Router LSAs (type 1) Router LSAs are sent by every OSPF router into each connected area. These advertisements describe the router's links within the area. Network LSAs (type 2) Network LSAs are sent by DRs, and describe the routers connected to the network from which the LSA was received. Summary LSAs for ABRs (type 3) Summary LSAs for ABRs are sent by ABRs. These advertisements describe inter- area routes for networks. They are also used to advertise summary routes. Summary LSAs for ASBRs (type 4) Summary LSAs for ASBRs are sent by ASBRs and ABRs. These advertisements describe links to ASBRs. Autonomous System External (ASE) LSAs (type 5) ASE LSAs are sent by ASBRs and ABRs. These advertisements describe net- works external to the autonomous system. They are sent everywhere, except to stub areas. Not So Stubby Area (NSSA) LSAs (type 7) NSSA LSAs are sent by ABRs. These advertisements describe links within the NSSA. OSPF separates networks into areas. The core area, which all other areas must con- nect with, is area zero. One of the perceived benefits of OSPF is that it forces you to design your network in such a way that there is a core with satellite areas. You can certainly build an OSPF network with only an area zero, but such a design usually doesn't scale well. There are two main types of areas: backbone and nonbackbone areas. Area zero is the backbone area; all other areas are nonbackbone areas. Nonbackbone areas are further divided into the following types: Normal area An OSPF area that is not area zero, and is not configured as one of the following types. No special configuration is required. Chapter 10: Routing Protocols Stub area An OSPF area that does not allow ASE LSAs. When an area is configured as a stub, no 0 E1 or 0 E2 routes will be seen in the area. Totally stubby area (TSA) An OSPF area that does not allow type-3, -4, or -5 LSAs, except for the default summary route. TSAs see only a default route, and routes local to the areas themselves. Not SO stubby area (NSSA) No type-5 LSAs are allowed in