Cisco WS-C4003 Software Guide - Page 475

Configuring NTP 35 C H A P T E R This chapter describes how to configure the Network Time Protocol (NTP) on the Catalyst enterprise LAN switches. Note For complete syntax and usage information for the commands used in this chapter, refer to the Command Reference-Catalyst 4000 Family, Catalyst 2948G, and Catalyst 2980G Switches. This chapter consists of these sections: • Understanding How NTP Works, page 35-1 • Default NTP Configuration, page 35-2 • Configuring NTP, page 35-2 Understanding How NTP Works NTP synchronizes timekeeping among a set of distributed time servers and clients. This synchronization allows events to be correlated when system logs are created and other time-specific events occur. An NTP server must be accessible by the client switch. NTP uses the User Datagram Protocol (UDP) as its transport protocol. All NTP communication uses Coordinated Universal Time (UTC), which is the same as Greenwich Mean Time. An NTP network usually gets its time from an authoritative time source, such as a radio clock or an atomic clock attached to a time server. NTP distributes this time across the network. NTP is extremely efficient; no more than one packet per minute is necessary to synchronize two machines to within a millisecond of one another. NTP uses a stratum to describe how many NTP hops away a machine is from an authoritative time source. A stratum 1 time server has a radio or atomic clock directly attached, a stratum 2 time server receives its time from a stratum 1 time server, and so on. A machine running NTP automatically chooses as its time source the machine with the lowest stratum number that it is configured to communicate with through NTP. This strategy effectively builds a self-organizing tree of NTP speakers. NTP has two ways to avoid synchronizing to a machine whose time might be ambiguous: • NTP never synchronizes to a machine that is not synchronized itself. • NTP compares the time reported by several machines and does not synchronize to a machine whose time is significantly different from the others, even if its stratum is lower. The communications between machines running NTP, known as associations, are usually statically configured; each machine is given the IP addresses of all machines with which it should form associations. Accurate timekeeping is possible by exchanging NTP messages between each pair of 78-12647-02 Software Configuration Guide-Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4 35-1