NTP Overview 951
■ Receive Timestamp: the local time at which the request arrived at the service
host.
■ Transmit Timestamp: the local time at which the reply departed the service host
for the client.
■ Authenticator: authentication information.
Operation Modes of NTP Switches running NTP can implement clock synchronization in one of the
following modes:
Server/client mode
Figure 279 Server/client mode
When working in the server/client mode, a client sends a clock synchronization
message to servers, with the Mode field in the message set to 3 (client mode).
Upon receiving the message, the servers automatically work in the server mode
and send a reply, with the Mode field in the messages set to 4 (server mode). Upon
receiving the replies from the servers, the client performs clock filtering and
selection, and synchronizes its local clock to that of the optimal reference source.
In this mode, a client can be synchronized to a server, but not vice versa.
Symmetric peers mode
Figure 280 Symmetric peers mode
A switch working in the symmetric active mode periodically sends clock
synchronization messages, with the Mode field in the message set to 1 (symmetric
active); the switch that receives this message automatically enters the symmetric
Network
ServerClient
Automatically works in
server /client mode and
sends a reply
Reply (Mode 4)
Performs clock filtering and
selection and synchronizes its
local clock to that of the
optimal reference source
Clock synchronization
message (Mode 3)
Network
Symmetric
passive peer
Symmetric
active peer
Clock synchronization
message (Mode 1)
Reply (Mode 2)
Synchronize
each other
Automatically
works in
symmetric
peers mode
and sends a
reply
Clock synchronization message
exchange (Mode 3 and Mode 4)
The symmetric peers mode is
established and the two devices
can synchronize or be
synchronized by each other
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