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Cisco IOS Cookbook, 2nd Edition by Ian Brown, Kevin Dooley

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Using NTP to Send Periodic Multicast Time Updates

Problem

You want to set up your router to use the NTP multicast mode so that devices do not need to query periodically for the time.

Solution

Use the ntp multicast interface command to enable server-side NTP multicast packets:

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 172.25.1.3
Router1(config)#interface FastEthernet 0/0
Router1(config-if)#ntp multicast 224.0.1.1 ttl 1
Router1(config-if)#end
Router1#

To enable NTP multicast client functionality on the router, use the following commands:

Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#clock timezone EST -5
Router2(config)#clock summer-time EDT recurring
Router2(config)#interface Ethernet0
Router2(config-if)#ntp multicast client 224.0.1.1
Router2(config-if)#ntp multicast version 3
Router2(config-if)#end
Router2#

NTP multicast support is available starting in IOS Version 12.1.

Discussion

On the surface, the ability to forward NTP broadcast packets and NTP Multicast packets on a LAN interface appear similar. However, there are some important differences. First, NTP sends broadcast packets to the 255.255.255.255 local broadcast address. This means that every device on the network must examine the NTP packet. If there are devices on the network ...

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