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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 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.
Protocol Overview
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 87. MSTP with Three VLANs Mapped to TWO Spanning Tree Instances
Multiple Spanning Tree Protocol (MSTP) 595