Network Time Protocol (NTP) is an application layer protocol used for clock synchronization between hosts on a TCP/IP network. The goal of NTP is to ensure that all computers on a network agree on the time, since even a small difference can create problems. For example, if there is more than 5 minutes difference on your host and the Active Directory domain controller, you will not be able to login into your AD domain.
NTP uses a hierarchical system of time sources. At the top of the structure are highly accurate time sources – typically atomic or GPS clocks. These clocks are known as stratum 0 servers. Stratum 1 servers are directly linked to stratum 0 servers and computers run NTP servers that deliver the time to stratum 2 servers, and so on (image source: Wikipedia):
NTP uses a client-server architecture; one host is configured as the NTP server and all other hosts on the network are configured as NTP clients. Consider the following example:
Host A is configured to use a public NTP server uk.pool.ntp.org. Host A will periodically send an NTP request to the NTP server. The NTP server will provide the accurate data and time, so Host A can synchronize its clock.
NTP uses a well-known UDP port 123. The current version is NTPv4, and it is backward compatible with NTPv3.
Prerequisites for 200-301
200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.
The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.
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