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NTP Overview

Network Time Protocol (NTP) is a widely used protocol used to synchronize the clocks of routers and other hardware devices on the Internet. Primary NTP servers are synchronized to a reference clock directly traceable to Coordinated Universal Time (UTC). Reference clocks include GPS receivers and telephone modem services, NTP accuracy expectations depend on the environment application requirements, however, NTP can generally maintain time to within tens of milliseconds over the public internet.

NTP is defined in the RFC 5905: Network Time Protocol Version 4: Protocol and Algorithms Specification

Junos devices can be configured to act as an NTP client, a secondary NTP server, or a primary NTP server. These variations are as follows:

• Primary NTP Server—Primary NTP servers are synchronized to a reference clock that is directly traceable to UTC. These servers then re-distribute this time data downstream to other Secondary NTP servers or NTP clients.
• Secondary NTP Server—Secondary NTP servers are synchronized to a primary or secondary NTP server. These servers then re-distribute this data downstream to other Secondary NTP servers or NTP clients.
• NTP Client—NTP clients are synchronized to a primary or secondary NTP server. Clients do not re-distribute this time data to other devices.

Note: The NTP subnet includes a number of widely accessible public primary time servers that can be used as a network’s primary NTP server. Juniper Networks strongly recommends that you authenticate any primary servers you use.

Each device on your network can be configured to run in one or more of the following NTP modes:

• Broadcast Mode—One or more devices is set up to transmit time information to a specified broadcast or multicast address. Other devices listen time sync packets on these addresses. This mode is less accurate than the client/server mode.
• Client/Server Mode—Devices are organized hierarchically across the network in client/server relationships.

  Note: QFX devices cannot be act as NTP servers, only clients.

• Symmetric Active (peer) Mode—Two or more devices are configured as NTP server peers to provide redundancy.

By default, if an NTP client time drifts so that the difference in time from the NTP server exceeds 128 milliseconds, the NTP client is automatically stepped back into synchronization. The NTP client will still synchronize with the server even if the offset between the NTP client and server exceeds the 1000-second threshold. You can manually request that a device synchronize with an NTP server by using the set date ntp operational command on the router. On devices running Junos OS that have dual Routing Engines, the backup Routing Engine synchronizes directly with the master Routing Engine.

For more details on the Network Time Protocol, go to the Network Time Foundation website at http://www.ntp.org.

Note: NTP is required for Common Criteria compliance. For more information on the Common Criteria certification, see Public Sector Certifications.

In Junos operating system (Junos OS) Release 11.2 or later, NTP supports IPv4 VPN routing and forwarding (VRF) requests. This enables an NTP server running on a provider edge (PE) router to respond to NTP requests from a customer edge (CE) router. As a result, a PE router can process any NTP request packet coming from different routing instances. In Junos OS Release 11.4 and later, NTP also supports IPv6 VRF requests.