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CCNA · 200-301Intermediate

OSPF: Passive Interface Surprise

A router advertises a subnet but never forms the intended adjacency. Diagnose the passive-interface scope and fix it yourself, live, right below.

Interactive simRoutingOSPF

Symptom

A router advertises a subnet but never forms the intended adjacency. Identify the passive-interface scope and restore only the required OSPF hello traffic.

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 ↓
OSPF: Passive Interface Surprise

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 · R2

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

R1 advertises its loopback but has no neighbor on the shared transit link because passive-interface default was configured and the transit interface was not exempted.

What you'll practice

  • Read OSPF protocol configuration
  • Distinguish advertisement from hello behavior
  • Use passive-interface safely
  • Verify a FULL neighbor

The topology

Two routers connect over one transit network and advertise loopbacks into a single OSPF area.

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.

Check OSPF neighbors
show ip ospf neighbor
Inspect OSPF settings
show ip protocols
Inspect OSPF interface
show ip ospf interface Ethernet0/0
OSPF: passive interface surpriseR1 advertises its loopback but sends no Hellos on its transit interface.R1Et0/0 · 10.0.12.1/30R2Et0/0 · 10.0.12.2/3010.0.12.0/30passive-interface default on R1R1's passive-interface scopeCompare what's advertised against where Hellos are actually permitted.

Frequently asked

Why keep passive-interface default at all instead of removing it?

passive-interface default is a sound hardening control: it silences Hellos everywhere by default, so only interfaces explicitly exempted send them. The fix isn't removing the default, it's exempting the one transit interface that actually needs adjacencies.

Ready to run this lab yourself?

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