65Configuring NTPSynchronize your device with a trusted time source by using the Network Time Protocol (NTP) orchanging the system time before you run it on a live network. Various tasks, including networkmanagement, charging, auditing, and distributed computing depend on an accurate system time setting,because the timestamps of system messages and logs use the system time.OverviewNTP is typically used in large networks to dynamically synchronize time among network devices. Itguarantees higher clock accuracy than manual system clock setting. In a small network that does notrequire high clock accuracy, you can keep time synchronized among devices by changing their systemclocks one by one.NTP runs over UDP and uses UDP port 123.NOTE:NTP is supported only on the following Layer 3 interfaces:• Layer 3 Ethernet interfaces• Layer 3 Ethernet subinterfaces• Layer 3 aggregate interfaces• VLAN interfaces• Tunnel interfacesHow NTP worksFigure 26 shows how NTP synchronizes the system time between two devices (Device A and Device B, inthis example). Assume that:• Prior to the time synchronization, the time of Device A is set to 10:00:00 am and that of Device Bis set to 11:00:00 am.• Device B is used as the NTP server. Device A is to be synchronized to Device B.• It takes 1 second for an NTP message to travel from Device A to Device B, and from Device B toDevice A.• It takes 1 second for Device B to process the NTP message.
