OSPF Is Innocent

OSPF is extremely stable. Most outages blamed on OSPF are actually design problems that only show themselves when the network grows.

The Flat Area Disaster

Everything in Area 0 feels easy at the beginning. No thinking. No planning. Just neighbors everywhere. Then LSAs explode, SPF calculations spike CPU, and convergence becomes slow exactly when you need it to be fast.

Areas Exist for a Reason

Areas are not academic theory. They are scaling tools. They limit LSA flooding and reduce SPF calculations. If you have more than a handful of routers and you are still flat, you are planting a time bomb.

ABR Abuse

Not every router should be an ABR. Random distribution layer switches acting as ABRs create unpredictable LSA paths and hard to troubleshoot routing.

A Simple Rule

  • Core is Area 0
  • Distribution is ABR
  • Access lives in non backbone areas

LSA Flooding and Why You Should Care

Every topology change floods LSAs. In large flat designs this becomes a storm. Routers spend time recalculating instead of forwarding packets.

When CPU is busy calculating routes, packets wait.

Timer Tuning Mistakes

People reduce hello and dead timers trying to get fast convergence. That often makes things worse. You create flapping adjacencies on imperfect links.

No Summarization Anywhere

If every subnet is visible everywhere, you are leaking detail into parts of the network that do not need it. Summarization at ABRs keeps the routing table sane.

Careless Redistribution

Redistributing static, BGP, or connected routes without filtering creates route feedback loops and ghost paths that are painful to trace.

A Healthy OSPF Design

  • Clear area hierarchy
  • ABRs only at distribution
  • Summarize aggressively
  • Default routes toward access
  • Minimal redistribution with filters

Conclusion

OSPF works beautifully when you respect its design model. Ignore that model and you create a slow moving outage waiting for traffic growth.