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FAQ-When NTP server down ,Why the Ntp client is still in synchronized status?

Publication Date:  2013-11-15 Views:  134 Downloads:  0
Issue Description
The configuration is : ntp-service unicast-server 10.1.104.236
the ntp server is not available,but the NTP server  is sitll in synchronized status.

<MEX_RTR_A>ping 10.1.104.236
  PING 10.1.104.236: 56 data bytes, press CTRL_C to break
    Request time out
    Request time out

<MEX_RTR_A>dis ntp sever verbose               
clock source: 10.1.104.236
clock stratum: 1
clock status: configured, master, sane, valid
reference clock ID: 71.80.83.0
local mode: client, local poll: 10
peer mode: server, peer poll: 10
   ...

<MEX_RTR_A>dis ntp status             
clock status: synchronized
clock stratum: 2
reference clock ID: 10.1.104.236
nominal frequency: 100.0000 Hz
actual frequency: 100.0000 Hz
clock precision: 2^18
...
Alarm Information
NONE
Handling Process
On this case we can see the local pool is 10 ,that mean it needs 8*2^10 seconds to lose  synchronization .

Root Cause
If the NTP client has 2 unicast servers configured as shown below.
ntp-service unicast-server 10.1.104.236 preference         NTP server master
ntp-service unicast-server 10.1.104.237                     NTP server backup
here are the procedure of NTP.
1.NTP client gets synchronized to the unicast server configured with preference.
2.When the clock status on the NTP client is synchronized and the system is stable, the local poll value on the NTP client        
         increases  from default poll interval of 2^6 seconds to maximum system poll value which is 2^17 seconds.
3.NTP client will keep polling the NTP server at an interval which is equal to the local poll value.
4.After every poll interval expires, NTP client will send a request to the NTP server and the server will send a response.
5.After the local poll value has increased from 2^6 seconds to 2^10 seconds, if the interface of the primary server is shutdown, then the NTP client will continue to poll the NTP server every 2^10 seconds. NTP client will send request to NTP server every 2^10 seconds and wait for response from the NTP server.
6.If the NTP client does not receive response from the NTP server for 6 consecutive packets, then the NTP client will switch from primary server to backup server.

If NTP client has only one unicast servers configured as shown below.
ntp-service unicast-server 10.1.104.236

It is similar that if the interface of the server is shutdown, it will take 8* current-poll seconds to lose  synchronization.
Suggestions
NONE

END