STP detects and prevents Layer 2 bridging loops from forming. Parallel paths can exist, but only one is allowed to forward frames.
STP is based on the IEEE 802.1D bridge protocol standard.
Switches run one instance of STP per VLAN with Per-VLAN Spanning Tree (PVST). PVST between switches requires the use of Inter-Switch Link (ISL) trunking.
For IEEE 802.1Q trunks, only a single instance of STP is allowed for all VLANs. The Common Spanning Tree (CST) is communicated over VLAN 1.
PVST+ is a Cisco proprietary extension that allows switches to interoperate between CST and PVST. PVST bridge protocol data units (BPDUs) are tunneled over an 802.1Q trunk. Catalyst switches run PVST+ by default.
Multiple Instance Spanning Tree Protocol