HP Procurve 2650 Management And Configuration Manual page 309

802.1w Rapid Spanning Tree Protocol (RSTP) and 802.1d Spanning Tree Protocol (STP)
Overview
Use spanning tree to ensure that only one active path at a time exists between
any two nodes on the network. In networks where there is more than one
physical, active path between any two nodes, enabling spanning tree ensures
a single active path between such nodes by blocking all redundant paths.
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 mes-
sages, leading to a "broadcast storm" that can bring down the network.
N o t e
You should enable spanning tree operation in any switch that is part of a
redundant physical link (loop topology). (It is recommended that you do so
on all switches belonging to a loop topology.) This topic is covered in more
detail under "How Spanning Tree Operates" on page 13-4.
As recommended in the IEEE 802.1Q VLAN standard, the Switches 2650 and
6108 use single-instance STP. (As a result, the switch generates untagged
Bridge Protocol Data Units—BPDUs.) This implementation creates a single
spanning tree to make sure there are no network loops associated with any of
the connections to the switch, regardless of whether multiple VLANs are
configured on the switch. Thus, these switches do not distinguish between
VLANs when identifying redundant physical links. If VLANs are configured on
the switch, see "Spanning Tree Operation with VLANs" on page 11-30.
13-3