# Networking: %OSPF-5-ADJCHG: Nbr 10.0.0.2 from EXSTART to DOWN — OSPF neighbor stuck in EXSTART/EXCHANGE (MTU mismatch)

**Code:** `OSPF neighbor stuck in EXSTART/EXCHANGE` · **Category:** Networking · **Seen on:** Cisco, Networking

Two OSPF routers see each other but never finish exchanging databases, cycling between EXSTART and DOWN.

## Meaning

The classic cause is different interface MTU on the two ends, because OSPF includes the MTU in Database Description packets. Make the MTU equal (or use `ip ospf mtu-ignore` as a workaround) and also check duplicate router IDs, unicast reachability and ACLs blocking IP protocol 89.

## Common causes

- Different interface MTU on the two routers
- Duplicate OSPF router ID
- ACL or firewall blocking protocol 89
- Unicast connectivity problem (only multicast works)
- Authentication or area mismatch

## Quick fix

1. Compare `show ip ospf interface` MTU on both ends
2. Make MTUs identical (or temporarily `ip ospf mtu-ignore`)
3. Check router IDs are unique

## Detailed fix by platform

### Cisco

1. 

```
show ip ospf neighbor
show interfaces Gi0/1 | include MTU
interface Gi0/1
 ip mtu 1500
```


## How to diagnose

1. **State** — EXSTART/EXCHANGE?
2. **MTU** — Equal?
3. **Router ID** — Unique?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/networking/ospf-neighbor-stuck-exstart-mtu-mismatch)