MST problems

Unanswered Question
Feb 5th, 2007

Hello,

Last week I had to implement new instances in MST_A (instance 15 and instance 16) for new customers.

Look at attached drawing.

After putting a command (on MST_A region):

spanning-tree mst config

instance 15 2800-2899

instance 16 2900-2999

the link between MST regions that was blocking for CST started to flapping (causing device to be unreachable) until I back-off the new instances 15 and 16.

(log:)

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 1 moving from blocking to forwarding

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 2 moving from blocking to forwarding

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 1 moving from forwarding to blocking

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 2 moving from forwarding to blocking

...... (it was for 2-3 minutes then I back-off config)

I read that instance 16 should not exist, becouse there is instance 0 (IST) and 15 MST instances (1-15), but Cat 6500 apply that command. Is it possible that instance 16 would be interprete as instance 0 or 1 or 2?

Do anyone have any idea what could happend?

Thanks for any output. If further information are needed, please ask.

Kind Regards:

Michal Grzelak

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Francois Tallet Mon, 02/05/2007 - 10:32

Hi Michal,

Your switches probably support the IEEE standard version of MST, which allows the configuration of up to 65 instances (CST + 64 MSTIs). As long as you expect the two regions MSTA and MSTB to be different, there should not be any problem with your changes. Did you apply the changes on all the MST switches in region MST A? Else, you could have created two MST A regions that would have make the CST topology a little bit more complex. That would not explain the instability though. I don't have an answer for you with the (already detailed) information you provided. Will you have a change to attempt configuring this again? I can suggest some few debugs command to enable on the relevant ports, but of course, it assumes you have a maintenance window.

In the end, if you configure all your switches in region MST A with the exact same config, I don't see how it could make a difference on the links between MST A and MST B.

Regards,

Francois

michal.grzelak Tue, 02/06/2007 - 00:22

Hi,

Thanks for replay. I didn't know that there are MST with 64 instances. So I am again in the "starting point" of troubleshooting :)

When I start adding the new instance 15 and 16 in MST_A region I only added it on core_1 switch and just a two or three seconds later the link between MST regions started to flap for instance 1 and 2. This is strange, cause between MST there should be RSTP so, why in logs there were instances changing?

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 2 moving from forwarding to blocking

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 1 moving from blocking to forwarding

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 2 moving from blocking to forwarding

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 1 moving from forwarding to blocking

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 2 moving from forwarding to blocking

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 1 moving from blocking to forwarding

Jan 30 09:01:18.014: %SPANTREE-SP-6-PORT_STATE: Port Po21 instance 2 moving from blocking to forwarding

Moreover is it possible that inconsistency in instances in two MST regions (look at drawing in first post) would influence each other. e.g. in MST_A vlan 30 would be in MST 1 and in MST_B vlan 30 would be in MST 2?

Thanks for any suggestion. Unfortunatly, debugging commands are out of the questions, only on service window.

Kind Regards:

Michal

Francois Tallet Tue, 02/06/2007 - 10:09

Hi Michal,

From the CST point of view, you had two bridges (MSTA and MSTB, a region looks like a virtual bridge to the CST) connected redundantly. By changing the configuration in one of your core bridges in MSTA, you have created a new region MSTA'. From the CST perspective, you now have three bridges in a ring: MSTA, MSTB and MSTA'. So yes, I expect your topology to change.

What I can't explain is this flapping. I cannot figure out the source from the message. Even the "show spanning-tree mst" ouput on the bridges at the different staged would help, if you had them.

Regards,

Francois

michal.grzelak Wed, 02/07/2007 - 08:13

Hi,

Thank You for information. I also think that would happen (new region MST was created). Hopefully in near future I will try to test the same configuration in lab and find what then happen.

Kind Regards:

Michal

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