Last year, I wrote an article about why Cisco devices should use Network Time Protocol (NTP) for their time synchronization needs, in which I explained how to configure NTP on your Cisco devices ("Synchronize a Cisco router's clock with Network Time Protocol (NTP)"). Using NTP is the ideal method for medium to large-scale networks.
However, if you have only a handful of routers, manually setting the clock may be the easiest way to properly configure your devices' times. Let's walk through the process.
If a Cisco router boots up before you've configured a local time or network time source, it will display the date as March 1, 1993. Here's an example:
Router> show clock *00:01:10.415 UTC Mon Mar 1 1993 Router>
This date's appearance on log files is a good indication that no one has set the router's time source or local time. This is much more likely than the router's log entries actually dating back to 1993.
Does setting the correct time on a router really matter? While proper time configuration isn't necessary for a router to fully operate, that doesn't mean you shouldn't set the right time. Here are some of the benefits of setting the correct time on a router:
- Log files display the proper time stamp, which can help when troubleshooting a problem.
- The correct time allows you to make connections between two different times.
- You can schedule commands using the router's kron scheduler.
Configure the time zone
When setting a router's (or switch's) correct time, the first step is configuring the proper time zone. This is the first step for a reason: If you set the time first and then try to set to the time zone, you'll have to reset the time again.
The key point to remember is that it's not enough to know that you're in the Eastern or Pacific time zone. You need to know how many hours you are from Greenwich Mean Time (GMT).
For example, if you're in the Eastern Standard Time zone in the United States, you're five hours behind GMT. You would indicate this to the router with -5. If you're unsure how many hours you are from GMT, the U.S. Navy's Web site offers a great resource—the World Time Zone Map.
After you've determined your time zone value, you can set the time zone. For example, I live in the Central Standard Time (CST) zone, so here's how I would configure the router:
Router(config)# clock timezone CST -6
Configure Daylight Saving Time
After setting the appropriate time zone, you need to configure the router to adjust for Daylight Saving Time. You can use the summer-time command to accomplish this. Using our CST zone example, here's how to configure the router to use Daylight Saving Time:
Router(config)# clock summer-time CDT recurring
The summer-time command tells the router to refer to Daylight Saving Time as Central Daylight Time (CDT), which will automatically occur according to predefined dates and times on the router. (You can use the same command to manually set the date and time for Daylight Saving Time.) The recurring option tells the router to use the accepted U.S. Daylight Saving Time rules for the annual time changes in April and October.
Configure the clock
After configuring the time zone and Daylight Saving Time, the last step is to configure the router's clock. You must do this while in Privileged Mode—not Global Configuration Mode.
If you've never done this before, the format can be a bit tricky. Here are some things to keep in mind:
- Use the clock set command.
- Use military time.
- Include seconds when setting the time.
- Specify the month using its three-letter abbreviation.
- Add the date and the year.
Here's an example:
Router# clock set 10:50:00 Oct 26 2006
View the time
After configuring the time zone, Daylight Saving Time, and the clock, you can view the clock using the show clock command. Here's an example:
Router# show clock 10:51:33.208 CDT Thu Oct 19 2006 Router#
Keep in mind that most Cisco routers and switches don't have internal clocks that store the time when you power them off. That means rebooting a device will lose the set local time. However, the time zone will remain set because the router stores it in its configuration.
For more information on Cisco IOS time configuration, check out Cisco's documentation for the various clock commands. How do you set the time on routers or switches? Do you set it manually or use NTP? What other router and switch topics would you like to see covered? Share your comments in this article's discussion.
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David Davis has worked in the IT industry for 12 years and holds several certifications, including CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of systems/network administrators for a privately owned retail company and performs networking/systems consulting on a part-time basis.