10-16-2012 07:10 AM
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
10-16-2012 11:38 AM
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
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.
10-17-2012 12:51 AM
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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