to enable NTP broadcast do
Router1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#clock timezone EST -5
Router1(config)#clock summer-time EDT recurring
Router1(config)#ntp server 172.25.1.1
Router1(config)#ntp server 172.25.1.2
Router1(config)#interface FastEthernet0/0
Router1(config-if)#ntp broadcast
Router1(config-if)#end
Router1#
Discussion
Usually NTP associations are configured in a master/slave relationship ( unicast NTP ) , but the server (router) can also send periodic time updates using broadcast messages. This is useful on LAN segments that contain a large number of devices requiring NTP synchronization. Instead of responding to a large number of unicast NTP packets through a single interface, the router can simply send a single broadcast packet at a regular interval.
NTP devices configured to accept NTP broadcast messages can synchronize their internal clocks without ever sending a single NTP request packet. However, this simplicity comes at the cost of reduced timing accuracy, since the traffic is one-way only. The accuracy improves slightly by configuring an estimated broadcast delay on the client side, using the ntp broadcastdelay configuration command, as in the client configuration example above.
