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selection criteria of NTP server

kthned
Level 3
Level 3

Hi

If 4 x NTP servers are configured on cisco router then what is the criteria that which server cisco picks first ? can some one share the selection criteria of NTP servers, same questions for NTP peer.

Thanks !

Regards,

Umair

2 Replies 2

ngoldwat
Level 4
Level 4

Hi,

"In a common client/server model, a client sends an NTP message to one or more servers and processes the replies as received. The server interchanges addresses and ports, overwrites certain fields in the message, recalculates the checksum, and returns  the message immediately. Information included in the NTP message allows the client to determine the server time with respect to local time and  adjust the local clock accordingly. In addition, the message includes information to calculate the expected timekeeping accuracy and reliability, as well as select the best server."

So in other words "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."

You can prefer a server:  ntp server prefer

You also can peer NTP servers.  If you have two or more peers then the most accurate peer acts as the server and the other peer(s) act as clients.

Here are some helpful commands:

show ntp associations

show ntp status

debug ntp events

debug ntp adjust

debug ntp core

Note: NTP server functionality is supported starting in Cisco NX-OS Release 5.2.

Hope this helps.


Hi

Thanks for your answer !

I understand that router measures "best time value" using the number of time sources configured on router (ntp server x.x.x.x). This "best time" is calculated based on offset, delay, dispersion etc which is reported by different NTP servers.

Cisco device compares it with the prefer NTP server which the router has selected. If the prefer NTP server deviates too much from the "best time", then router mark this time source as "insane" and selects the second best server which has closest time (offset, delay, etc) with the "best time".

is my understanding is correct ?

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