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CCNA · 200-301Intermediate

NTP: Wrong Source Interface

A router sends NTP requests from an unexpected interface address. Diagnose it and fix it yourself, live, right below.

Interactive simIP ServicesNTPManagement

Symptom

Clients can reach the time server, but authentication or ACL checks reject synchronization. Set a stable NTP source and verify stratum.

Your mission

Prove the cause, then fix it

Format

Guided CLI investigation

Start interactive labSee the investigation plan ↓Need a hint? Reveal the root cause ↓
NTP: Wrong Source Interface

Investigate before you configure.

Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.

Observe the symptom and link state.

Diagnose by comparing the protocol evidence.

Verify the expected device state and confirm the original symptom is resolved.

Console access: R1

This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.

Need a hint?

Reveal the root cause when you're ready.

Try the investigation first, then use this as your escape hatch, not a dead end.

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Lab debrief

Turn the session into a repeatable troubleshooting pattern.

Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.

The problem

A router sends NTP requests from an unexpected interface address. The upstream NTP server permits only the management subnet, so the router remains unsynchronized despite basic IP reachability.

What you'll practice

  • Read NTP association state
  • Identify packet source addresses
  • Configure a stable source interface
  • Verify synchronization and stratum

The topology

A router has separate management and transit interfaces and reaches a centralized NTP server through the management network.

Commands to run yourself

The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.

Inspect NTP status
show clock detail
Inspect NTP associations
show ntp associations
Inspect interface addresses
show ip interface brief
NTP: wrong source interfaceR1 has reachability to the NTP server but its requests leave from an address the server won't permit.R1Lo0 · 10.255.0.1/32NTP Server198.51.100.10management pathpermitted by server ACLtransit pathGi0/0 · 203.0.113.1/30R1's NTP source addressCompare which interface sends NTP packets against which the server permits.

Frequently asked

Why doesn't ping working mean NTP will work too?

Ping only proves basic reachability between two addresses you chose. NTP's source address is picked automatically by the router based on its outbound route, and that address might not be the one the server's access policy permits, even though the same router can reach the server for other traffic.

Ready to run this lab yourself?

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