Alcatel-Lucent 6850-48 Supplementary Manual

Configuring Network Time Protocol (NTP)

NTP Overview

OmniSwitch AOS Release 6 Switch Management Guide

page 4-7

Examples of these are shown in the simple network diagram below:

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.