Discussion

Chapter 6
CHAPTER 6
NTP6
6.0 Introduction
Having the correct time and time zone on your routers and having the time be synchronized
across your network is useful to accurately track events that occur on the
router and network and to correlate events that occur on different routers. The time
and date are used to mark when the files stored on the router, including the active
and previous configuration files, the system logging files, and the tracing files, were
created or last updated. The messages in JUNOS logging and tracing files are
timestamped to mark when events, errors, and problems occurred. All these dates
and times are based on the router’s time. A number of JUNOS commands, including
the ones you use to check the status and uptime of the router, report the current
time and the time at which events occurred.
On JUNOS routers, you can configure the time manually by simply setting it. This is
generally sufficient for smaller or less complex networks. However, for larger or
more global networks, you should use Network Time Protocol (NTP) to set the time
for you. NTP is an IETF standard described in RFC 1305 that synchronizes time
across computers and routers on the Internet. The router synchronizes the system
time with an NTP server and periodically accesses the server to maintain the correct
time. NTP uses a hierarchical system of clock strata to derive time. The top-level
stratum 1 clock, also called the primary NTP server, is a computer that is connected
to a high-precision accurate clock, such as an atomic clock, or to a radio clock, such
as a GPS, Loran, or WWVB, which is the NIST time signal radio station. NTP stratum
2 systems derive their time from a stratum 1 system and are one hop away from
a stratum 1 system; NTP stratum 3 systems derive their time from a stratum 2 clock,
and are one hop from the stratum 2 machine and two hops from the stratum 1
machine. Note that these hops are NTP hops, not network hops. The systems can be
any number of network hops apart. NTP can have up to 16 strata.
This is the Title of the Book, eMatter Edition
Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.
181