4000 Router Series Network Management and Monitoring Configuration Guide

many NTP hops away a device is from the primary time server. A stratum 2 time server receives its time

from a stratum 1 time server, and so on.

To ensure time accuracy and availability, you can specify multiple NTP servers for a device. The device

selects an optimal NTP server as the clock source based on parameters such as stratum. The clock that

the device selects is called the reference source. For more information about clock selection, see the

related protocols and standards.

If the devices in a network cannot synchronize to an authoritative time source, you can select a device that

has a relatively accurate clock from the network, and use the local clock of the device as the reference

clock to synchronize other devices in the network.

Association modes

NTP supports the following association modes:

• Client/server mode

• Symmetric active/passive mode

• Broadcast mode

• Multicast mode

Table 2 NTP association modes

Mode Workin

rocess

Princi

lication scenario

Client/server

On the client, specify the IP

address of the NTP server.

A client sends a clock

synchronization message to the

NTP servers. Upon receiving the

message, the servers

automatically operate in server

mode and send a reply.

If the client can synchronize to

multiple time servers, it selects an

optimal clock and synchronizes its

local clock to the optimal

reference source after receiving

the replies from the servers.

A client can synchronize

to a server, but a server

cannot synchronize to a

client.

As Figure 26 sh

ows, this

mode is intended for

configurations where

devices of a higher stratum

are synchronized to

devices with a lower

stratum.