OSPF - Stuck in ExStart or Exchange states

When creating an OSPF neighbor adjacency, there are several states through which those neighbors progress to establish their communication.

If the OSPF neighbors are stuck in the ExStart or Exchange states, it usually indicates a problem during the Database Description (DBD) exchange process. Here's a quick troubleshooting guide for resolving this issue:

Common Causes

  1. MTU Mismatch:

    • An MTU mismatch between neighbors is one of the most common reasons for being stuck in the ExStart/Exchange state.
    • Check the MTU settings on both interfaces. If the MTU size is not the same, OSPF will not properly form a full adjacency.
    • Solution: Set the ip ospf mtu-ignore command on the interface or adjust the MTU to match on both sides.

  2. Mismatched OSPF Network Types:

    • Ensure that the OSPF network type on both sides of the link is compatible. Different OSPF network types (Broadcast, Non-Broadcast, Point-to-Point) can prevent proper neighbor relationships.
    • Solution: Use the show ip ospf interface command to verify and set a consistent network type using the ip ospf network command on the interface.

  3. Router ID Conflicts:

    • If two routers have the same OSPF Router ID, they cannot establish a complete OSPF adjacency.
    • Solution: Check and manually set unique Router IDs using the router-id command in OSPF configuration.

  4. ACL or Firewall Restrictions:

    • Ensure that there are no Access Control Lists (ACLs) or firewalls blocking OSPF packets (Multicast addresses 224.0.0.5 and 224.0.0.6).
    • Solution: Review ACLs or firewall configurations and permit OSPF traffic.

  5. Authentication Mismatch:

    • If OSPF authentication is enabled, a mismatch in authentication keys will prevent the exchange process.
    • Solution: Verify the authentication configuration using the show ip ospf interface and correct the authentication keys.

  6. Incompatible OSPF Timers:

    • Hello and Dead interval mismatches can also cause issues with OSPF adjacency formation.
    • Solution: Use show ip ospf interface and verify that Hello and Dead timers match on both sides.

Quick Verification Commands

  • show ip ospf neighbor – Check the current state of OSPF neighbors.
  • show ip ospf interface [interface] – Verify MTU, OSPF timers, and other interface settings.
  • debug ip ospf adj – Get detailed logs for adjacency formation issues.

https://networklessons.com/ospf/ospf-packets-and-neighbor-discovery