First, we’ll sync the router time against another NTP time source. It’s as easy as specifying an IP:

phbrtr#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
phbrtr(config)#ntp server <IP address>
phbrtr(config)#exit
phbrtr#

Incidentally, you can find a list of public NTP servers at:

http://support.ntp.org/bin/view/Servers/WebHome 

To tidy things up, you might want to prevent NTP synchronisation attempts on a per-interface basic. To do this, you can ‘ntp disable’ on the relevant interface(s):

phbrtr#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
phbrtr(config)#interface fa1/0
phbrtr(config-if)#ntp disable
phbrtr(config-if)#exit
phbrtr(config)#exit

Finally, heres how to check that everything’s working as it should:

 phbrtr#sh ntp status
Clock is synchronized, stratum 2, reference is 129.6.15.28
nominal freq is 250.0000 Hz, actual freq is 250.0005 Hz, precision is 2**24
reference time is CE89F9F3.27B5E326 (21:21:55.155 GMT Wed Oct 21 2009)
clock offset is -13.4327 msec, root delay is 99.40 msec
root dispersion is 21.27 msec, peer dispersion is 7.83 msec
phbrtr#

Are you looking to learn more about Cisco equipment? Well, here’s a selection of a few books that I’ve found useful over the years:

Cisco: A Beginner’s Guide

CCNA – Cisco Certified Network Associate Study Guide

Cisco Networking for Dummies

Cisco IOS in a Nutshell – O’Reilly