Chapter 13 Spanning Tree Protocol
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blocked as STP and RSTP allow only one link in the network and block the 
redundant link. 
Figure 54   STP/RSTP Network Example 
With MSTP, VLANs 1 and 2 are mapped to different spanning trees in the network. 
Thus traffic from the two VLANs travel on different paths. The following figure 
shows the network example using MSTP. 
Figure 55   MSTP Network Example 
13.1.4.2  MST Region 
An MST region is a logical grouping of multiple network devices that appears as a 
single device to the rest of the network. Each MSTP-enabled device can only 
belong to one MST region. When BPDUs enter an MST region, external path cost 
(of paths outside this region) is increased by one. Internal path cost (of paths 
within this region) is increased by one when BPDUs traverse the region.