Spanning tree issue

Answered Question

We had an intersting situation last night. One of the guys I work with created a new VLAN on two 6509s, configured one interface on both 6509s for the new VLAN and at the end he added the new VLAN to instance 6 on both 6509s. That is when everything went down. On one of the 6509s the VLAN interfaces, whcihc belong to different instances, started to flap so did EIRGP. Here is what the spanning tree configuration looks like;

USPA833COREA:

spanning-tree mst configuration

instance 1 vlan 50, 55, 60, 80, 110, 120, 130, 160-161, 166, 169, 300
instance 1 vlan 650, 700, 840
instance 2 vlan 90
instance 3 vlan 34, 95, 100
instance 4 vlan 399
instance 5 vlan 400-401, 410, 420-421, 430-431, 440-441, 450-451, 460
instance 6 vlan 498-499
instance 7 vlan 500-501, 510-511, 520-521, 530-531, 540-541, 550, 560-561
instance 7 vlan 570-571, 580-581, 590-591
instance 8 vlan 990, 999
!        
spanning-tree mst 1,3,5,7 priority 4096
spanning-tree mst 2,4,6,8 priority 8192

USPA833COREB:

spanning-tree mst configuration

instance 1 vlan 50, 55, 60, 80, 110, 120, 130, 160-161, 166, 169, 300
instance 1 vlan 650, 700, 840
instance 2 vlan 90
instance 3 vlan 34, 95, 100
instance 4 vlan 399
instance 5 vlan 400-401, 410, 420-421, 430-431, 440-441, 450-451, 460
instance 6 vlan 498-499
instance 7 vlan 500-501, 510-511, 520-521, 530-531, 540-541, 550, 560-561
instance 7 vlan 570-571, 580-581, 590-591
instance 8 vlan 990, 999
!
spanning-tree mst 1,3,5,7 priority 8192
spanning-tree mst 2,4,6,8 priority 4096

I did not thing that adding a VLAN to an instance, that don't have an interface defined on the 6509s, could cause this kind of issues. Did any one of you ever had the same problem?

Thank you for your help.

I have this problem too.
0 votes
Correct Answer by Jerry Ye about 6 years 4 months ago

This is the normal behavior of MST and it is clearly stated in the MST Configuration Restrictions and Guildlines from the 4500 configuration guide (I am not able to find this on the 6500 configuration guide):

Complete any MST configuration that incorporates a  large number of either existing or new logical VLAN ports during a  maintenance window because the complete MST database gets reinitialized  for any incremental change (such as adding new VLANs to instances or  moving VLANs across instances).

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/54sg/configuration/guide/spantree.html#wp1083326

The MST best practice is predefine and populate all VLAN's before the switch goes in production.

Regards,

jerry

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Correct Answer
Jerry Ye Wed, 08/04/2010 - 07:04

This is the normal behavior of MST and it is clearly stated in the MST Configuration Restrictions and Guildlines from the 4500 configuration guide (I am not able to find this on the 6500 configuration guide):

Complete any MST configuration that incorporates a  large number of either existing or new logical VLAN ports during a  maintenance window because the complete MST database gets reinitialized  for any incremental change (such as adding new VLANs to instances or  moving VLANs across instances).

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/54sg/configuration/guide/spantree.html#wp1083326

The MST best practice is predefine and populate all VLAN's before the switch goes in production.

Regards,

jerry

Jerry Ye Wed, 08/04/2010 - 11:19

It depends. What was the original reason you are going with MST? If you will only use Cisco equipment and will not interop with another vendor, you can use Rapid PVST+, it is more straight forward and a typical 6500 will be able to handle it.

Regards,

jerry

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