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I have a question about NTP. We want to setup our core switches as NTP master for our network.

We have three data centers.

  1. Site A : Two Nexus as core
  2. Site B : two Nexus as core
  3. Site C : two 6800 (in VSS, so it's kind of one logical switch) as core

In sites A and B, we have servers. In site C, we have servers and clients.

We want our network devices (servers, clients, etc) to get their time from these switches.

I was thinking about setting external NTP servers from the internet on these switches and connect them as NTP peers.

  1. Should we connect all cores switches (from all three sites) as "NTP peers" all together (kind of full mesh setup)?

    Or

    Should each site have their local switches connected as peers, Except Site C, which a one logical switch?

  2. Should all clients/servers be configured to use all NTP servers (so their list of NTP servers will be all switches from all sites over WAN)?

    Or

    Clients/servers should only have their local NTP servers configured?

  3. Should all core switches acting as NTP masters have their clock sync with NTP servers on the internet?

    Or

    Only some NTP masters should be connected to the internet for external sync? If so, would you chose a dedicated site for this or one switches in each site?

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Unless you have a very good NTP source, you should set up your NTP servers as clients of low-stratum NTP servers elsewhere (e.g. Internet). You can, and probably should, use multiple, external servers as NTP servers for your NTP servers, but not as peers since the low-stratum NTP servers will not actually peer with you (peers should be at the same stratum).

You could set up your servers at the various sites as peers, but this may add more complexity, latency, and prove unnecessary if they are getting their time from the same external servers. You probably do want to do this with your NTP servers within a single site.

If your core switches are true core switches (not routing, just dedicated to high-speed switching), you probably don't want to burden them with NTP requests from every device at a site. You should use the distribution routers/switches for NTP servers. The end-devices on each VLAN would get their time from the server dedicated for the VLAN on which they are addressed. The VLAN NTP servers could be the distribution or access switches for the VLAN.

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I might be running an older code train that doesn't support this but I think if you go an do some checking you will find that Cisco Nexus devices will not act as an NTP server.

That being said, you would probably benefit greatly from looking at (3) devices that does some form of Stratum 1 (GPS, CDMA/GSM, Rubidum crystal oscillator, radio, etc). With 3 devices or more, you end up with quorum which is ideal for maintaining very synchronized time.

If costs are a great barrier, there are some Open Source type projects.

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    Nexus devices can act as NTP servers, see this doc for example, and even PTP – JFL Apr 26 '16 at 22:29

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