Prerequisites For Implementing Ntp On Cisco Ios Xr Software - Cisco ASR 9000 Series User Configuration Manual
Aggregation services router
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Also See for ASR 9000 Series:
- Command reference manual (1138 pages) ,
- Routing configuration manual (702 pages) ,
- Reference manual (696 pages)
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Prerequisites for Implementing NTP on Cisco IOS XR Software
Prerequisites for Implementing NTP on Cisco IOS XR Software
You must be in a user group associated with a task group that includes the proper task IDs. The command
reference guides include the task IDs required for each command. If you suspect user group assignment is
preventing you from using a command, contact your AAA administrator for assistance.
Information About Implementing NTP
NTP synchronizes timekeeping among a set of distributed time servers and clients. This synchronization
allows events to be correlated when system logs are created and other time-specific events occur.
NTP uses the User Datagram Protocol (UDP) as its transport protocol. All NTP communication uses
Coordinated Universal Time (UTC). An NTP network usually receives its time from an authoritative time
source, such as a radio clock or an atomic clock attached to a time server. NTP distributes this time across
the network. NTP is extremely efficient; no more than one packet per minute is necessary to synchronize two
machines to within a millisecond of each other.
NTP uses the concept of a "stratum" to describe how many NTP "hops" away a machine is from an authoritative
time source. A "stratum 1" time server typically has an authoritative time source (such as a radio or atomic
clock, or a GPS time source) directly attached, a "stratum 2" time server receives its time via NTP from a
"stratum 1" time server, and so on.
NTP avoids synchronizing to a machine whose time may not be accurate, in two ways. First, NTP never
synchronizes to a machine that is not synchronized itself. Second, NTP compares the time reported by several
machines and does not synchronize to a machine whose time is significantly different than the others, even
if its stratum is lower. This strategy effectively builds a self-organizing tree of NTP servers.
The Cisco implementation of NTP does not support stratum 1 service; in other words, it is not possible to
connect to a radio or atomic clock (for some specific platforms, however, you can connect a GPS time-source
device). We recommend that time service for your network be derived from the public NTP servers available
in the IP Internet.
If the network is isolated from the Internet, the Cisco implementation of NTP allows a machine to be configured
so that it acts as though it is synchronized via NTP, when in fact it has determined the time using other means.
Other machines can then synchronize to that machine via NTP.
Several manufacturers include NTP software for their host systems, and a publicly available version for
systems running UNIX and its various derivatives is also available. This software also allows UNIX-derivative
servers to acquire the time directly from an atomic clock, which would subsequently propagate time information
along to Cisco routers.
The communications between machines running NTP (known as associations) are usually statically configured;
each machine is given the IP address of all machines with which it should form associations. Accurate
timekeeping is made possible by exchanging NTP messages between each pair of machines with an association.
The Cisco implementation of NTP supports two ways that a networking device can obtain NTP time information
on a network:
• By polling host servers
• By listening to NTP broadcasts
In a LAN environment, NTP can be configured to use IP broadcast messages. As compared to polling, IP
broadcast messages reduce configuration complexity, because each machine can simply be configured to send
Cisco ASR 9000 Series Aggregation Services Router System Management Configuration Guide, Release 5.1.x
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Implementing NTP
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