12.7. The Multiple Spanning Tree Protocol

The Multiple Spanning Tree Protocol (MSTP) is an IEEE standard that is based on the Cisco proprietary protocol known as Multiple Instances Spanning Tree Protocol (MISTP). MSTP largely operates like RSTP (see Chapter 5, "Loop Resolution"). As a matter of fact, MSTP can be considered an extension of the RSTP protocol. The main difference is the MSTP protocol drills down to the VLAN level, allowing a separate Spanning Tree instance within a grouping of VLANs.

MSTP retains all of its spanning tree information in a single BPDU. Thus it is backward compatible with both STP and RSTP. MSTP attaches region and instance (more on this shortly) information in the BPDU. This information is inserted at the end of the traditional RSSTP BPDU. Thus RSTP bridges are able to receive the BPDU and recognize it as a RSTP BPDU. There are no configuration changes to make in order to get these stations communicating with one another.

Simply put, if you understand and know how RSTP works, then MSTP should be a breeze.

12.7.1. So Exactly What Are They Trying to Accomplish Here?

With all of the growth and changes in the data world, there are always new conformances and standards (for example, the demands for speed, bandwidth, application changes ... lions and tigers and bears — oh my!) that ridges within a LAN are expected to meet; the need for MSTP was apparent early in its development. With specific goals in mind, and the foresight to see that goals were obtained ...

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