Route filtering in OSPF isn't just theory—it’s a vital part of large-scale enterprise design. While OSPF was designed to be a link-state protocol with complete topological visibility, real-world implementations often require strategic control over route propagation.
Why Filter OSPF Routes?
In multi-area OSPF designs, especially in large environments with dozens of areas, it’s not always desirable—or scalable—to allow every route to flood throughout the OSPF domain. Route filtering helps reduce routing table size, simplify troubleshooting, and improve convergence in unstable networks.
Common Use Cases
- ABR Route Filtering: Prevent specific intra-area routes from being advertised between areas.
- LSA Type 3 Filters: Limit external redistribution to specific areas using prefix lists and distribute-lists.
- Stub and NSSA Areas: Used in conjunction with filters to avoid unnecessary Type 5 LSAs.
Configuration Tips
Use area X filter-list prefix carefully—it’s only effective on ABRs and applies to Type 3 LSAs. In Cisco IOS, use distribute-list out on redistribution points to control external routes from leaking into OSPF.
Gotchas
OSPF is not designed to have per-router customized views like BGP.
Too much filtering can break SPF calculations or make troubleshooting
hellish. Always validate filters with show ip ospf database and simulate changes in lab before deploying.