Alcatel-Lucent OmniSwitch AOS Release 6 Manual page 157

Configuring Network Time Protocol (NTP)
Examples of these are shown in the simple network diagram below:
NTP
Servers
NTP
Server/Clients
NTP
Clients
Servers 1a and 1b receive time information from, or synchronize with, a UTC time source such as a radio
clock. (In most cases, these servers would not be connected to the same UTC source, though it is shown
this way for simplicity.) Servers 1a and 1b become stratum 1 NTP servers and are peered with each other,
allowing them to check UTC time information against each other. These machines support machines 2a
and 2b as clients, and these clients are synchronized to the higher stratum servers 1a and 1b.
Clients 2a and 2b are also peered with each other for time checks, and become stratum 2 NTP servers for
more clients (3a and 3b, which are also peered). In this hierarchy, the stratum 1 servers synchronize to the
most accurate time source available, then check the time information with peers at the same stratum. The
stratum 2 machines synchronize to the stratum 1 servers, but do not send time information to the stratum 1
machines. Machines 2a and 2b in turn provide time information to the stratum 3 machines. It is important
to consider the issue of robustness when selecting sources for time synchronization.
It is suggested that at least three sources should be available, and at least one should be "close" to you in
terms of network topology. It is also suggested that each NTP client is peered with at least three other
same stratum clients, so that time information crosschecking is performed.
When planning your network, it is helpful to use the following general rules:
•
It is usually not a good idea to synchronize a local time server with a peer (in other words, a server at
the same stratum), unless the latter is receiving time updates from a source that has a lower stratum
than from where the former is receiving time updates. This minimizes common points of failure.
OmniSwitch AOS Release 6 Switch Management Guide
UTC Time Source
1a
2a
3a
1b
2b
3b
July 2010
NTP Overview
Stratum 1
Stratum 2
Stratum 3
page 4-7