router and dst

Unanswered Question
Oct 19th, 2007

Hi, the router currently is using external NTP. For Nov 4 DST, do we need change anything on the router to reflect the new time?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 4 (1 ratings)
Richard Burts Fri, 10/19/2007 - 09:43

Whether you need to do anything for the Nov 4 DST depends on what version of code you are running and on whether you did something this spring when DST began.

Cisco has supplied logic in recent versions of IOS that do implement the changes in begin and end dates for DST. If you are running the new versions of code then you do not need to do anything. If you are not running the new version of code, did you do anything in the spring when DST started? On the clock summertime command there are optional parameters in which you can specify the begin and end dates for DST. If you fixed that in the spring then it should be good in the fall. If you did not already fix it then you probably need to fix it now.

There is a fairly easy way to check on whether you need to do something or not. Do the show clock detail command. The output will show the beginning and ending dates for DST. If it does not show Nov 4 as the ending date then you need to do something.

Perhaps it will help if we clarify one point: DST has nothing to do with NTP. Time is transmitted by NTP using only GMT (or UTC which ever name you prefer) and NTP has no concept of DST. The changes for DST are done in the local router and not in NTP.



ciscoforum Mon, 10/22/2007 - 07:34

Thanks Rick.

the show clock detail does show summer time ends 02:00:00 EDT Sun Nov 4 2007. So we are fine with it.

Next question I have is: There are a few windows servers using this router(actually this is 6509 layer 3 switch) as NTP server. Since the switch has DST propoerly configured. will the windows get update automatically? I guess the answer is no since I see your statement that DST has nothing to do with NTP. The windows still need to have its mechnism to understand the DST on Nov 4,right? The way I understand from you describerd is that any device will get clock from the NTP server first, then will base on its own DST setting to offset the clock from the clock learned from external NTP, correct? But wait a sec, in this specific case, since the switch will change the clock 1 hour back on Nov 4, then windows server learns this new clock, window should auto updated too. If that's the case, then windows does not need to know Nov 4 DST if they never applied patch in spring? A little confused.

Richard Burts Mon, 10/22/2007 - 07:40

It is good that the show clock detail does show that your router (actually a switch - it works the same on both) shows that it will process the DST change according to the new schedule.

Your understanding is correct that Windows servers (or PCs) learning time from the NTP server will learn that time in UTC (or GMT) which has no concept of DST and they will need to make their own local mechanism for DST. And your understanding of the general principle is correct that any device that learns time from an NTP server will apply its own local DST setting to offset the clock from the external NTP.




This Discussion