NTP server/client configuration in switches

[Sihanu N]

Hi Experts,

We had core(4503), distribution(3750), and access switches(2960) in our environment. Currently we configured the clock manually in each switch, but a reboot of the switch resets the clock also. We are planning to make a single switch as a NTP servers and others are clients to synchronise  the correct time even after a reboot of the access switches.

Kindly provide valuable suggessions and advise regarding the requirement.

Thanks and Regards,

Sihanu N

[Thotsaphon Lueangwattanaphong · Accepted solution]

Hello,

   C4500 should be configured  ntp server to point to the external clock sources and ntp master to ack as a clock source for other devices. The rest of devices just point ntp server to C4500.

Ref: http://www.cisco.com/en/US/docs/ios/12_1/configfun/configuration/guide/fcd303.html#wp1001170

HTH,

Toshi

[Sihanu N]

Hi Toshi,

Many thanks for the reply.

1) Is there any way to set a hardcoded clock for swithes/routers so that a reboot doesn't affect the clock in a single device  environment with no internet connectivity?

Thanks and Regards,

Sihanu N

[cadet alain]

Hi,

if your NTP master has a hardware clock then even if it loses connectivity with public NTP server it will still continue to sync time on other machines.

Regards.

Alain

[Sihanu N]

Hi Alain,

Many many thanks for the information.

We have configured successfully the NTP in our environment as core switch as ntp master (currently not using the public NTP server, manually configured clock in the core switch) because the core switch having redundant power supply (least chance of a downtime to the switch). But as a worst case scenario if the entire power outage occurs, a chance of wrong time replication will happen.

1) Is there any way we can configure a hardware clock in the core switch(retain the date and time even after the reboot)?

Thanks and Regards,

Sihanu N

[hobbe]

Hi

The problem with the clock is that the crystalls are not equal and the same, but they are within tolerances ie they are never exactly the same. This will make time go different in each of the units.

so with time your 4500 will not be the correct time.

So if you have the money, buy a GPS or radiocontrolled device. they are not cheap but they are not extremely expensive either. and they make life a little bit easier.

and its a good thing to have more than one timesource if the one you have fails and gives wrong information many strange and interesting things might happen.

Good luck

HTH

[Richard Burts]

Toshi

If a switch is configured with ntp server to get NTP time from an external server (which I believe is what you are suggesting) then there is no need to configure ntp master. Once the switch has learned authoritative time from the external server then the switch will automatically be able to act as ntp server to other switches in the network.

If you do configure a switch with the command ntp master then the switch will assert that it has authoritative time and will offer NTP time to other devices even if it has not learned time from the external server. There is a scenario which illustrates what could happen:

- assume that you have configured the core 4503 as ntp master.

- if there is also configured ntp server, then assume that something has happened to disrupt connectivity from the 4503 to the external server.

- something causes the 4503 to reboot.

- when it comes up the 4503 will be using the arbitrary Cisco start time and date.

- since the switch is ntp master it will start announcing its time to clients in the network.

- now all the network devices are using that arbitrary Cisco date.

So I suggest that you be very careful about configuring ntp master on any device that does not have a reliable source for time.

HTH

Rick