3Com Switch 4500 26-Port, Configuration Manual

Page: 297 / 396

Overview 295
■
When receiving a response packet, LS_A inserts a new timestamp 10:00:03 am
(T
4
) into it.
At this time, LS_A has enough information to calculate the following two
parameters:
■
Delay for an NTP packet to make a round trip between LS_A and LS_B:
Delay = (T
4
-T
1
)-(T
3
-T
2
).
■
Time offset of LS_A relative to LS_B:
Offset = ((T
2
-T
1
) + (T
3
-T
4
))/2.
LS_A can then set its own clock according to the above information to synchronize
its clock to that of LS_B.
For detailed information, refer to RFC1305.
NTP Implementation
Modes
According to the network structure and the position of the local Ethernet switch in
the network, the local Ethernet switch can work in multiple NTP modes to
synchronize the clock.
Figure 82 Client/sever mode
Figure 83 Peer mode
In the peer mode, the local Switch 4500 Ethernet switch serves as the active peer
and sends clock synchronization request packets first, while the remote server
serves as the passive peer automatically.
If both of the peers have reference clocks, the one with a smaller stratum number
is adopted.
Network
Client Server
Clock synchronization
request packet
Response packet
Filters and selects
a clocks and
synchronize the
local clock to that of
the preferred server
Works in server mode
automatically and send
a response packet
Network Network Network
Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network Network
Clock synchronization
request packet
Network Network Network Network Network Network Network Network Network
Active peer
Network Network
Works in passive peer
mode automatically
Passive peer
Response packet
Synchronize
In peer mode, both
sides can be
synchronized to each
other