OSPF neighbor stuck in EXSTART/EXCHANGE Networking Origin: Network OSI layer 3 · Network Last updated 5 Nov 2026

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

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

Exact message you may see OSPF neighbor stuck in EXSTARTOSPF stuck in EXCHANGE%OSPF-5-ADJCHG: Process 1, Nbr from EXSTART to DOWNOSPF MTU mismatchNbr 10.0.0.2 Adjacency stuck in EXSTART
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Seen on: Cisco Networking

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. text
    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?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Network. Errors that are often confused with or caused by this one (click to open):

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