Hello Karen
The MAC address table aging time will be reduced to 15 seconds only when a switch receives a BPDU with the Tc bit set to 1. Therefore, the aging time is reduced only once. Also, the TCA is not involved in the reduction of the aging time. A TCA is a BPDU with the TCA bit set (which is different than the Tc bit). This is simply an acknowledgment response to a TCN BPDU. When a switch sends a TCN BPDU towards the root bridge to indicate a topology change, the upstream switch (and eventually the root bridge) will acknowledge this notification by setting the TCA bit.
You are indeed correct. Take a look at this NetworkLessons note on the topic for more clarification. I will talk to Rene to have him consider revising the lesson accordingly.
Some additional NetworkLessons notes that deal with and clarify the issues involved in this process are listed below:
- STP topology change process
- STP - Aging time reduction with a TCN
- STP BPDU Types
- STP Contents of a BPDU
I hope this has been helpful!
Laz