29Multiple Spanning Tree Protocol (MSTP)Multiple spanning tree protocol (MSTP) — specified in IEEE 802.1Q-2003 — is a rapid spanning treeprotocol (RSTP)-based spanning tree variation that improves per-VLAN spanning tree plus (PVST+). MSTPallows multiple spanning tree instances and allows you to map many VLANs to one spanning treeinstance 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 treevariation that improves on per-VLAN spanning tree plus (PVST+). MSTP allows multiple spanning treeinstances and allows you to map many VLANs to one spanning tree instance to reduce the total numberof required instances.In contrast, PVST+ allows a spanning tree instance for each VLAN. This 1:1 approach is not suitable if youhave 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 howyou can use MSTP to achieve load balancing.Figure 88. MSTP with Three VLANs Mapped to TWO Spanning Tree Instances578 Multiple Spanning Tree Protocol (MSTP)
