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

ulf.norberg at banverket.se ulf.norberg at banverket.se
Tue Jun 26 09:22:27 UTC 2007

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
Any risk of synchronisation loops or other strange behaviour?

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

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.

Thanks in advance.

Best regards
Uffe Norberg

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