Technical data

FastIron Ethernet Switch Administration Guide 27
53-1002637-02
Network Time Protocol Version 4 (NTPv4)
2
NTP broadcast client
An NTP broadcast client listens for NTP packets on a broadcast address. When the first packet is
received, the client attempts to quantify the delay to the server, to better quantify the correct time
from later broadcasts. This is accomplished by a series of brief interchanges where the client and
server act as a regular (non-broadcast) NTP client and server. Once interchanges occur, the client
has an idea of the network delay and thereafter can estimate the time based only on broadcast
packets.
NTP associations
Networking devices running NTP can be configured to operate in variety of association modes when
synchronizing time with reference time sources. A networking device can obtain time information
on a network in two ways-by polling host servers and by listening to NTP broadcasts. That is, there
are two types of associations-poll-based and broadcast-based.
NTP poll-based associations
The following modes are the NTP polling based associations:
1. Server mode
2. Client mode
3. Symmetric Active/Passive
The server mode requires no prior client configuration. The server responds to client mode NTP
packets. Use the master command to set the device to operate in server mode when it has lost the
synchronization.
When the system is operating in the client mode, it polls all configured NTP servers and peers. The
device selects a host from all the polled NTP servers to synchronize with. Because the relationship
that is established in this case is a client-host relationship, the host will not capture or use any time
information sent by the local client device. This mode is most suited for file-server and workstation
clients that are not required to provide any form of time synchronization to other local clients. Use
the server and peer to individually specify the time server that you want the networking device to
consider synchronizing with and to set your networking device to operate in the client mode.
Symmetric active/passive mode is intended for configurations where group devices operate as
mutual backups for each other. Each device operates with one or more primary reference sources,
such as a radio clock, or a subset of reliable NTP secondary servers. If one of the devices lose all
reference sources or simply cease operation, the other peers automatically reconfigures. This
helps the flow of time value from the surviving peers to all the others.
When a networking device is operating in the symmetric active mode, it polls its assigned
time-serving hosts for the current time and it responds to polls by its hosts. Because symmetric
active mode is a peer-to-peer relationship, the host will also retain time-related information of the
local networking device that it is communicating with. When many mutually redundant servers are
interconnected via diverse network paths, the symmetric active mode should be used. Most
stratum 1 and stratum 2 servers on the Internet adopt the symmetric active form of network setup.
The FastIron device operates in symmetric active mode, when the peer information is configured
using the peer command and specifying the address of the peer. The peer is also configured in
symmetric active mode in this way by specifying the FastIron device information. If the peer is not
specifically configured, a symmetric passive association is activated upon arrival of a symmetric
active message.