UplinkFast Dummy Multicast Frames Behavior During Link Recovery

The UplinkFast mechanism used by Spanning Tree Protocol (STP) in networking is utilized to enhance rapid convergence in Layer 2 networks when a primary uplink failure occurs. Upon such a failure, a redundant uplink transitions to the Forwarding state, and dummy multicast frames are sent to update the MAC address tables in upstream switches. This ensures that data traffic can be quickly rerouted without significant delay.

When the previously failed primary uplink is restored, the network's STP does not immediately switch it to the forwarding state. Instead, it executes its standard process to avoid potential temporary loops or incorrect forwarding. During this phase, dummy multicast frames are not resent because the redundant uplink, which ensures data flow, remains active. As the restored uplink eventually transitions back to forwarding, normal Unicast traffic enables upstream switches to relearn MAC addresses naturally, maintaining optimal data transmission without the need for additional dummy frames.

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