NTP questions

[sarahr202]

Hi every body.

Please consider the following example:

R5 199.199.199.5 ( NTP Client)-----SW--- 199.199.199.2 ( R2)-----R1( MASTER : 199.199.199.1)

                                                                ---199.199.199.3 ( R3)-----R1( MASTER: 199.199.199.1)

                                                                ---199.199.199.4 (R4)------R1 ( MASTER:199.199.199.1)

R5 is a client and is configured with NTP server 199.199.199.2, 199.199.199.3, 199.199.4, all these servers in turn get their time from master 199.199.199.1 ( R1)

Below you can see R5 chose to syn with R3:

R5#show ntp associations

address ref clock st when poll reach delay offset disp
~199.199.199.2 199.199.199.1 2 55 128 3 8.131 -23.677 0.977
*~199.199.199.3 199.199.199.1 2 39 128 377 23.583 -19.536 3.077
~199.199.199.4 199.199.199.1 2 3 128 37 5.571 -7.131 2.862

Questions:

1) How does R5 determine which NTP server it should sync with?

2) How does the polling interval is determined?

3) Does the number of NTP servers configured on R5 have any bearing how quickly will R5 sync its time,  in other words if R5 has only one NTP server configured versus three ntp servers configured, will it determine the how frequent R5 sync its time?

Thanks and have a nice day!!

[Georg Pauwen · Accepted solution]

Hello,

1) When more than one NTP server is configured (as in your scenario), the NTP client router measures the "best time value" derived from these servers.
"Best time" is calculated based on offset, delay, dispersion etc which is reported by different NTP servers. Those values are never exactly the same on different servers, so one will always be selected as 'best'.

2) The maximum and minimum poll interval are set to 6 and 4, respectively, those can be changed to anything from 4 to 16 seconds.

3) Good question. The client polls all configured servers at specified intervals, but synchronizes only with one. Obviously when only one server is configured compared to several, in case one of the servers is deemed 'insane', a new one will be chosen at the next polling interval. So, the initial sync time is not affected by the number of servers configured...