HP ProCurve 9304M Installation And Configuration Manual page 282

Installation and Basic Configuration Guide
Previous releases allow an HP device to interoperate with IEEE 802.1Q devices only when the HP device is
configured for Single STP (SSTP). In this case, the HP device is operating as an IEEE 802.1Q device but cannot
run multiple spanning trees. The current release and previous releases allow the HP device to interoperate with
PVST when the HP device is configured for MSTP.
NO TE: HP ports automatically detect PVST+ BPDUs and enable support for the BPDUs once detected. You do
not need to perform any configuration steps to enable PVST+ support. However, to support the IEEE 802.1Q
BPDUs, you might need to enable dual-mode support.
HP's support for Cisco's Per VLAN Spanning Tree plus (PVST+), allows an HP device to run multiple spanning
trees (MSTP) while also interoperating with IEEE 802.1Q devices. HP ports automatically detect PVST+ BPDUs
and enable support for the BPDUs once detected. The enhancement allows a port that is in PVST+ compatibility
mode due to auto-detection to revert to the default MSTP mode when one of the following events occurs:
• The link is disconnected or broken
• The link is administratively disabled
• The link is disabled by interaction with the link-keepalive protocol
This enhancement allows a port that was originally interoperating with PVST+ to revert to MSTP when connected
to an HP device.
O v e r v i e w o f PVST a n d PVST +
Per VLAN Spanning Tree (PVST) is a Cisco proprietary protocol that allows a Cisco device to have multiple
spanning trees. The Cisco device can interoperate with spanning trees on other PVST devices but cannot
interoperate with IEEE 802.1Q devices. An IEEE 802.1Q device has all its ports running a single spanning tree.
PVST+ is an extension of PVST that allows a Cisco device to also interoperate with devices that are running a
single spanning tree (IEEE 802.1Q).
The enhanced PVST+ support in release 07.6.04 allows an HP device to interoperate with PVST spanning trees
and the IEEE 802.1Q spanning tree at the same time.
IEEE 802.1Q and PVST regions cannot interoperate directly but can interoperate indirectly through PVST+
regions. PVST BPDUs are tunneled through 802.1Q regions, while PVST BPDUs for VLAN 1 (the IEEE 802.1Q
VLAN) are processed by PVST+ regions. Figure 8.34 shows the interaction of IEEE 802.1Q, PVST, and PVST+
regions.
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