HP ProCurve Switch 6120G/XG Management Manual page 103

Multiple Instance Spanning-Tree Operation
Overview
4-4
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
10, 11, 12 Yes
20, 21, 22 No
Instance 2
No
Yes