Quick Reference Guide

PowerConnect B-Series TI24X Configuration Guide 151
53-1002269-02
PVST/PVST+ compatibility
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Support for Cisco's Per VLAN Spanning Tree plus (PVST+), allows a device to run multiple spanning
trees (MSTP) while also interoperating with IEEE 802.1Q devices. Dell 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 device.
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 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. Figure 27 shows the interaction of IEEE
802.1Q, PVST, and PVST+ regions.