[olug] a funny MDI/MDI-X autodetection story

[Craig Wolf]

If it is any sympathy, we have replaced all our Intel switches with 3com
and they have spanning tree turned off and we have the SAME loop
problem.  Teachers will see a plug hanging out and "help" by plugging
that free end into the wall...loop city until we find it.  Takes down
the whole network in that building.  Sucks to find!  Start unplugging
switches and see when the problem goes away...

Craig Wolf
Linux Web Server Support
Desktop/Network Specialist
402-894-6283

>>> neal at lists.rauhauser.net 10/21/05 11:31 PM >>>

 An enterprise customer of mine, who shall remain nameless, has a number

of locations and 336 managed switch ports, primarily Cisco Catalyst 2924

and 2950 series switches.

 Their facilities are older, they're growing quickly, and there are  a 
few places where they choose to install those small Linksys or Netgear 
desktop switches. A steady diet of unmanaged devices in a network that 

size will give you an ulcer, but a few dozen ports done that way is no 
big deal ... or so we thought.

 We don't like DHCP problems so we've set spanning-tree portfast on all 
the Cisco boxes except for the few places where we've got L2 redundancy 
in the network. Instead of fifty seconds of spanning tree the stations 
get instant service so their DHCP requests don't time out on them.

 One of the smaller locations has a show floor and a small common area 
where executives will sit with their laptops when they're on site. It 
was served by a small desktop switch. One day a nameless executive from 
the nameless enterprise finished his work and, wanting to keep things 
tidy, coiled up his network cable, plugged the free end into the 
autodetecting desktop switch port, and walked away for the day.

  Four hours later after much driving on my part, much grumbling from 
the staff at the location, replacement of their switch, replacement of 
their router, and much other horsing around, we finally isolated the 
source of the 142% CPU utilization on the switch :-)

 
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