[ntp:questions] Peering and synching over multiple interfaces and subnets.

Danny Mayer mayer at ntp.isc.org
Fri Jun 29 03:04:58 UTC 2007

ulf.norberg at banverket.se wrote:
> Hi all.
> I have 2 questions about synching and peering over multiple interfaces
> and subnets.
> For redundancy reasons our supplier of a SCADA system wants to get
> NTP-time over multiple interfaces. The routers act as stratum-3 servers.
> (they could possibly be raised to stratum-2 if necessary) 
> Question 1
> The SCADA servers is serving 4 different subnets with services and
> getting it's NTP time from the default gateway on each subnet. These 4
> subnets are served by 2 physical routers handling 2 subnets each, so
> what the SCADA system believes are 4 routers are actually 2.
> Is there any problem asking the same NTP source twice from different
> subnets?
> Any risk of synchronisation loops or other strange behaviour?

You won't get loops since those servers aren't synching to each other.
They'd need to be on the same subnet to even be an issue in the first
place and refid's do a good job of loop prevention if needed. I'm not
sure where you think you are getting redundancy since the redundancy
comes from picking a sufficient number of servers to provide time
service to the SCADA systems. You can skip the routers. They don't need
to be involved here.

> Question 2
> My second question is similar but is about peering.
> I want to "clean up" the existing NTP-configuration and introduce
> peering between the SCADA server because they are mutual dependent on
> each other and there's a need to keep their clocks as close to each
> other as possible.
> If I want to peer the SCADA servers with each other, is it any problem
> peering them over 2 subnets? 

It doesn't matter. You can peer across the other side of the world if
you want (though the quality would be poor across such a distance).

> Each SCADA server will then have 2 peering instances per neighbour in
> the conf files. Will this cause any strange behaviour in the
> NTP-synchronisation?


> If anyone needs a picture to follow my description above, I can send it
> as PDF to anyone willing to help me solve this issues.

Not necessary.


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