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| 3.5 |  | Evolution of Spanning Tree |
| 3.5.3 |  | MST (802.1 s) |

The acronym MST stands for two things:

- Mono Spanning Tree
- Multiple Spanning Tree

Be careful to differentiate between the two in context. The multiple spanning tree feature was originally released for Catalyst 4000 and 6000 series switches. MST is specified in IEEE 802.1s, an amendment to IEEE 802.1Q. MST extends the IEEE 802.1w rapid spanning tree (RST) algorithm to multiple spanning-trees, as opposed to the single CST of the original IEEE 802.1Q specification.

This extension provides for both rapid convergence and load balancing in a VLAN environment. Cisco implementation of MST is backward compatible with 802.1D STP, the 802.1w Rapid Spanning-Tree Protocol (RSTP), and the Cisco PVST+ architecture. MST can be thought of as a standards-based Multiple Instance of Spanning-Tree Protocol (MISTP). The IEEE 802.1w specification, Rapid Spanning-Tree Protocol, provides for subsecond reconvergence of STP after failure of one of the uplinks in a bridged environment.802.1w provides the structure on which the 802.1s feature operates.

The IEEE 802.1s multiple spanning tree specification allows a user to build multiple spanning trees over VLAN trunks. Fast convergence of the MST topology is achieved using a modified version of the RSTP protocol. VLANs can be grouped and associate to spanning tree instances. Each instance can have a topology independent of other spanning tree instances. This new architecture provides multiple forwarding paths for data traffic and enables load balancing. Network fault tolerance is improved, because a failure in one instance does not affect other instances.

In large networks, having different VLAN spanning tree instance assignments located in different parts of the network makes it easier to administer and utilize redundant paths. However, a spanning tree instance can exist only on bridges that have compatible VLAN instance assignments. Therefore, MST requires the configuration of a set of bridges with the same MST configuration information, allowing them to participate in a given set of spanning tree instances. Interconnected bridges that have the same MST configuration are called MST regions. MST, like MISTP, provides interoperability with PVST+ regions.