STP Topology Change (TC) flag
In an STP topology, when a bridge receives a BPDU with the Topology Change (TC) flag bit set, it reduces its MAC address table aging time from 300 seconds to 15 seconds. This ensures a relatively quick flush of stale information.
According to the official documentation, it is only when a switch receives the TC BPDU from the root that the aging timer for the MAC address table is reduced, and not when it receives a TCN from a downstream switch. The only exception is the switch on which the topology change was detected. In such a case, the age timer of the MAC address table on the originator of the TCN is immediately reduced.
The TC flag plays a vital role in the STP topology change process.