From: Jay Hennigan (jay@xxxxxxxx)
Date: Thu Jul 18 2002 - 12:00:11 GMT-3
On Thu, 18 Jul 2002 Giveortake@aol.com wrote:
> I need some clarification on NTP UTC time. A lab (Not the CCIE LAB) asks
> me to configure a router as a NTP Master and use the clock on my PC to set
> the time.
Once, or should it track the clock on your PC? If it should track your PC,
then your PC needs NTP server capability and to be networked to the lab
environment.
> So if it was say 12 noon and I lived in the Eastern Time zone I
> would actually set my clock 5 hours ahead and then adjust for daylight
> savings. Then I would enter the "clock timezone" command and adjust it
> back 5 hours???
The NTP master should reference UTC, and the clients (which can be anywhere
in the world) will apply their local timezone offset.
> Is the above the correct way to do it?
There are two ways to do it.
One is to not apply a timezone (or summer-time recurring) to the reference
router and to set its clock to UTC. The clients then can have their clock
offset to the proper timezone. This can be confusing to someone casually
looking at that router's clock as it will show UTC, and the lab might not
want this.
The other way is to set the timezone and summer-time recurring if applicable
to the reference router. Then set its clock to local time, and it will
"back out" the offset and send the correct UTC time to its NTP peers.
What is NOT right is to set the reference to local time without specifying
the timezone. This will send the offset local time to its peers. Not good
unless you're in London.
There are some hardware and IOS quirks that can get you into trouble as
well. Some routers have no battery-backed clock/calendar and will lose
the time-of-day on reload or power failure. Others have a battery inside
that maintains the time-of-day. See also the "clock calendar-valid" and
"clock update-calendar" commands as appropriate for your hardware.
> Then I have a second router that resides in the Central timezone which is one
> hour behind Eastern. Do I then use the "clock timezone" command on the
> Central router and adjust it back -6 to show correct time (assuming not
> adjusted for Daylight savings)?
Don't just set it -6. Do the following:
clock timezone CST -6
clock summer-time CDT recurring
Now it will automagically switch to the correct offset for daylight savings
time.
> Specifically this example came out of IPEpxert lab 20-3 but I want to check
> the solution. It is showing that I would do a "clock timezone CT -1" but
> that just doesn't feel accurate! Seems like it should be -6!!!
Your scenario may have been written by someone in the -1 timezone, or it
may be wrong. In any event, your NTP master wants to be set to UTC [1]
unless there's a clearly defined lab requrement for it to violate RFC1305
which has more gory detail about timekeeping and calendar systems than you
really want to know.
[1] Actually the number of standard seconds since January 1, 1900 UTC
with appropriate corrections for leap-seconds.
When someone asks you for the time, point them to RFC1305 and ask them
to be more specific.
-- Jay Hennigan - CCIE #7880 - Network Administration - jay@west.net NetLojix Communications, Inc. - http://www.netlojix.com/ WestNet: Connecting you to the planet. 805 884-6323
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