OSPF : Neighbor Down: Too many retransmissions

[rvillavi]

Hi All,

I'm having a strange issue, I have some OSPF neighbor ships that are going down. When I look into the loggs I’m seeing:

Sep 15 19:00:45: %OSPF-5-ADJCHG: Process 10, Nbr 172.18.164.20 on Vlan104 from FULL to DOWN, Neighbor Down: Too many retransmissions

Sep 15 19:01:45: %OSPF-5-ADJCHG: Process 10, Nbr 172.18.164.20 on Vlan104 from DOWN to DOWN, Neighbor Down: Ignore timer expired

This issue happens every 12 hours more or less. When I check the show ip ospf traffic, and for this interface I’m seeing:

  Length 0, Checksum 0, Version 0, Bad Source 0,

  No Virtual Link 0, Area Mismatch 0, No Sham Link 0,

  Self Originated 0, Duplicate ID 0, Hello 0,

  MTU Mismatch 0, Nbr Ignored 40794, LLS 0,

  Unknown neighbor 0, Authentication 0,

Do anyone knows what could make the to many retransmissions? Or if the high Nbr Ignore is producing this problem?

I’m attaching the show tech, show log and show ip ospf traffic.

Have a great day and thanks for your time and help.

[Reza Sharifi]

Your issue seems to be related to VLAN 104.

Is there a reason for configuring dampening under VLAN 104?

I would remove it on both sides of the connections and see if OSPF still flaps.

dampening is usually used in a service provider network where they punish the customer if the EBGP link flaps.

Also, make sure the MTU size is the same on both sides of the connection.

I would also reaserch to see if there is a bug in IOS you are using.

HTH

Reza

[Seifeddine-Tlili]

Your retransmission problem is mainly caused by an MTU mistmatch u can check this on both equippement using show system mtu and u can disable MTU check for OSFP using ip ospf mtu-ignore under the interface connected to the other equippement remember to do it in  both equippememnt.