What is the difference between an RSTP (Rapid Spanning Tree Protocol) alternate port and a backup port?

An alternate port provides a backup path to the root bridge via a different switch, while a backup port is a redundant path through a shared medium (like a hub) on the same switch.

Alternate port:

• Receives a superior BPDU (Bridge Protocol Data Unit) from another switch
• Provides an alternative path to the root bridge
• Can quickly transition to become the root port if the current root port fails
• This is the common scenario in modern networks

Backup port:

• Receives a superior BPDU from its own switch (via a shared medium like a hub)
• Both ports connect to the same segment through a hub
• Much less common today because hubs are legacy devices

In the diagram: S2 has an alternate port (blocked) providing a backup path to root bridge S1 via S3. S3 has a backup port because two of its ports connect to the same hub — both reach the same segment.

Go deeper:

• RSTP port roles — Spanning Tree Protocol (Wikipedia) — an alternate (path to root via another bridge) vs a backup (redundant port to a segment this bridge already serves).