Installing and Administering Internet Services

298 Chapter7

Conﬁguring the Network Time Protocol (NTP)

Advanced NTP Topics

Stratum Levels and Time Server Hierarchy

An NTP synchronization subnet is a network of timekeeping systems,

called time servers. These time servers are a subset of the systems on a

network or an internetwork. Each time server synchronizes to Universal

Coordinated Time (also known by the acronym UTC). Each server

measures the time difference between its local system clock and the

system clocks of its neighbors. These servers are automatically assigned

stratum values, which indicate how close the time server is to the time

source.

Stratum-1 Time Servers

Time servers are organized into levels, or strata. Stratum-1 servers are

directly connected to an external time source. The stratum-1 server

relies on the external source of time to provide the correct time, and

synchronizes its system clock to that external time source. The external

time source can be a device such as a radio receiver. Figure 7-2 shows the

relationship between the GPS receiver time source and the stratum-1

server associated with it.

Figure 7-2 Stratum-1 Time Servers

Stratum-2 and -3 Time Servers

Stratum-2 time servers use stratum-1 servers as their time source.

Likewise, stratum-3 servers use stratum-2 servers as their time sources.

Stratum 1

Server

Stratum 2

Server

Stratum 2

Server

external

clock

--Stratum 0

Server