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New Member

NTP configuration


I would like one CISCO 3550 to be the NTP server and all my aother switches (1 3550 and 12 2950) to be listening to it.

I found the CISCO NTP configuration www page, but I found it confusing ?

NTP server should be a third server and I would like my 3550 to be the "master".

Which command should I introduce ? (I know that there are global command and command on 1 interface possible).

Could you write me an example of such a configuration?

Many Thanks

New Member

Re: NTP configuration

on the 3550 go to conf t and type ntp master (stratum number) example 3

at the client type ntp server (ip address of naster)

New Member

Re: NTP configuration

Do not have ntp master command ???

under Conf t I have


- access group

- authenticate

- broadcastdelay

- clock period

- max-association

- peer

- server

- source

- trusted key....

and other command if I am under interface configuration....

Can someone as an example ?


New Member

Re: NTP configuration

Any other comments ?

Thanks in advance

New Member

Re: NTP configuration

Since the switch does not support the ntp master command; it cannot be a ntp server for other devices.


Re: NTP configuration

My routers are acting as NTP servers without the "NTP master" command just fine. I think all you need is the "NTP server" command. I believe "NTP master" has to do with the device continuing to act as an NTP server even if it doesn't have it's own time source; e.g., even if it can't sync to a higher-stratum NTP server. I'm not positive on this, however.

New Member

Re: NTP configuration


This would then mean that I have a command like

ntp server (to enable this one to be the one who sends the broadcast ntp ?

What do I put on the client side ?

Many Thanks

New Member

Re: NTP configuration

That, quite simply, isn't true.

Even though the switch can't be NTP master, there's nothing preventing it from being NTP server. (You would need it to sync to some other upstream NTP server, of cause).


New Member

Re: NTP configuration

Ok, reading up the doc again I got it.

My netwrok is done by 2 3550 and 12 2950. So I can't decide now that one of the 2 3550 is the NTP server as I can't sync it so far.

Can you confirm that external clock is needed and that I can't just decide that one 3550 is the NTP broadcast and that all the other ones will listen to it to tune their own clock ?

I guess there is no other way to sync a minimum such an isolated network, or I should find the way to run NTP on one of my NT 4.0 server ?

Many thanks for any hints


New Member

Re: NTP configuration

You can 'trick' the router (or, as in this case, switch) into responding to NTP request by configuring it with it's own address as NTP server, even if is neither NTP master nor sync'ed to an upstream NTP server.

So if the router is, set the clock manually and configure 'ntp server'. Then, on the other switches, also configure 'ntp server' to get them to sync to it.

However, since the 3550 has no hardware clock, there really isn't much point in configuring it as NTP server, as the clock will be reset upon boot.

What I would suggest instead is that you set the clock manually on both your 3550s and then peer them to each other (ntp peer a.b.c.d). Then configure all your 2950s with two NTP servers.

The best thing would be to sync to some external source, of cause. There are excellent appliance-style NTP servers that sync to a radio source or to GPS, so you don't need to be connected to the Net.



PS: You specifically ask for NTP braodcast, but I find the unicast model more generally applicable.

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