Overview of MSTP

Without spanning tree, having more than one active path between a pair of nodes causes loops in the network, which can result in duplication of messages leading to a "broadcast storm" that can bring down the network.


MSTP cannot protect against loops when there is an unmanaged device on the network that drops spanning tree packets, or may fail to detect loops where this is an edge port configured with client authentication (802.1X, Web and MAC authentication). To protect against the formation of loops in these cases, you can use the loop protection feature.

Multiple-Instance spanning tree operation (802.1s) ensures that only one active path exists between any two nodes in a spanning tree instance. A spanning tree instance comprises a unique set of VLANs, and belongs to a specific spanning tree region. A region can comprise multiple spanning tree instances (each with a different set of VLANs), and allows one active path among regions in a network. Applying VLAN tagging to the ports in a multiple-instance spanning tree network enables blocking of redundant links in one instance while allowing forwarding over the same links for non-redundant use by another instance.

VLAN/Instance groupings

Suppose that there are three switches in a region configured with VLANs grouped into two instances, as follows:


Instance 1

Instance 2

10, 11, 12



20, 21, 22



The logical and physical topologies resulting from these VLAN/Instance groupings result in blocking on different links for different VLANs:

A multiple spanning tree application