What is RSTP? | Rapid Spanning-Tree

What is RSTP?

Rapid Spanning Tree Protocol, abbreviated as RSTP, is an enhancement of the traditional Spanning Tree Protocol (STP). It was developed to overcome the limitations and shortcomings of STP, such as slow convergence times. RSTP is an IEEE 802.1w standard that plays a crucial role in preventing loops and ensuring redundancy in Ethernet networks.

RSTP is a network protocol that allows redundant paths in a LAN to provide backup links in case of link failure. It ensures that there is only one active path between any two network devices, preventing loops and improving network stability.

What’s New in RSTP?

RSTP was introduced to overcome the limitations of STP, including slow convergence and inefficient use of network resources. The key improvement in RSTP is the faster convergence time, which reduces downtime and improves network resiliency.

The primary enhancement is the significantly faster convergence time. While STP can take up to 50 seconds to converge, RSTP can do it in a matter of seconds, making it more suitable for modern, high-speed networks.

Additionally, RSTP introduces the concept of port roles and states, which provide more granular control over network traffic and path selection. This improves network efficiency and stability.

How Does RSTP Work?

RSTP uses an algorithm called the Rapid Spanning Tree Protocol Algorithm to determine the active path in the network. This algorithm calculates the shortest path to the root bridge and selects the best path based on criteria such as path cost, port priority, and port role.

Here’s a simplified overview of how RSTP works:

1. Bridge Election: RSTP starts by electing a root bridge, which serves as the central reference point for all path calculations within the network. The root bridge is the bridge with the lowest bridge ID, which is a combination of a bridge’s priority value and its MAC address.
2. Port Roles and States: RSTP assigns roles and states to each port on every bridge in the network. These roles and states help determine the significance of each port in the network topology.
   • Root Port: This is the port on a non-root bridge with the lowest path cost to the root bridge. It becomes the primary path for forwarding traffic toward the root bridge.
   • Designated Port: These ports are part of the active topology and have the responsibility of forwarding network traffic. They are crucial for ensuring efficient data transmission within the network.
   • Blocking Port: Ports in this state are effectively disabled to prevent loops. However, they can quickly transition to the forwarding state if the active path fails, ensuring network redundancy.
3. Path Cost Calculation: RSTP calculates the path cost for each port, taking into account factors like the bandwidth and delay on the link. Ports with lower path costs are preferred for forwarding traffic.
4. Port Activation: RSTP activates the best path by transitioning ports to the forwarding state, allowing them to actively forward network frames. At the same time, it keeps redundant paths in the blocking state to avoid loops.

RSTP’s rapid convergence and precise control over port roles and states make it a valuable protocol for modern Ethernet networks. It ensures that data flows efficiently through the network while preventing the issues associated with network loops.

RSTP Algorithm

RSTP enhances the older Spanning Tree Protocol (STP) by introducing faster convergence mechanisms. In RSTP, bridges are assigned roles such as Root Bridge and Designated Bridge based on their Bridge ID, consisting of a configurable Bridge Priority and MAC address.

The protocol quickly selects a Root Bridge and Designated Ports for each network segment, minimizing the risk of loops. RSTP achieves faster convergence through efficient port state transitions, reducing downtime caused by topology changes. It also introduces features like Bridge Assurance and PortFast to further enhance network stability.

• Bridge selection to determine the Root Bridge.
• Election of Root Ports and Designated Ports on each bridge.
• Calculation of Alternate and Backup Ports.
• Transitioning ports through various states for rapid convergence.

Types of Ports in RSTP

RSTP introduces three types of ports: root ports, designated ports, and alternate or backup ports. Root ports are the ports that offer the shortest path to the root bridge. Designated ports are the ports that are selected as the primary path for forwarding data. Alternate or backup ports are the ports that provide a backup path in case the designated port fails.

In RSTP, there are three primary types of ports:

1. Root Port: This is the port on a non-root bridge with the lowest path cost to the root bridge.
2. Designated Port: These are the ports that are part of the active topology and have the responsibility of forwarding traffic.
3. Blocking Port: Ports in this state are effectively disabled to prevent loops but can quickly transition to forwarding if the active path fails.

Port Roles in RSTP

RSTP defines three port roles: root port, designated port, and alternate port. The root port is the port that offers the shortest path to the root bridge and is responsible for forwarding data. The designated port is the port that is selected as the primary path for forwarding data to a specific segment. The alternate port is the backup port that comes into play if the designated port fails.

The roles assigned to ports in RSTP are vital for determining how traffic flows within the network:

1. Root Port: This port has the lowest path cost to the root bridge and is the primary path for forwarding traffic.
2. Designated Port: These ports are responsible for forwarding traffic towards the root bridge.
3. Blocking Port: Ports in this state are not part of the active topology and are in standby mode.

Port States in RSTP

RSTP introduces three port states: discarding, learning, and forwarding. The discarding state is the initial state of a port, where it receives but does not forward any data. The learning state allows the port to learn MAC addresses and populate its forwarding table. The forwarding state is the final state where the port actively forwards data.

RSTP defines several port states to manage network convergence:

1. Discarding State: This is the initial state where a port does not participate in frame forwarding.
2. Learning State: In this state, a port listens and learns MAC addresses but does not yet forward frames.
3. Forwarding State: Ports in this state actively forward frames as part of the active topology.

RSTP vs STP

RSTP offers several advantages over STP. It provides faster convergence time, reducing downtime in case of link failure. RSTP also allows for better use of network resources by enabling faster switching between ports. However, RSTP requires more processing power and can be more complex to configure compared to STP.

Aspect	RSTP (Rapid Spanning Tree Protocol)	STP (Spanning Tree Protocol)
Convergence Time	Rapid convergence, typically within seconds.	Slow convergence, potentially up to 50 seconds.
IEEE Standard	IEEE 802.1w	IEEE 802.1D
Port States	Discarding, Learning, Forwarding	Blocking, Listening, Learning, Forwarding
Port Roles	Root Port, Designated Port, Blocking Port	Root Port, Designated Port
Path Selection Criteria	Based on path cost (bandwidth and delay).	Based on Bridge ID (priority and MAC address).
Compatibility	Backward compatible with STP (802.1D).	Legacy protocol without RSTP enhancements.
Efficiency	Efficient utilization of network resources.	Can lead to inefficient use of network bandwidth.
Network Disruption	Minimizes network disruptions during topology changes.	Longer convergence time may lead to network interruptions.
Processing Power	Requires more processing power and memory due to added complexity.	Relatively simpler, requiring less processing power.
Configuration	More complex to configure and manage, especially in large networks.	Simpler configuration but can be challenging to optimize.

RSTP offers rapid convergence, better resource utilization, and improved efficiency compared to the older STP protocol. While it requires more processing power and can be more complex to configure, its advantages in terms of network stability and performance make it the preferred choice for modern Ethernet networks.

Advantages and Disadvantages of RSTP

The advantages of RSTP include faster convergence, better network performance, and improved network resiliency. However, RSTP requires more resources and can be more challenging to configure and troubleshoot compared to STP.

Advantages

• Rapid convergence ensures minimal downtime during network changes.
• Granular control over port roles and states improves network efficiency.
• Compatibility with legacy STP bridges.

Disadvantages

• Requires more processing power and memory compared to STP.
• Configuration and management can be complex in large networks.

Rapid Spanning Tree Protocol (RSTP) is an enhancement of the Spanning Tree Protocol that offers faster convergence and better network performance. By understanding how RSTP works, the different port roles and states, and its advantages and disadvantages, network administrators can make informed decisions when implementing and managing RSTP in their networks.