Installing and Administering Internet Services

Chapter 7 215
Configuring NTP
Configuration
Guidelines for Configuration
The following are guidelines that you should consider when planning
your configuration:
• Every NTP hierarchy must have at least one stratum-1 server. You
may configure your administrative domain to have outside sources of
synchronization which ultimately link to stratum-1 server(s), or you
may implement your own hierarchy of NTP time servers with one or
more stratum-1 servers. For example, an NFS-Diskless cluster may
be configured as its own NTP hierarchy. In this topology, the
NFS-Diskless server is configured as a stratum-1 NTP server, and
may use its own system clock as the time server.
• Configure at least three time servers in your administrative domain.
It is important to provide multiple, redundant sources of
synchronization, as NTP is specifically designed to select an optimal
source of synchronization from several potential candidates. Each
time server should be a peer with each of the other time servers. In
Figure 7-8, each of these servers are depicted as a “Stratum 2 Server”
within the administrative domain.
• For each time server, select 1-3 outside sources of synchronization.
This assures a relative degree of reliability in obtaining time,
especially if you can select sources that do not share common paths.
The sources should operate at a stratum level that is one less than
the local time servers. In Figure 7-8, there are two stratum-1 sources
shown for each server in the administrative domain.
NOTE An enterprise may implement its own hierarchy of NTP time servers,
including stratum-1 servers. If your administrative domain is part of an
enterprise-wide internet, you should check for available NTP resources
in your enterprise. If your administrative domain does not have access to
lower-stratum time servers, there are NTP servers on the Internet that
are willing to provide public time synchronization. (Many stratum-1 and
stratum-2 servers can be used only by permission of the administrator of
the system; you should always check with the administrator before using
an NTP server on the Internet.) The list of servers is available by
anonymous ftp in the file pub/ntp/doc/clock.txt on Internet host
louie.udel.edu (Internet address 128.175.1.3).