Foundry Configuration Guide for the FESX, FSX, and FWSX
7 - 62
© Foundry Networks, Inc.
December 2005
Foundry’s support for Cisco's Per VLAN Spanning Tree plus (PVST+), allows a Foundry device to run multiple
spanning trees (MSTP) while also interoperating with IEEE 802.1Q devices. Foundry 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 Foundry 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).
The enhanced PVST+ support 1allows a Foundry 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 7.28 shows the interaction of IEEE 802.1Q, PVST, and PVST+
regions.
Figure 7.28
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+.
PVST Region
IEEE 802.1Q Region
802.1D BPDUs
PVST BPDUs tunneled through
the IEEE 802.1Q region
Do not
connect
dual mode
port
dual mode
port
802.1D BPDUs
PVST BPDUs
(over ISL trunks)
PVST+ Region
PVST BPDUs
(over ISL trunks)
PVST+ Region
Содержание FastIron Edge Switch X424
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