952 CHAPTER 77: NTP CONFIGURATION
passive mode and sends a reply, with the Mode field in the message set to 2
(symmetric passive). By exchanging messages, the symmetric peers mode is
established between the two switches. Then, the two switches can synchronize, or
be synchronized by, each other. If the clocks of both switches have been already
synchronized, the switch whose local clock has a lower stratum level will
synchronize the clock of the other switch.
Broadcast mode
Figure 281 Broadcast mode
In the broadcast mode, a server periodically sends clock synchronization messages
to the broadcast address 255.255.255.255, with the Mode field in the messages
set to 5 (broadcast mode). Clients listen to the broadcast messages from servers.
After a client receives the first broadcast message, the client and the server start to
exchange messages, with the Mode field set to 3 (client mode) and 4 (server
mode) to calculate the network delay between client and the server. Then, the
client enters the broadcast client mode and continues listening to broadcast
messages, and synchronizes its local clock based on the received broadcast
messages.
Multicast mode
Figure 282 Multicast mode
In the multicast mode, a server periodically sends clock synchronization messages
to the user-configured multicast address, or, if no multicast address is configured,
to the default NTP multicast address 224.0.1.1, with the Mode field in the
Network
Client
Server
After receiving the first
broadcast message the client
sends a request
Clock synchronization message
exchange (Mode 3 and Mode 4)
Periodically broadcasts clock
synchronization messages (Mode 5)
Calculates the network delay
between client and server and
enters the broadcast client mode
Periodically broadcasts clock
synchronization messages (Mode 5)
Receives broadcast messages
and synchronizes its local
clock
ˈ
Network
Client
Server
After receiving the first
multicast message the client
sends a request
Clock synchronization message
exchange (Mode 3 and Mode 4)
Periodically multicasts clock
synchronization messages (Mode 5)
Calculates the network delay
between client and server and
enters the multicast client mode
Periodically multicasts clock
synchronization messages (Mode 5)
Receives multicast messages
and synchronizes its local
clock
ˈ
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