Dell FCX624-S Laptop User Manual


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276 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
PVST/PVST+ compatibility
8
NOTE
Dell PowerConnect 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.
Support for Cisco's Per VLAN Spanning Tree plus (PVST+), allows a Dell PowerConnect device to run
multiple spanning trees (MSTP) while also interoperating with IEEE 802.1Q devices. Dell
PowerConnect 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 Dell PowerConnect 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 Dell PowerConnect 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 53 shows the interaction of IEEE
802.1Q, PVST, and PVST+ regions.
1. Cisco user documentation for PVST/PVST+ refers to the IEEE 802.1Q spanning tree as the
Common Spanning Tree (CST).