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 that is associated with that VLAN to all other devices in the network. Because each device has the same information about the network, this process ensures that the network topology is maintained. • Rapid PVST+—Rapid PVST+ is the default STP mode on your device. 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. Rapid PVST+ uses the same configuration as PVST+ (except where noted), and the device 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 Multiple Spanning Tree Protocol (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 that are required to support many 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. 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.
Supported Spanning-Tree Instances Starting with Cisco IOS XE Amsterdam 17.2.1 release, in PVST+ or Rapid PVST+ mode, the device or device stack supports up to 300 spanning-tree instances. In MSTP mode, the device or device stack supports up to 65 MST instances. The number of VLANs that can be mapped to a particular MST instance is unlimited. Spanning-Tree Interoperability and Backward Compatibility In a mixed MSTP and PVST+ network, the common spanning-tree (CST) root must be inside the MST backbone, and a PVST+ device cannot connect to multiple MST regions.
When a network contains devices running Rapid PVST+ and devices running PVST+, we recommend that the Rapid PVST+ devices and PVST+ devices be configured for different spanning-tree instances. In the Rapid PVST+ spanning-tree instances, the root switch must be a Rapid PVST+ device. In the PVST+ instances, the root switch must be a PVST+ device. The PVST+ devices 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). Table 2:
PVST+, MSTP, and Rapid-PVST+ Interoperability and Compatibility Configuring Spanning Tree Protocol Supported Spanning-Tree Instances 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 Configuring Spanning Tree Protocol 9