Types of STP Protocols

Since STPs development in the 1980s, it has received many improvements to satisfy modern network requirements. Some improvements were required to speed up slow convergence or simplify the operational approach for switches with limited hardware resources. Also, some upgrades addressed network segmentation due to common multi-VLAN implementations.

There are several variations of STP, each with its own unique features and benefits, such as faster convergence times, better load balancing, and greater control over the network. Several different types of STPs exist today:

• Spanning Tree Protocol (STP)
• Per VLAN Spanning Tree Protocol (PVST+)
• Rapid Spanning Tree Protocol (RSTP)
• Rapid Per VLAN Spanning Tree Protocol (Rapid PVST+)
• Multiple Spanning Tree Protocol (MSTP)

Spanning Tree Protocol (STP)

STP is the legacy standard protocol that provides a loop-free topology in a network with redundant links. STP creates a single Common Spanning Tree (CST) instance for the entire bridged network, regardless of the number of VLANs configured on the switches.

As a result of using just a single STP process (instance), only one root bridge is elected, and the STP calculation applies to all VLANs. Therefore, traffic for all VLANs travels over the same path, regardless of whether it provides the most optimal traffic flow.

However, on the flip side, fewer CPU and memory resources are required because only one instance is used. Because this is the oldest protocol, it is also the slowest one, with a convergence time between 30 and 50 seconds, which is considered unacceptable today.

As you can see from the image above, PC 1, which belongs to VLAN 3, uses a suboptimal path through SW1 (the root bridge) to reach the server instead of going directly through SW3 because of the disabled port by STP.

Per VLAN Spanning Tree Protocol (PVST+)

PVST+ is a Cisco proprietary protocol that allows for separate instances of STP to run on each VLAN, improving the limitations of suboptimal traffic flows, providing better performance, and greater flexibility in network design. 

PVST+ allows per-VLAN root bridge election, so a different spanning tree is built for each VLAN, resulting in the most optimal paths always being used within VLANs.

As you can see from the image above, PC 1 now uses the most optimal path through SW3 (root bridge for VLAN 3) to reach the server instead of going through SW1 (root bridge for VLAN 2) like in the previous example.

Although using PVST+ provides the ability to load balance traffic per VLAN, using an instance for each VLAN increases the CPU and memory requirements. Therefore, you should be cautious with this STP type, especially when the network has many VLANs.

Rapid Spanning Tree Protocol (RSTP)

The Rapid Spanning Tree Protocol was developed to fulfill the requirements for faster convergence and more efficient use of network bandwidth. RSTP allows for faster failover times when a link or switch fails, allowing for faster network recovery.

However, just like standard STP, RSTP uses only a single instance regardless of the number of VLANs configured on the switches, which leads to suboptimal traffic flow issues.

On the positive side, RSTP provides a much faster convergence time than STP and PVST+, so instead of waiting 30 to 50 seconds for a disabled port to activate and take over, the switch needs less than a second for the same task.

RSTP is designed to be backward compatible with STP, so it can work with both RSTP and STP-enabled devices in a network. It uses fewer ports and fewer states than STP, and it provides faster convergence times when topology changes occur in the network.

Just like the STP protocol, RSTP uses the root and designated port roles. However, unlike STP, RSTP relies on “alternate” and “backup” port roles instead of STP’s nondesignated port roles. 

These replacement port roles allow RSTP to define a standby switch port before a failure or topology change. The state of the alternate and backup ports is always "discarding," which is equivalent to blocking in STP.

Rapid Per VLAN Spanning Tree Protocol (PVST+)

Cisco developed the Rapid PVST+ protocol to overcome the instance limitations of RSTP. It combines the convergence functionality of RSTP and the multi-instance approach of PVST+.

This provides optimal traffic flow by using a different spanning-tree instance for each VLAN configured on the switches and converging fast when a port fails. Because of these functionalities, Rapid PVST+ has the largest CPU and memory requirements among all STP variants.

Multiple Spanning Tree Protocol (MSTP)

Cisco developed the Multi-Instance STP to overcome the limitations of the PVST+, which later became a standard STP protocol called the Multiple Spanning Tree Protocol. The main goal of this protocol is to reduce the number of STP instances that can lead to full resource consumption when many VLANs are configured on the switches.

MSTP enables you to map many VLANs into the same spanning-tree instance, for which only a single root bridge is required. As you can see from the image above, SW1 operates as a root bridge for the first instance containing VLANs 2, 3, 4, and 5, while SW3 operates as a root bridge for the second instance with VLANs 10, 11, and 12. As a result, instead of having seven instances running simultaneously, this implementation of MSTP uses only two.

When implementing MSTP on Cisco switches, you can create up to 16 instances containing as many VLANs as you need. Because of this approach, the CPU and memory requirements of MSTP are lower than the requirements of PVST+.

In most cases, the convergence time of MSTP is in the order of a few seconds or less, depending on the size and complexity of the network. Usually, MSTP is used in extremely large networks, such as campuses or service providers, where the number of VLANs is counted in the hundreds or even thousands.