The Network Time Protocol (NTP) synchronizes the time of day among a set of distributed time servers and clients so that you can correlate events when you receive system logs and other time-specific events from multiple network devices. NTP uses the User Datagram Protocol (UDP) as its transport protocol.

All NTP communications use Coordinated Universal Time (UTC).

An NTP server usually receives its time from an authoritative time source, such as a radio clock or an atomic clock attached to a time server, and 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 within a millisecond of each other.

NTP uses a stratum to describe the distance between a network device and an authoritative time source:

• A stratum 1 time server is directly attached to an authoritative time source (such as a radio or atomic clock or a GPS time source).

• A stratum 2 NTP server receives its time through NTP from a stratum 1 time server.

Before synchronizing, NTP compares the time reported by several network devices and does not synchronize with one that is significantly different, even if it is a stratum 1. Because Cisco NX-OS cannot connect to a radio or atomic clock and act as a stratum 1 server, we recommend that you use the public NTP servers available on the Internet. If the network is isolated from the Internet, Cisco NX-OS allows you to configure the time as though it were synchronized through NTP, even though it was not.

Note	You can create NTP peer relationships to designate the time-serving hosts that you want your network device to consider synchronizing with and to keep accurate time if a server failure occurs.

The time kept on a device 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.

Other devices can configure it as a time server. You can also configure the device to act as an authoritative NTP server, enabling it to distribute time even when it is not synchronized to an outside time source.

Cisco Fabric Services (CFS) distributes the local NTP configuration to all Cisco devices in the network.

After enabling CFS on your device, a network-wide lock is applied to NTP whenever an NTP configuration is started. After making the NTP configuration changes, you can discard or commit them.

In either case, the CFS lock is then released from the NTP application.

Clocks are resources that need to be shared across different processes.

Multiple time synchronization protocols, such as NTP and Precision Time Protocol (PTP), might be running in the system.

Stateless restarts are supported for NTP. After a reboot or a supervisor switchover, the running configuration is applied.

You can configure NTP peers to provide redundancy in case an NTP server fails.

NTP has the following configuration guidelines and limitations:

• NTP server functionality is supported.

• You should have a peer association with another device only when you are sure that your clock is reliable (which means that you are a client of a reliable NTP server).

• A peer configured alone takes on the role of a server and should be used as a backup. If you have two servers, you can configure several devices to point to one server and the remaining devices to point to the other server. You can then configure a peer association between these two servers to create a more reliable NTP configuration.

• If you have only one server, you should configure all the devices as clients to that server.

• You can configure up to 64 NTP entities (servers and peers).

• If CFS is disabled for NTP, NTP does not distribute any configuration and does not accept a distribution from other devices in the network.

• After CFS distribution is enabled for NTP, the entry of an NTP configuration command locks the network for NTP configuration until a commit command is entered. During the lock, no changes can be made to the NTP configuration by any other device in the network except the device that initiated the lock.

• If you use CFS to distribute NTP, all devices in the network should have the same VRFs configured as you use for NTP.

• If you configure NTP in a VRF, ensure that the NTP server and peers can reach each other through the configured VRFs.

• You must manually distribute NTP authentication keys on the NTP server and Cisco NX-OS devices across the network.

• Use NTP broadcast or multicast associations when time accuracy and reliability requirements are modest, your network is localized, and the network has more than 20 clients. We recommend that you use NTP broadcast or multicast associations in networks that have limited bandwidth, system memory, or CPU resources.

Note	Time accuracy is marginally reduced in NTP broadcast associations because information flows only one way.

1.    switch# configure terminal

2.    switch(config)# [no] feature ntp

3.    (Optional) switch(config)# show ntp status

4.    (Optional) switch(config)# copy running-config startup-config

switch# configure terminal
switch(config)# no feature ntp

1.    switch# configure terminal

2.    [no] ntp master [stratum]


3.    (Optional) show running-config ntp


4.    (Optional) switch(config)# copy running-config startup-config

switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
switch(config)# ntp master 5

1.    switch# configure terminal

2.    (Optional) switch(config)# show ntp peers

3.    (Optional) switch(config)# copy running-config startup-config

1.    switch# configure terminal

2.    switch(config)# [no] ntp authentication-key number md5 md5-string

3.    (Optional) switch(config)# show ntp authentication-keys

4.    switch(config)# [no] ntp trusted-key number

5.    (Optional) switch(config)# show ntp trusted-keys

6.    switch(config)# [no] ntp authenticate

7.    (Optional) switch(config)# show ntp authentication-status

8.    (Optional) switch(config)# copy running-config startup-config

switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
switch(config)# ntp authentication-key 42 md5 aNiceKey
switch(config)# ntp trusted-key 42
switch(config)# ntp authenticate
switch(config)# copy running-config startup-config
[########################################] 100%
switch(config)#

1.    switch# configure terminal

2.    switch(config)# [no] ntp access-group {peer | serve | serve-only | query-only} access-list-name

3.    switch(config)# show ntp access-groups

4.    (Optional) switch(config)# copy running-config startup-config

switch# configure terminal
switch(config)# ntp access-group peer accesslist1
switch(config)# show ntp access-groups
Access List Type
-----------------------------
accesslist1 Peer
switch(config)# copy running-config startup-config
[########################################] 100%
switch(config)#

1.    switch# configure terminal

2.    [no] ntp source ip-address

switch# configure terminal
switch(config)# ntp source 192.0.2.2

1.    switch# configure terminal

2.    [no] ntp source-interface interface

switch# configure terminal
switch(config)# ntp source-interface ethernet

1.    switch# configure terminal

2.    switch(config)# [no] ntp logging

3.    (Optional) switch(config)# show ntp logging-status

4.    (Optional) switch(config)# copy running-config startup-config

switch# configure terminal
switch(config)# ntp logging
switch(config)# copy running-config startup-config
[########################################] 100%
switch(config)#

1.    switch# configure terminal

2.    switch(config)# [no] ntp distribute

3.    (Optional) switch(config)# show ntp status

4.    (Optional) switch(config)# copy running-config startup-config

switch# configure terminal
switch(config)# ntp distribute
switch(config)# copy running-config startup-config

1.    switch# configure terminal

2.    switch(config)# ntp commit

1.    switch# configure terminal

2.    switch(config)# ntp abort

1.    switch# configure terminal

2.    switch(config)# clear ntp session