OSFP LSA types 1-7

I'm going through the process of decertifying my CCNP and one thing I can never recall from memory is a good description of each of the OSPF LSA types. Therefore it gets a quick blog post to aid the old memory.

LSA Type 1 (Router LSA):

Routers each create a type 1 LSA for each area they connect to in order to represent themselves within an area. Therefore the LSDB for an area will contain one type 1 LSA  for each router in the area. 

LSA Type 2 (Network LSA):

The type 2 LSA is created by a DR to detail the subnet and connected router interfaces in that subnet.

LSA Type 3 (Network Summary LSA):

The type 3 LSA is used to advertise subnets listed in one area to another area. The is created by an ABR. 

LSA Type 4 (ASBR Summary LSA):

The type 4 LSA is created by an ABR when it receives a type 5 LSA from a ASBR. This LSA is required in order to support the tie breaking logic for best path selection for routers, internal to an area, when calculating the best path to an external network in another area. This is required because E2 external routes do not increment the metric when travelling through the network, therefore a router in an area could have 2 (or more) paths to the external network. The tie break logic says that even though the metrics tie the router should put into the routing table the best route to reach the ASBR. So if it had a fast Ethernet connection and a serial connection even though the metrics would be the same (20 by default for an E2) instead of load balancing the router looks up the best path to reach the ASBR and puts this route into the routing table.

Now the above logic works fine if the router and the ASBR are in the same area. If not then the router can calculate the best path to the ABR but beyond this it has no awareness of the topology. This is where the type 4 LSA comes in. A type 4 LSA is generated by an ABR and details the ABR cost to reach the ASBR. This solves the above problem because where a internal area router has 2 ABRs out of the area, it now knows the cost of each ABR to reach the ASBR in the other area and so can make the best path selection accordingly.

LSA Type 5 (AS External LSA):

The type 5 LSA is used to advertise external routes into an area, this is created by the ASBR.

LSA Type 6 (Group Membership):

Unused on Cisco IOS so I'm ignoring it for the time being

LSA Type 7 (NSSA External LSA):

The type 7 LSA is created by an ASBR in a stub area to advertise external routes from an ASBR within that stub area. Stub areas suppress type 5 LSAs therefore an ASBR in a stub area uses type 7 LSAs within the stub which are then converted to type 5 LSAs at the ABR. Stub areas with an ASBR are NSSAs (Not So Stubby Areas)

I might well update this as I read and learn more but for the time being it's a good reference for me.