Dell S4820T, Configuration Manual

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Multiple Spanning Tree Protocol (MSTP)
Multiple spanning tree protocol (MSTP) is supported on Dell Networking OS.
Protocol Overview
MSTP — 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 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 traffic takes a different path than VLAN 200 and 300 traffic. The behavior demonstrates how
you can use MSTP to achieve load balancing.
Figure 90. MSTP with Three VLANs Mapped to TWO Spanning Tree Instances
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Multiple Spanning Tree Protocol (MSTP)