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OSPF Neighbor Down: Too many retransmissions

hi guys,

I would appreciate some assistance here. 

every 3 weeks or so, we have an issue with remote sites connecting to our head office. it occurred again this morning, which led to me capturing logs from WAN router melrtrw001, core router melcore001 as well as non-cisco accelerator sitting between the two routers

 

melcore001#sh log

Jul 24 23:22:12.041: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.10.83 on Vlan254 from FULL to DOWN, Neighbor Down: Too many retransmissions

Jul 24 23:23:12.048: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.10.83 on Vlan254 from DOWN to DOWN, Neighbor Down: Ignore timer expired

Jul 24 23:24:46.851: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.10.83 on Vlan254 from EXSTART to DOWN, Neighbor Down: Dead timer expired

Jul 24 23:27:50.297: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.10.83 on Vlan254 from LOADING to FULL, Loading Done

 

melrtrw001#sh log

Jul 25 09:22:50 AEST: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.10.84 on GigabitEthernet0/0 from 2WAY to DOWN, Neighbor Down: Dead timer expired

Jul 25 09:23:24 AEST: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.252.2 on GigabitEthernet0/0 from 2WAY to DOWN, Neighbor Down: Dead timer expired

Jul 25 09:25:23 AEST: %OSPF-5-ADJCHG: Process 1, Nbr 10.3.254.5 on GigabitEthernet0/0 from 2WAY to DOWN, Neighbor Down: Dead timer expired

Jul 25 09:26:01 AEST: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.252.2 on GigabitEthernet0/0 from 2WAY to DOWN, Neighbor Down: Dead timer expired

Jul 25 09:26:39 AEST: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.10.84 on GigabitEthernet0/0 from 2WAY to DOWN, Neighbor Down: Dead timer expired

Jul 25 09:27:49 AEST: %OSPF-5-ADJCHG: Process 1, Nbr 202.10.51.9 on GigabitEthernet0/0 from LOADING to FULL, Loading Done

Jul 25 09:27:50 AEST: %OSPF-5-ADJCHG: Process 1, Nbr 202.10.51.8 on GigabitEthernet0/0 from LOADING to FULL, Loading Done

 

i have checked mtu (set at 1500 on both devices). 

any ideas?

thanks, rama

6 REPLIES
Cisco Employee

Hi Rama, OSPF flap due to too

Hi Rama,

 

OSPF flap due to too many retransmission happens when DBD packets (OSPF updates) from one router are not able to reach to the other router. Most f the time it happens due to MTU issue and low MTU on any L2device in the path can also cause the problem. To check this, you can ping to other side ip address with size 1500 and DF bit set. But if that would be the case then it will keep on flapping since due to low MTU in the path DBD packet will never reach to the other side.

 

In your case it is flapping in every 3 weeks or so. This could also be due to some layer1 issue in the path which is causing packet drop and BDB packets are not reaching to the other side. Check interface statistics for CRC, input errors on both sides . During issue run ping for 1000 packets with size 1500 and check if there is any packet drop. 

 

-Akash

Cisco Employee

Hello Akash,Your analysis is

Hello Akash,

Your analysis is good but I am afraid it does not fully apply in this case: if you read Rama's post carefully you will notice that his OSPF routers complain about the retransmissions in the Full state and revert from the Full state into the Down state. The DBD packets and MTU issues would have already manifested themselves in the ExStart and Exchange states, and the routers would never make it to the Full state.

In fact, all OSPF reliable packets are retransmitted if not acknowledged - DBDs, LSRs, and LSUs. The issue with too many retransmissions can occur in any state that allows reliable packets to be exchanged between OSPF neighbors, i.e. ExStart, Exchange, Loading and Full

However, as this issue occurs infrequently and over WAN links, I would indeed suggest keeping an eye on these circuits, both their utilization and reliability. Perhaps the service provider can provide some kind of testing of the circuits to see if they are operating within negotiated parameters.

I do not suppose that there is STP involved over those WAN circuits - but if there is I would also have a look whether there can be any topology change going around the same time when the OSPF adjacencies are torn down. STP topology changes can cause disrupted connectivity for up to 50 seconds. That could easily bring OSPF neighors down.

Best regards,
Peter

 

Cisco Employee

Hi Peter, I agree with you,

Hi Peter,

 

I agree with you, that is why in 2nd paragraph i mentioned that it is less likely MTU issue but possibly due to packet drop issue on the network.

 

Regards,

Akash

New Member

Hi Akash/Peter,Thanks for

Hi Akash/Peter,

Thanks for your input. MTU has been checked and verified to be OK. 

Issue likely to be isolated to core router IMO, as the cpu utlisation is consistently quite high >90% during BH. Would this cause packet loss on network?

It seems to be the most likely issue.

Intrusive testing of WAN link from carrier is also being organised.

Rama

Super Bronze

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Posting

Where exactly does the WAN accelerator reside in the topology?

What I'm wondering with whether that accelerator is delaying or causing OSPF packets to be lost.

New Member

Hi Joseph,It resides in

Hi Joseph,

It resides in between the core router & WAN router. I've opened a seperate case with the vendor to analyse that.

Rama

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