Dell Force10 MXL Blade - Multiple Spanning Tree Protocol (MSTP); Overview

Multiple Spanning Tree Protocol (MSTP) | 337
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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
R2
R3
1/21
3/11
3/21
1/31
2/11
2/31
MSTI 1 root
MSTI 1: VLAN 100
MSTI 2: VLAN 200, VLAN 300 MSTI 2: VLAN 200, VLAN 300
MSTI 2 root
Forwarding
Blocking