OSPF Adjacency Issue

[d01afj]

Hey guys, hoping you can shed a bit of light on something for me. Hopefully you can get an idea of what's going on without knowing the full network layout.

Basically, we have 2 sites linked by a dedicated line with 3750 switches at either end. Recently, the link between the 2 site went down due to a problem at one of the sites (for the sake of reference, site A).

This caused an error in the logs for the switch at site B as follows:

%OSPF-5-ADJCHG: Process 1, Nbr 172.16.15.1 on GigabitEthernet1/0/24 from FULL to DOWN, Neighbor Down: Dead timer expired

This makes sense - the neighbour adjacency is lost due to the failure on the line. Then however, the switch placed the port in err-disable state due to a loop being detected in the network as follows:

%ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on GigabitEthernet1/0/24.

%PM-4-ERR_DISABLE: loopback error detected on Gi1/0/24, putting Gi1/0/24 in err-disable state

%LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/24, changed state to down

%LINK-3-UPDOWN: Interface GigabitEthernet1/0/24, changed state to down


Now this doesn't make a lot of sense to me. I'm assuming this is a layer 2 issue and the only layer 3 route to the 172.16.0.0/16 subnet is out of that interface (gi1/0/24).

Can you think of any reason why a loss of an OSPF adjacency would result in a network loop?

Thanks.

 

[Minue]

Hi try to look at the problem the other way around,starting from the lower layers.First layer 2 sense a loop and bring the link down.Then the OSPF packets didn't get it hello packets,so the OSPF process was brought down.
Regards

 

[burtsbees]

Yes---it is the loop that caused the link to go into err-disable, shutting it down. The "hello" could not be sent/received as a result.

/

tim@tim-laptop ~ $ sudo apt-get install windows
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package windows...Thank Goodness!

 

[d01afj]

Hey, thanks for your replies. The only thing is, the adjacency error occurred a good minute or so before the loop error.

Also, I'm now aware that there was definitely a problem with a power outage in site A which seems to have caused the problem.

 

[Minue]

Hello
The OSPF "dead interval" is 40 seconds by default.This means that the link was down for about a minute before the switch sense and log the error message.If read the logs you maybe able to reconstuct the events.

Regards

 

[unclerico]

is the connection between switches fiber or copper??

I hate all Uppercase... I don't want my groups to seem angry at me all the time! =)
- ColdFlame (vbscript forum)

 

[d01afj]

Hey, the connection is fiber.

 

[VolatileAcid]

Perhaps this was related to your issue:

https://supportforums.cisco.com/docs/DOC-3876

Power loss to the router at the other site might have caused this keepalive packet to loop back, causing the err-disable of the port, and thus the subsequent failure of OSPF.

Just speculating.

 

[burtsbees]

But he said, "The only thing is, the adjacency error occurred a good minute or so before the loop error."

/

tim@tim-laptop ~ $ sudo apt-get install windows
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package windows...Thank Goodness!