Thursday, May 10, 2012

Making Changes to MST Config with Care

Multiple Spanning Tree (MST) is great for VLAN spanning tree management and load-balancing. In concept, you can have multiple VLANs and group them up into 2 regions. Instead of managing every individual VLAN spanning tree, you would just need to manage the two MST regions regardless of the number of VLANs.

However, for two or more switches to be in the same MST region, they must have the identical MST name,
VLAN-to-instance mapping, and MST revision number. Any changes applied to existing MST configuration on one switch but not on others would cause network disruptions. To minimize such disruptions, learn to make MST changes but only commit them until all switches are 'standardized' in the new MST configuration.

This is how to enter MST configuration sub-mode on the switch:
switch# configure terminal
switch(config)# spanning-tree mst configuration

This shows how to leave MST-submode configuration on the switch without committing the
switch(config-mst)# abort

This shows how to commit the changes and leave MST configuration sub-mode on the switch:
switch(config-mst)# exit

For more information, download this Cisco "Configuring MST" doc.

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