STP TCN at root port role change

Question

I am facing a situation in our work topology, and I will try to simplify the question as much as possible. I have this non-root switch (SW1) that has two ports connected to separate switches in a tringle-like manner. All of them are running basic STP. One of SW1's ports is a root port (p1), the other one is alternate (p2). If I lower the cost on the alternate port (p2), it becomes root port and it will enter listening, then learning state and finally, forwarding state.

What I have noticed is that the new root port (p2) will send a TCN BPDU after it is placed in forwarding state. This will trigger about 35 seconds of Conf + TC BPDUs from the root bridge, which considering the time it takes SW1 to change the port roles, it amounts to +65s of STP topology reconvergence time.

From documentation over the internet I understand that a TCN BPDU will be sent immediately after a topology change occurs, and it can be sent on the Root Port only. But how do I interpret this information?

• Should the old root port (p1) send the TCN when the cost is lowered, before it is changed to alternate, thus satisfying the "send the TCN immediately" condition?
• Is it normal that the TCN is sent after 30 seconds by p2 and the reconvergence time is 65+ seconds?

Answer 1

Classic STP may be rather slow to converge, even in a simple network, and has been obsoleted for a reason (in 2004, twenty years ago).

You should really update your gear to use RSTP or MSTP which can reconverge in under a second (in your scenario).

There's a rather large amount of half-truths about xSTP floating around the Internet. If you want the real thing, get IEEE 802.1Q-2022 from https://ieeexplore.ieee.org/browse/standards/get-program/page/series?id=68 (free after registration).

The last version of classic STP was in IEEE 802.1D-1998, if you really want to stick with that.

Answer 2

Should the old root port (p1) send the TCN when the cost is lowered, before it is changed to alternate, thus satisfying the "send the TCN immediately" condition?

No, because it is no longer the root port.

Is it normal that the TCN is sent after 30 seconds by p2 and the reconvergence time is 65+ seconds?

It is certainly possible. That's one of the main reasons it is considered obsolete.