Users Guide
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Multiple Spanning Tree Protocol (MSTP)
Multiple spanning tree protocol (MSTP) — specied 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 — specied 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 trac takes a
dierent path than VLAN 200 and 300 trac. The behavior demonstrates how you can use MSTP to achieve load balancing.
Figure 95. MSTP with Three VLANs Mapped to TWO Spanning Tree Instances
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Multiple Spanning Tree Protocol (MSTP)