Open Shortest Path First (OSPF)
Also known as: link-state routing, intra-domain routing
OSPF is a link-state interior gateway protocol (IGP) for routing within an autonomous system. Introduced by John Moy in 1998, OSPF converges faster than distance-vector protocols and supports equal-cost multipath (ECMP). It remains widely deployed in enterprise and ISP networks for intra-domain routing, though IS-IS is increasingly preferred in large backbones.
Key highlights
- Fast convergence: detects link changes and reconverges in seconds
- Optimal paths: Dijkstra ensures shortest path routing
- ECMP support: equal-cost paths used simultaneously for load balancing
- Hierarchical design: areas reduce flooding and state
- No hop-count limit: metric is configurable link cost, not hop count
Intuition
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How it works
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When to use it
Use OSPF for enterprise and mid-size ISP networks. OSPF convergence is faster than RIP and scales better. For very large networks (>1000 routers), IS-IS is often preferred. OSPF areas provide hierarchical scaling but add configuration complexity.
Strengths & limitations
- Fast convergence: detects link changes and reconverges in seconds
- Optimal paths: Dijkstra ensures shortest path routing
- ECMP support: equal-cost paths used simultaneously for load balancing
- Hierarchical design: areas reduce flooding and state
- No hop-count limit: metric is configurable link cost, not hop count
- CPU/memory overhead: running Dijkstra on large topologies is expensive
- Flooding overhead: LSA flooding consumes bandwidth, especially with many routers
- Area design complexity: poorly designed areas can cause routing inefficiency
- Not suitable for policy-based routing; use BGP for that
- Limited to intra-domain routing; must use BGP for inter-domain
Common pitfalls
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Applications
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Frequently asked
What is an area in OSPF?
An area is a contiguous network segment. Area 0 (backbone) connects all areas. Routers flood LSAs only within areas, reducing overhead. Inter-area routing is handled by Area Border Routers (ABRs).
How does OSPF handle interface cost?
OSPF metric is the sum of link costs. By default, cost = 100 Mbps / interface bandwidth. Link cost is tunable, allowing operators to influence path selection.
What is ECMP in OSPF?
Equal-Cost Multipath: if multiple paths have the same cost, OSPF uses all of them, distributing traffic across interfaces. This provides load balancing without explicit configuration.
What are stub and NSSA areas?
Stub areas reduce LSA flooding by not receiving external routes (LSA type 5). NSSA (Not-So-Stubby Area) is a variant allowing selective external route advertisement.
Sources
- 1.Moy, J. T. (1998). OSPF Version 2. RFC 2328.
- 2.Coltun, R., Ferguson, D., Moy, J., & Lindem, A. (2008). OSPF for IPv6. RFC 5340.
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Cite this page
ScholarGate. (2026, June 3). OSPF. ScholarGate. https://scholargate.app/telecommunications/ospf