[ntp:questions] a few questions about broadcast

Danny Mayer mayer at ntp.isc.org
Mon Oct 24 03:40:55 UTC 2005

vrkid0 at gmail.com wrote:
> Hi
> I want to move my subnet from using unicast to using broadcast. The
> current setup is as follows: 2 local servers poll (unicast) from 2
> external (on the internet) servers at stratum 2 (making my 2 local
> servers stratum 3). All the other systems poll from the 2 local servers
> and peer with each other (making them stratum 4).
> I want to move to broadcast to simplify client setup, add reliability
> and accuracy. I setup my 2 main servers to broadcast and 1 client to be
> broadcast client (for testing) and have the following questions:
> 1. Does having 2 broadcast server add to the accuracy of the clients?

It means you have more than one source which is a good thing. However
you should set up at least 3 sources whether they are broadcast or
server or multicast to enable your clients to decide which ones to
believe. 4 is preferably. Each should be broadcasting on the same
address and each will be seen as separate servers by the client. If all
of your systems on the network are using ntp then broadcast is
definitely the way to go. If only some of the systems need it consider
using multicast as the packets will only be routed to those systems that
are requesting it.

> 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?

Don't. If you lose connectivity to the servers it will continue on with
the last set of adjustments. It won't "drift" it will just continue with
the information it has and will continue to be pretty accurate.

> 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. Why?

We'd need to see your setup and the output of ntpq -p to be able to
answer that.


> TIA 
> Paolo

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