Users 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 81. 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
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