3Com 4500G Family Configuration Manual page 1147

In this mode, a client can be synchronized to a server, but not vice versa.
Symmetric peers mode
Figure 1-4 Symmetric peers mode
A device working in the symmetric active mode periodically sends clock synchronization messages,
with the Mode field in the message set to 1 (symmetric active); the device that receives this message
automatically enters the symmetric 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 devices. Then, the two devices can synchronize, or be synchronized by each other. If
the clocks of both devices have been already synchronized, the device whose local clock has a lower
stratum level will synchronize the clock of the other device.
Broadcast mode
Figure 1-5 Broadcast mode
Server
Network
Periodically broadcasts clock
synchronization messages (Mode 5)
Clock synchronization message
exchange (Mode 3 and Mode 4)
Periodically broadcasts clock
synchronization messages (Mode 5)
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.
Client
After receiving the first
broadcast message, the
client sends a request
Calculates the network delay
between client and the server
and enters the broadcast
client mode
Receives broadcast
messages and synchronizes
its local clock
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