Cisco Cookbook

Chapter 14. NTP and Time

14.0. Introduction

Many engineers overlook the importance of accurate timekeeping on a router. It is often extremely useful to be able to accurately pinpoint when a particular event occurred. You may want to compare network event messages from various routers on your network for fault isolation, troubleshooting, and security purposes. This is impossible if their clocks are not set to a common source. In fact, merely setting the clocks to a single common standard is not enough, because some clocks run a little bit fast and others run a little bit slow. So router clocks need to be continuously adjusted and synchronized.

Network Time Protocol (NTP) is the de facto standard for Internet time synchronization. The current standard for NTP is Version 3, which is defined in RFC 1305. The IETF is currently developing a new version.

The protocol allows devices to communicate over UDP port 123 to obtain time from an authoritative time source such as a radio clock, atomic clock, or GPS-based time source. An NTP server connected directly to one of these known reliable time sources is called a Stratum 1 timeserver. Stratum 2 timeservers receive their time via NTP from a Stratum 1 server, and so forth, up to a maximum of Stratum 16. Stratum numbers are analogous to hop counts from the authoritative time source. NTP generally prefers lower stratum servers to higher stratum servers unless the lower stratum server’s time is significantly different.

The algorithm is able to detect ...