I had the same problem year ago.
STP or RSTP looping can cause the OpenBSD hardware to freeze.
I could not figure out why suddenly my firewall is not working.
Then I noticed that the switch led is blinking crazily.
I remove the cable, and my firewall was normal again.

Brgds,
Riwan

Christian Taube wrote:
quoted text
> Am Fri, 16 Jul 2010 23:26:18 +0200
> schrieb Janusz Gumkowski <Janusz.Gumkowski@am.torun.pl>:
>
>   
>> Are you sure the switch didn't put this port in some 'disabled'
>> state that would require your action to manually re-enable it ?
>>     
>
> I'm rather sure about this.
>
> Network looks like this:
>
>                      +---------+ re0 -----[ Netgear switch ]
>  DSL/PPPoE ----- em0 | OpenBSD | 
>                      +---------+ re1 -----[ HP switch ]
>                         com0
>                          |
>                          |
>                   (serial console)
>
>
> Looping the HP switch resulted in clients on the Netgear switch to be
> unable to connect to any internet services provided via em0 and vice
> vesa.
>
> Anyway, the serial console didn't work either and the machine doesn't
> respond to the power-off button. Need to get that paper clip for
> rebooting. So I'm rather sure it really locked up.
>
>
> I'm thinking about reconfiguring the machine like this:
>
>                      +---------+ re0 -----[ Netgear switch ]
>  DSL/PPPoE ----- re2 | OpenBSD | 
>                      +---------+ em0 -----[ HP switch ]
>                         com0
>
> The re* interfaces are onboard. The em0 interface is on a miniPCI card.
> I would like to know if the machine locks up when the broadcasts come
> in through the em0 interface.
>
>
> BTW: This is the hardware OpenBSD is running on:
>   http://www.flepo.de/minipc-delta.html
> The text is in german, but there is a link to a PDF-datasheet down
> in the text which is written in english language.