types of spanning tree protocol

Spanning-Tree Protocol (STP) ensures network availability with redundant paths without a loop. Several types of spanning-tree protocols have appeared since the original IEEE 802.1D. The default spanning-tree mode for Cisco Catalyst switches is PVST+, enabled on all switch ports. The different types of spanning-tree protocols are:-

Spanning-Tree Protocol (STP)

The STP is the original IEEE 802.1D version, providing redundant network connectivity without a loop. It is also known as the CST (Common Spanning Tree). Despite the number of VLANs, it assumes one spanning tree instance for the entire bridged network.

The whole network has only one Root Bridge, so all the traffic flows over the same path. The STP is very slow and takes a long time for convergence. The time of convergence for STP is 32 seconds. The updated version of the standard spanning-tree protocol is 802.1D-2004

Because of using only one Root Bridge, the CPU and memory requirements for CST are lower than for the other protocols. However, because there are only one Root Bridge and one tree, therefore traffic for all VLANs flows over the same path, which can lead to suboptimal traffic flows.

Per VLAN Spanning Tree + (PVST+)

PVST+ is the improved version of STP, which provides a dedicated 802.1D spanning-tree root bridge for every VLAN configured in the network. It is the default version of STP. It provides compatibility with the Common Spanning Tree (CST). It is slower than Common Spanning Tree (CST). PVST+ consumes less bandwidth than CST and provides more network performance optimization than CST. PVST+ also required more CPU and memory.

The speed of convergence is comparable to the original STP. Still, the difference is the separate instance and Root Bridge, which support Port Fast, Uplink Fast, Backbone Fast, BPDU filter, BPDU guard, loop guard, and root guard.

Port roles are the same as those in RSTP. Separate instance and separate root bridge for each VLAN increases the CPU and memory requirements. PVST+ allows the spanning-tree optimization for the traffic of each VLAN. The convergence of this PVST+ is similar to 802.1D. Though, it provides per-VLAN convergence.

Rapid Spanning Tree Protocol (RSTP)

RSTP is also known as IEEE 802.1w. This advanced version of STP provides faster convergence than CST but holds with the same single root bridge in the topology. The bridge resources needed in RSTP are higher than CST but less than PVST+. RSTP resolve many convergence issues, except it still provides a single instance of STP, it does not address the suboptimal traffic flow issues. Due to faster convergence, more CPU and memory are required for this version than CST but less than Rapid PVST+.

Rapid Per VLAN Spanning Tree + (Rapid PVST+)

Rapid PVST+ is a Spanning Tree standard providing faster convergence than PVST+ and a separate instance of 802.1w per VLAN but with much more CPU and memory requirements than other STP standards. The separate instance supports Port Fast, BPDU guard, BPDU filter, root guard, and loop guard. It resolves the convergence issues and suboptimal traffic flow issues.

Multiple Spanning-Tree Protocols (MSTP)

MSTP is the IEEE 802.1s standard, inspired from the earlier Cisco proprietary MISTP implementation. It reduces the number of required STP instances using the mapping of multiple VLANs that have the same traffic flow requirements into the same spanning-tree instance.

Multiple Spanning Tree (MST)

MST is Cisco proprietary and equal to MSTP. It provides up to 16 instances of RSTP (802.1w) and combines many VLANs with the same physical and logical topology into a common RSTP instance. Each RSTP instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard. This version’s CPU and memory requirements are less than Rapid PVST+ but more than RSTP.

Note: New Cisco switches, such as Catalyst 2960 with IOS 15.0, run PVST+ by default. They include several of IEEE 802.1D-2004’s specifications.