The simple answer is that if your configure on the 1721 the command ntp master, then the 1721 will act as an NTP time server. If devices send NTP requests to it, it will respond with what it believes is the time. You can configure the 1721 to broadcast NTP onto its LAN interface(s) if you want to do that.
One point about this is that you need to have a way for the 1721 to know what time it is. If the 1721 is sending requests to some device and learning the correct NTP time then you do not need the ntp master command. If the 1721 has learned time from an authoritative source then it will send time in response to requests. If the 1721 is not learning NTP from some other device then you do need the ntp master command and it is important that you have some way to get accurate time onto the 1721. (any time that the router reboots is an opportunity for inaccuracy in the time to be introduced)
HTH
Rick
HTH
Rick
