Configuring Multiple Spanning-Tree Protocol Prerequisites for Multiple Spanning Tree Protocol, on page 1 Restrictions for Multiple Spanning-Tree Protocol, on page 1 Information About Multiple Spanning Tree Protocol, on page 2 How to Configure Multiple Spanning Tree Protocol and Parameters, on page 16 Feature History for Multiple Spanning Tree Protocol, on page 28 Prerequisites for Multiple Spanning Tree Protocol • For two or more devices to be in the same multiple spanning tree (MST) region, they must have the same VLAN-to-instance map, the same configuration revision number, and the same name.
• For load-balancing across redundant paths in the network to work, all VLAN-to-instance mapping assignments must match; otherwise, all traffic flows on a single link. • For load-balancing between a per-VLAN spanning tree plus (PVST+) and an MST cloud or between a rapid-PVST+ and an MST cloud to work, all MST boundary ports must be forwarding. MST boundary ports are forwarding when the root of the internal spanning tree (IST) of the MST cloud is the root of the common spanning tree (CST).
If the MST cloud consists of multiple MST regions, one of the MST regions must contain the CST root, and all of the other MST regions must have a better path to the root contained within the MST cloud than a path through the PVST+ or rapid-PVST+ cloud. You might have to manually configure the devices in the clouds. Restrictions for Multiple Spanning-Tree Protocol • The switch stack supports up to 65 MST instances. The number of VLANs that can be mapped to a particular MST instance is the maximum active VLAN supported by a given switch. • PVST+, Rapid PVST+, and MSTP are supported, but only one version can be active at any time.
(For example, all VLANs run PVST+, all VLANs run Rapid PVST+, or all VLANs run MSTP.) • VLAN Trunking Protocol (VTP) propagation of the MST configuration is not supported. However, you can manually configure the MST configuration (region name, revision number, and VLAN-to-instance mapping) on each device within the MST region by using the command-line interface (CLI) or through the Simple Network Management Protocol (SNMP) support. Configuring Multiple Spanning-Tree Protocol 1