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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 20 Configuring STP
Understanding Spanning-Tree Features
forward delay and by quickly transitioning root ports and designated ports to the forwarding state.
In a switch stack, the cross-stack rapid transition (CSRT) feature performs the same function as
RSTP. You cannot run MSTP without RSTP or CSRT.
The most common initial deployment of MSTP is in the backbone and distribution layers of a
Layer 2 switched network. For more information, see Chapter 21, “Configuring MSTP.”
For information about the number of supported spanning-tree instances, see the next section.
Supported Spanning-Tree Instances
In PVST+ or rapid-PVST+ mode, the switch or switch stack supports up to 128 spanning-tree instances.
In MSTP mode, the switch or switch stack supports up to 65 MST instances. The number of VLANs that
can be mapped to a particular MST instance is unlimited.
For information about how spanning tree interoperates with the VLAN Trunking Protocol (VTP), see the
“Spanning-Tree Configuration Guidelines” section on page 20-13.
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 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.
All stack members run the same version of spanning tree (all PVST+, all rapid PVST+, or all MSTP).
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.
Tab le 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