OSPF Area Types and LSA's

Link State Advertisement (LSA) Types have never been my strong suite. I made a visual representation of how they are forwarded to help me get a better grasp on them.

When a type 5 LSA enters a stubby area, it’s entered into the RIB as a default route, striping the type 5 LSA routes. Type 5 LSAs are also not forwarded to not-so-stubby areas. To get external routes INTO an not-so-stubby area, the area has to be converted to a totally not-so-stubby-area, which will remove all summaries and inject a default route.

As you can see from the diagram, above, there are six area types.

All areas have to be directly connected to area 0, which is the backbone area. All routers that exist within only area 0 or within just a single area are called backbone routers. Routers that connect to multiple OSPF areas are called ABRs. Routers that are connected to an OSPF area AND connected to an external routing protocol (EIGRP, ISIS, RIP, BGP, static routes, etc) are called ASBRS.

Stubby areas and totally stubby areas are areas that have one way in and one way out of an area. That is, they can’t be connected to area 0 and any other area or external network at the same time, as a stubby and totally stubby area ABR will not forward type 5 LSAs to other areas. A totally stubby area will only have a default route injected into its network.

Not-so-stubby and totally not-so-stubby areas can be connected to external networks and will forward the type 7 LSAs beyond their area. When the type 7 LSAs reach an ABR, the ABR will convert them to type 5 and forward them to the other areas.