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The NTP deamon if often used in its simplest mode, which is client/server: you specify one or more server directives in your ntp.conf and your clients will use those servers.

In addition to that, when you run your own NTP servers, it is good practice to peer them together, so if one of them looses connectivity to its upstream servers, it will get time from its peers.

But NTPd can also work with broadcast and/or multicast distribution of time data, with the documentation stating:

broadcast and multicast modes are intended for configurations involving one or a few servers and a possibly very large client population

The documentation also says elsewhere:

It is possible and frequently useful to configure a host as both broadcast client and broadcast server. A number of hosts configured this way and sharing a common broadcast address will automatically organize themselves in an optimum configuration based on stratum and synchronization distance.

I can see one obvious administrative benefit: you don't have to manually specify and update your list of NTP servers in the clients ntp.conf, so to me it looks tempting to use broadcast mode even for a small client population (say 5+ clients with 3~4 servers). I expect network traffic to be a little higher with broadcasts instead of client/server associations, but given the usual gigabit ethernet LAN the impact should be negligible unless you have a very very large number of hosts in the same broadcast domain.

At the end of the day, when should broadcast mode be used or avoided? Are there pros and cons I haven't seen?

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up vote 1 down vote accepted

In my opinion, broadcast/broadcastclient should be avoided at all times.

I've looked into this option myself and didn't find any suitable way to configure client just to accept those broadcasts from "official" servers only.

And the next point is: How compatible is this broadcasting with Computers running Windows/MacOS?

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suitable way to configure client trust = autokey – dfc Nov 5 '12 at 2:30

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