Christmas Calendar: OSPF Stub Areas and OSPF Network Types

Day 1️⃣6️⃣: OSPF stub areas are used to control the advertisement of external routes into an area. A default route is generated instead. A totally stubby area also suppresses inter-area routes.

Day 1️⃣7️⃣: OSPF not-so-stubby area (NSSA) and totally NSSA have the capability to import external routes, which is not allowed in other stub area types. NSSAs use a special Type 7 LSAs for the external routes, which are translated to Type 5 LSAs by the ABR.

Day 1️⃣8️⃣: This table depicts the flooding scope of OSPF LSAs. In other words, it illustrates which LSAs are allowed in each OSPF area type.

Day 1️⃣9️⃣: OSPF defines distinct types of networks (based on their physical link types). OSPF operates differently for each network type, including how neighbors are discovered, if multiple routers can exist on the same subnet, and if a DR/BDR is elected.

Day 2️⃣0️⃣: When OSPF adjacency is formed, a router goes through several state changes before it becomes fully adjacent with its neighbor. This diagram summarizes OSPF neighbor states and illustrates the state changes.


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