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 treevariation that improves per-VLAN spanning tree plus (PVST+). MSTP allows multiple spanning tree instances and allows you to map manyVLANs to one spanning tree instance to reduce the total number of required instances.Protocol OverviewMSTP — 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 treeinstance 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 eachspanning 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 differentpath than VLAN 200 and 300 traffic. The behavior demonstrates how you can use MSTP to achieve load balancing.Figure 93. MSTP with Three VLANs Mapped to TWO Spanning Tree InstancesTopics:• Spanning Tree Variations• Configure Multiple Spanning Tree Protocol29Multiple Spanning Tree Protocol (MSTP) 557
