Stop internal loop traffic 1

[disturbedone]

Yesterday was interesting! We have a stack of 2x 5520s in one area of our school and someone put a patch cable from a port on one switch to a port on the other. This brought down the entire school

We have 2x Cisco Catalyst 4507 core routers with various Nortel & Cisco switches and most of them were affected at some point. They would go down then come up randomly.

I had a look at the Catalyst that the 5520s in question was connected to and saw a broadcast storm coming from it so I disconnected it and all settled. Then I found this loop cable on the 5520s.

The question is how do I stop this from happening again? I believe this has happened before my time here and a Nortel tech came to make a change that would keep any loop like this inside the switch and not affect any other switches but it appears this was not done on this switch.

Any ideas?

 

[fr38]

I met a similar problem on my network, you can activate STP on port that don't participate to trunk, this should block the port in case of bad connection.

 

[biv343]

Make sure STP is enabled on the ports that face the edge/users. STP should throw one port into a blocking state when a loop is created. SLPP can be used on uplinks when running in a SMLT configuration but I assume you aren't running that based on the Cisco cores.

 

[disturbedone]

Great. Thanks guys.

I had a look at the switches in 'Spanning Tree Port Configuration' and see that both port 1/31 and 2/12 (which was where the loop was) are set to 'Disable'. Most other ports are set to 'Fast Learning'. There are also numerous others set to 'Disabled'. Should these all be set to 'Fast Learning'??

I've had a look at other switches and they're the same. It's a mixture of 'Disabled' and 'Fast Learning'.

Do I leave the trunk port as 'Disabled' or 'Normal Learning'? Or do I also make it 'Fast Learning'??

 

[biv343]

Either or is fine. I usually set them to fast learning, but the important thing is to have it enabled.

A trick with Nortel/Avaya switches: Any time you remove all VLANs from a port it disables spanning tree on that port. It does not get turned back on when you add the port back to a VLAN. I'll add the new VLANs to the port before removing the existing VLAN membership, that way STP doesn't get disabled on the port and I don't have to remember to check the STP configuration each time I make a change.

 

[biv343]

I clicked submit too fast. I typically leave the trunk ports set to normal learning unless running SMLT to a pair of Avaya/Nortel split cores. In your case, that is dependent on how the Cisco cores are configured

 

[disturbedone]

Great. That makes sense. I suspect the Nortel tech enabled it on all but that was years ago and we will have been changing VLANs all over the place unaware that it was disabling STP as we did it.

Now we've replaced the Passport 8600s with Catalyst 4507s at the core we'll eventually replace the switches with Ciscos but that will take time.