Watch the full Intro to OSPF Tech Talk here to explore these concepts in greater depth.
In this tech talk, Patrick Cavell walked through the fundamentals of Open Shortest Path First, or OSPF, one of the most widely used dynamic routing protocols in modern networks. The goal of the session was to demystify how OSPF works, explain why it is so effective at scale, and highlight best practices for real world implementation.
What Is OSPF and Why It Matters
OSPF is a link state routing protocol designed to calculate the most efficient path for traffic across an IP network. Rather than exchanging entire routing tables, OSPF shares detailed information about network links using Link State Advertisements, or LSAs. Each router uses this information to build a complete view of the network and independently calculate the best paths.
This approach allows OSPF to converge quickly and remain highly consistent, even in complex environments.
How OSPF Shares and Maintains Network State
A key concept emphasized during the session is that OSPF shares interface and link information, not individual routes. LSAs are flooded throughout an area, so every router maintains an identical link state database. This shared understanding is critical, as routing stability depends on all routers seeing the same network topology.
Because each router calculates routes locally from the same data set, OSPF can adapt efficiently to changes while minimizing disruptions.
Areas, Scalability, and Design Choices
To support large networks, OSPF allows administrators to divide the network into multiple areas. Areas help control routing table size and limit LSA flooding, improving performance and scalability. All areas connect through the backbone, known as Area 0, which ensures proper route of exchange.
Stub areas were also discussed as a way to simplify routing in parts of the network by limiting which routes are advertised, reducing overhead for routers that do not need full visibility into external networks.
Designated Routers and Adjacencies
On broadcast networks, OSPF uses a designated router and backup designated router to reduce unnecessary traffic. Instead of every router forming adjacencies with every other router, full adjacencies are centralized.
In the example shared during the webinar, router two was elected as the designated router (DR); router three served as the backup designated router (BDR), and all other routers operated as DROTHER routers, having full adjacencies only to the DR and the BDR. This model keeps routing efficient while maintaining full network awareness.
Understanding OSPF LSAs and External Routes
Patrick also reviewed the most common OSPF LSA types and how they are used to share routing information within and between areas. The most encountered types include:
- Type 1 and Type 2 LSAs, which describe routers and networks within an area
- Type 3 and Type 4 LSAs, which allow routes to be shared between different OSPF areas
- Type 5 LSAs, which advertise external networks from Autonomous System Boundary Routers into the OSPF domain
Type five LSAs are particularly important because they allow OSPF to learn routes from non-OSPF domains. These LSAs are flooded throughout the network, except into stub areas, where external routes are intentionally limited.
Reading the Routing Table and Avoiding Pitfalls
An OSPF routing table can reveal whether a network is operating in a multi-area design and whether external routes are being introduced. The session also reinforced the importance of administrative distance, noting that connected routes always take precedence over OSPF learned routes.
One key caution highlighted was route filtering. Because all routers in an area must receive identical LSAs, improper filtering can lead to inconsistencies, routing loops, or traffic black holes.
What’s Next
This session provided a strong foundation for understanding OSPF and how it supports scalable, resilient network design. If you are responsible for building or managing networks, mastering these fundamentals is essential.
Be sure to join us for our next tech talk on January 15, co-presented by Lockstep and Glean, to learn how AI is reshaping enterprises across all industries, including Public Sector, Education, and Healthcare.
