You want to limit the number of NTP peers the router will accept.
Use the ntp max-associations configuration command to limit the number of NTP associations the router will accept:
Router#configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router(config)#ntp max-associations 30 Router(config)#end Router#
To prevent oversubscribing valuable router resources by NTP associations, Cisco provides the ability to limit the number of associations that a router will accept. While the ntp max-associations command limits the number of inbound NTP associations, it does so without prejudice. The first 30 NTP associations, using the example above, will be permitted regardless of whom they are.
Other methods of controlling NTP associations mentioned in this chapter provide greater control and granularity than just limiting the number.