HP ProCurve Series 4100GL Management And Configuration Manual page 327

802.1w Rapid Spanning Tree Protocol (RSTP) and 802.1d Spanning Tree Protocol (STP)
Spanning Tree Operation with 802.1Q VLANs. As recommended in the
IEEE 802.1Q VLAN standard, when spanning tree is enabled on the switch, a
single spanning tree is configured for all ports across the switch, including
those in separate VLANs (that is, single-instance spanning tree, which gener­
ates untagged BPDUs). This means that if redundant physical links exist in
separate VLANs, spanning tree will block all but one of those links. However,
if you need to use spanning tree on the switch in a VLAN environment with
redundant physical links, you can prevent blocked redundant links by using
a port trunk. The following example shows how you can use a port trunk with
802.1Q (tagged) VLANs and spanning tree without unnecessarily blocking any
links or losing any bandwidth.
Problem:
STP enabled with 2
separate (non-trunked)
links blocks a VLAN link.
Nodes 1 and 2 cannot
communicate because
STP is blocking the link.
Figure 14-2. Example of Using a Trunked Link with STP and VLANs
For more information, refer to "Spanning Tree Operation with VLANs" on page
12-30.
How Spanning Tree Operates
Solution:
STP enabled with
one trunked link.
Nodes 1 and 2 can communicate because STP
sees the trunk as a single link and 802.1Q (tagged)
VLANs enable the use of one (trunked) link for
both VLANs.
14-5