Pvst/Pvst+ Compatibility; Overview Of Pvst And Pvst; Vlan Tags And Dual Mode - Dell PowerConnect B-RX Configuration Manual

PVST/PVST+ compatibility

Brocade's support for Cisco's Per VLAN Spanning Tree plus (PVST+) allows the device to run
multiple spanning trees (MSTP) while also interoperating with IEEE 802.1Q devices
automatically detect PVST+ BPDUs and enable support for the BPDUs once detected.
When it is configured for MSTP, the device can interoperate with PVST.

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).
The PVST+ support allows the 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.
802.1Q, PVST, and PVST+ regions.
FIGURE 31

VLAN tags and dual mode

To support the IEEE 802.1Q (Common Spanning Tree) portion of PVST+, a port must be a member
of VLAN 1. Cisco devices always use VLAN 1 to support the IEEE 802.1Q portion of PVST+.
BigIron RX Series Configuration Guide
53-1002253-01
Interaction of IEEE 802.1Q, PVST, and PVST+ regions
802.1D BPDUs
PVST+Region
dual mode
port
PVST BPDUs
(over ISL trunks)
1. Cisco user documentation for PVST/PVST+ refers to the IEEE 802.1Q spanning tree as the
Common Spanning Tree (CST).
PVST/PVST+ compatibility
Figure 31
PVST BPDUs tunneled through
the IEEE 802.1Q region
IEEE 802.1Q Region
Do not
connect
PVST Region
1
. Brocade ports
shows the interaction of IEEE
802.1D BPDUs
PVST+Region
dual mode
port
PVST BPDUs
(over ISL trunks)
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