OSPFv2 LSA Types

Part of what makes OSPF hard to understand and troubleshoot is all the different types of Link-State Advertisements (LSAs). Some areas have different LSA types allowed and some can only be originated by specific routers and so on - it's a mess! Take into account the fact that the LSAs are referred to by numbers and in the Cisco CLI they are referred to by names, which is then just another thing you will have to get familiar with when working with OSPF.

Below is a list of OSPF LSA types along with a description of their function in the OSPF domain and the command to show them in the Link-State Database (LSDB) of a Cisco router.

LSA Type 1 - Router LSA
Each router in an area will generate a type 1 LSA - one for each area it is connected to. It contains the routers RID and all the routers IP addresses for interfaces attached to the specific area. It is not flooded beyond the area in which it was originated.
The show ip ospf database router command will show the type 1 LSAs in a routers LSDB.

LSA Type 2 - Network LSA
The Designated Router (DR), on shared segment, originates type 2 LSAs containing the interface IP address of the DR and a list of the DRs connected neighbors in the area. The type 2 LSA is only propagated in the area it is originated in and only by the DR.
The show ip ospf database network command will show the type 2 LSAs in the routers LSDB.

LSA Type 3 - Summary LSA
The summary LSA is originate by an Area Border Router (ABR) and advertises a prefix from one area into another. It will advertise the destination prefixes from a non-backbone area into the backbone including the metric from the ABR to the destination. It will do this for each prefix known, but it can instructed to replace the individual prefixes with a less-specific summary address and thus the ABR is one of the few places, in an OSPF network, that allows for network summarization.
The show ip ospf database summary command will show the type 3 LSAs in the routers LSDB.

LSA Type 4 - ASBR Summary LSA
Originated by an ABR, this type of LSA contains the host address of an Autonomous System Border Router (ASBR) in an area and the cost to reach it from the ABR. This is sent along with the LSA type 5 to allow routers outside the area, of the ASBR, to find a path to the ASBR, redistributing an external route into OSPF. This is not needed for routers in the same area as the ASBR due to the presence of the type 1 and 2 LSAs.
The show ip ospf database asbr-summary command will show the type 4 LSAs in the routers LSDB.

LSA Type 5 - AS-External LSA
The type 5 LSA is originated by an ASBR and contains the E1 or E2 external route information for a prefix redistributed into the OSPF process from another AS (for example BGP, EIGRP or maybe just a connected interface, to name but a few options). Routers not residing in the same area as the ASBR will need an ABR to originate a type 4 LSA containing the information needed to compute the SPF tree to the ASBR.
The show ip ospf database external command will show the type 5 LSAs in the routers LSDB.

LSA Type 6 - Group Membership
Used in Multicast OSPF. The feature is unsupported in Cisco IOS (as far as I know).

LSA Type 7 - NSSA External
The type 7 LSA is flooded in an area by an ASBR in a Not-So-Stubby-Area (NSSA). An ABR will convert the type 7 LSA into a type 5 LSA for other areas. The type 7 LSA allows other routers in the same NSSA area to learn the external routes advertised by the ASBR as one of the features of an NSSA type area is to filter out type 5 LSAs.
The show ip ospf database nssa-external command will show the type 5 LSAs in the routers LSDB.

Those are the most common OSPF LSA types. There are LSA types 8, 9, 10 and 11 as well, but they are bit outside my scope right now. I will probably update this post later on in my studies, when I get to OSPFv3 and MPLS, where I expect I will be working with these types of LSAs in some way.