What happens to a port that is neither a root port nor a designated port?

It becomes an alternate (blocked) port, placed in blocking state to prevent loops.

An alternate port is essentially a backup path to the root bridge. It's in discarding/blocking state, meaning it:

• Does NOT forward user data frames
• Still receives BPDUs (Bridge Protocol Data Units) (so it knows the topology)
• Is ready to transition to forwarding if the active path fails

In the example topology:

• S3's port F0/2 connects to Trunk2 toward S2
• F0/2 on S3 is neither the root port (that's F0/1) nor a designated port
• So F0/2 becomes the alternate port and is blocked

This is the actual loop-prevention mechanism. By blocking one port on the redundant link, STP (Spanning Tree Protocol) breaks the physical loop while keeping the wire available for failover.

Tip: Alternate ports are the "standby" links. They're listening and ready, just not forwarding.

Go deeper:

• Spanning-Tree Port Roles (NetworkAcademy.IO) — the blocked/alternate port as the loop-breaking standby that still listens to BPDUs.