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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 20 Configuring STP
Information About Configuring STP
Spanning-Tree Interoperability and Backward Compatibility
Table 20-2 lists the interoperability and compatibility among the supported spanning-tree modes in a
network.
In a mixed MSTP and PVST+ network, the common spanning-tree (CST) root must be inside the MST
backbone, and a PVST+ switch cannot connect to multiple MS T regions.
When a network contains switches running rapid PVST+ and switches running PVST+, we recommend
that the rapid-PVST+ switches and PVST+ switches be configured for different spanning-tree instanc es.
In the rapid-PVST+ spanning-tree instances, the root switch must be a rapid-PVST+ switch. In the
PVST+ instances, the root switch must be a PVST+ switch. The PVST+ switches shoul d be at the edge
of the network.
STP and IEEE 802.1Q Trunks
The IEEE 802.1Q standard for VLAN trunks imposes some limitations on the spanning-tree strat egy for
a network. The standard requires only one spanning-tree instance for all VLANs allowed on the trunks.
However, in a network of Cisco switches connected through IEEE 802.1Q trunks, the switches maintain
one spanning-tree instance for each VLAN allowed on the trunks.
When you connect a Cisco switch to a non-Cisco device through an IEEE 802.1Q trunk, the Cisco switch
uses PVST+ to provide spanning-tree interoperability. If rapid PVST+ is enabled, the switch uses it
instead of PVST+. The switch combines the spanning-tree instance of the IEEE 802.1Q VLAN of the
trunk with the spanning-tree instance of the non-Cisco IEEE 802.1Q switch.
However, all PVST+ or rapid-PVST+ information is maintained by Cisco switches separated by a cloud
of non-Cisco IEEE 802.1Q switches. The non-Cisco IEEE 802.1Q cloud separa ting the Cisco switches
is treated as a single trunk link between the switches.
PVST+ is automatically enabled on IEEE 802.1Q trunks, and no user configuration is required. The
external spanning-tree behavior on access ports is not affected by PVST+.
VLAN-Bridge Spanning Tree
Cisco VLAN-bridge spanning tree is used with the fallback bridging feature (bridge groups), whi ch
forwards non-IP protocols such as DECnet between two or more VLAN bridge domains or ro uted ports.
The VLAN-bridge spanning tree allows the bridge groups to form a spanning tree on top of the individual
VLAN spanning trees to prevent loops from forming if there are mult iple connections among VLANs.
It also prevents the individual spanning trees from the VLANs being bridged from collapsing into a
single spanning tree.
To support VLAN-bridge spanning tree, some of the spanning-tree timers are i ncreased.
Tab l e 20-2 PVST+, MSTP, and Rapid-PVST+ Interoperability
PVST+ MSTP Rapid PVST+
PVST+ Yes Yes (with restrictions) Yes (reverts to PVST+)
MSTP Yes (with restrictions) Yes Yes (reverts to PVST+)
Rapid PVST+ Yes (reverts to PVST+) Yes (reverts to PVST+) Yes