Spanning Tree Topology Change Notification (TCN)

Thank you so much for your clear explanation.

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Hi Team ,

I’m a bit confused about this lesson.

I can’t understand why the aging time goes from 300s to 15s.
when the topology changes, it only takes 50s for the port that was BLK not to be in FWD mode - is this the problem?

Hello David

Take a look at our initial topology:

STP has chosen to block Fa0/19 on SW2. As frames are sent to and from our hosts, all switches will populate their MAC address tables. SW2 will have an entry for the MAC address of H2 (000c.29e2.03ba) like so:

Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    000c.29e2.03ba    DYNAMIC     Fa0/14

So whenever H1 sends a frame destined for H2, SW2 will do a lookup in the MAC address table and will send that frame out of the Fa0/14 port.

Now imagine that the link between SW1 and SW3 fails. What happens then? The STP topology will reconverge, and the link between SW2 and SW4 will eventually remove the block and start operating. As you said, this may take up to 50 seconds for standard STP. This however is not the problem.

The problem is that entry in the MAC address table of SW2. Because of that, all frames sent from H1 to H2 will be forwarded out of the Fa0/14 port. This will be the case for 300 seconds (or five minutes) until the entry expires. Even after the STP topology reconverges and stabilizes 50 seconds later, SW2 will continue to send frames destined for H2 out of Fa0/14. When SW1 receives them, it will simply drop them.

Thus the aging time goes from 300s to 15s simply to quickly age out any entries that would cause such a behavior. Does that make sense?

I hope this has been helpful!


Hello ,

Understood now

Thanks for your feedback

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Based on your picture Switch has received TCN and TCA BPDU.
Does it mean that Switch decreased Aging time 2 times? First when he received TCN and second TCA BPDU?
I don’t think that article is correct , I don’t think that when switch received TCN it will decrease aging time from 300 to 15 , I think this will happen ONLY when switch received BPDU with TC Bit Flag set.

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:

I hope this has been helpful!


Hi There,

I have a question,

When you have a host that is connected to a switch, when the host side of the cable is plugged out, it will send TCN right? I mean is there a difference mechanism for TCN regarding which side cable unplugged?
Another question if we keep the cable connected on the both side but if we completely shut down the host does this mean that the switch will perceive this change as if the interface is down and send TCN ?

Thank you.

Hello Görgen

First of all, it is best practice to enable portfast on all ports where hosts are connected. When portfast is enabled, a switch will never generate a TCN. This resolves any issues concerning TCNs being sent every time a PC is connected or disconnected or turned off.

Now having said that, let’s take a look at your question assuming that we are looking at switches being unplugged rather than end devices.

No, there is no difference which side of the link is unplugged. The result is that there will no longer be any voltage (assuming a copper RJ-45 cable) coming into the port, so the port is considered down. A TCN will be generated regardless of which end is unplugged.

Yes that is the case. Whenever a network device is shut down, it typically shuts down the network interface as well. When no voltage is sent across the wire, the switch detects the port as down and acts accordingly with the generation of a TCN.

I hope this has been helpful!


Hi Laz,

I am working on aruba 2930F-48G-PoE switches used at access and trying to optimize STP (MSTP). Based on this lesson I am trying to ensure that TCN’s are not generated on ports that are connected to end hosts such as laptops, printers etc. As we can see from this lesson, Portfast feature on cisco would help me achieve my requirement.

Could you please help with me with a feature or command that will help me achieve this on above switches. I have found command “admin-edge” for the aruba switches but I am not 100% sure this will help in my case.

I tried to see if I can test this in my lab but so far I am not able to find virtual OS for this switch model. It seem virtual OS available for aruba CX switches but 2930 comes under arubaos-switch.

Hello Rahul

After doing a bit of research, I have found that the admin-edge command in ArubaOS is indeed equivalent to the PortFast feature in Cisco. This command is used to enable or disable the administratively configured edge port state for the specified ports. When you enable the admin-edge port state, the switch does not send TCNs when the port transitions up or down. This is exactly what you’re looking for.

As for ArubaOS virtual switch, you’re correct. The Aruba Virtual Switching Framework (VSF) is available only on ArubaOS-CX switches and not on ArubaOS-Switch like the 2930F series.

I hope this has been helpful!


Thanks for your response Laz. It seems I will perhaps also need tcn-guard command to be enabled on the switches. Its kind of hard to figure this out especially when you can’t even test this in any emulator since I am working on HP procurve switches which have ArubaOS-Switch running on them.

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Hello Rahul

Yes, you are correct. The ‘tcn-guard’ command is useful in preventing topology change notifications (TCN) from being sent out to the rest of the network, which can be beneficial in certain network setups.

As for working with HP Procurve switches running ArubaOS, I believe that Aruba provides an AOS-CX switch simulator where you can test these topologies. I don’t have personal experience with it, but from the little I’ve read, it seems that there is a comprehensive simulator with much documentation that you can use. Do a search to find out more about it.

Thanks for keeping us up to date with your progress on this one.

I hope this has been helpful!