Hi,
Actually, we shall be talking about TC, TCN and TCA TCN is a BPDU, TC and TCA are flags.
In 802.1D, it works as follows: The TCN is an entire BPDU of a different type that is sent via the root port towards the root bridge. The receiving switch will acknowledge this BPDU with its own normal (i.e. configuration or hello) BPDU that has the TCA (acknlowedgement) flag set. This way, the TCN will "bubble up" to the root switch. The root switch then starts informing all switches about the topology change by sending its normal BPDUs with the TC flag set.
In essence, the TCN conveys the information about the presence of a topology change, TCA is used to confirm the arrival of TCN, and TC is used to force switches to shorten the aging time of MAC table entries to 15 seconds.
So in legacy 802.1D, only the root switch actually had the possibility of informing the entire network that the topology has changed, and the reason was that the TCN message traversed only from the point of the topological change via the shortest path to the root, and was not flooded to the entire network. Therefore, the root switch had to set a special flag in its BPDUs that are flooded over the network and thereby inform all switches about a topology change.
In RSTP, the information about a topology change is actually immediately flooded, so no TCN/TCA are necessary, only the TC remained in use.
I am not sure if this is understandable... please ask further!
Best regards,
Peter
