90
Figure 3-12: Single STP Topology
For VLAN 10 this single STP topology is fine and presents no limitations or inefficiencies. On the other hand,
VLAN 20's traffic pattern is inefficient. All frames from Switch B will have to traverse a path through Switch A
before arriving at Switch C. If the Port 2 on Switch B and Switch C could be used, these inefficiencies could be
eliminated. MSTP does just that, by allowing the configuration of MSTIs based upon a VLAN or groups of
VLANs. In this simple case, VLAN 10 could be associated with Multiple Spanning Tree Instance (MSTI)1 with
an active topology similar to Figure 16 and VLAN 20 could be associated with MSTI 2 where Port 1 on both
Switch A and Switch B begin discarding and all others forwarding. This simple modification creates an active
topology with a better distribution of network traffic and an increase in available bandwidth.
The logical representation of the MSTP environment for these three switches is shown in Figure 13.
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