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I have a few different types of managed switches, including mostly 3com Super Stacks, Baselines (for example, one specifically is a 3com baseline pwr-plus 2928).

Most of them are configured with two VLANs, one for data, one for voice (VOIP).

It all works fine, but spanning tree is OFF completely, and I would like to turn it on, to help protect the network.

I have found when I turn it on, the phones end up going down. I believe what is happening, is that they are not able to get DHCP with STP on.

Doing some searching, I found an article talking about a different model switch, and that you have to turn on PortFast (or possibly turn it off). However, I do not see anything like that, looking through the various settings screens in my switches.

First, does that make sense, or is there possibly a different solutions? If it makes sense, what might I be looking for, for terminology turn on/off PortFast?

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up vote 3 down vote accepted

Yes, that makes sense. When spanning tree is enabled, the switch has to make sure a port doesn't form a loop before it can pass traffic over that port. This takes about thirty seconds. Some devices, broken in my opinion, don't try DHCP long enough. So they don't get an IP address if spanning tree is enabled.

The most common workaround is to enable portfast on ports that will only be used for edge devices. Portfast, essentially, disables spanning tree on that port, eliminating the 30 second delay. Almost all switches that support spanning tree support portfast or some similar mechanism.

For the 3Com 2928, see page 290 of the user guide. Under "port setup", set "protection" to "edge port". Note that BPDU protection should be used along with edge ports to prevent spurious topology changes. Read the manual.

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Thanks, I like your answer. – Scott Szretter Jan 18 '13 at 18:39

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