[c-nsp] switch port "shutdown" and "no shutdown"- what exactly happens?

[Andrew Koch]

On Thu, Jun 2, 2011 at 06:07, Martin T <m4rtntns at gmail.com> wrote:
> Hi,
> rather stupid subject, but the thing is, that I have one Cisco
> WS-C2950-24 switch, which has one Motorola radio device connected to
> one of fast-ethernet ports:
>
> WS-C2950-24[Fa0/1] <-> [eth]Motorola_radio_device
>
> Problem is, that about once every 24h this connection stops forwarding
> traffic. I have changed the fast-ethernet ports in WS-C2950-24 switch
> and even replaced the switch with a new one, I have replaced the
> Motorola_radio_device, there are no interface flaps and errors on
> switch port, nothing interesting in the Motorola_radio_device log
> file. Both devices are behind the proper UPS. Once the traffic between
> those two devices stops, simple "shutdown" and "no shutdown" to the
> switch port helps. Cat5e cable between the devices is 75m(246 feet)
> long. One might suspect the cable/connectors, but on the other hand
> there really are no errors on the switch port. What might cause the
> issue where reinitialization of the Ethernet port reallows traffic
> forwarding? Might there be a possibility to reproduce such situation
> in the lab?

Hi Martin,

You mention this Motorola device is a radio.  Is this a receiver only,
or a transmitter also?  Is it putting out a signal of considerable
strength?  I have seen all sorts of odd troubles when RFI gets induced
into a device that is not intended of receiving it, including
completely seizing up.  Assuming this is a transceiver, I wonder if
you have proper grounding of the radio so that it puts its RF to
ground rather than to that nice cable you strung for your ethernet
connectivity.  Also, have you isolated the radio from the switch -
used fiber or an isolator block?

This only happens on the 24th?  Is the radio setup to do something on
that day, such as a pre-scheduled test?

Being that the cable is 75m, it may also be acting as an antenna.
Again, changing this to a fiber connection would eliminate this
potential pickup of noise.  Using a properly terminated shielded cable
may also be a choice.  If neither of these are feasible, you could run
the ethernet cabling through a ferrite core at each end that would
prevent some noise from entering the switch and radio.

As far as the specifics in the switch during a shut/no shut, I am not
certain.  However, if the radio is inducing bits to flip in registers
or overloading the ethernet port receiver, resetting the port may
refire the receiver correctly.

HTH,
Andy Koch
KC9GXN