Avaya Configuring IP Utilities Uživatelský manuál stáhnout pdf (Strana 44)

Configuring IP Utilities

2-14 114071 Rev. A

Figure 2-5. Time Servers Forming a Synchronization Subnet

Distributing Time Throughout the NTP Network

Time is distributed through a hierarchy of NTP servers, with each server adopting

a “stratum,” which deﬁnes how many NTP “hops” away a particular secondary

time server is from a primary time server on the network. A “stratum 1” time

server has a wire or radio clock directly attached and is located at the top of the

hierarchy (or bottom, depending on your point of view); a “stratum 2” time server

receives its time via NTP from a “stratum 1” time server; a “stratum 3” time

server receives its time via NTP from a “stratum 2” time server; and so forth. A

time server running NTP automatically chooses as its time source the server with

the lowest stratum number that it is conﬁgured to communicate with via NTP.

This strategy effectively builds a self-organizing tree of NTP speakers. The

number of strata is limited to 15 to avoid long-lived synchronization loops.

NTP tries not to synchronize to a server whose time might not be accurate. It

avoids doing this in two ways. First, NTP never synchronizes to a time server that

is not in turn synchronized itself. Second, NTP compares the time reported by

several servers, and will not synchronize to a server whose time is markedly

different from the others, even if its stratum is lower.

TCP0007A

Stratum 1

Stratum 2

Secondary

servers

Stratum 3

Primary

server

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Avaya Configuring IP Utilities Uživatelský manuál Strana 44