospf adjacency issues wheninteroperating withother vendors routers. The OSPF adjacencies will be stuck in Exstart/Exchange state.

Configuring no rfc1583-compatibility command underOSPF allowstheInterface MTUsize to besentout. When no rfc1583-compatibilitycommand is executed router will now operate withnew OSPFRFCstandard.

!
router ospf
area 0
no rfc1583-compatibility
!


Below is the wireshark output.

- Open Shortest Path First
+ OSPF Header
- OSPF DB Description
Interface MTU: 9000
+ Options: 0x42 (0, E)
+ DB Description: 0x07 (I, M, MS)
DD Sequence: 11798327


NOTE: Using the commandip ospf mtu-ignore under the interfacecan be used as a workaround and will work in some cases.
Additional notes

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