Upon the departure of the NTP message, Router B adds its timestamp
11:00:02am (T
3
) again.
Upon the receipt of the response, Router A adds a new timestamp, that is,
10:00:03am (T
4
).
In this way, Router A obtains adequate information for calculating two essential
parameters. They are:
Roundtrip delay of a NTP message, that is, Delay = (T
4
-T
1
) – (T
3
-T
2
).
The clock offset of Router A relative to Router B, that is, offset = ( (T
2
-T
1
) +
(T
3
-T
4
) ) / 2.
With these two parameters, Router A can synchronize its clock with that maintained
by Router B.
The NTP operating fundamentals described in this section provides only a broad
outline. NTP provisioned in RFC1305 provides a comprehensive algorithm to ensure
the accuracy in clock synchronization.
4.2 NTP Configuration Tasks
NTP is used for the time synchronization on a network. Perform the following tasks to
configure NTP.
Configure NTP operating mode
Set the roundtrip delay between the local router and the NTP broadcast server
Set NTP authentication
Set NTP authentication key
Set a specified key to be a reliable key
Set the local NTP message sending interface
Set the external reference clock or local clock to be the NTP master clock
Enable/Disable the interface to receive NTP messages
Control the access to the services of the local router
Set the number of sessions allowed at the local
4.2.1 Configure NTP Operating Mode
You may set the operating mode of the local router in NTP depending on the location
of the router in the network and the structure of the network. For example, a) you can
set a remote server as the local time server in which case the local router is working
in client mode; b) set the remote server as the peer of the local router in which case
the local router is working in symmetric active mode; c) set the local router to use an
interface to send NTP broadcast packets in which case it is working in broadcast
client mode, d) set it to use an interface to send NTP multicast packets in which case
it is working in multicast mode, or e) you can set it to receive NTP multicast packets in
which case it is working in multicast client mode.
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