STP Convergence Question

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Jul 31st, 2007
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Hello all,

Just a quick question as i'm troubleshooting an issue for one of my clients. They have a mixed network of 3com switches and cat express 500's. They also have a 3com phone system. We did not sell nor implement any of the 3com solution and they are having problems where their phones (which are strictly L2 phones, according to the vendor/reseller) will lose ability to dial out. It happens weekly and only affects certain phones.

The vendor/reseller wants us to disable STP on the Cat500's as he feels that it is causing the issue. I would rather not do that as i would feel it is compromising the redundancy of the network and is not a permanent solution. The main question I had that I can't seem to find the answer to is if anything beyond a new switch being installed and sending and receiving BPDU's would cause STP to reconverge.

My question stems from the vendor/reseller saying that anyone who plugs in ANYTHING to the network (be it 4-port hub, wireless AP, or PC/laptop) will cause STP to reconverge and can cause packets (from the phones) to be dropped as ports transition.

I think he's about you guys?



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Overall Rating: 4 (2 ratings)
graemeporter Tue, 07/31/2007 - 03:39
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He's crazy :)

Assuming all the user access ports on the Cat500 are running Spanning Tree PortFast, plugging any user device into any of these ports will not cause any reconvergence. Even switches (which is why we don't use PortFast on ports connected to other switches).

Access Points can be connected to PortFast ports, as new wireless users connecting to the AP will only affect the AP itself, and no reconvergence period is required.

I hope this helps!

Kind regards,


Jon Marshall Tue, 07/31/2007 - 03:42
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It all depends on whether you have portfast running on your switches. If you have portfast turned on when that port becomes active it does not generate a topology change notification (TCN) so there should be no spanning-tree recalculation.

See attached link for more details



goldbergj Tue, 07/31/2007 - 03:45
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Thanks Jon and Graeme,

So even if we were having TC issues and STP was flooding BPDU's to reconverge, i still don't think that any packets would be lost...i run STP on all of my clients that have Cisco IPT phone systems and never have issues like it maybe a 3com caveat only?

Thanks so much for the quick responses!


graemeporter Tue, 07/31/2007 - 03:51
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It could be 3com-related; I've never worked with their networking kit.

Actually, I don't know if 3com network switches provide PortFast. Have you got the model numbers for the 3com switches involved?

Kind regards


goldbergj Tue, 07/31/2007 - 03:55
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No I don't as i'm not on-site. I'll get that info and google it to see if they support PF on STP.

Thanks again!


Francois Tallet Tue, 07/31/2007 - 08:50
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Exactly, there is a difference between topology change notification and STP reconvergence. When there is a physical change in the topology and the STP needs to converge to a different logical topology, there are TC indication advertised along the process, in order to update the cam table. However, it is not because your STP is updating the cam entries by sending TC that the network is reconverging. TC just have an effect on the cam entries in the network, and cam entries only help filtering traffic. Flushing them or aging them fast has no impact on connectivity (in fact, it increases connectivity).



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