RSTP Port States and Types Explained

There are three RSTP port states: Discarding, Learning, and Forwarding. During these states, RSTP assigns a role to each port. There are five RSTP port roles: root, alternate, designated, backup, and disabled. Based on the connected device, RSTP classifies ports into three categories: point-to-point, point-to-edge, and shared. This tutorial explains the meaning of these states, roles, and types in detail.

RSTP Port States

All starts all ports in the Discarding state. In this state, it accepts and processes only BPDUs. It discards all incoming user frames. From incoming BPDUs, it learns the network topology, selects the root bridge, and assigns a role to each port. In the Learning state, it flushes all CAM table entries. It accepts user frames but does not forward them. It uses them to learn the MAC addresses. It saves the MAC addresses in the CAM table. In the Forwarding state, it accepts and forwards user frames. A port in this state is considered fully functional.

RSTP Port Roles

Not all ports move to the forwarding state. Only root and designated ports move to the forwarding state. Only non-root bridge switches have root ports. The root port connects the non-bridge switch to the root bridge switch. The root bridge switch does not use a root port. The designated port connects the switch to a local segment. All non-root bridges select only designated ports for each segment. On the root bridge, all ports are designated ports.

The role of the alternate port

An alternate port is a backup port for the root port. If a non-root bridge switch has multiple paths to reach the root bridge, it compares all paths and selects the port connected to the best path as the root port and the port connected to the second-best path as the alternate port. If the link attached to the root port fails, the switch immediately changes the alternate port to the root port.

The switch does not use the alternate port until the root port is up. If the root port is up, it keeps the alternate port in the discarding state. It will use the alternate port only if the root port fails.

The role of the backup port

A backup port is a backup port for the designated port. If a non-root bridge switch has multiple paths to reach the local segment, it compares all paths and selects the port connected to the best path as the designated port and the port connected to the second-best path as the backup port. If the link connected to the designated port fails, the switch immediately changes the backup port to the designated port.

A backup port is required only when the switch connects to a hub. A hub allows a switch to have multiple connections in the same collision domain. Hubs are outdated. Their use in the modern network is rare. The switch does not use the backup port until the designated port is up. If the designated port is up, it keeps the backup port in the discarding state. It will use the backup port only if the designated port fails.

RSTP port types

Based on the connected devices, there are three types of RSPT ports. These types are point-to-point ports, point-to-point edge ports, and shared ports. A point-to-point port connects to another switch. A point-to-point edge port connects to an end device, such as a PC or a server. A shared port connects to a hub.

Conclusion

RSTP improves network efficiency by defining and using port states, roles, and types. Understanding how ports transition between discarding, learning, and forwarding states helps you design loop-free network topologies. Additionally, recognizing port types based on connected devices aids in proper configuration and troubleshooting. Together, these concepts are essential for optimizing and maintaining a stable and robust switched network.