Explain Types of Spanning-Tree Protocols

Spanning-Tree Protocol (STP) is used to ensure network availability with redundant path without 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 ports of the switch. The different types of spanning-tree protocols are:-

Spanning-Tree Protocol (STP)

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

The whole network has only one Root Bridge, so all the traffic flows over the same path. The STP is very slow and taking 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 providing 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 Common Spanning Tree (CST). It is slower than Common Spanning Tree (CST). It consumes less bandwidth than CST and providing more optimization on the performance of the network then CST. PVST+ also required more CPU and memory.

The speed of convergence is comparable to the original STP but the difference is the separate instance and separate 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 they are 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 is an advance version of STP providing faster convergence than CST but holds with the same a single root bridge in the topology. The bridge resources needed in RSTP is 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 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 also a separate instance of 802.1w per VLAN but with much more CPU and memory requirement 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 the suboptimal traffic flow issues.

Multiple Spanning Tree Protocol (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 the Cisco proprietary and equal to MSTP, which 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. The CPU and memory requirements of this version are less than Rapid PVST+ but more than RSTP.

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