Multiple Spanning Tree (MST)

Multiple Spanning Tree (MST)
Multiple Spanning Tree (MST) builds upon the proprietary PVST+ standard. With MST, a number
of spanning tree instances can be created, but they are not mapped one-to-one to VLANs. The
reason for this is that in most networks—even those supporting hundreds of VLANs—there are
a small number of optimal topologies. As each instance of STP demands its own root and all the
associated BPDU activity, the processing overhead can be unnecessarily high if you allow each
VLAN to have its own spanning tree. Better to create the STP instances and then map VLANs to
those instances.
MST features include switches that are grouped together in MST “regions”—interconnected
bridges that have the same MST configuration. Each switch in an MST region maintains three
attributes: a configuration name, a revision number, and a table associating each of the VLANs
supported per MST instance (up to the 4096 maximum). These attributes are common across
a domain and must be shared by all switches. Different attributes signify a different domain,
which changes the switch-to-switch relationship. Finally, different instances of STP have several
VLANs mapped to them, creating the opportunity for VLANs to operate with optimal topology,
but reducing the overhead associated with PVST.
MST was approved by the IEEE as 802.1s in June 2003, so a standards-based implementation
of this protocol is likely to figure extensively in the future.
Readers wishing to know more about 802.1s than is covered in the CCNP program
should visit
www.cisco.com/warp/public/473/147.html
.