Strange case of Spanning-tree (MSTP)

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Aug 8th, 2010
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Hi all,


I have a strage spanning-tree case seeking your help, the background is as follow:


Currently we have 3 physical sites, site A (production data centre), site B (DR data centre) and site C (BCP office), those 3 sites has Cisco 6506E, a pair of stacked 3750 and a 3560 switch respectively. The service provider using QinQ tunnel to provide the WAN services to us like the following simple picture:


site A (switch A) -------trunk in my switch and SP using VLAN ID 701-------site B (switch B)-------trunk in my switch and SP using VLAN ID 709 --------site C (switch C)


all these 3 switches are running the same MSTP region and the configuration are the same, same region name, same revision number, and instance (instance 0 for VLAN 1 and VLAN 2-4094 in instance 1), and we have manually configure the switch A being the root bridge for all of the instance.


However, recently, the switch B say it is the root bridge of instance 1, so there are 2 root bridge in the network for instance 1 and switch C also said switch B is the root of instance 1. Switch B is successful to receive BPDU from switch A, and it say switch A is root for instance 0. But from the debug spanning-tree all message, it said the trunk port connecting to switch A (sanity: mrecord missing).

I have searched in Internet but have no luck. Now we don't have spanning-tree issue because the connection is not a loop in our network. But I will feel more comfortable if switch B can get BPDU normally. Would you please help in this case and how can I solve the problem of mrecord missing?


Thank you


Best Regards,


Terry Chow

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Tharak Abraham Sun, 08/08/2010 - 18:45
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Terry,


How did you assign Switch A as root ?


Could you do a "spanning-tree mst 0 priority 0" and "spanning-tree mst 1 priority 0" on Switch A?

Reset the revision number to a +1 higher than whats in Switch B


Check the Status on both


Please carry out these changes in non-production hours or weekends..

MST can be a real PITA if not handled properly..-:)

chowterry Sun, 08/08/2010 - 19:26
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Hi Tharak,


I am using the command "spanning-tree mst 0-1 priority 24576" to set switch A as the root, and other switches using the default 32768.

Should I also change the revision number in switch B and C also?


Thank you.

Tharak Abraham Mon, 08/09/2010 - 04:38
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Yes, you need to set the revision number as same in all the switches in the same region...say 10.

This parameter wont get incremented on changes.

chowterry Mon, 08/09/2010 - 18:06
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Thank you Tharak,


I will schedule a maintenance window to test it.


Thank you.


Best Regards,


Terry Chow

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