Configure R1 as an NTP server with authentication, source NTP packets from Loopback0, set NTP stratum 2, and permit NTP only from client 209.165.200.225. Which configuration achieves this?
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Correct answer: ntp authenticate ntp authentication-key 2 md5 CISCO123 ntp source Loopback0 ntp access-group server-only 10 ntp master 2 ! access-list 10 permit 209.165.200.225.
Why this is the answer
The correct configuration enables NTP authentication, specifies MD5 for the key, and sources NTP packets from Loopback0. The ntp master 2 command sets the router as an authoritative NTP server with a stratum of 2. The ntp access-group server-only 10 command restricts NTP server access to clients permitted by access-list 10. The access-list 10 permit 209.165.200.225 entry correctly allows only the specified client. The first incorrect option uses sha1 instead of md5 for the authentication key and specifies udp host ... any eq 123 in the ACL, which is overly specific and not required for a simple host permit. The second incorrect option uses ntp interface Loopback0 which is not a valid command for sourcing NTP packets, and ntp stratum 2 is not the correct command to set the stratum for a master server; ntp master 2 is used. The fourth incorrect option uses ntp stratum 2 instead of ntp master 2 and includes the unnecessary udp host ... any eq 123 in the ACL.
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