Configuring NTP

This chapter contains the following sections:

Information About NTP

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.

NTP as a Time Server

the Cisco NX-OS device can use NTP to distribute time. 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.

Distributing NTP Using CFS

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.

Clock Manager

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

The clock manager allows you to specify the protocol to control the various clocks in the system. Once you specify the protocol, the system clock starts updating.

Virtualization Support

NTP recognizes virtual routing and forwarding (VRF) instances. NTP uses the default VRF if you do not configure a specific VRF for the NTP server and NTP peer.

Guidelines and Limitations for NTP

NTP has the following configuration guidelines and limitations:

  • To configure NTP, you must have connectivity to at least one server that is running NTP.

  • NTP operates when the clock protocol is set to NTP. Configuring PTP and NTP together is not 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, then 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.

Default Settings

Table 1. Default NTP Parameters

Parameters

Default

NTP authentication

disabled

NTP access

enabled

NTP logging

disabled

Configuring NTP

Configuring NTP Server and Peer

You can configure an NTP server and peer.

Before you begin

Make sure you know the IP address or DNS names of your NTP server and its peers.

If you plan to use CFS to distribute your NTP configuration to other devices, then you should have already completed the following:

  • Enabled CFS distribution.

  • Enabled CFS for NTP.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# [no] ntp server {ip-address | ipv6-address | dns-name} [key key-id] [maxpoll max-poll] [minpoll min-poll] [prefer] [use-vrf vrf-name]
  3. switch(config)# [no] ntp peer {ip-address | ipv6-address | dns-name} [key key-id] [maxpoll max-poll] [minpoll min-poll] [prefer] [use-vrf vrf-name]
  4. (Optional) switch(config)# show ntp peers
  5. (Optional) switch(config)# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# [no] ntp server {ip-address | ipv6-address | dns-name} [key key-id] [maxpoll max-poll] [minpoll min-poll] [prefer] [use-vrf vrf-name]

Forms an association with a server.

Use the key keyword to configure a key to be used while communicating with the NTP server. The range for the key-id argument is from 1 to 65535.

Use the maxpoll and minpoll keywords to configure the maximum and minimum intervals in which to poll a server. The range for the max-poll and min-poll arguments is from 4 to 16 (configured as powers of 2, so effectively 16 to 65536 seconds), and the default values are 6 and 4, respectively (maxpoll default = 64 seconds, minpoll default = 16 seconds).

Use the prefer keyword to make this the preferred NTP server for the device.

Use the use-vrf keyword to configure the NTP server to communicate over the specified VRF. The vrf-name argument can be default, management, or any case-sensitive alphanumeric string up to 32 characters.

Note

 

If you configure a key to be used while communicating with the NTP server, make sure that the key exists as a trusted key on the device.

Step 3

switch(config)# [no] ntp peer {ip-address | ipv6-address | dns-name} [key key-id] [maxpoll max-poll] [minpoll min-poll] [prefer] [use-vrf vrf-name]

Forms an association with a peer. You can specify multiple peer associations.

Use the key keyword to configure a key to be used while communicating with the NTP peer. The range for the key-id argument is from 1 to 65535.

Use the maxpoll and minpoll keywords to configure the maximum and minimum intervals in which to poll a server. The range for the max-poll and min-poll arguments is from 4 to 17 (configured as powers of 2, so effectively 16 to 131072 seconds), and the default values are 6 and 4, respectively (maxpoll default = 64 seconds, minpoll default = 16 seconds).

Use the prefer keyword to make this the preferred NTP server for the device.

Use the use-vrf keyword to configure the NTP server to communicate over the specified VRF. The vrf-name argument can be default, management, or any case-sensitive alphanumeric string up to 32 characters.

Step 4

(Optional) switch(config)# show ntp peers

(Optional)

Displays the configured server and peers.

Note

 

A domain name is resolved only when you have a DNS server configured.

