How to configure NTP and SNTP on the Cisco AP

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Jun 22, 2009 4:42 PM
Jun 22nd, 2009


Introduction

How to configure NTP and SNTP on the Cisco AP

Resolution

A 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 devices to within a millisecond of one another.

Before trying to configure NTP check for the ping test with NTP server.If NTP doesn't work upgrade the IOS to 12.3 (4)JA and use SNTP.

In Cisco IOS Release 12.3(4)JA, access points and bridges support SNTP instead of NTP. This change improves the reliability of the system time on access points and bridges, allows access points and bridges to synchronize with any NTP server, and prevents client devices from synchronizing to an access point or bridge clock that might not be accurate.

Note:Starting Cisco IOS  Software Release 12.3(4)JA, APs and bridges support SNTP instead of NTP. For more information, refer to the SNTP Replaces NTP section of Release Notes for Cisco Aironet Access Points, Cisco IOS Release 12.3(4)JA.

Understanding Simple Network Time Protocol

Simple Network Time Protocol (SNTP) is a simplified, client-only version of NTP. SNTP can only receive the time from NTP servers; it cannot be used to provide time services to other systems. SNTP typically provides time within 100 milliseconds of the accurate time, but it does not provide the complex filtering and statistical mechanisms of NTP.

You can configure SNTP to request and accept packets from configured servers or to accept NTP broadcast packets from any source. When multiple sources are sending NTP packets, the server with the best stratum is selected. Click this URL for more information on NTP and strata: http://www.cisco.com/en/US/products/hw/switches/ps708/products_installation_and_configuration_guides_list.html

If multiple servers are at the same stratum, a configured server is preferred over a broadcast server. If multiple servers pass both tests, the first one to send a time packet is selected. SNTP will only choose a new server if it stops receiving packets from the currently selected server, or if a better server (according to the above criteria) is discovered.

Configuring SNTP

SNTP is disabled by default. To enable SNTP on the access point, use one or both of these commands in global configuration mode:

Table 5-4 SNTP Commands

Command

Purpose

sntp server {address | hostname}
[version number]

Configures SNTP to request NTP packets from an NTP server.

sntp broadcast client

Configures SNTP to accept NTP packets from any NTP broadcast server.

Enter the sntp server command once for each NTP server. The NTP servers must be configured to respond to the SNTP messages from the access point.

If you enter both the sntp server command and the sntp broadcast client command, the access point will accept time from a broadcast server but prefers time from a configured server, assuming the strata are equal. To display information about SNTP, use the show sntp EXEC command.

To configure the AP with the Command Line Interface (CLI) for Network Time Protocol (NTP), refer to the Understanding Simple Network Time Protocol section of Administering the Access Point.

Simple Network Time Protocol (SNTP) is a simplified, client-only version of NTP. SNTP can only receive the time from NTP servers; it cannot be used to provide time services to other systems. SNTP typically provides time within 100 milliseconds of the accurate time, but it does not provide the complex filtering and statistical mechanisms of NTP.

Note:An access point running 12.3(4)JA, when configured with "sntp broadcast client", will ignore NTP broadcasts from a v4 server.  If that server is reconfigured to report itself as v3, the SNTP client will synchronize with no difficulty. So you need to do the following.

1. Configure the sntp client as a unicast client (sntp server )

2. Reconfigure the NTP server as v3 rather than v4

Reference

For Simple Network Time Protocol (SNTP), refer to theConfiguring SNTP section of Administering the Access Point.

To configure the AP with VxWorks for SNTP, refer to the Time Server Setup Page section of the Cisco Aironet 1200 Series Access Point Software Configuration Guide for VxWorks.


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