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Forum Replies

  1. Excellent!

  2. Hi Thomas,

    I haven’t tested this but from what I’ve read, NTP prefers low stratum servers over high stratum servers unless the time difference between the local clock and the low stratum server is quite different.

    On Cisco IOS, you can also use the “prefer” parameter to tell the router what NTP server to use as the primary:

    ntp server prefer


  3. Hello Guy!

    The first complete specification of NTP, that is, Version 1, appeared in 1988 (RFC 1059) which provided simple symmetric and client server mode operation.

    Version 2 appeared in 1989 (RFC 1119) and added symmetric key authentication using DES-CBC.

    Version 3, which is the version that is most used today was first described in 1992 (RFC 1305) and has been systematically improved over the years. It introduced formal correctness principles, revised algorithms and broadcast mode . This is the default version that is available in most Cisco devices using the 12.X IOS version, and the recommended minimum version you should use.

    Version 4 extends the support of NTP to IPv6 and is available on any Cisco device that supports IPv6.

    I hope this has been helpful!


  4. Hello Laz,
    A few questions.

    1. Let’s say I have a router that is configured to receive the ntp information from a ntp server located in the internet. I have also configured the time locally by using clock set command. Which time will have more preference? In another words, which time the router will use?
    2. What is the command to change time-zone in a router?
    3. Let’s say a router is configured to sync its time from a ntp server and the ntp server is feeding UTC time to the router. However, I like the router to show EST time in the clock or let’s say in syslog message as well. How can I do it?
    4. What is the difference between hardware and software clock in a router/switch?

    Thank you so much as usual for your great help .

    Best Regards,
    Azm Uddin

  5. Hello AZM

    Question 1
    When NTP is configured on a device, there is what is called a poll interval. This interval is dynamic and as client and server become better synced, and there aren’t any dropped packets, this interval increases to a maximum of 1024 seconds. If you change the time using the clock set command, the time you set will become the new time. However, when the poll interval is exhausted, the device will re-sync with the NTP server. So any changes you make manually will be over-ridden at the next poll interval.

    Question 2
    To change the time zone of a router, use the clock timezone _zone hours-offset _ command where

    • zone is the name of the zone to be displayed - this is just a label that you can define
    • hours-offset is how many hours difference from UTC

    You can find more information about this command here.

    Question 3
    NTP always communicates time in UTC. If you have a time zone configured on your Cisco device and it is configured as an NTP client, then it will receive the time in UTC and will convert the clock to the local time zone. However, SYSLOG messages will always indicate UTC time by default, even if you have configured a different time zone. In order to have SYSLOG messages display the local time instead of UTC time, you can achieve this with the following command: service timestamps log datetime localtime

    Question 4
    The hardware clock of a Cisco device is a hardware chip on the motherboard of the device with a rechargeable backup battery. The hardware clock functions separately from the software clock and its main purpose is to retain the time and date information after a reboot or an extended period of time where the device is powered down.

    The software clock is the clock that functions during normal operation. It is maintained in memory using the CPU as a “timekeeper”. It is this clock that can be updated and synchronised with an NTP server on the network.

    These two clocks work together to maintain the most accurate time. The software clock can be updated periodically from NTP, and it in turn updates the hardware clock at regular intervals. When the device is rebooted, the software clock is synchronised with the hardware clock to obtain its initial time when it begins to function.

    The two clocks can be managed separately however. More information about related commands can be found here.

    I hope this has been helpful!


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