Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 1666)

NTP Introduction and Configuration
Software Reference for x310 Series Switches
61.2 AlliedWare Plus TM Operating System - Version 5.4.4C C613-50046-01 REV A
Introduction
This chapter describes the Network Time Protocol (NTP) service provided by the switch,
and how to configure and monitor NTP on the switch.
NTP is a protocol for synchronizing the time clocks on a collection of network devices
using a distributed client/server mechanism. NTP uses UDP (User Datagram Protocol) as
the transport mechanism. NTP evolved from the Time Protocol (RFC 868) and the ICMP
Timestamp message (RFC 792).
NTP provides protocol mechanisms to specify the precision and estimated error of the
local clock and the characteristics of the reference clock to which it may be synchronized.
For detailed information about the commands used to configure NTP, see Chapter 62,
NTP Commands .
Overview
NTP uses a subnetwork with primary reference clocks, gateways, secondary reference
clocks, and local hosts. These are organized into a hierarchy with the more accurate clocks
near the top and less accurate ones near the bottom.
A number of primary reference clocks, synchronized to national standards, are connected
to widely accessible resources (such as backbone gateways or switches) operating as
primary time servers. The primary time servers use NTP between them to crosscheck
clocks, to mitigate errors due to equipment or propagation failures, and to distribute time
information to local secondary time servers. The secondary time servers redistribute the
time information to the remaining local hosts.
The hierarchical organization and distribution of time information reduces the protocol
overhead, and allows selected hosts to be equipped with cheaper but less accurate clocks.
NTP provides information which organizes this hierarchy on the basis of precision or
estimated error.
■ An NTP entity may be in one of the following operating modes; however, the switch’s
implementation of NTP supports two modes: client and server.
■ An NTP entity operating in a client mode sends periodic messages to its peers,
requesting synchronization by its peers.
■ An NTP entity enters the server mode temporarily when it receives a client request
message from one of its peers, and remains in server mode until the reply to the
request has been transmitted.
■ An NTP entity operating in symmetric active mode sends messages announcing its
willingness to synchronize and be synchronized by its peers.
■ An NTP entity enters symmetric passive mode in response to a message from a peer
operating in Symmetric Active mode. An NTP entity operating in this mode
announces its willingness to synchronize and be synchronized by its peers.
■ An NTP entity operating in broadcast mode periodically sends messages announcing
its willingness to synchronize all of its peers but not to be synchronized by any of
them.