[ntp:questions] Re: a few questions about broadcast

Steve Kostecke kostecke at ntp.isc.org
Mon Oct 24 01:59:56 UTC 2005

On 2005-10-23, vrkid0 at gmail.com <vrkid0 at gmail.com> wrote:

> 1. Does having 2 broadcast server add to the accuracy of the clients?


broadcast clients receive a timestamp every 64 seconds from each server

unicast clients poll each server every 64 to 1024 seconds

Operating in broadcastclient mode may improve your clock stability by
reducing the amount of time between "polls".

> 2. I tried to have on the client in addition to the "broadcastclient"
> directive the "server" and "fudge" directives for local synchronization
> in case there is a network outage (slim chance, but none the less) and
> it didn't work. Why?

Do you mean you used:

fudge stratum NN

Please keep in mind that it will take as much as 8 minutes before ntpd
will consider the undisciplined local clock (127.127.1.x) for

> 3. I noticed that on the client when I run `ntpq -p` it sees all the
> servers on the subnet even though only 2 servers broadcast, and while
> only 2 servers are stratum 3 and all the others are on stratum 4, the
> broadcast client moved between the servers in stratum 4 ignoring the
> broadcast servers in stratum 3.

As has been said elsewhere, the broadcastclient / multicastclient modes
require the use of NTP Authentication. You can disable the requirement
for authenticaton, but doing so allows unauthorized (broadcast) time
servers on your LAN to change your clients' clocks.

Please see http://ntp.isc.org/Support/ConfiguringAutokey for detailed
information about setting up NTP Authentication using Autokey.

BTW: It has been pointed out that two upstream time servers is
not a good configuration. That is true but is not the reason why
broadcastclient mode does not work. The problem with using only two
sources of time is that ntpd has no way of deciding which source is
correct. You need to use three (some say four), or more, time sources to
give ntpd enough input to detect a "false-ticker".

Steve Kostecke <kostecke at ntp.isc.org>
NTP Public Services Project - http://ntp.isc.org/

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