If you're running NTP and SNTP on the same router, SNTP will never synchronize with a configured SNTP server, as the NTP process captures the reply packet before SNTP has the chance to react.

SNTP multicast/broadcast client mode works in combination with NTP

NTP process could be running even if your running configuration has no NTP-related commands. It starts automatically whenever you enter NTP-related configuration (ntp logging configuration command is enough) and is not stopped when the last NTP-related configuration command is removed. You have to reload the router to kill it.

On my test router, I've switched from NTP to SNTP by removing all NTP configuration and configuring SNTP server:

b2#show run | inc ntp
sntp logging
sntp server 192.168.0.5

However, SNTP would not synchronize as the NTP process was still running (check with the show processes | inc NTP command):

b2#show process | inc NTP
 259 Mwe 41E568BC   8  3233    2 3624/6000   0 NTP
 260 Mwe 4228BA9C   8    86   93 3660/6000   0 SNTP

NTP process was obviously receiving the NTP reply packets, as the debug sntp packet command did not display the return packets, even though the debug ntp packet peer command on the NTP server (192.168.0.5) displayed received and transmitted packets:

Sending SNTP packet to 192.168.0.5
 xmt CB66B3C0.8EC48A8E (14:12:16.557 UTC Wed Feb 20 2008)

After the reload, SNTP started working immediately and generated a syslog message indicating the time has been adjusted:

00:00:45: %SYS-6-CLOCKUPDATE: System clock has been updated from 15:00:59 UTC Wed Feb 20 2008 to 14:15:10 UTC Wed Feb 20 2008, configured from SNTP by 192.168.0.5.

The debugging printout also confirmed that the NTP reply packets were received by SNTP.

Sending SNTP packet to 192.168.0.5
 xmt CB66B4B8.D94D8B90 (14:16:24.848 UTC Wed Feb 20 2008)
Received SNTP packet from 192.168.0.5, length 48
 leap 0, mode 4, version 1, stratum 8, ppoll 16

 rtdel 00000C87, rtdsp 0000E5F1, refid 0A000005 (10.0.0.5)
 ref CB66B33F.2E02462D (14:10:07.179 UTC Wed Feb 20 2008)
 org CB66B4B8.D94D8B90 (14:16:24.848 UTC Wed Feb 20 2008)
 rec CB66B4B9.4C2B68CA (14:16:25.297 UTC Wed Feb 20 2008)
 xmt CB66B4B9.4C6FF670 (14:16:25.298 UTC Wed Feb 20 2008)
 inp CB66B4B8.DA2137A9 (14:16:24.852 UTC Wed Feb 20 2008)