OSPF Stuck in EXSTART: show ip ospf neighbor When MTU Kills the Adjacency

Cisco routing · Techclick CLI troubleshooting lab

OSPF neighbor stuck EXSTART/EXCHANGE. ping works. Area and timers match.

This is a live-ticket style lesson from Techclick Infosec. You will isolate the fault with CLI first, then prove the fix with one clean packet or log.

What you will be able to do

• Name the first CLI command to run on Cisco routing for this ticket

• Separate the layer that is actually broken from the layer people blame

• Apply one scoped fix instead of a global disable

• Prove the ticket closed with a session, log, or counter

Quick answer

EXSTART that never finishes is almost always MTU. ip ospf mtu-ignore is a bandage; matching MTU is the fix.

Why this ticket shows up

DBD packets must fit. A 1500 vs 9000 mismatch loops EXSTART forever.

Symptoms operators actually see

• EXSTART or EXCHANGE forever

• ping OK

• One side is a switch SVI with jumbo

• Recently enabled jumbo frames

CLI first — copy this block

Run these on the Cisco routing device or console named in the ticket. Do not skip the first show command — it tells you which later command matters.

show ip ospf neighbor
show ip ospf interface GigabitEthernet0/1
show interface Gi0/1 | include MTU
debug ip ospf adj
ping 10.0.0.2 size 1500 df-bit

How to read that output

If large ping with DF fails, MTU is the problem. Neighbor output will sit in EXSTART.

Triage order

1. Compare IP MTU both sides

2. Align MTU or use mtu-ignore only as a temporary

3. Clear ospf process in a window

4. Confirm FULL

5. Turn debug off

Classic traps

What you see	Real cause	Fix
Change area id	ping already works	It is MTU
Leave jumbo on one side	EXSTART will return	Match both
Reload the router	MTU is still wrong	Fix the interface

Proof the ticket is closed

• Neighbor FULL

• Large ping with DF works

• LSAs exchange

Say this in an interview

OSPF EXSTART with a good ping is MTU until the interface counters prove otherwise.

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