Brocade BigIron RX Series Configuration Guide - Manual (Page 423)

BigIron RX Series Configuration Guide 345
53-1002484-04
PVST/PVST+ compatibility
12
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
1
. Brocade ports
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.
Figure 31 shows the interaction of IEEE
802.1Q, PVST, and PVST+ regions.
FIGURE 31 Interaction of IEEE 802.1Q, PVST, and PVST+ regions
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+.
1. Cisco user documentation for PVST/PVST+ refers to the IEEE 802.1Q spanning tree as the
Common Spanning Tree (CST).
PVST+Region IEEE 802.1Q Region PVST+Region
PVST Region
PVST BPDUs
(over ISL trunks)
PVST BPDUs
(over ISL trunks)
Do not
connect
dual mode
port
dual mode
port
802.1D BPDUs 802.1D BPDUs
PVST BPDUs tunneled through
the IEEE 802.1Q region