NTP in large networks

[ksowell]

In a large network that is tiered from core to distribution to assess layers, and gets it's time from an external Internet source, is the 'ntp master' config statement be used on the core routers to keep the network in sync in the event that the Internet time servers become unavailable? Or will they serve ntp updates based on core router's hardware calendar during Internet outages?

[rsissons]

The NTP master command is designed to provide clock syncronization when internet connectivity is unavalable. From the doco on the web -

If the system has ntp master configured, and it cannot reach any clock with a lower stratum number, the system will claim to be synchronized at the configured stratum number, and other systems will be willing to synchronize to it via NTP.

So you need to configure it with a high stratun value so the intenet source will take priority.