OSPF LSA Types

I was working through Narbik's OSPF labs and ran across a few notes that I took in the class. I believe way too many network engineers get stuck when it comes to LSA types and Narbik did a great job teaching us the concepts. Here is a nice visio I did to capture the concepts! Let me know how these help you out. Let me first try to explain the different LSA types and then you can take a look at the diagram for better understanding.

• 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.