STP

Spanning-Tree Protocol 

STP prevents loops from being formed when switches or bridges are interconnected via multiple paths. Spanning-Tree Protocol implements the 802.1D IEEE algorithm by exchanging BPDU messages with other switches to detect loops, and then removes the loop by shutting down selected bridge interfaces.
This algorithm guarantees that there is one and only one active path between two network devices

 

Cisco and STP Variants

There are many types or variants of STP. Some of these variants are Cisco proprietary and others are IEEE standards.

Per-VLAN spanning tree protocol (PVST) - Maintains a spanning-tree instance for each VLAN configured in the network. It uses the Cisco proprietary ISL trunking protocol that allows a VLAN trunk to be forwarding for some VLANs while blocking for other VLANs. Because PVST treats each VLAN as a separate network, it can load balance traffic at Layer 2 by forwarding some VLANs on one trunk and other VLANs on another trunk without causing a loop.

Per-VLAN spanning tree protocol plus (PVST+) - Cisco developed PVST+ to provide support for IEEE 802.1Q trunking. PVST+ provides the same functionality as PVST, including the Cisco proprietary STP extensions. PVST+ is not supported on non-Cisco devices. PVST+ includes the PortFast enhancement called BPDU guard, and root guard. 

Rapid per-VLAN spanning tree protocol (rapid PVST+) - Based on the IEEE 802.1w standard and has a faster convergence than STP (standard 802.1D). Rapid PVST+ includes Cisco-proprietary extensions such as BackboneFast, UplinkFast, and PortFast.

Rapid spanning tree protocol (RSTP) - It provides faster spanning-tree convergence after a topology change. RSTP implements the Cisco-proprietary STP extensions, BackboneFast, UplinkFast, and PortFast, into the public standard.

Multiple STP (MSTP) - Enables multiple VLANs to be mapped to the same spanning-tree instance, reducing the number of instances needed to support a large number of VLANs. MSTP was inspired by the Cisco-proprietary Multiple Instances STP (MISTP) and is an evolution of STP and RSTP. MSTP provides for multiple forwarding paths for data traffic and enables load balancing.