- 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.
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:
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.
