Which commands will you use to configurationure the router?

You have 2 NTP servers in your network – 10.1.1.1 and 10.1.1.2. You want to configurationure a Cisco router to use 10.1.1.2 as its NTP server before falling back to 10.1.1.1. Which commands will you use to configurationure the router?

A.
ntp server 10.1.1.1
ntp server 10.1.1.2

B.
ntp server 10.1.1.1
ntp server 10.1.1.2 primary

C.
ntp server 10.1.1.1
ntp server 10.1.1.2 prefer

D.
ntp server 10.1.1.1 fallback
ntp server 10.1.1.2

Explanation:
Preferred server

A router can be configured to prefer an NTP source over another. A preferred server’s responses are discarded only if they vary dramatically from the other time sources. Otherwise, the preferred server is used for synchronization without consideration of the other time sources. Preferred servers are usually specified when they are known to be extremely accurate. To specify a preferred server, use the prefer keyword appended to the ntp server command. The following example tells the router to prefer TimeServerOne over TimeServerTwo:

Router#config terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#ntp server TimeServerOne prefer
Router(config)#ntp server TimeServerTwo
Router(config)#^Z

http://oreilly.com/catalog/hardcisco/chapter/ch10.html

http://www.cisco.com/en/US/docs/ios/netmgmt/command/reference/nm_10.html#wp1123176