Step 5

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to configure an NTP server and peer:

switch# config t
Enter configuration commands, one per line. End with CNTL/Z.
switch(config)# ntp server 192.0.2.10 key 10 use-vrf Red
switch(config)# ntp peer 2001:0db8::4101 prefer use-vrf Red
switch(config)# show ntp peers
--------------------------------------------------
Peer IP Address Serv/Peer
--------------------------------------------------
2001:0db8::4101 Peer (configured)
192.0.2.10 Server (configured)
switch(config)# copy running-config startup-config
[########################################] 100%
switch(config)#

Configuring NTP Authentication

You can configure the device to authenticate the time sources to which the local clock is synchronized. When you enable NTP authentication, the device synchronizes to a time source only if the source carries one of the authentication keys specified by the ntp trusted-key command. The device drops any packets that fail the authentication check and prevents them from updating the local clock. NTP authentication is disabled by default.

Before you begin

Make sure that you configured the NTP server with the authentication keys that you plan to specify in this procedure.

SUMMARY STEPS

  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

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

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

Defines the authentication keys. The device does not synchronize to a time source unless the source has one of these authentication keys and the key number is specified by the ntp trusted-key number command.

Step 3

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

(Optional)

Displays the configured NTP authentication keys.

Step 4

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

Specifies one or more keys that a time source must provide in its NTP packets in order for the device to synchronize to it. The range for trusted keys is from 1 to 65535.

This command provides protection against accidentally synchronizing the device to a time source that is not trusted.

Step 5

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

(Optional)

Displays the configured NTP trusted keys.

Step 6

switch(config)# [no] ntp authenticate

Enables or disables the NTP authentication feature. NTP authentication is disabled by default.

Step 7

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

(Optional)

Displays the status of NTP authentication.

Step 8

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to configure the device to synchronize only to time sources that provide authentication key 42 in their NTP packets: 

switch# config t
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)#

Configuring NTP Access Restrictions

You can control access to NTP services by using access groups. Specifically, you can specify the types of requests that the device allows and the servers from which it accepts responses.

If you do not configure any access groups, NTP access is granted to all devices. If you configure any access groups, NTP access is granted only to the remote device whose source IP address passes the access list criteria.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# [no] ntp access-group {peer | serve | serve-only | query-only} access-list-name
  3. (Optional) switch(config)# show ntp access-groups
  4. (Optional) switch(config)# copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

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

Creates or removes an access group to control NTP access and applies a basic IP access list.

The access group options are scanned in the following order, from least restrictive to most restrictive. However, if NTP matches a deny ACL rule in a configured peer, ACL processing stops and does not continue to the next access group option.

  • The peer keyword enables the device to receive time requests and NTP control queries and to synchronize itself to the servers specified in the access list.

  • The serve keyword enables the device to receive time requests and NTP control queries from the servers specified in the access list but not to synchronize itself to the specified servers.

  • The serve-only keyword enables the device to receive only time requests from servers specified in the access list.

  • The query-only keyword enables the device to receive only NTP control queries from the servers specified in the access list.

Step 3

(Optional) switch(config)# show ntp access-groups

(Optional)

Displays the NTP access group configuration.

Step 4

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to configure the device to allow it to synchronize to a peer from access group “accesslist1”:

switch# config t
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)#

Configuring the NTP Source IP Address

NTP sets the source IP address for all NTP packets based on the address of the interface through which the NTP packets are sent. You can configure NTP to use a specific source IP address.

To configure the NTP source IP address, use the following command in global configuration mode:

SUMMARY STEPS

  1. switch(config)# [no] ntp source ip-address

DETAILED STEPS

Command or Action Purpose

switch(config)# [no] ntp source ip-address

Configures the source IP address for all NTP packets. The ip-address can be in IPv4 or IPv6 format.

Example

This example shows how to configure NTP to a source IP address:

switch(config)# ntp source 192.0.2.1

Configuring the NTP Source Interface

You can configure NTP to use a specific interface.

To configure the NTP source interface, use the following command in global configuration mode:

SUMMARY STEPS

  1. switch(config)# [no] ntp source-interface interface

DETAILED STEPS

Command or Action Purpose

switch(config)# [no] ntp source-interface interface

Configures the source interface for all NTP packets. Use the ? keyword to display a list of supported interfaces.

Example

This example shows how to configure NTP to a specific interface:

switch(config)# ntp source-interface
ethernet 2/1

Configuring NTP Logging

You can configure NTP logging in order to generate system logs with significant NTP events. NTP logging is disabled by default.

