RSTP Proposal and Agreement

Rapid Spanning Tree Protocol (RSTP, IEEE 802.1w) is an enhancement of the original Spanning Tree Protocol (STP, IEEE 802.1D) that provides faster convergence time and improved efficiency in Ethernet networks. RSTP uses a handshake mechanism between bridges (switches) in the network, involving proposal and agreement messages, to speed up the process of selecting the active topology and moving to the forwarding state.

Here's an overview of how proposal and agreement work in RSTP:

Root Bridge: First, the root bridge is elected based on the lowest bridge ID in the network, just like in the original STP. The root bridge sends Bridge Protocol Data Units (BPDUs) out of all its designated ports.
Proposal: When a non-root bridge receives a BPDU on its root port (the port with the best path to the root bridge), it updates its port roles and states according to RSTP rules. Then, it sends a BPDU with the "proposal" flag set on its designated ports. The proposal message indicates that the bridge is proposing the designated port to be part of the active topology.
Agreement: When a downstream bridge receives the proposal message on one of its ports, it verifies whether the received BPDU has better path information than its current one. If the received BPDU is better, the downstream bridge blocks its other ports, changes the state of the receiving port to the root port, and sends a BPDU with the "agreement" flag set back to the upstream bridge. The agreement message indicates that the downstream bridge accepts the proposal and acknowledges the designated port to be part of the active topology.
Rapid transition to forwarding state: Upon receiving the agreement message, the upstream bridge rapidly transitions its designated port to the forwarding state, without going through the listening and learning states as in the original STP. This allows for faster convergence and reduces the time it takes for the network to become stable.
Synchronization: The proposal and agreement process continues until all bridges in the network have synchronized their port roles and states. Once the synchronization is complete, the network converges to a loop-free active topology.

The proposal and agreement handshake mechanism in RSTP helps speed up the convergence process and minimizes the time it takes for the network to reach a stable state. This is a significant improvement over the original STP, which often required long waiting times for network convergence.

RSTP and MST Proposal and Agreement Process Caveats

The proposal and agreement handshake mechanism in Rapid Spanning Tree Protocol (RSTP) provides faster convergence and improved efficiency compared to the original Spanning Tree Protocol (STP). However, there are still some potential caveats to consider:

Network complexity: In large or complex networks with many switches and redundant links, the proposal and agreement process may become more intricate, leading to increased processing overhead on the switches. Although RSTP converges faster than STP, the overall convergence time could still be affected in such networks.
Compatibility issues: When RSTP is deployed in a mixed network with both RSTP and STP switches, it can operate in a compatibility mode to interact with the legacy STP switches. However, this may result in a reduced convergence speed, as the RSTP switches must fall back to the slower STP convergence mechanisms when interacting with STP switches.
Configuration errors: Incorrectly configured switches, port priorities, or path costs can lead to suboptimal network performance or even network loops. Proper planning, design, and configuration are essential to ensure the effectiveness of the proposal and agreement process and to achieve the desired network stability and convergence speed.
Misbehaving devices: If a switch in the network does not properly participate in the proposal and agreement process, it may cause instability, delays in convergence, or incorrect port roles and states. This could happen due to hardware or software issues, misconfigurations, or even malicious activity.
Transient loops: While RSTP significantly reduces the time it takes for the network to converge, it is still possible for transient loops to occur during the convergence process. These loops are short-lived and resolved once the proposal and agreement process completes, but they may cause temporary disruptions in network traffic.

Despite these caveats, RSTP offers significant advantages over the original STP in terms of convergence speed and network efficiency. By understanding these potential caveats and taking appropriate measures during network planning, design, and configuration, the effectiveness of the RSTP proposal and agreement process can be maximized.

MST Proposal and Agreement Process

The proposal and agreement handshake mechanism is also applicable in Multiple Spanning Tree Protocol (MSTP, IEEE 802.1s). MSTP is an extension of Rapid Spanning Tree Protocol (RSTP) that allows for multiple spanning tree instances in a network, helping to further optimize network resources and traffic flow.

MSTP maintains the RSTP's rapid convergence capabilities, including the proposal and agreement process, while introducing the concept of Multiple Spanning Tree Instances (MSTIs) and a Common Spanning Tree (CST) to handle multiple VLANs.

MSTP groups VLANs into distinct MSTIs, allowing each MSTI to have its own spanning tree topology. This enables better load balancing and more efficient use of network resources, as different MSTIs can forward traffic over different paths. The Common Spanning Tree (CST) interconnects all the MSTIs and handles communication between MST regions.

The proposal and agreement handshake mechanism in MSTP works similarly to RSTP, ensuring rapid convergence within each MSTI and the CST. When MSTP operates in a mixed environment with RSTP or STP switches, it maintains compatibility by interacting with those switches using the appropriate protocol mechanisms, although this may result in slower convergence when communicating with legacy STP switches.

In summary, MSTP incorporates the rapid convergence benefits of RSTP, including the proposal and agreement process, while providing additional optimization through the use of multiple spanning tree instances for different VLANs.

Layer 2 Networking Articles

RSTP and MST proposal and agreement process