[ntp:questions] Re: Peer configuration question?

Richard B. Gilbert rgilbert88 at comcast.net
Thu Jan 29 23:57:03 UTC 2004

The difference between peers and servers is very simple!

Peers have a symmetric association; either one can supply time to the 
other.  If two peers have different opinions as to what time it is, they 
will try to determine who has the more accurate time and both will use 
that time.

Servers are asymmetric; a server can supply time to you but will not 
accept time from you.

For maximum reliability, you would have your two stratum 1 servers 
configured as peers.  If everything is working properly, these two 
servers should track each other and the "correct" time within a few 
microseconds.  If either GPS receiver fails, the server it is connected 
to will obtain the correct time from its peer and continue to serve its 
clients.  If you have a network neighbor who also operates stratum 1 
servers you might wish to peer with his servers as well.  A setup like 
that gives you both a defense in depth against failures of reference 
clocks, or servers.

Configure your stratum 2 servers to peer with each other and to be 
served by both stratum 1 servers.  All clients should be served by the 
stratum 2 servers.

Your single point of failure would be the GPS satellite system itself.  
Highly unlikely but the DOD could just pull the plug on all those 
satellites. . . .

Nicholas Garfield wrote:

>Hi All,
>I have two stratum 1 servers which are each connected to Trimble Accutime
>GPS receivers.  Then two more servers use these sources as stratum 2
>I have been reading the NTP FAQ Section 6.2. Advanced Configuration, however
>I am unsure of the configuration.
>(1) For the stratum 1 servers, should there be a peering relationship
>between them?
>ie on server 1 config is
>server prefer minpoll 4 maxpoll 4 #ACCUTIME
>peer version 4 minpoll 4 maxpoll 4 #SERVER 2
>On server 2 config is
>server prefer minpoll 4 maxpoll 4 #ACCUTIME
>peer version 4 minpoll 4 maxpoll 4 #SERVER 1
>Also, would someone be kind enough to clarfiy whether or not the "prefer"
>keyword is necessary here.  I figure (probably incorrectly) that I would
>prefer the time to come from the accutime, but how would this work with the
>idea of selecting the "best" clock.
>(2) Similarly the config for the stratum 2 servers uses peering.
>I am not 100% clear what the difference between a "server" declaration and a
>"peer" declaration is.  When a server and a peer is specified how does ntp
>choose which is preferred?
>Thankyou in advance to anyone who can explain clearly when/where the peer
>and prefer statements should/should not be used.

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