Administrator Guide

Multiple Spanning Tree Protocol (MSTP)

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

tree variation that improves per-VLAN spanning tree plus (PVST+). MSTP allows multiple spanning tree instances and allows you to map

many VLANs to one spanning tree instance to reduce the total number of required instances.

Protocol Overview

MSTP — specified in IEEE 802.1Q-2003 — is a rapid spanning tree protocol (RSTP)-based spanning tree variation that improves on per-

VLAN spanning tree plus (PVST+). MSTP allows multiple spanning tree instances and allows you to map many VLANs to one spanning

tree instance to reduce the total number of required instances.

In contrast, 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 the following illustration, 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 demonstrates how you can use MSTP to achieve load balancing.

Figure 85. MSTP with Three VLANs Mapped to TWO Spanning Tree Instances

Topics:

• Spanning Tree Variations

• Configure Multiple Spanning Tree Protocol

• Enable Multiple Spanning Tree Globally

• Adding and Removing Interfaces

• Creating Multiple Spanning Tree Instances

• Influencing MSTP Root Selection

• Interoperate with Non-Dell Bridges

• Changing the Region Name or Revision

• Modifying Global Parameters

472 Multiple Spanning Tree Protocol (MSTP)