3-2
Multiple Instance Spanning-Tree Operation
Overview
Note 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 (see “Loop Protection” on page 3-72).
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.
For example, suppose you have three switches in a region configured with
VLANs grouped into two instances, as follows:
VLANs Instance 1 Instance 2
10, 11, 12 Yes No
20, 21, 22 No Yes
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