Ciscos Per VLAN Spanning Tree Plus (PVST+) creates an STP instance for each and every VLAN. By allowing STP to have it’s own instance for each VLAN, you can assign different switches to be root per VLAN. This allows you to have multiple loop free topologies and in instances where ports would otherwise be blocked to maintain a loop free environment, those ports can still be forwarding frames for other VLAN instances of STP.
This allows for better LAN load balancing where you can send some VLANs in one path across the network and other VLANs along a redundant loop free path.
Non-Cisco switches that only support the IEEE 802.1D STP standard only support a Common Spanning Tree, where there is a single loop free network in which ALL VLANs use the same loopfree path…This does not allow you to load balance the same way you could with PVST+.
PVST+ running on trunks to non-Cisco switches use a VLAN 1 STP instance to communicate wit hthe non-Cisco switches and their STP.
VLAN 1’s instance of the PVST+ topology merges with the CST instance creating a single loopfree topology throughout the vendor neutral LAN.
for VLANs 2-4094, BPDUs sent across trunks that connect to non-PVST+ devices treat their BPDUs differently. The instead send their BPDU’s to the Multicast address of 0100.0CCC.CCCD instead of 0180.C200.0000 and they tag their BPDUs wit hthe corresponding VLAN ID in a special TLV record called a Port VLAN ID TLV or PVID TLV.
This PVID TLV is processed by PVST+ switches to determine which VLAN the BPDU is associated with, and these encapsulated BPDUs are simply tunneled through CST switches and are not processed since it is being transmitted as normal multicast traffic.
So to PVST+ topologies they are simply interconnected by shared CST segments to create a loop free topology.