OSPF LSA TYPES AND AREAS

• LSA 1 (Router LSA)

Generated by all routers in an area to describe their directly attached links (Intra-area routes). These do not leave the area.

• LSA 2 (Network LSA)

Generated by the DR of a broadcast or Nonbroadcast segment to describe the neighbors connected to the segment. These do not leave the area.

• LSA 3 (Summary LSA)

Generated by the ABR to describe a route to neighbors outside the area. (Inter-area routes)

• LSA 4 (Summary LSA)

Generated by the ABR to describe a route to an ASBR to neighbors outside the area.

• LSA 5 (External LSA)

Generated by ASBR to describe routes redistributed into the area. These routes appear as E1 or E2 in the routing table. E2 (default) uses a static cost throughout the OSPF domain as it only takes the cost into account that is reported at redistribution. E1 uses a cumulative cost of the cost reported into the OSPF domain at redistribution plus the local cost to the ASBR.

• LSA 6 (Multicast LSA)

Not supported on Cisco routers.

• LSA 7 (NSSA External LSA)

Generated by an ASBR inside a NSSA to describe routes redistributed into the NSSA. LSA 7 is translated into LSA 5 as it leaves the NSSA. These routes appear as N1 or N2 in the ip routing table inside the NSSA. Much like LSA 5, N2 is a static cost while N1 is a cumulative cost that includes the cost upto the ASBR.


Area types

STUB AREA
Instead of propagating external routes (type 5 LSAs) into the area, the ABR injects a type 3 LSA containing a default route into the stub area.

For an area to become a stub, all routers belonging to it must be configured to operate as such. Stub routers and non-stub routers will not form adjacencies.

Router(config-router)# area 10 stub

Totally stubby areas 

can only contain type 1 and 2 LSAs, and a single type 3 LSA. The type 3 LSA describes a default route,
substitutes all external and inter-area routes.

Router(config-router)# area 10 stub no-summary

NSSA

AN ASBR cannot form an adjacency in a stub area.  To work around this NSSA were created. An NSSA makes use of type 7 LSAs, which are essentially type 5 LSAs in disguise. This allows an ASBR to advertise external links to an ABR, which converts the type 7 LSAs into type 5 before flooding them to the rest of the OSPF domain.

An NSSA can function as either a stub or totally stubby area. To designate a normal (stub) NSSA, all routers in the area must be so configured:

Router(config-router)# area 10 nssa 

To expand an NSSA to function as a totally stubby area, eliminating type 3 LSAs (bar 1), all of its ABRs must be configured with the no-summary parameter:

Router(config-router)# area 10 nssa no-summary 

Summary

• Standard areas can contain LSAs of type 1, 2, 3, 4, and 5, and may contain an ASBR. The backbone is considered a standard area.
• Stub areas can contain type 1, 2, and 3 LSAs. A default route is substituted for external routes.
• Totally stubby areas can only contain type 1 and 2 LSAs, and a single type 3 LSA. The type 3 LSA describes a default route, substituted for all external and inter-area routes.
• Not-so-stubby areas implement stub or totally stubby functionality yet contain an ASBR. Type 7 LSAs generated by the ASBR are converted to type 5 by ABRs to be flooded to the rest of the OSPF domain.