I'm trying to find information on whether it is possible to enable an NTP cluster containing multiple routers, that are configured with VRRP to share the same address.

I am aware that with many devices it is possible to configure multiple NTP servers. Unfortunately, there are some devices in the corporate network that only allow one DNS entry.

My question now is whether you can solve this with a virtual router and what problems you might encounter.

Thanks in Advance

  • 1
    You can resolve this with DNS round robin. – JFL Oct 11 '18 at 7:48
  • @JFL Will end devices notice and try to resolve the dns entry again if the chosen router is offline? – cyacedev Oct 11 '18 at 7:57
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    No but the next time they try to update the date with NTP they will have a different IP and try to reach a different NTP server. Depending on which accuracy you need it may be enough. You need to set a low TTL on the DNS records. – JFL Oct 11 '18 at 7:59

I can see a few ways of doing this:

  • Round robin DNS with multiple A records (as suggested in comments by JFL). This would require the clients to properly re-look up the addresses when required, and get rotated A records (small TTL would probably help) and/or properly try the different addresses themselves.
  • VRRP/HSRP (per your original question) This looks like a perfectly good way
  • Anycast with multiple NTP servers, perhaps with SLA tracking to decide which one to use
  • Unsolicited Broadcast client where client just listens for broadcast time, and you have multiple broadcasters
  • SNTP Broadcast client might be also suitable, where the client has a broadcast address as its "server", and accepts whatever answers come.

All of these will have the same condition that the far server, as seen by the client, might change to a different server altogether. This means that you should expect trouble if you were using peer-to-peer rather than client-serve modes of NTP. Given the description of your devices, I'd guess they are simple clients anyways.

I'd suggest the VRRP/HSRP and anycast solutions are going to be better than DNS round-robin because they don't require anything of the client. Choosing between these two would depend on your other networking criteria.

If the clients can support it, you might also look at making them broadcast clients or multicast clients. Rereading RFC 1769 SNTP it looks like at least one of the broadcast modes would specifically cover what you're trying to do, and might be lots simpler than getting clever with the router.

  • Thanks for the good summary, I'll be sure to also check out Anycast – cyacedev Oct 11 '18 at 9:19
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    I was rereading SNTP RFC and looks to me like the broadcast client would be just what you need, if you have no other need for VRRP. – jonathanjo Oct 11 '18 at 9:27

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