STP - UplinkFast

UplinkFast is a Cisco-specific optimization for Spanning Tree Protocol (STP) designed to enhance the recovery time in case of a direct link failure, particularly in access-layer switches with redundant links to the distribution layer. Normally, a switch port goes through listening and learning states to avoid creating loops. However, when UplinkFast is enabled, the switch keeps track of alternate paths to the root bridge. If the primary path fails, the switch can transition the blocking port to forwarding almost immediately, bypassing the listening/learning states.

UplinkFast modifies the BPDUs sent out by the switch to prevent loops. It increases the path cost in the BPDUs, making the switch less attractive to downstream switches as a path to the root bridge. This ensures that downstream switches do not select the recovering switch as their new path to the root bridge, thereby avoiding potential loops. It also sets the local bridge priority to 49152 in an attempt at preventing the switch from becoming the root bridge.

When the original root port that failed recovers, the topology does not immediately reconverge, but waits for the root port delay timer to expire first.

UplinkFast should be deployed carefully and is ideally applied only on access layer switches, not on distribution or core layer switches.

Links

https://networklessons.com/spanning-tree/spanning-tree-uplinkfast