ntp server [ip]
Configures the router to synchronize its system clock with an NTP server, ensuring accurate time for logging, authentication, and network protocols.
ntp server [ip]When to Use This Command
- Set up NTP on a router to synchronize time with a central NTP server for consistent logging across the network.
- Configure multiple NTP servers for redundancy in case the primary server becomes unreachable.
- Use NTP to ensure accurate timestamps for security logs and digital certificates in a corporate network.
- Synchronize time on routers in a branch office with a head-end NTP server to maintain consistent time for routing protocols like OSPF.
Command Examples
Basic NTP server configuration
ntp server 192.168.1.1This command configures the router to synchronize its clock with the NTP server at IP address 192.168.1.1. No output is displayed upon successful configuration.
Verifying NTP association after configuration
show ntp associationsaddress ref clock st when poll reach delay offset disp *~192.168.1.1 .GPS. 1 23 64 377 1.234 0.123 0.456 +~10.0.0.1 .GPS. 2 12 64 377 2.345 0.234 0.567
The output shows NTP associations. The asterisk (*) indicates the current synchronization source. 'address' is the NTP server IP, 'ref clock' is the reference clock source (e.g., GPS), 'st' is stratum level (lower is better), 'when' is seconds since last sync, 'poll' is polling interval, 'reach' is reachability (377 means all 8 attempts successful), 'delay' is round-trip delay in ms, 'offset' is time difference in ms, 'disp' is dispersion in ms. Good values: low delay, offset near 0, high reach.
Understanding the Output
The 'show ntp associations' command displays the status of NTP peers. The asterisk (*) marks the server currently used for synchronization. The 'st' column shows stratum; stratum 1 is most accurate (directly connected to a reference clock). 'when' indicates seconds since the last update; a low value is good. 'poll' is the polling interval in seconds. 'reach' is an octal value; 377 (all 8 bits set) means the last 8 polls were successful. 'delay' and 'offset' should be low for accurate time; high values may indicate network issues. 'disp' is the dispersion, representing the maximum error; lower is better. If no server is selected (no asterisk), check reachability and network connectivity.
CCNA Exam Tips
CCNA exam tip: NTP uses UDP port 123; ensure ACLs allow this traffic.
CCNA exam tip: The 'ntp server' command is configured in global config mode; verify with 'show ntp status'.
CCNA exam tip: Stratum levels: 1 is most accurate, 16 is unsynchronized; routers typically operate at stratum 2-10.
CCNA exam tip: Use 'ntp peer' for symmetric active mode (not in CCNA scope), but know the difference.
Common Mistakes
Mistake: Forgetting to configure 'ntp update-calendar' to update the hardware clock; the system clock resets after reboot.
Mistake: Using an incorrect or unreachable NTP server IP; verify connectivity with 'ping'.
Mistake: Not allowing NTP through ACLs; the router will not synchronize.
Related Commands
show ntp associations
Displays the status of NTP associations configured on the device, used to verify NTP synchronization and identify time sources.
show ntp status
Displays the current NTP synchronization status, including clock stratum, reference clock, and synchronization state, used to verify NTP operation and clock accuracy.
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