STP—Defined in IEEE 802.1D, this is the original standard that provided a loop-free topology in a network with redundant links. Also called Common Spanning Tree (CST), it assumed one spanning-tree instance for the entire bridged network, regardless of the number of VLANs.

Per-VLAN Spanning Tree (PVST+)—PVST+ is a Cisco enhancement of STP that provides a separate 802.1D spanning-tree instance for each VLAN configured in the network.

Rapid Spanning Tree Protocol (RSTP)—RSTP is defined in IEEE 802.1w. It is an evolution of STP that provides faster convergence than STP.

Rapid Per-VLAN Spanning Tree (Rapid PVST+)—Rapid PVST+ is a Cisco enhancement of RSTP that uses PVST+ and provides a separate instance of 802.1w for each VLAN.

Multiple Spanning Tree Protocol (MSTP)—MSTP, defined in IEEE 802.1s, maps multiple VLANs into the same spanning-tree instance. The Cisco implementation of MSTP is often referred to as Multiple Spanning Tree (MST).