SUMMARY STEPS

  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

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# [no] ntp logging

Enables or disables system logs to be generated with significant NTP events. NTP logging is disabled by default.

Step 3

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

(Optional)

Displays the NTP logging configuration status.

Step 4

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to enable NTP logging in order to generate system logs with significant NTP events:

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

Enabling CFS Distribution for NTP

You can enable CFS distribution for NTP in order to distribute the NTP configuration to other CFS-enabled devices.

Before you begin

Make sure that you have enabled CFS distribution for the device.

SUMMARY STEPS

  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

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# [no] ntp distribute

Enables or disables the device to receive NTP configuration updates that are distributed through CFS.

Step 3

(Optional) switch(config)# show ntp status

(Optional)

Displays the NTP CFS distribution status.

Step 4

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

(Optional)

Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Example

This example shows how to enable CFS distribution for NTP:

switch# config t
Enter configuration commands, one per
line. End with CNTL/Z.
switch(config)# ntp distribute
switch(config)# copy running-config
startup-config

Commiting NTP Configuration Changes

When you commit the NTP configuration changes, the effective database is overwritten by the configuration changes in the pending database and all the devices in the network receive the same configuration.

SUMMARY STEPS

  1. switch# configure terminal
  2. switch(config)# ntp commit

DETAILED STEPS

  Command or Action Purpose

Step 1

switch# configure terminal

Enters global configuration mode.

Step 2

switch(config)# ntp commit

Distributes the NTP configuration changes to all Cisco NX-OS devices in the network and releases the CFS lock. This command overwrites the effective database with the changes made to the pending database.

Example

This example shows how to commit the NTP configuration changes:

switch(config)# ntp commit

Discarding NTP Configuration Changes

After making the configuration changes, you can choose to discard the changes instead of committing them. If you discard the changes, Cisco NX-OS removes the pending database changes and releases the CFS lock.

To discard NTP configuration changes, use the following command in global configuration mode:

SUMMARY STEPS

  1. switch(config)# ntp abort

DETAILED STEPS

Command or Action Purpose

switch(config)# ntp abort

Discards the NTP configuration changes in the pending database and releases the CFS lock. Use this command on the device where you started the NTP configuration.

Example

This example shows how to discard the NTP configuration changes:

switch(config)# ntp abort

Releasing the CFS Session Lock

If you have performed an NTP configuration and have forgotten to release the lock by either committing or discarding the changes, you or another administrator can release the lock from any device in the network. This action also discards pending database changes.

To release the session lock from any device and discard any pending database changes, use the following command in global configuration mode:

SUMMARY STEPS

  1. switch(config)# clear ntp session

DETAILED STEPS

Command or Action Purpose

switch(config)# clear ntp session

Discards the NTP configuration changes in the pending database and releases the CFS lock.

Example

This example shows how to release the CFS session lock:

switch(config)# clear ntp session

Verifying the NTP Configuration

To display the NTP configuration, perform one of the following tasks:

Use the clear ntp session command to clear the NTP sessions.

Use the clear ntp statistics command to clear the NTP statistics.

SUMMARY STEPS

  1. show ntp access-groups
  2. show ntp authentication-keys
  3. show ntp authentication-status
  4. show ntp logging-status
  5. show ntp peer-status
  6. show ntp peers
  7. show ntp pending
  8. show ntp pending-diff
  9. show ntp rts-update
  10. show ntp session status
  11. show ntp source
  12. show ntp source-interface
  13. show ntp statistics {io | local | memory | peer {ipaddr {ipv4-addr | ipv6-addr} | name peer-name}}
  14. show ntp status
  15. show ntp trusted-keys
  16. show running-config ntp

DETAILED STEPS

  Command or Action Purpose

Step 1

show ntp access-groups

Displays the NTP access group configuration.

Step 2

show ntp authentication-keys

Displays the configured NTP authentication keys.

Step 3

show ntp authentication-status

Displays the status of NTP authentication.

Step 4

show ntp logging-status

Displays the NTP logging status.

Step 5

show ntp peer-status

Displays the status for all NTP servers and peers.

Step 6

show ntp peers

Displays all the NTP peers.

