OSPF neighbor status is stuck in **ExStart** state

Products

• VMware NSX

Issue/Introduction

• OSPF neighbor status on NSX edge T0/T1 is stuck in ExStart state
• Log messages in syslog or ospfd.log show MTU discrepancies:
  Packet[DD]: Neighbor #### MTU #### is larger than [uplink-####:####]'s MTU ####.

Environment

VMware NSX 4.x / 9.x

Cause

MTU Mismatch: One peer is sending Database Description (DD) packets larger than the other peer can receive. Note: NSX does not support `mtu-ignore`.

• Log messages in syslog, /var/log/frr/frr.log or ospfd.log show MTU discrepancies:
  Packet[DD]: Neighbor #### MTU #### is larger than [uplink-####:####]'s MTU ####.

Resolution

Check MTU Alignment:

• Confirm the MTU on the NSX Edge uplink profile matches the physical TOR switch port.
• Review syslog for "MTU is larger than" errors. Both peers must use the exact same MTU.
• Maximum supported MTU for NSX edge links is 9000. If uplink OSPF peer is sending MTU higher than 9000, adjust the MTU configuration on uplink peer for lower than 9000.

Additional Information

For more troubleshooting details for OSPF adjacency issues follow KB Troubleshooting NSX OSPF