RSTP – Rapid Spanning Tree Protocol Explained

Introduction

One of the challenges of network switching is to avoid loops in the network topology. Loops can cause problems such as broadcast storms, multiple frame copies, and MAC address table instability. To prevent loops, switches use a protocol called Spanning Tree Protocol (STP). STP creates a logical tree structure of the network by blocking some of the redundant links between switches.

STP also detects changes in the network topology and adjusts the tree accordingly. STP has some limitations, such as slow convergence time, low bandwidth utilization, and lack of load balancing. A new protocol called Rapid Spanning Tree Protocol (RSTP) was developed in order to overcome these limitations. RSTP protocol is an enhancement of STP that provides faster and more efficient network switching.

What is RSTP?

RSTP stands for Rapid Spanning Tree Protocol. It is an enhanced version of the Spanning Tree Protocol (STP) that enhances the performance of the spanning tree. Similar to STP, the RSTP protocol can generate a single spanning tree (also known as instance 0). As a result, it is not equipped to consider VLANs in its management of redundant paths.

The enhancements in the performance of RSTP typically make it a better choice than STP in environments that do not involve VLANs. The 802.1w standard is recognized by the Institute of Electrical and Electronics Engineers (IEEE) as the standard for Rapid Spanning Tree Protocol.

How does RSTP work?

RSTP complies with a strict set of guidelines that allows switches to determine the optimal method for forwarding network traffic without any unnecessary redundancy. When it is turned on for a network, the spanning tree algorithm will make all of the necessary decisions automatically about the configuration of the spanning tree.

In Rapid Spanning Tree Protocol, the topmost bridge in the network is designated as the Root bridge. Its primary responsibility is to send network topology information to other switches within the network. This plays a critical role in the case of hardware malfunctions or any changes in topology. The optimal alternative routes are established without any delay.

RSTP Port Roles

RSTP protocol defines four port roles. These are:

1. Root ports are ports that connect to the root bridge, which is the switch with the lowest bridge ID in the network. Root ports are always in a forwarding state and forward all traffic to and from the root bridge.
2. Designated ports are ports that connect to other switches and are responsible for forwarding traffic to and from their segments. Designated ports are also always in a forwarding state and are selected based on the lowest path cost to the root bridge.
3. Alternate ports are ports that provide an alternative path to the root bridge in case of a failure of a root or designated port. Alternate ports are in a discarding state by default and do not forward any traffic unless they become root or designated ports.
4. Backup ports are ports that provide a redundant link to the same segment as a designated port. Backup ports are also in a discarding state by default and do not forward any traffic unless they become designated ports.

Port States in RSTP

RSTP assigns different port states to each port on a bridge, depending on its role and status in the network topology. The port states in RSTP are:

1. Discarding: The port does not forward or receive any data frames. It only listens for BPDU (Bridge Protocol Data Unit) messages that contain information about the network topology. This is the initial state of all ports and the state of non-designated ports.
2. Learning: The port learns the MAC (Media Access Control) addresses of the devices connected to it and adds them to its MAC address table. It still does not forward or receive any data frames, but it continues to listen for BPDU messages.
3. Forwarding: The port forwards and receives data frames normally. It also learns MAC addresses and listens for BPDU messages. This is the state of designated ports and root ports.

Note: While the learning state is utilized in Rapid Spanning Tree Protocol, its duration is comparatively short compared to STP.

Advantages of RSTP

One of the key benefits of using the RSTP over STP is that it reduces the convergence time. In case of any changes in network topology, STP displays convergence time somewhere between 40-50 seconds. In a modern networking environment, there is a high demand for enhanced speed, with a 40-50 seconds latency typically considered unacceptable. In contrast, Rapid Spanning Tree Protocol decreases the time required for convergence, typically to a range of 5 to 10 seconds. Hence, it’s a better option than STP.

Here are some other benefits of RSTP Protocol –

• Rapid network reconfiguration: It quickly transitions ports to the forwarding state, minimizing network downtime.
• Improved network performance: It maximizes bandwidth utilization and minimizes packet loss.
• Simple implementation: It is easy to implement and configure, maintaining a similar logical topology to STP.
• Scalability: Rapid Spanning Tree Protocol supports networks of various sizes, multiple VLANs, and multiple spanning trees (MST) for load balancing and performance improvement.

Disadvantages of RSTP

Here are the disadvantages of Rapid Spanning Tree Protocol –

• Increased resource consumption: It demands more processing power and memory compared to STP.
• Potential for suboptimal path selection: Its fast convergence may lead to less optimal path choices in certain scenarios.
• Dependency on network hardware: Its effectiveness relies on the capabilities and implementation of the underlying network equipment.

It’s important to consider these disadvantages alongside the benefits of Rapid Spanning Tree Protocol when evaluating its suitability for a particular network environment.

Frequently Asked Questions

Q1 – What is the difference between STP and RSTP?

The main difference is that RSTP is faster and more efficient than STP, as it can switch to an alternate path without waiting for the network to converge.

Q2 – Why is RSTP faster than STP?

RSTP is faster than STP because it does not depend on forwarding delay timers. It allows seamless port transitions from discarding to forwarding states, thereby proving advantageous in a dynamic network environment where switches frequently change their roles.

Q3 – Is RSTP an open standard?

RSTP (Rapid Spanning Tree Protocol) is an open standard that defines a network protocol for preventing loops in a bridged LAN. RSTP is defined in IEEE 802.1w and is compatible with legacy STP devices.

Q4 – What is RSTP or MSTP?

RSTP stands for rapid spanning tree protocol, a faster version of STP. MSTP stands for multiple spanning tree protocol. It is a modern networking protocol that serves to combine Virtual Local Area Networks (VLANs) into Instances. MSTP is designed to be compatible with both the Spanning Tree Protocol (STP) and the Rapid Spanning Tree Protocol.

Conclusion

RSTP protocol provides fast and reliable network switching by preventing loops in a network of switches. It can handle multiple topology changes simultaneously and reduce the convergence time from tens of seconds (in STP) to sub-seconds (in RSTP). Rapid Spanning Tree Protocol is compatible with STP and can interoperate with legacy switches that use STP.

You can join CCNA Training Course to learn Rapid Spanning Tree Protocol and STP in detail.