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Spanning Tree questions ...


There are four 6500 switches configured as below:


& Trunk & STP

SW1 --------- SW2 ----PC1

NO PortChannel | |No

& No TRunk | |PortChannel

& NO STP | |& No Trunk

| |& No STP &

PC2--------SW3 ---------- SW4

PortChannel &

Trunk & NO STP

Between SW1 & SW2, there is a STP enabled on each PVST. For other connections, there is NO STP enabled.

THe configuration as below:

SW1-SW2 : PortChannel, Trunk & STP enabled

SW1-SW3 : No PortChannel, Trunk & STP disabled

SW2-SW4 : No PortChannel, Trunk & STP disabled

SW3-SW4 : PortChannel & Trunk enanabled, STP disabled

In this configuraiton, the SW2 will block the connection between SW1 & SW2 to prevent loop.

Also, there is only ~20 seconds network interruption on particular connection if any one of connections broken.

I would like to know whether above configuration is supported. Is there any impact or unpredicatble issue?

In addition, I would like to know whether we can enable BackBone Fast feature if SW1 & SW2 are third party switches.

Thanks in advance.


Iavn Cheng

VIP Purple

Re: Spanning Tree questions ...


I have tried to draw your setup up, but I am not sure if I got it right: SW1 has a redundant connection to SW2 through SW3 and SW4, while SW4 has a redundant connection to SW3 through SW2 and SW1, right ? I would not turn off spanning tree on any of the links: you have redundant connections, and in that scenario, spanning tree is your best option for loop prevention.

As to BackboneFast, this is a Cisco-proprietary feature, so it is only supported on Cisco switches...



Re: Spanning Tree questions ...

Hi Iavn,

I guess that in your diagram SW1 and SW2 are running STP but SW3 and SW4 are not.

First, especially if you use third party devices, I want to warn you that there is no standard defining what "disabling STP" means, so interpretation may vary. In Cisco's PVST, we flood BPDUs, which allow you indeed to break the loop between SW1 & SW2 (SW1 and SW2 just see redundant point to point links between them).

Your configuration will however -1- be less efficient that running STP everywhere, plus -2- it breaks one of the basic asumption of STP.

-1- When a link fails between two hosts that don't run STP, the reconvergence can only be based on timer. In your example, if the link between SW3 and SW4 breaks, you will need max_age + 2xforward_delay to recover with STP. Even with RSTP your convergence time will depend on timer, which is not efficient.

-2- But the real problem, which is related to the same scenario is that when the network has converged after the failure of the link SW3-SW4, bringing back up this link will result in a temporary bridging loop. This is because SW3 and SW4 are not running STP and will put their ports directly to forwarding. Even temporary, loops are bad in a L2 network:-(

At last, backbonefast is absolutely helpless if you don't run STP on each switch. Backbonefast is supposed to save you waiting max_age when a bridge that does not have any alternate port loses its root port. In the case where you only have two bridges running STP (SW1 and SW2), you will never get into this situation anyway. If you run STP on all 4 switches, then it makes sense. As already mentioned by Georg, Backbonefast is a proprietary feature.

I highly recommend running RSTP (whether Rapid-PVST or MST) on all switches if possible.



Re: Spanning Tree questions ...

Forgot to mention that you can indeed enable uplinkfast on one of your two switches. I guess that's the way you get 20 seconds (max_age) reconvergence time already. But I still don't recommend this config for reason -2- at least. If temporary bridging loops were not a big issue, STP would be much simpler! If you are able to accept that, you could configure portfast on all your ports for the same price;-)