Linux: Inspect and Refresh the Neighbour Cache
The gateway's cached MAC is stale. Inspect neighbour state, delete one entry, trigger resolution, and verify. Work through it yourself in the live host terminal below.
Symptom
The gateway's cached MAC is stale. Inspect neighbour state, delete one entry, trigger resolution, and verify.
Your mission
Inspect, recover, then verify
Format
Linux shell
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: Linux
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.
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
After a router replacement, the host still associates 10.20.30.1 with the old gateway MAC and cannot send off-subnet traffic.
What you'll practice
- Display IPv4 and IPv6 neighbour entries
- Interpret STALE, REACHABLE, INCOMPLETE, and FAILED
- Delete one neighbour entry
- Trigger and verify fresh resolution
The topology
One Linux host shares Ethernet with gateway 10.20.30.1. The router was replaced and now uses MAC 00:50:56:aa:00:02.
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.
ip neigh showTopology diagram
Fact-checked references
The commands and behaviour in this lab were checked against these primary references.
Frequently asked
Is STALE always broken?
No. STALE means a valid entry has not been confirmed recently. The old MAC in this scenario, not the state label alone, is the decisive evidence.
Why ping after deleting?
Traffic to the same-link gateway causes the kernel to resolve its link-layer address again.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.