In RSTP, which port type functions as a backup?

In Rapid Spanning Tree Protocol (RSTP), the backup port is a specific port type that takes on the role of a redundant path. It is primarily used to provide a backup connection to the same segment as the designated port. When the designated port fails, the backup port can take over to ensure continued connectivity and minimize downtime.

The concept of a backup port is particularly important in switch configurations where network availability is critical. In this role, the backup port remains in a blocking state unless the designated port goes down, at which point it can transition to a forwarding state. This feature helps in reducing the time it takes to recover from a failure by preemptively setting up potential paths that can be activated as needed.

In contrast, the root port is the port on a switch that is closest to the root bridge and is used to forward traffic towards it, while the designated port is the one that is elected to forward traffic to and from a given network segment. The alternate port provides a backup path to the root bridge but is not designated for use on the same segment as a backup for the designated port. Thus, only the backup port explicitly serves the unique function of acting as a secondary path for traffic on the same link as the designated port.