NTP synchronizes networked devices to a common time source. Consistent clocks support security functions and make logs from multiple devices easier to correlate. Manual clock settings drift, so they do not scale operationally. section 2.0 section 2.1.1 section 2.2
NTP clients send requests to servers over UDP port 123. The protocol uses a hierarchy: reference clocks are stratum 0, directly connected primary servers are stratum 1, and successive secondary servers have higher stratum values. Lower stratum is normally preferred; stratum 16 is unreliable and is not accepted for synchronization. A device can be both an NTP client and server, while peers exchange time information at the same level. section 2.2 section 2.4
R1# show logging . . . Aug 28 16:34:07.860: %OSPF-5-ADJCHG: Process 1, #1 Nbr 10.0.0.2 on GigabitEthernet0/0 from LOADING to #1 FULL, Loading Done #1 Aug 28 16:35:07.080: %OSPF-5-ADJCHG: Process 1, #2 Nbr 10.0.0.2 on GigabitEthernet0/0 from FULL to #2 DOWN, Neighbor Down: Dead timer expired #2 #1 An OSPF neighbor entered the Full state. #2 The same OSPF neighbor entered the Down state due to an expired dead timer.
Compare OSPF log events with mismatched device clocks to see why synchronized time matters for troubleshooting. section 2.1.1
Cisco IOS client configuration and verification
Configure R1 with two servers, favoring one: `ntp server 216.239.35.0 prefer` and `ntp server 20.43.94.199`. Verify relationships with `show ntp associations`; `*` marks the selected sys.peer and `+` marks a reliable candidate. Verify synchronization with `show clock detail` and `show ntp status`. A synchronized client using a stratum 1 server reports itself as stratum 2. Synchronization may take 15–20 minutes, though setting the correct time manually first can accelerate it. section 2.3.1
Cisco IOS devices support NTP client, server, and peer modes. The `ntp server ip-address [prefer]` command configures client/server operation; multiple servers provide redundancy and fault tolerance. NTP can also be secured with authentication. section 2.3 section 2.4
