Avaya Configuring IP Utilities Uživatelský manuál stáhnout pdf (Strana 55)

Overview of IP Utilities

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How NTP Distributes Time Within the Subnet

NTP distributes time through a hierarchy of primary and secondary time servers,

with each server adopting a “stratum” (see Figure 2-5

on page 2-16). A “stratum”

defines how many NTP “hops” away a particular secondary time server is from an

authoritative time source (primary time server) in the synchronization subnet.

A “stratum 1” time server, located at the top of the hierarchy, is directly attached

to an external time source, typically a wire or radio clock; 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.

Each NTP client in the synchronization subnet 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 remote time server whose time might not be

accurate. It avoids doing this in two ways. First, NTP never synchronizes to a

remote time server that is not in turn synchronized itself. Second, NTP compares

the time reported by several remote time servers, and will not synchronize to a

remote time server whose time is markedly different from the others, even if its

stratum is lower.

Synchronizing with the Best Available Time Server

Unlike other implementations of time synchronization protocols, NTP does not

attempt to synchronize the remote time server’s internal clocks to each other.

Rather, NTP achieves time synchronization by synchronizing their clocks to

universal standard time using the “best” available time source and transmission

paths to that time source.

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Avaya Configuring IP Utilities Uživatelský manuál Strana 55