What is a designated port in STP (Spanning Tree Protocol), and how are designated ports determined?

A designated port is the port on each network segment that has the best (lowest-cost) path to the root bridge.

* Port roles on a small network: every root-bridge port is designated; each non-root switch picks one root port; each segment keeps exactly one designated port, and the loser blocks (alternate). *

Rules for designated ports:

• Every segment between two switches has exactly one designated port
• All ports on the root bridge are designated ports (they have cost 0 to the root)
• If one end of a segment is a root port, the other end is a designated port
• All ports connected to end devices (PCs, servers) are designated ports
• On segments between two non-root switches, the port on the switch with the lower root path cost becomes designated

Think of it this way:

• Root port = "my best way TO the root" (one per non-root switch)
• Designated port = "I'm the best way FROM this segment toward the root" (one per segment)

Every active port in STP is either a root port, a designated port, or an alternate/blocked port. There's no other option.

Go deeper:

• Spanning-Tree Port Roles (NetworkAcademy.IO) — one designated port per segment, why all root-bridge ports are designated, lower-cost-wins.