NTP guidelines

If your network is small, consisting of around a dozen hosts, you need only a single NTP time server on your network. Locate three time servers elsewhere on the Internet from which your time server can get the correct time. If possible, pick two stratum 1 servers and one stratum 2 server. Ideally, you should choose to get the time from servers that are closest to your host in terms of roundtrip delay. In other words, you should prefer servers for which the amount of time it takes to send a request for the current time and the time it takes to receive a response is smallest. However, there is no great harm if you select servers for which you do not know the roundtrip delay.

If, however, your network is large, consisting of dozens or hundreds of hosts, you should set up a larger number of time servers. A good rule of thumb is to have one time server for every dozen hosts. In this case, you first establish a few stratum 2 time servers, then you establish the remaining time servers as stratum 3 time servers. Finally, all of the remaining hosts, which constitute the vast majority of the hosts on the network, become stratum 4 hosts.

Following are guidelines for setting up NTP on a network of medium size.

1. Choose three of your local hosts to operate as stratum 2 servers. Then choose six stratum 1 servers and configure each of your stratum 2 servers to poll a distinct pair of the stratum 1 servers. If possible, see to it that each stratum 2 server polls at least one stratum 1 server for which the roundtrip delay is small. You should also configure each of the stratum 2 servers to poll both of the other stratum 2 servers. Thus, each stratum 2 server is configured to poll two stratum 1 servers elsewhere on the Internet and two stratum 2 servers connected to the local network.

2. From the remaining hosts, choose those you would like to operate at stratum 3. This might be all of the rest if you want to synchronize at most a couple of dozen hosts, or you might choose hosts such as file servers or those with good clocks if you need to synchronize many more hosts. Configure each stratum 3 server to poll the three stratum 2 servers.

3. Configure all remaining hosts to either poll or listen for broadcast packets from three or four of the nearby stratum 3 servers.

4. Avoid configuring hosts beyond stratum 4. Doing so makes the network unnecessarily complex and harder to manage.

5. Do not configure a host to poll another host at the same stratum, unless the latter is directly polling lower stratum servers that the former does not.

6. Do not configure hosts to poll other hosts at higher strata.