Multi-Instance Spanning Tree (MSTP), IEEE 802.1.q-2005,  to allow for additional features in Rapid Spanning Tree Protocol, such as “Per Vlan” STP as is found in PVST+.

The concept behind MSTP is that Layer 2 devices could be further subdivided into regions and vlans could be added to the regions as instances.  The “Per Vlan” STP would be an instance reducing the number BPDUs created and processed by the switches to just their region’s instances.  This feature increased MSTP’s efficiency over PVST+, but also caused it have a greater complexity in configuration and maintenance.

A region is a group of switches that adhere to the same STP profile and elect a root within their region.  When multiple regions are enabled as pictured below.  Each region used the Common Instance Spanning Tree (CIST) to come up with a loop free topology for the entire Layer 2 domain.  This region feature in 802.1s allows for granularity in how traffic can flow at Layer 2 while maintaining a loop-free topology.

Each region only has one switch and thus each switch designates itself as root for that region.  The regions are the default configuration for MSTP with the Switch MAC address being defined as the region.  The CIST is the STP that defines how the Layer 2 Regions interact to create a loop-free topology.

The regions are colored with the respective name being defined as the switch MAC address.

• Green
• Orange
• Yellow

The CIST is colored gray and covers all the regions.  The regions determine their respective STP roots and a CIST root is chosen based on STP algorithm with each region appearing as a switch.

Which port will block in the scenario below?

The link between 5800A and 5800B will be Alternate mode on the 5800B side of the link.

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