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Configuring STP
Information About Configuring STP
Spanning-Tree Modes and Protocols
The switch supports these spanning-tree modes and protocols:
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 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.
Rapid PVST+—This spanning-tree mode is the same as PVST+ except that is uses a rapid convergence based on the
IEEE 802.1w standard. To provide rapid convergence, the rapid PVST+ immediately deletes dynamically learned
MAC address entries on a per-port basis upon receiving a topology change. By contrast, PVST+ uses a short aging
time for dynamically learned MAC address entries.
The rapid PVST+ uses the same configuration as PVST+ (except where noted), and the switch needs only minimal
extra configuration. The benefit of rapid PVST+ is that you can migrate a large PVST+ install base to rapid PVST+
without having to learn the complexities of the MSTP configuration and without having to reprovision your network.
In rapid-PVST+ mode, each VLAN runs its own spanning-tree instance up to the maximum supported.
MSTP—This spanning-tree mode is based on the IEEE 802.1s standard. You can map multiple VLANs to the same
spanning-tree instance, which reduces the number of spanning-tree instances required to support a large number
of VLANs. The MSTP runs on top of the RSTP (based on IEEE 802.1w), which provides for rapid convergence of the
spanning tree by eliminating the forward delay and by quickly transitioning root ports and designated ports to the
forwarding state. You cannot run MSTP without RSTP.
The most common initial deployment of MSTP is in the backbone and distribution layers of a Layer 2 switched
network. For more information, see Configuring MSTP, page 333
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 supports up to 128 spanning-tree instances.
In MSTP mode, the switch 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 Changing the
Spanning-Tree Mode, page 327.
Spanning-Tree Interoperability and Backward Compatibility
Table 38Spanning-Tree Interoperability and Backward Compatibility, page 323 lists the interoperability and compatibility
among the supported spanning-tree modes in a network.
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