MST is built on the concept of mapping one or more VLANs to a single STP instance. Multiple instances of STP can be used (hence the name MST), with each instance supporting a different group of VLANs.
For the network shown in Figure 11-3, only two MST instances would be needed. Each could be tuned to result in a different topology so that Instance 1 would forward on the left uplink, whereas Instance 2 would forward on the right uplink. Therefore, VLAN A would be mapped to Instance 1, and VLAN B would be mapped to Instance 2.
To implement MST in a network, you need to determine the following:
■ The number of STP instances needed to support the desired topologies
■ Whether to map a set of VLANs to each instance
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