OSPF

Course at a glance

• 63 lessons in 5 units, plus a hands-on OSPF lab.
• Exams: covers the OSPF topics of CCNA 200-301, CCNP ENCOR 350-401, CCNP ENARSI 300-410, and CCIE Enterprise Infrastructure.
• Format: self-paced lessons with Cisco IOS configurations, packet captures, and video.
• Last updated: October 2026.

What is OSPF?

OSPF (Open Shortest Path First) is a link-state interior gateway protocol (IGP). It routes traffic within a network that is under your own administrative control. Each OSPF router describes its own links in LSAs and floods them to its neighbors, so every router in an area ends up with an identical link-state database. Each router then runs Dijkstra’s SPF (Shortest Path First) algorithm on that database to find the path with the lowest cost to every destination.

Distance vector protocols only know what their neighbors tell them. OSPF is different: every router has a complete map of its area, which makes OSPF loop-free and quick to converge. To keep that map manageable in large networks, OSPF uses areas. Area 0 is the backbone, and all other areas connect to it through Area Border Routers (ABRs). OSPF runs directly on top of IP (protocol 89), uses multicast to find neighbors, and only sends updates when something changes.

Who uses OSPF? It’s one of the most popular IGPs in enterprise and campus networks, and service providers use it in their core, often together with MPLS and BGP. Because it’s an open standard, it works between routers from different vendors. OSPFv2 is used for IPv4, and OSPFv3 for IPv6.

OSPF Learning Roadmap

This roadmap follows the course units. Each topic has a short explanation and links to the lessons where we cover it in detail.

OSPF Fundamentals

OSPF calculates a cost for each link, based on its bandwidth, and picks the path with the lowest total cost. To keep large networks manageable, we split them into areas, connected to the backbone area 0. In this unit you configure single and multi-area OSPF, tune the reference bandwidth for fast links, secure OSPF with authentication, and advertise a default route.

Lessons: OSPF explained, Basic OSPF configuration, Multi-area OSPF, Reference bandwidth, MD5 authentication, HMAC-SHA authentication, Default route

OSPF LSAs and the LSDB

OSPF routers don’t exchange routes; they exchange LSAs (link-state advertisements) that describe routers, links, and networks. All routers in an area build the same link-state database (LSDB) from these LSAs. There are different LSA types for intra-area, inter-area, and external information, and once you know them, you can read the OSPF database like a map of your network.

Lessons: OSPF LSA types, Type 1 Router LSA, Type 2 Network LSA, Type 3 Summary LSA, Type 5 External LSA, LSAs and LSDB flooding, How to read the OSPF database

OSPF Neighbor Adjacency

OSPF routers discover their neighbors automatically with hello packets. Before two routers become neighbors, parameters like the area, hello and dead interval, subnet, and authentication have to match. They then go through several states, from Down to Full, to synchronize their databases. On multi-access networks like Ethernet, a DR and BDR are elected to reduce the number of adjacencies.

Lessons: OSPF packets and neighbor discovery, Hello and dead interval, Router ID, DR/BDR election, Passive interface, Troubleshooting OSPF neighbor adjacency

OSPF Network Types

The OSPF network type of an interface decides whether OSPF discovers neighbors automatically, whether it elects a DR and BDR, and which hello and dead timers it uses. Broadcast and point-to-point are the most common, but you also need to understand non-broadcast and point-to-multipoint for hub and spoke networks like DMVPN. The network type also affects the next hop address of your routes.

Lessons: OSPF network types, Broadcast, Non-broadcast, Point-to-point, Point-to-multipoint over DMVPN, Loopback, Next hop with network types

OSPF Stub Areas

Not every router needs to know every network. Stub areas reduce the size of the LSDB and routing table by replacing external and inter-area routes with a default route. There are four flavors: stub, totally stubby, NSSA (not-so-stubby area), and totally NSSA. The NSSA variants let you redistribute external routes into the area anyway, with type 7 LSAs.

Lessons: Introduction to stub areas, Stub area, Totally stubby area, NSSA, Totally NSSA, NSSA P-bit

OSPF Path Selection, Summarization and Filtering

OSPF doesn’t only look at cost. It always prefers intra-area routes over inter-area routes, and inter-area routes over external routes, before it compares the cost. Because all routers in an area need the same LSDB, you can only summarize and filter at area borders (ABR) and redistribution points (ASBR). When an area can’t connect directly to area 0, a virtual link fixes it.

Lessons: OSPF path selection, Summarization, Distribute-list filtering, Type 3 LSA filtering, Type 5 LSA filtering, Virtual link, Troubleshooting OSPF route advertisement

OSPF Convergence and High Availability

When a link fails, OSPF floods new LSAs and every router runs SPF again. SPF and LSA throttling let you decide how quickly routers react, and incremental SPF reduces the work. Loop-Free Alternate (LFA) Fast Reroute pre-computes a backup path, so traffic switches over almost immediately. For planned maintenance, the stub router feature, graceful shutdown, and graceful restart keep traffic flowing while a router reloads.

Lessons: SPF throttling, LSA throttling, Incremental SPF, LFA fast reroute, Remote LFA, Stub router, Graceful restart, Prefix suppression

Practice: OSPF Lab

Reading about OSPF is one thing; configuring it is another. The OSPF professional lab lets you put everything together: neighbors, areas, network types, stub areas, and troubleshooting.

Lab: OSPF Professional Lab 1

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