Hello David
In normal STP operation, a switch port goes through listening and learning states to avoid creating loops, as you mentioned. However, UplinkFast is a Cisco-specific optimization designed for quick recovery in case of a direct link failure. It’s typically used in access-layer switches that have redundant links to the distribution layer.
When UplinkFast is enabled, a switch keeps track of alternate paths to the root bridge. If the primary path fails, the switch already knows about an alternate path and can transition the blocking port to forwarding almost immediately, skipping the listening/learning states.
This doesn’t create a loop because UplinkFast also modifies the BPDUs that the switch sends out. The switch makes its other ports appear less attractive to downstream switches by artificially increasing the path cost in the BPDUs. This prevents downstream switches from choosing the recovering switch as their new path to the root bridge, avoiding potential loops.
Even so, UplinkFast should be deployed carefully. Ideally, it should only be applied on access layer switches and not on distribution or core layer switches.
I hope this has been helpful!
Laz