Cisco IOS Cookbook, 2nd Edition

Configuring NTP Redundancy

Problem

You want to configure more than one NTP server for redundancy.

Solution

You can improve NTP reliability by configuring several redundant servers. The reliability is better still if the router uses different paths to reach these servers:

Router#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#clock timezone EST -5
Router(config)#clock summer-time EDT recurring
Router(config)#ntp server 172.25.1.1
Router(config)#ntp server 10.121.33.231
Router(config)#ntp peer 192.168.12.12
Router(config)#end
Router#

Discussion

The NTP algorithms have built-in sanity checks to help choose the best time source. The NTP client chooses the most accurate time source and synchronizes its internal clock to that server. The algorithm continuously performs sanity checks to ensure that it synchronizes to the best possible server. It is also common for a router to change its preferred NTP server many times during a day.

Configuring multiple time sources improves reliability as well as the accuracy of a router’s clock. Although NTP is a remarkably stable protocol, device and link failures can disrupt timing services to your router. Providing the router with a choice of NTP servers ensures accurate time synchronization and provides resilience in case of failure. Be sure to choose redundant NTP servers that provide alternate network paths and hardware.

An NTP network is a hierarchy of servers and clients configured in a redundant topology. ...