Multiple Spanning Tree (MST)
With Multiple Spanning Tree (MST), you can group VLANs and run one
instance of Spanning Tree for a group of VLANs. This cuts down on the
number of root bridges, root ports, designated ports, and BPDUs in your
network. Switches in the same MST Region share the same configuration
and VLAN mappings. Configure MST with these commands:
(config)# spanning-tree mode mst
(config)# spanning-tree mst configuration
(config-mst)# name region_name
(config-mst)# revision number
(config-mst)# instance number vlan vlan_range
(config-mst)# end
To be compatible with 802.1Q trunking, which has one common Spanning
Tree (CST) for all VLANs, MST runs one instance of an Internal Spanning
Tree (IST). The IST appears as one bridge to a CST area and is MST
instance number 0. The original MST Spanning Trees (called M-Trees) are
active only in the region—they combine at the edge of the CST area to form
one.