3com 8807 User Guide

MSTP stands for Multiple Spanning Tree Protocol, which is compatible with 
Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP).

STP is not fast in state transition. Even on a point-to-point link or an edge port, it 
has to take an interval twice as long as forward delay before the port transits to 
the forwarding state.

RSTP converges fast, but has the following drawback like STP: all the network 
bridges in a LAN share one spanning tree and the redundant links cannot be 
blocked based on VLANs. Packets of all VLANs are forwarded along one spanning 
tree.

MSTP makes up for the drawback of STP and RSTP. It not only converges fast, but 
also allows the traffic of different VLANs to be distributed along their respective 
paths, which provides a better load-balance mechanism for the redundant links.

MSTP keeps a VLAN mapping table to associate VLANs with their spanning trees. 
Using MSTP, you can divide one switching network into multiple regions, each of 
which can have multiple spanning trees with each one independent of others. 
MSTP prunes the ring network into a loopfree tree to avoid the generation of 
loops and infinite circulations. It also provides multiple redundant paths for data 
forwarding to implement the load-balance mechanism of the VLAN data.

There are 4 MST regions in Figure 23. Each region consists of four switches, all of 
which run MSTP. The following introduces the concept of MSTP with the help of 
this figure.