Dell Force10 MXL Blade MXL 10/40GbE Switch IO Module Configuration Guide - Page 339

19 Multiple Spanning Tree Protocol (MSTP) Overview Multiple spanning tree protocol (MSTP)-specified in IEEE 802.1Q-2003-is an rapid spanning tree protocol (RSTP)-based spanning tree variation that improves on PVST+. MSTP allows multiple spanning tree instances and allows you to map many virtual local area networks (VLANs) to one spanning tree instance to reduce the total number of required instances. In contrast, per-VLAN spanning tree plus (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 Figure 19-1, 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 in Figure 19-1 demonstrates how you can use MSTP to achieve load balancing. Figure 19-1. MSTP with Three VLANs Mapped to Two Spanning Tree Instances R1 MSTI 1: VLAN 100 MSTI 2: VLAN 200, VLAN 300 1/21 2/11 R2 MSTI 1 root 1/31 2/31 Blocking Forwarding 3/11 3/21 R3 MSTI 2 root Multiple Spanning Tree Protocol (MSTP) | 337