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Which two statements about NTP operation are true? (Choose two.)
A. If multiple NTP servers are configured, the one with the lowest stratum is preferred
B. By default, NTP communications use UDP port 123.
C. If multiple NTP servers are configured, the one with the highest stratum is preferred.
D. Locally configured time overrides time received from an NTP server.
E. “Stratum” refers to the number of hops between the NTP client and the NTP server.
Correct Answer: AB
Explanation/Reference:
NTP Overview :
NTP is designed to synchronize the time on a network of machines. NTP runs over the User Datagram Protocol (UDP), using port 123 as both the source and destination, which in turn runs over IP. NTP Version 3 RFC 1305 leavingcisco.com is used to synchronize timekeeping among a set of distributed time servers and clients. A set of nodes on a network are identified and configured with NTP and the nodes form a synchronization subnet, sometimes referred to as an overlay network. While multiple masters (primary servers) may exist, there is no requirement for an election protocol.
An NTP network usually gets its time from an authoritative time source, such as a radio clock or an atomic clock attached to a time server. NTP then distributes this time across the network. An NTP client makes a transaction with its server over its polling interval (from 64 to 1024 seconds) which dynamically changes over time depending on the network conditions between the NTP server and the client. The other situation occurs when the router communicates to a bad NTP server (for example, NTP server with large dispersion); the router also increases the poll interval. No more than one NTP transaction per minute is needed to synchronize two machines. It is not possible to adjust the NTP poll interval on a router.
NTP uses the concept of a stratum to describe how many NTP hops away a machine is from an authoritative time source. For example, a stratum 1 time server has a radio or atomic clock directly attached to it. It then sends its time to a stratum 2 time server through NTP, and so on. A machine running NTP automatically chooses the machine with the lowest stratum number that it is configured to communicate with using NTP as its time source. This strategy effectively builds a self-organizing tree of NTP speakers. NTP performs well over the non-deterministic path lengths of packet-switched networks, because it makes robust estimates of the following three key variables in the relationship between a client and a time server.
