Newbie question re back-to-back serial cables

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I've just started working in my Cisco lab and I heard somewhere that
you shouldn't disconnect back-to-back serial cables from the
interfaces while the routers are powered on. Is there any truth to

Thanks for any info.

Re: Newbie question re back-to-back serial cables
On Oct 17, 1:33 pm, wrote:
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Not sure what you mean by back to back serial cables, but I see no
reason why this warning is legit, and have never encountered such a
problem in any of my labs back in the day.  Perhaps Merv or Mike has
some additional input.

Re: Newbie question re back-to-back serial cables
With old routers you may burn the port. Also, if you plug the cable
"on-fly", your controller may fail - cable itself defines what type of
interface to initialize. For example, if you connect V.35 cable, the same
controller will initialize the same port as V.35 interface, but if you plug
RS-232 cable, the port will be RS-232 port. Also there is a difference if
controller is initialized as DTE or DCE. And all of this is determined
during bootup, depending on that is connected to the serial port.

Good luck,

CCNP, CCDP, CCSP, Cisco Voice, MCSE W2K, MCSE+I, Security+, etc.
CCIE R&S (in progress), CCIE Voice (in progress)
Headset Adapters for Cisco IP Phones

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Re: Newbie question re back-to-back serial cables
The word "should" puts uncertianty into this.  I lean more towards this
being a relatively safe activity.

In ten years of working with 2500 and 2600 series up to 3600 and 7200, I
have plugged/unplugged serial interface cables while both ends were powered
on and not burned up any equipment.  I would have cared less about the 2500
series than controllers for the 3600 and 7200.  When the links are not up, a
software shutdown/no-shutdown often clears links if problems are
experienced.  I have even witnessed a serial cable getting plugged in upside
down which bent the plug housing, but after reverting the cable back the
connection still worked.

It is probably not a bad idea to keep devices off when connecting and
disconnecting cables unless it creates unneccesary inconvenience.  If you
are wanting to connect or disconnect a serial cable and might have to wait
until network downtime or a maintenance period, I would support just
connecting a cable or disconnecting a cable while the device is powered on
to get things done.  It depends on how prudent and cautious you and your
company want to be in facing the risk.  In my experience, I have had no more
trouble with connecting/disconnecting while powered on than I have with a
PS/2 style keyboard or mouse cable on a PC.


     Scott Perry
Indianapolis, Indiana
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