PVST+ is a Cisco proprietary extension to 801.2d STP that provides a separate 802.1 spanning-tree instance for each VLAN configured on your switches.
All of Cisco proprietary extensions were created to improve convergence times, which is 50 seconds by default.
Creating a per-VLAN STP instance for each VLAN is worth the increased CPU and memory requirements, because it allows for per-VLAN root bridges. This feature allows the STP tree to be optimized for the traffic of each VLAN by allowing you to configure the root bridge in the center of each of them.
This root bridge placement clearly enables faster convergence as well as optimal path determination. This version’s convergence is really similar to 802.1 CST’s, which has one instance of STP no matter how many VLANs you have configured on your network. The difference is that with PVST+, convergence happens on a per-VLAN basis, with each VLAN running its own instance of STP.
To allow for the PVST+ to operate, there’s a field inserted into the BPDU to accommodate the extended system ID so that PVST+ can have a root bridge configured on a per-STP instance. The bridge ID actually becomes smaller—only 4 bits—which means that we would configure the bridge priority in blocks of 4,096 rather than in increments of 1 as we did with CST. The extended system ID (VLAN ID) is a 12-bit field.