11-9
Catalyst 2940 Switch Software Configuration Guide
78-15507-02
Chapter 11 Configuring STP
Understanding Spanning-Tree Features
Spanning-Tree Modes and Protocols
The switch supports PVST+. This spanning-tree mode is based on the IEEE 802.1D standard and Cisco
proprietary extensions. It is the default spanning-tree mode used on all Ethernet, Fast Ethernet, and
Gigabit Ethernet port-based VLANs. The PVST+ runs on each VLAN on the switch up to the maximum
supported, ensuring that each has a loop-free path through the network.
The PVST+ provides Layer 2 load balancing for the VLAN on which it runs. You can create different
logical topologies by using the VLANs on your network to ensure that all of your links are used but that
no one link is oversubscribed. Each instance of PVST+ on a VLAN has a single root switch. This root
switch propagates the spanning-tree information associated with that VLAN to all other switches in the
network. Because each switch has the same information about the network, this process ensures that the
network topology is maintained.
Supported Spanning-Tree Instances
In PVST+, the switch supports up to 4 spanning-tree instances.
For information about how spanning tree interoperates with the VLAN Trunking Protocol (VTP), see
the “STP Configuration Guidelines” section on page 11-11.
Spanning-Tree Interoperability and Backward Compatibility
Table 11-2 lists the interoperability and compatibility among the supported spanning-tree modes in a
network.
In a mixed Multiple STP (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.
The Catalyst 2940 switch does not support MSTP or rapid PVST+.
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 802.1Q trunks, the switches maintain one
spanning-tree instance for each VLAN allowed on the trunks.
Table 11-2 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
