The original spanning tree protocol, developed in the 1980s, served its purpose well at the time. However, with the increasing demand on networks in the 1990s and beyond, a faster and more efficient solution was needed. Enter Rapid Spanning Tree Protocol (RSTP), an improved version that aimed to enhance network change detection and handling. In this article, we will explore the changes introduced by RSTP and how it achieves faster network convergence.
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What’s Changed in RSTP?
While there are still similarities between RSTP and the original spanning tree protocol, there have been significant improvements.
Firstly, in RSTP, all switches actively send Bridge Protocol Data Units (BPDUs), not just the root bridge. This change allows for better communication and information sharing between switches.
Furthermore, the BPDU format has been enhanced in RSTP, including more information to facilitate better decision-making. Additionally, new port states have been introduced to supplement the blocking state, leading to quicker port activation.
Moreover, Cisco has added its own per-VLAN technology to RSTP, making it VLAN-aware and allowing for better configuration and optimization.
In this article, we will delve deeper into each of these changes and explore how they contribute to the improved performance of RSTP.
How to Determine Your Switch’s Spanning Tree Version
Knowing which version of spanning tree your switch is running can be crucial for configuration and troubleshooting purposes. To determine the version, you can use the ‘show spanning-tree’ command.
On Cisco switches, the default mode is usually some type of RSTP. However, it is still important to confirm the version. Running the ‘show spanning-tree’ command will provide you with the information you need. For example, if you see “IEEE” as your spanning tree version, it means you are running the original spanning tree protocol (STP).
To change the version being used, you can use the ‘spanning-tree mode’ command and select the desired option. Rapid PVST (Per-VLAN Spanning Tree) is Cisco’s version of RSTP, which includes additional features not found in the standard RSTP. By changing the version, you can optimize your network’s performance and stability.
Faster Network Convergence with RSTP
One of the key advantages of RSTP over the original spanning tree protocol is its faster network convergence. In the original protocol, it would take approximately 50 seconds for a port to come online after a network change. This is unacceptable in modern networks where downtime can have a significant impact.
RSTP addresses this issue by introducing a new set of port states: discarding, learning, and forwarding. These states replace the previous blocking, listening, learning, and forwarding states, eliminating the need for timers and relying instead on a negotiation procedure.
When an interface first comes up, it enters the discarding state to prevent loops before they are detected. Then, through a negotiation process called SYNC, switches exchange information and determine the best path to the root bridge. Once the negotiation is complete, ports transition to the forwarding state, allowing for rapid network convergence.
The absence of timers in RSTP ensures that interfaces come up faster, allowing the protocol to respond swiftly to changes in the network topology. This enhanced agility and speed make RSTP an ideal choice for modern networks.
FAQs
Q: How can I determine which version of spanning tree my switch is running?
A: You can use the ‘show spanning-tree’ command to check the spanning tree version. On Cisco switches, the default version is usually some form of RSTP.
Q: Can I change the spanning tree version being used on my switch?
A: Yes, you can change the spanning tree version by using the ‘spanning-tree mode’ command. Select the desired option based on your network’s requirements.
Q: What are the key advantages of RSTP over the original spanning tree protocol?
A: RSTP offers faster network convergence, thanks to its discarding, learning, and forwarding states. It eliminates the need for timers and relies on a negotiation procedure to quickly bring up interfaces after network changes.
Conclusion
Rapid Spanning Tree Protocol (RSTP) has revolutionized network convergence by significantly reducing the time it takes for interfaces to come online after network changes. Its enhanced capabilities and faster response times make it an essential tool for modern networks. By understanding the changes introduced by RSTP and its advantages over the original spanning tree protocol, network engineers can optimize their networks for improved performance and stability.
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