The Network Time Protocol (NTP) is a protocol designed to time-synchronize a network of machines. NTP runs over UDP, which in turn runs over IPv4. NTP Version 4 (NTPv4) is an extension of NTP version 3. NTPv4 supports both IPv4 and IPv6 and is backward-compatible with NTPv3.

NTPv4 provides the following capabilities:

• NTPv4 supports IPv6, making NTP time synchronization possible over IPv6.
• Security is improved over NTPv3. The NTPv4 protocol provides a whole security framework based on public key cryptography and standard X509 certificates.
• Using specific multicast groups, NTPv4 can automatically calculate its time-distribution hierarchy through an entire network. NTPv4 automatically configures the hierarchy of the servers in order to achieve the best time accuracy for the lowest bandwidth cost. This feature leverages site-local IPv6 multicast addresses.

NTPv4 works in much the same way as does NTP. An NTP network usually gets its time from an authoritative time source, such as a radio clock or an atomic clock attached to a time server. NTP then distributes this time across the network. NTP is extremely efficient; no more than one packet per minute is necessary to synchronize two machines to the accuracy of within a millisecond of one another.

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 in turn synchronized itself. Second, NTP compares the time reported by several machines, and will 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).

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.

A number of 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 IPv4 or IPv6 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.

• IPv6 Multicast Mode
• NTP Access Groups versus Symmetric Key Authentication
• DNS Support for IPv6 in NTPv4

Networking devices running NTPv4 can be configured to operate in variety of association modes when synchronizing time with reference time sources. There are two ways that a networking device can obtain time information on a network: by polling host servers and by listening to NTPv4 broadcasts.

The following are two most commonly used poll-based association modes:

• Client mode
• Symmetric active mode

When a networking device is operating in the client mode, it polls its assigned time serving hosts for the current time. The networking device will then pick a host from all the polled time servers to synchronize with. Because the relationship that is established in this case is a client-host relationship, the host will not capture or use any time information sent by the local client device. This mode is most suited for file-server and workstation clients that are not required to provide any form of time synchronization to other local clients. Use the ntp server command to individually specify the time serving hosts that you want your networking device to consider synchronizing with and to set your networking device to operate in the client mode.

When a networking device is operating in the symmetric active mode, it polls its assigned time serving hosts for the current time and it responds to polls by its hosts. Because this is a peer-to-peer relationship, the host will also retain time-related information about the local networking device that it is communicating with. This mode should be used when there are several mutually redundant servers that are interconnected using diverse network paths. Most Stratum 1 and stratum 2 servers on the Internet today adopt this form of network setup. Use the ntp peer command to specify individually the time serving hosts that you want your networking device to consider synchronizing with and to set your networking device to operate in the symmetric active mode.

The specific mode that you should set each of your networking devices to depends primarily on the role that you want it to assume as a timekeeping device (server or client) and its proximity to a stratum 1 timekeeping server.

• Configuring Symmetric Active Mode
• Configuring Client Mode

• Configuring an Interface to Send NTPv4 Multicast Packets
• Configuring an Interface to Receive NTPv4 Multicast Packets

1.    enable

2.    configure terminal

3.    ntp access-group {query-only | serve-only | serve | peer} {access-list-number | access-list-name} [kod

1.    enable

2.    configure terminal

3.    ntp authenticate

4.    ntp authentication-key number md5 value

5.    ntp trusted-key key-number

1.    enable

2.    configure terminal

3.    ntp disabl e [ipv4 | ipv6

1.    enable

2.    configure terminal

3.    ntp source type number

1.    enable

2.    configure terminal

3.    ntp master [stratum]

Note

Use the ntp mastercommand with caution. It is very easy to override valid time sources using this command, especially if a low stratum number is configured. Configuring multiple machines in the same network with the ntp master command can cause instability in timekeeping if the machines do not agree on the time.

1.    enable

2.    configure terminal

3.    ntp update-calendar

1.    enable

2.    ntp drift clear

1.    enable

2.    show clock [detail]

3.    show ntp associations [detail

4.    s how ntp status

5.    debug ntp {adjust | authentication | events | loopfilter | packets | params | refclock | select | sync | validity}