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:
-
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.
- An MTU mismatch between neighbors is one of the most common reasons for being stuck in the
-
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 theip ospf network
command on the interface.
-
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.
-
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.
-
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.
-
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.
Links:
https://forum.networklessons.com/t/introduction-to-ospf/878/201?u=lagapidis
https://networklessons.com/ospf/ospf-packets-and-neighbor-discovery