Rapid Spanning Tree Protocol (RSTP)

Rapid Spanning Tree Protocol (RSTP), defined in IEEE 802.1w, is an enhanced version of the original Spanning Tree Protocol (STP).

Its main purpose is to drastically reduce convergence time, making it ideal for modern networks that require high availability and fast failover.

In the previous course, we highlighted STP’s slow convergence as a major drawback, especially for real-time applications. RSTP overcomes this by cutting recovery times from about 50 seconds to just a few hundred milliseconds.

RSTP also keeps backward compatibility with IEEE 802.1D STP, allowing for integration with older networks while delivering performance improvements.

Key Features of RSTP:

1. Backward Compatibility: Works seamlessly with traditional STP networks.

2. Same Algorithm: Uses the same spanning tree algorithm for determining port roles and topology.

3. Faster Convergence: Can recalculate the spanning tree in just a few hundred milliseconds in optimized configurations.

4. Hello Mechanism: Unlike STP, where the root bridge generates Hello messages, RSTP allows all switches to send their own Hellos.

RSTP simplifies the port states used in STP by merging some of them:

STP vs. RSTP Port States

Listening is not a port state included in RSTP, it goes right from discarding to learning.

Figure 1 – Comparison of STP and RSTP Port States

While STP defines two primary port roles root and designated ports.

RSTP introduces two additional roles:

1. Alternate Port: Provides an alternative path to the root bridge.

   • Example: A redundant connection between switches remains ready for use if the primary link fails.

2. Backup Port: Acts as a backup for a shared medium like a hub.

   • Note: This role is less common today due to the rarity of hubs in modern networks.

Figure 2 – Comparison of STP and RSTP Port Roles

Let me illustrate this with a practical example to help you better understand how Rapid Spanning Tree Protocol RSTP determines port roles in modern switching environments.

Take a look at the topology below to see how RSTP assigns port roles :

Figure 3 – RSTP Port Role Assignment in a Network Topology

Assigning Port Roles

Let’s break down how RSTP assigns port roles step by step, so you can clearly understand how each role contributes to network efficiency and redundancy.