3Com Switch 7750 Configuration Guide Guide

Introduction to NTP 697
■ Before this NTP packet leaves LS_B, LS_B inserts its own timestamp once again,
which identifies 11:00:02am (noted as T
3
).
■ When receiving the response packet, LS_A inserts a new timestamp, which
identifies 10:00:03am (noted as T
4
), into it.
At this time, LS_A has enough information to calculate the following two
parameters:
■ The 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
).
■ The time offset of LS_A with regard 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 the detailed information, refer to RFC1305.
NTP Implementation
Mode
To accommodate networks of different structures and switches in different
network positions, NTP can operate in multiple modes, as described in the
following.
Client/Server mode
Figure 176 NTP implementation mode: client/Sever mode
Peer mode
Figure 177 NTP implementation mode: peer mode
In peer mode, the active peer sends clock synchronization packets first, and its
peer works as a passive peer automatically.
If both of the peers have reference clocks, the one with smaller stratum is
adopted.
Netw ork
Client Server
Clock synchronization
request packet
Response packetFilter and select clocks
and synchronize its
ow n clock to that of
the selected server
Work as a server
automatically and
send response
packets
Netw ork
Client Server
Response packetFilter and select clocks
and synchronize its
ow n clock to that of
the selected server
Work as a server
automatically and
send response
packets
Netw ork
Client Server
Clock synchronization
request packet
Response packetFilter and select clocks
and synchronize its
ow n clock to that of
the selected server
Work as a server
automatically and
send response
packets
Netw ork
Client Server
Response packetFilter and select clocks
and synchronize its
ow n clock to that of
the selected server
Work as a server
automatically and
send response
packets
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
In peer mode, both
sides are synchronized
to the clock with
smaller stratum
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
Active peer Passive peer
Netw ork
Clock synchronization
request packet
Operates in the passive
peer mode automatically
Netw ork
Response packet
Sy nc hr oniz e
In peer mode, both
sides are synchronized
to the clock with
smaller stratum