Reference Guide

Multiple Spanning Tree Protocol (MSTP) | 601

Multiple Spanning Tree Protocol (MSTP)

Overview

Multiple spanning tree protocol (MSTP)—specified in IEEE 802.1Q-2003—is an rapid spanning tree

protocol (RSTP)-based spanning tree variation that improves on PVST+. MSTP allows multiple spanning

tree instances and allows you to map many virtual local area networks (VLANs) to one spanning tree

instance to reduce the total number of required instances.

In contrast, per-VLAN spanning tree plus (PVST+) allows a spanning tree instance for each VLAN. This

1:1 approach is not suitable if you have many VLANs, because each spanning tree instance costs

bandwidth and processing resources.

In Figure 32-1, three VLANs are mapped to two multiple spanning tree instances (MSTI). VLAN 100

traffic takes a different path than VLAN 200 and 300 traffic. The behavior in Figure 32-1 demonstrates

how you can use MSTP to achieve load balancing.

Figure 32-1. MSTP with Three VLANs Mapped to Two Spanning Tree Instances

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MSTI 1 root

MSTI 1: VLAN 100

MSTI 2: VLAN 200, VLAN 300 MSTI 2: VLAN 200, VLAN 300

MSTI 2 root

Forwarding

Blocking