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S5700 fail to synchronize with NTP server

Publication Date:  2013-12-13 Views:  132 Downloads:  0
Issue Description
Customer said that Windows 2003 PC works as a NTP server  ,but 5700  NTP cannot  synchronize with it ,but firewall can . 
Alarm Information
Handling Process
1. check configuration ,the server is alive with ping test from S5700:
[Quidway] ntp-service unicast-server

2. by checking NTP debug message ,It is normal.
Jan 4 2012 13:29:52.310.2+08:00 Quidway NTP/7/debug_NTP_packet_xmt:
packet to
leap: 3, version: 3, mode: 3
stratum: 0, ppoll: 128, precision: 2**18
rdel: 0.000, rdsp: 29.022, refid:
reftime: 00:00:00.000 UTC Jan 1 1900(00000000.00000000)
orgtime: 05:27:41.543 UTC Jan 4 2012(D2AE61CD.8B2B020C)
rectime: 05:27:42.339 UTC Jan 4 2012(D2AE61CE.56E631F8)
xmttime: 05:29:52.322 UTC Jan 4 2012(D2AE6250.52A5B530)
Jan 4 2012 13:29:52.330.1+08:00 Quidway NTP/7/debug_NTP_packet_rcv:
packet from to on Vlanif1
leap: 0, version: 3, mode: 4
stratum: 7, ppoll: 128, precision: 2**6
rdel: 31.250, rdsp: 16000.000, refid:
reftime: 15:29:59.012 UTC Jan 3 2012(D2AD9D77.033B645A)
orgtime: 05:29:52.322 UTC Jan 4 2012(D2AE6250.52A5B530)
rectime: 05:29:51.543 UTC Jan 4 2012(D2AE624F.8B2B020C)
xmttime: 05:29:51.543 UTC Jan 4 2012(D2AE624F.8B2B020C)
inptime: 00:00:00.000 UTC Jan 1 1900(5062AED2.B376DB55)
By checking rdsp (Dispersion between the local clock and the root reference clock ) in NTP packets ,its value is bigger than 16 seconds(this value is defined in RFC),causing that the Switch thinks the NTP server is not reliable ,So switch dropped the NTP packets  and NTP synchronize fail.

Root Cause
Probable  reason:
1. configuration problem
2. NTP messages send/receive abnormally
3. The NTP packets were dropped by some reason.
RFC rules: Server is Windows, We receive NTP packet from server with root dispersion as "rdsp: 16000.000, refid:"The root dispersion should be less than 16 seconds for a synchronized server as per RFC. RFC-1305 , page 26•
So  If  switch  find rsdp value in  NTP is  equal or greater than 16s, it will drop  it because   the NTP source is not reliable .