Which of the following protocols synchronize the time of different systems?

(A) NTP
(B) SMTP
(C) UDP
(D) CDP

NTP is the correct answer.

NTP (network time protocol) is a protocol for clock synchronization in computer systems. NTP is a built-on UDP, where port 123 is used for NTP server communication and NTP clients use port 1023 (for example, a desktop). Unfortunately, like many legacy protocols, NTP suffers from security issues.

It is possible to spoof NTP packets, causing clocks to set to various times (an issue for certain services that run periodically). There are several cases of NTP misuse and abuse where servers are the victim of DoS attacks. As a result, if clock synchronization is needed, it may be better to provide an internal NTP server (master clock) that synchronizes the remaining clocks in the internal network.

Cisco supplies two ntp configuration commands that dictate how NTP works on a router or switch, as follows:

• ntp master {stratum-level}: NTP server mode—the device acts only as an NTP server, and not as an NTP client. The device gets its time information from the internal clock on the device.
• ntp server {address | hostname}: NTP client/server mode—the device acts as both client and server. First, it acts as an NTP client, to synchronize time with a server. Once synchronized, the device can then act as an NTP server, to supply time to other NTP clients.