28.7.1 Overview

The following explains the features of Multiple Spanning Tree. MST instances can be used to perform load balancing. MST regions can be used to divide large network configurations into smaller configurations, to simplify network design. The following gives a functional overview of how Multiple Spanning Tree can be used to achieve these goals.

<Structure of this section>

(1) MST instances

Multiple Spanning Tree allows Spanning Tree Protocols to be built for each group that aggregates multiple VLANs, called MST instances or MSTI, enabling load balancing for each MST instance. For load balancing using PVST+, a tree is needed for each VLAN, but with Multiple Spanning Tree, MST instances can be used to use only the trees needed through planned load balancing. Therefore, unlike PVST+, increases in CPU load and network load can be kept to a minimum for each increase in VLAN count. The switch allows as many as 16 MST instances to be set for the Switch.

The following figure shows an example MST instance setup.

Figure 28-9 MST instance image

(2) MST region

Multiple Spanning Tree allows multiple switches to be grouped and handled as an MST region. To belong in the same MST region, the region name, revision number, MST instance ID, and VLAN correspondence must be the same. These are set by configuration. Trees are built separately between MST regions and within MST regions, and the topology within an MST region can be built per MST instance.

The following explains Spanning Tree Protocols that run both between MST regions and within MST regions.

CST

The Common Spanning Tree (CST) controls connections between MST regions, and bridges using Single Spanning Tree. Because this topology performs calculations by physical port as with Single Spanning Tree, it cannot perform load balancing.
IST

The Internal Spanning Tree (IST) refers to a topology that runs by default within an MST region for connecting outside of the MST region, and for which an MST instance ID of 0 is assigned. The port connecting outside of the MST region is called the boundary port. Note that a unique MST instance is used to send and receive BPDUs within and between regions. The topology information for all MST instances is encapsulated in an MST BPDU for reporting.
CIST

The Common and Internal Spanning Tree (CIST) refers to a topology that combines ISTs and CSTs.

The following figure provides an overview of Multiple Spanning Tree.

Figure 28-10: Overview of Multiple Spanning Tree