Implementing the Multiple Spanning Tree Protocol – MSTP
What is MST?

The main purpose of MST is to reduce the total number of spanning tree instances to match the physical topology of the network and thus reduce the CPU loading of a switch. The instances of spanning tree are reduced to the number of links (i.e. active paths) that are available. If the example in the diagram were implemented via PVST+, there could potentially be 4094 instances of spanning tree, each with their respective BPDU conversations, root bridge election and path selections.

In this example, the goal is to achieve load distribution with VLANS 1-500 using one path and VLANS 501-1000 using the other path, with only two instances of spanning tree. The two ranges of VLANs are mapped to two MST instances (MSTI) respectively. Rather than maintaining 1000 spanning trees, each switch needs to maintain only two. Implemented in this fashion, MST converges faster than Per VLAN Spanning Tree+ (PVST+) and is backward compatible with 802.1D STP, 802.1w (RSTP), and the Cisco PVST+ architecture. Implementation of MST is not required if the Enterprise Composite Model is being employed, as the number of active VLAN instances, and hence, the STP instances would be small in number and very stable due to the design.

MST allows you to build multiple spanning trees over trunks by grouping and associating VLANs to spanning tree instances. Each instance can have a topology independent of other spanning tree instances. This architecture provides multiple active forwarding paths for data traffic and enables load balancing. Network fault tolerance is improved over CST because a failure in one instance (forwarding path) does not necessarily affect other instances. This VLAN to MST grouping must be consistent across all bridges within an MST region.

In large networks, you can more easily administer the network and use redundant paths by locating different VLAN and spanning tree assignments in different parts of the network. A spanning tree instance can exist only on bridges that have compatible VLAN instance assignments.

You must configure a set of bridges with the same MST configuration information, which allows them to participate in a specific set of spanning tree instances. Interconnected bridges that have the same MST configuration are referred to as an "MST region." Bridges with different MST configurations or legacy bridges running 802.1D are considered separate MST regions.

In a Cisco PVST+ environment, the spanning tree parameters are tuned so that half of the VLANs are forwarding on each uplink trunk. This is easily achieved by electing bridge D1 to be the root for VLAN501–1000, and bridge D2 to be the root for VLAN1–500. In this configuration, the following is true:

  • Optimum load balancing is achieved.
  • One spanning tree instance for each VLAN is maintained, which means 1000 instances for only two different logical topologies. This consumes resources for all the switches in the network (in addition to the bandwidth used by each instance sending its own BPDUs).

MST (IEEE 802.1s) combines the best aspects from both PVST+ and 802.1Q. The idea is that several VLANs can be mapped to a reduced number of spanning-tree instances because most networks do not need more than a few logical topologies.