Easy question on STP and impact on VLAN

Unanswered Question
Oct 8th, 2009

Imagine that I have ApplicationCritical devices on SwitchA, Vlan 300. SwitchA is connected to Layer3Switch via trunk.

Then NonCriticalApplication devices are connected to SwitchB, Vlan 300. SwitchB is connected to Layer3Switch via trunk.

Devices on SwitchA and SwitchB have no dependency other than sharing same VLAN.

Imagine I need to reboot SwtichB for maintenance.

Do you see how normal STP recalculations generated by the fact I am power cycling SwitchB could affect ApplicationCritical devices functionality on SwitchA? I do not think that is expected, but I am just checking.

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simontibbitts Thu, 10/08/2009 - 10:04


Do you have a diagram?

If you have just a ONE Layer3Switch then it shouldn't cause any problem at all as there are no blocking ports. The only thing it will create is a TCN situation where all L2 tables will be put to fast aging. It won't cause an outage tho.

If you have dual Layer3Switches then hopefully you have one of these as the root bridge and then again - nothing will affect SwtichA (critical)


Francois Tallet Thu, 10/08/2009 - 10:09

I guess that Layer3Switch is connecting via switchports to A and B, else A and B are in different bridged domains...

Are you running Rapid-PVST or MST ? If so, you could experience a sync if B is the root.

news2010a Thu, 10/08/2009 - 10:16

Sorry, forgot to mention it is rapid-pvst.

Yes, SwitchA-trunk vlan300-->Layer3Switch.

SwtichB-trunk vlan300->Layer3Switch.

news2010a Thu, 10/08/2009 - 13:17

I checked and in this case SwitchA (Critical Applications) is the root for vlan 300.

So if I reboot SwitchA, SwitchB will remain as the root.

Francois Tallet Thu, 10/08/2009 - 13:21

If you reboot switch B, switch A will remain the root;-) This should not introduce any change on the link between switch A and the layer3switch, so you are safe. Sure, you'll get a topology change (which will result in a short flooding phase), but this will have no impact on the applications.



news2010a Thu, 10/08/2009 - 13:29

Thanks much! That is what I thought.

And I meant, if SwitchB is the one rebooted, SwitchA will remain as is, the root.


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