Step 7

show ntp pending

Displays the temporary CFS database for NTP.

Step 8

show ntp pending-diff

Displays the difference between the pending CFS database and the current NTP configuration.

Step 9

show ntp rts-update

Displays the RTS update status.

Step 10

show ntp session status

Displays the NTP CFS distribution session information.

Step 11

show ntp source

Displays the configured NTP source IP address.

Step 12

show ntp source-interface

Displays the configured NTP source interface.

Step 13

show ntp statistics {io | local | memory | peer {ipaddr {ipv4-addr | ipv6-addr} | name peer-name}}

Displays the NTP statistics.

Step 14

show ntp status

Displays the NTP CFS distribution status.

Step 15

show ntp trusted-keys

Displays the configured NTP trusted keys.

Step 16

show running-config ntp

Displays NTP information.

Configuration Examples for NTP

This example shows how to configure an NTP server and peer, enable NTP authentication, enable NTP logging, and then save the configuration in startup so that it is saved across reboots and restarts: 

switch# config terminal
Enter configuration commands, one per line. End with CNTL/Z.
switch(config)# ntp server 192.0.2.105 key 42
switch(config)# ntp peer 2001:0db8::4101
switch(config)# show ntp peers
--------------------------------------------------
     Peer IP Address            Serv/Peer
--------------------------------------------------
     2001:db8::4101         Peer (configured)
     192.0.2.105           Server (configured)
switch(config)# ntp authentication-key 42 md5 aNiceKey
switch(config)# show ntp authentication-keys
-----------------------------
  Auth key        MD5 String
-----------------------------
     42           aNicekey
switch(config)# ntp trusted-key 42
switch(config)# show ntp trusted-keys
Trusted Keys:
42
switch(config)# ntp authenticate
switch(config)# show ntp authentication-status
Authentication enabled.
switch(config)# ntp logging
switch(config)# show ntp logging
NTP logging enabled.
switch(config)# copy running-config startup-config
[########################################] 100%
switch(config)#

This example shows an NTP access group configuration with the following restrictions:

  • Peer restrictions are applied to IP addresses that pass the criteria of the access list named “peer-acl.”

  • Serve restrictions are applied to IP addresses that pass the criteria of the access list named “serve-acl.”

  • Serve-only restrictions are applied to IP addresses that pass the criteria of the access list named “serve-only-acl.”

  • Query-only restrictions are applied to IP addresses that pass the criteria of the access list named “query-only-acl.”

switch# config terminal
switch(config)# ntp peer 10.1.1.1
switch(config)# ntp peer 10.2.2.2
switch(config)# ntp peer 10.3.3.3
switch(config)# ntp peer 10.4.4.4
switch(config)# ntp peer 10.5.5.5
switch(config)# ntp peer 10.6.6.6
switch(config)# ntp peer 10.7.7.7
switch(config)# ntp peer 10.8.8.8
switch(config)# ntp access-group peer peer-acl
switch(config)# ntp access-group serve serve-acl
switch(config)# ntp access-group serve-only serve-only-acl
switch(config)# ntp access-group query-only query-only-acl

switch(config)# ip access-list peer-acl
switch(config-acl)# 10 permit ip host 10.1.1.1 any
switch(config-acl)# 20 permit ip host 10.8.8.8 any

switch(config)# ip access-list serve-acl
switch(config-acl)# 10 permit ip host 10.4.4.4 any
switch(config-acl)# 20 permit ip host 10.5.5.5 any

switch(config)# ip access-list serve-only-acl
switch(config-acl)# 10 permit ip host 10.6.6.6 any
switch(config-acl)# 20 permit ip host 10.7.7.7 any

switch(config)# ip access-list query-only-acl
switch(config-acl)# 10 permit ip host 10.2.2.2 any
switch(config-acl)# 20 permit ip host 10.3.3.3 any

Related Documents for NTP

Related Topic

Document Title

NTP CLI commands

 Cisco Nexus 3548 Switch NX-OS System Management Command Reference Guide

Feature History for NTP

This table includes only the updates for those releases that have resulted in additions or changes to the feature.

Feature Name Releases Feature Information

NTP

5.0(3)A1(1)

This feature was introduced.