Traditional Spanning Tree Protocol, which is the IEEE 802.1D standard, supports two flavors of STP implementation, which are Common Spanning Tree Protocol (CST) and Per VLAN Spanning Tree Protocol Plus (PVST+).
CST, which is also referred to as 802.1Q, runs a single STP instance for all VLANs configured on the switch. The problem with running a single instance of STP is that any blocked link is unable to actively participate in the forwarding of data and thus the path becomes a wasted resource. This is illustrated below in Figure 4-17:
Fig. 4 . . .
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