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NTP Server election

Hi ,

    I have 4 NTP server configured on the switch, how does NTP server chosen among the 4. which value takes the precedence for server election. It keeps varying among four.

Which value is considered for election

Stratum

ref clock

delay

offset

 

 

4 Replies 4

Diana Karolina Rojas
Cisco Employee
Cisco Employee

You have to take many thing into account:

 

Before synchronizing, NTP compares the time reported by several network devices and does not synchronize with one that is significantly different, even if it is a stratum 1.

If all your servers have a similar reported time so the selection criteria will be based on the straum.

 

You can prefer a server:  ntp server prefer (this in order to avoid your devices keep varying among the four NTPs)

Please do not forget to rate useful post.

Best Regards,

Dear Diana,

                Thanks for your response, how to understand difference in reference time from below output. I haven't configured any prefer in my configuration. Before applying prefer command, need to decide which server is providing accurate time which is every closet to reference time 

 

cisco#sh ntp associations

 

  address         ref clock       st   when   poll reach  delay  offset   disp

+~xx.250.113.149  xx.12.1.16       2     92    128   377 68.171   7.514  5.891

~xx.99.5.19    xx.99.20.200     3    115    128     1  1.753   6.745 187.55

*~xx.168.44.25  xx.71.80.83      1     22    128   377  0.971   7.150  3.891

~xx.99.5.10     10.9.xx.200     3     78    128     1  1.819   6.730 187.57

* sys.peer, # selected, + candidate, - outlyer, x falseticker, ~ configured

 

 

 

cisco#sh ntp status

Clock is synchronized, stratum 2, reference is xx.168.44.25

nominal freq is 250.0000 Hz, actual freq is 249.9972 Hz, precision is 2**22

ntp uptime is 622204100 (1/100 of seconds), resolution is 4016

reference time is DF1D601A.368FEB28 (00:05:14.213 AEST Wed Aug 15 2018)

clock offset is 7.1501 msec, root delay is 0.97 msec

root dispersion is 15.50 msec, peer dispersion is 3.89 msec

loopfilter state is 'CTRL' (Normal Controlled Loop), drift is 0.000011070 s/s

system poll interval is 128, last update was 429 sec ago.

Dear Diana,

                Thanks for your response, how to understand difference in reference time from below output. I haven't configured any prefer in my configuration. Before applying prefer command, need to decide which server is providing accurate time which is every closet to reference time 

 

Cisco# sh ntp associations

 

  address         ref clock       st   when   poll reach  delay  offset   disp

+~xx.250.113.149  xx.12.1.16       2     92    128   377 68.171   7.514  5.891

~xx.99.5.19    xx.99.20.200     3    115    128     1  1.753   6.745 187.55

*~xx.168.44.25  xx.71.80.83      1     22    128   377  0.971   7.150  3.891

~xx.99.5.10     10.9.xx.200     3     78    128     1  1.819   6.730 187.57

* sys.peer, # selected, + candidate, - outlyer, x falseticker, ~ configured

 

 

 

cisco#sh ntp status

Clock is synchronized, stratum 2, reference is xx.168.44.25

nominal freq is 250.0000 Hz, actual freq is 249.9972 Hz, precision is 2**22

ntp uptime is 622204100 (1/100 of seconds), resolution is 4016

reference time is DF1D601A.368FEB28 (00:05:14.213 AEST Wed Aug 15 2018)

clock offset is 7.1501 msec, root delay is 0.97 msec

root dispersion is 15.50 msec, peer dispersion is 3.89 msec

loopfilter state is 'CTRL' (Normal Controlled Loop), drift is 0.000011070 s/s

system poll interval is 128, last update was 429 sec ago.

Leo Laohoo
Hall of Fame
Hall of Fame
Stratum value is, by default, takes priority.
Whoever has "prefer" takes precedence (or priority).
IF two (or more) has "prefer" then stratum will decide.
And I think, if two (or more) has "prefer" and have the same stratum value, it's round robin.
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