Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 318)

Support for Cisco's Per VLAN Spanning Tree plus (PVST+), allows a Brocade device to run multiple
spanning trees (MSTP) while also interoperating with IEEE 802.1Q devices. Brocade 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 a Brocade device.
1 Cisco user documentation for PVST/PVST+ refers to the IEEE 802.1Q spanning tree as the Common Spanning Tree (CST).
Overview of PVST and 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).
Enhanced PVST+ support allows a Brocade 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 tunnelled through 802.1Q regions, while PVST BPDUs for VLAN 1
(the IEEE 802.1Q VLAN) are processed by PVST+ regions. The following figure shows the interaction
of IEEE 802.1Q, PVST, and PVST+ regions.
FIGURE 67 Interaction of IEEE 802.1Q, PVST, and PVST+ regions
Overview of PVST and PVST+
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