SNTP Configuration

Articles » Network Time Protocol (NTP) In a Nutshell » SNTP Configuration

The SNTP configuration in Cisco IOS is (as one would expect) much simpler than the NTP configuration:

  • You can configure the SNTP-related logging with the sntp logging configuration command;
  • Upstream NTP server is configured with the sntp server ip-address configuration command. You can configure multiple servers for redundancy purposes.
You cannot configure the SNTP process running on a Cisco IOS router to update the internal clock, as you’re supposed to use SNTP solely on the low-end models with no battery backed-up clock.

The SNTP process will not synchronize to the configured SNTP servers if you’ve previously entered any NTP-related configuration commands on the router (ntp logging is enough), as the NTP process in Cisco IOS receives replies that should be received by the SNTP process (remember: NTP and SNTP use the same UDP port number). The only way to fix this problem is to reload the router.

A router using SNTP synchronization cannot be used to provide NTP services to downstream clients, as these services would require the NTP process to run, thus blocking the SNTP synchronization.

Configuration Example

We’ll configure SNTP on the WAN edge router (S1) used on a non-redundant small site small-site:

Simple remote site using SNTP synchronization

Simple remote site using SNTP synchronization

The configuration commands are as simple as they can get:

SNTP configuration
sntp logging
sntp server C1
sntp server C2
sntp server NTP-Server

The SNTP process should quickly acquire the correct time from the NTP servers and generate syslog messages as it synchronizes with the servers (see the following listing). Multiple synchronizations might occur if the SNTP process reaches a high-stratum NTP server before a low-stratum one.

SNTP synchronizations on the Site router
00:00:56: %SYS-6-CLOCKUPDATE: System clock has been updated from 14:09:51 UTC Mon Feb 25 2008 to 13:29:59 UTC Mon Feb 25 2008, configured from SNTP by 10.0.0.5.
00:01:59: %SYS-6-CLOCKUPDATE: System clock has been updated from 13:31:02 UTC Mon Feb 25 2008 to 13:31:02 UTC Mon Feb 25 2008, configured from SNTP by 10.0.0.10

The show sntp command can be used to display the current synchronization status:

SNTP status on the Site router
Site#show sntp
SNTP server     Stratum   Version    Last Receive
10.0.0.5           7         1        00:00:56
10.0.0.10          6         1        00:01:03    Synced
10.0.0.6           7         1        00:00:15

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