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Configuring STP
Information About Configuring STP
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 MST 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 instances. 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 should 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 strategy 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 separating 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), which forwards non-IP
protocols such as DECnet between two or more VLAN bridge domains or routed 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 multiple 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 increased.
Table 38 Spanning-Tree Interoperability and Backward Compatibility
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
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