Ntp-Ptp Interworking - Cisco NCS 5000 Series Manual

NTP-PTP Interworking

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 three ways that a networking device can obtain NTP time
information on a network:
• By polling host servers
• By listening to NTP broadcasts
• By listening to NTP multicasts
• By polling host servers
• By listening to NTP broadcasts
In a LAN environment, NTP can be configured to use IP broadcast or multicast messages. As compared to
polling, IP broadcast or multicast messages reduce configuration complexity, because each machine can
simply be configured to send or receive broadcast or multicast messages. However, the accuracy of timekeeping
is marginally reduced because the information flow is one-way only.
An NTP broadcast client listens for broadcast messages sent by an NTP broadcast server at a designated IPv4
address. The client synchronizes the local clock using the first received broadcast message.
An NTP multicast server periodically sends a message to a designated IPv4 or IPv6 local multicast group
address. An NTP multicast client listens on this address for NTP messages.
The time kept on a machine is a critical resource, so we strongly recommend that you use the security features
of NTP to avoid the accidental or malicious setting of incorrect time. Two mechanisms are available: an access
list-based restriction scheme and an encrypted authentication mechanism.
When multiple sources of time (VINES, hardware clock, manual configuration) are available, NTP is always
considered to be more authoritative. NTP time overrides the time set by any other method.
NTP-PTP Interworking
NTP-PTP interworking provides the ability to use PTP, as well as other valid time of day (TOD) sources such
as Data over Cable Service Interface Specification (DOCSIS) Timing Interface (DTI) and global positioning
System Management Configuration Guide for Cisco NCS 5000 Series Routers, IOS XR Release 6.2.x
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Implementing NTP