OSPF: Passive Interface Surprise
Passive interfaces are a sound OSPF hardening control—until applied to a transit interface. The diagnosis is learning what is advertised versus where hellos are permitted.
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.
show ip ospf neighborshow ip protocolsshow ip ospf interfaceTopology diagram
Frequently asked
Is this aligned to CCNA practice?
Yes. It focuses on configuration, verification, and troubleshooting skills covered by the CCNA-level technology domain named on this lab.
Ready to run this lab yourself?
No setup, no image sourcing. Book a session or ask for a live demo.