[ntp:questions] Rejecting Good Peers

Steve Kostecke kostecke at ntp.org
Wed Dec 3 16:09:51 UTC 2008


Cal Webster said:

>Please explain what a "peer loop" is or point me to the doc page that
>explains it. I don't see the disadvantage of having common peers.

"peers" unfortunately has multiple meanings. The remote time servers
that an ntpd is synced to are often refered to as peers. But these ntpds
could be in a peer (bi-directional) or server (uni-directional)
association.

Systems which are in a peer association exchange their notion of time
with each other.

In a server association the "client" requests the time from the
"server".

>So, the new behavior is to select no candidates if they have common
>peers?

A peer association will be discarded if both peers are have selected the
same "sys_peer" (i.e. they are both synced to the same time source).

>This would seem counter-intuitive from a reliability/redundancy
>standpoint. What will this ver 4.2.4 server do when it can no longer
>reach the master server? My guess is that it will either stop serving
>time or die since it has no other reference (local undisciplined clocks
>not configured). If true, this is an undesirable condition, and seems
>especially wasteful when there are other time sources available even if
>they may not be as precise as the master.

An ntpd keeps running and continues to discipline the system clock
with the last known frequency correction when all time sources become
unreachable. So, no, it won't fall over. But what will happen is this
ntpd will no longer be able to serve time to others unless it is
configured with (a) the Undisciplined Local Clock or (b) Orphan Mode.

One thing you need to keep in mind is that the associations are not
static. ntpd evaluates them at each poll and will select (as sys_peer or
candidate) or discard associations as conditions change.

In your case the remaining servers will select the reachable server with
the lowest stratum Undisciplined Local Clock when the master (server
A) is unreachable. Assuming that you restarted ntpd with the revised
configuration files.

-- 
Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project http://support.ntp.org/
Public Key at http://support.ntp.org/Users/SteveKostecke



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