276 PowerConnect B-Series FCX Configuration Guide53-1002266-01PVST/PVST+ compatibility8NOTEDell PowerConnect ports automatically detect PVST+ BPDUs and enable support for the BPDUs oncedetected. 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 runmultiple spanning trees (MSTP) while also interoperating with IEEE 802.1Q devices. DellPowerConnect ports automatically detect PVST+ BPDUs and enable support for the BPDUs oncedetected. The enhancement allows a port that is in PVST+ compatibility mode due toauto-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 protocolThis enhancement allows a port that was originally interoperating with PVST+ to revert to MSTPwhen 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 havemultiple spanning trees. The Cisco device can interoperate with spanning trees on other PVSTdevices but cannot interoperate with IEEE 802.1Q devices. An IEEE 802.1Q device has all its portsrunning a single spanning tree. PVST+ is an extension of PVST that allows a Cisco device to alsointeroperate with devices that are running a single spanning tree (IEEE 802.1Q).Enhanced PVST+ support allows a Dell PowerConnect device to interoperate with PVST spanningtrees and the IEEE 802.1Q spanning tree at the same time.IEEE 802.1Q and PVST regions cannot interoperate directly but can interoperate indirectly throughPVST+ 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 IEEE802.1Q, PVST, and PVST+ regions.1. Cisco user documentation for PVST/PVST+ refers to the IEEE 802.1Q spanning tree as theCommon Spanning Tree (CST).
