Types Of Spanning Tree Protocol (STP)

The Spanning Tree Protocol (STP) is a Layer 2 network protocol designed to prevent loops in Ethernet networks by creating a loop-free logical topology. When multiple physical paths exist between network switches, STP blocks redundant links to prevent broadcast storms and ensures only one active path between any two network devices.

STP operates by electing a Root Bridge and calculating the best path to reach it from every switch in the network. All other paths are blocked but remain as backup links that can be activated if the primary path fails.

Types of Spanning Tree Protocol

There are four main variants of STP, each with different capabilities and convergence times:

• IEEE 802.1D Original Spanning Tree Protocol

• PVST+ Cisco's Per-VLAN Spanning Tree Plus

• IEEE 802.1w (RSTP) Rapid Spanning Tree Protocol

• RPVST+ Cisco's Rapid Per-VLAN Spanning Tree Plus

IEEE 802.1D (Original STP)

The original IEEE 802.1D standard creates a single spanning tree for the entire network. It selects one Root Bridge and blocks all redundant paths, taking up to 50 seconds to converge after a topology change. The protocol uses five port states: disabled, blocking, listening, learning, and forwarding.

Advantages

• Low memory and CPU requirements

• Simple configuration and troubleshooting

Disadvantages

• Slow convergence time (30-50 seconds)

• No load balancing across VLANs

PVST+ (Per-VLAN Spanning Tree Plus)

PVST+ is Cisco's proprietary enhancement that runs a separate STP instance for each VLAN. This allows different VLANs to use different paths, enabling load balancing across the network infrastructure.

Advantages

• Load balancing across VLANs

• Per-VLAN root bridge selection

• Supports Cisco enhancements like PortFast and BPDU Guard

Disadvantages

• Higher memory and CPU usage

• Slow convergence similar to 802.1D

IEEE 802.1w (RSTP)

Rapid Spanning Tree Protocol (RSTP) improves upon 802.1D by reducing convergence time to seconds rather than minutes. It uses only three port states: discarding, learning, and forwarding. RSTP introduces alternate and backup port roles for faster failover.

Advantages

• Fast convergence (2-6 seconds)

• Backward compatible with 802.1D

• Improved network stability

Disadvantages

• No per-VLAN load balancing

RPVST+ (Rapid Per-VLAN Spanning Tree Plus)

RPVST+ combines the benefits of RSTP's fast convergence with PVST+'s per-VLAN spanning trees. It provides rapid convergence while maintaining separate STP instances for each VLAN, enabling optimal load balancing.

Advantages

• Fast convergence with per-VLAN load balancing

• Supports all Cisco STP enhancements

Disadvantages

• Highest resource consumption

• Cisco proprietary protocol

Comparison of STP Types

Conclusion

STP variants provide essential loop prevention in switched networks, with newer protocols like RSTP and RPVST+ offering faster convergence and better load balancing. The choice of STP type depends on network requirements, convergence needs, and vendor compatibility.