* Time service. This way there is no need to set up any SNTP and the router is always in sync with the hosts.
Off topic, just wanted to make a quick comment on this one: most hypervisors are pretty tough on the internal clock. We've found our Hyper-V servers (and ESX before them) tended to drift around a bit depending on system load. Our environment (SCADA) is very sensitive to clock clock changes. We disabled timesync on all of our VMs (easy to do in bulk via powershell):
Disable-VMIntegrationService -name 'Time Synchronization' -Computername HYPERVHOST -Vmname *
and then plonked this registry entry on our guest VMs so they could sync directly against our timesource via NTP/SNTP.
Windows Registry Editor Version 5.00