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In addition to the root and designated port roles, RSTP has two additional port roles you need to know: alternate and backup.

 Alternate—An alternate port provides an alternate path to the root. If a root port fails, the switch will place the best (lowest cost) alternate port into a forwarding state. This lets the topology reconverge in a matter of milliseconds.

 Backup—You're unlikely to ever see a backup port role, except on an exam. Imagine that two ports are connected to a hub and hence are on the same segment. The port with the lowest cost—or if the costs are equal, the port with the lowest designated port priority—will be the designated port. The other port connected to the segment will be the backup.