Sean:
Here is something I wrote for one of my clients.
2.5 Clock and NTP settings
The Network Time Protocol (NTP) is the defacto standard for Internet time synchronization. The current standard for NTP is version 3, which is defined in RFC 1305.
The protocol allows devices to communicate over UDP port 123 to obtain time information from an authoritative source. An NTP server connected directly to that source is considered a stratum 1 time server. A device that receives time synchronization from a stratum 1 server is a stratum 2 source - and so on, up to 16 hops.
Every network device that is NTP-capable must have it configured to aid in troubleshooting and forensics.
2.5.1 Synchronizing the Internal Clocks
Every Cisco router has an internal system clock. When the router boots, the internal system clock starts to maintain the current date and time, which is useful information to have in logging events.
Most high-end routers also have an internal battery-powered clock called a calendar. It maintains the correct date and time even when the router is turned off. Upon booting, the calendar automatically synchronizes the internal clock to its own clock to give the correct time.
The command to see if your router has a calendar is:
Router# show calendar
(If you receive an error message, your router does not have a calendar)
Sometimes the calendar clock and the system clock drift after synchronization, but there is a command to use to resynchronize them.
Router# clock update-calendar
(set the calendar time to internal clock)
Router# clock read-calendar
(set the internal clock to calendar time)
If the router does not have a battery-powered calendar to synchronize the internal clock, you will need to set the clock time manually or you can configure the router to time off of a centralized NTP server.
2.5.2 Setting the Time
Use this command to configure the clock manually:
Router(config)# clock set hh:mm:ss
(The date and time will revert to the default after the router is rebooted if there is no NTP server to synchronize to)
2.5.3 Synchronizing to an NTP Server
The time taken from a time server is in UTC (Greenwich Mean Time). You may want the time to reflect the time zone in which the router exists, which will be a number of hours offset from UTC.
Use the following commands to synchronize to an NTP time server:
Router(config)# clock timezone EST -5
(EST is 5 hours back from UTC).
Router(config)# ntp source loopback0
(source the loopback interface for NTP traffic)
Router(config)# ntp server
(Repeat as needed for multiple NTP servers).
Router(config)# clock summer-time CDT recurring
(enable the device to adjust for daylight savings time)
HTH
Victor
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