This question is simply a mental exercise I was running through. When I wasn't 100% sure of the answer I thought..."NetPro!"

(see attached pic)

Assume I have a stable STP tree, then I add gig link between SwitchC and SwitchD. My question revolves around exactly what SwitchC does once port 1/3 hits the learning state, bare with me here….

From switch C's perspective:

The link between SwitchC and SwitchD is brought up.

Switch C generates a TCN towards the root bridge on the designated port. The designated switch (SwitchB) generates TCA to SwitchC and sends the TCN towards the root bridge on port 1/1. The root bridge receives the TCN and generates a configuration BPDU with the TCN bit set to all downstream switches. Switches that receive the config BPDU from the root bridge set their CAM table timeout to the value of forward_delay as advertised in the config BPDU.

At the same time all of this happening SwitchC has port 1/3 in the blocking state. Eventually port 1/3 will hit the learning state. Once it does hit the learning state it is going to receive a configuration BPDU from switch D with a root path cost of 8, which is less then SwitchC's current root port path cost.

How does SwitchC respond at this point? My gut is telling me that SwitchC immediately moves the current RP into the blocking state, and proceeds with moving port 1/3 through learning and eventually forwarding.