OSPF routers generate LSAs that are flooded within an area, between areas, and throughout the entire autonomous system. OSPF defines different LSA types for participating routers, DRs, ABRs, and ASBRs. Understanding the LSA types can help you with other OSPF concepts. Table 4-4 describes the major LSA types. For the ENSLD 300-420 exam, you need to know OSPF LSAs by type code (number) and by type name. Note that there are other LSA types that are not covered in this book.

Type 1 and Type 2 LSAs are intra-area LSAs that have an area-flooding scope. Type 3 LSAs provide summaries of destinations outside the local area but within the OSPF domain. Information carried in OSPF type 3 LSAs includes the network address, the network mask, and a metric. Type 4 LSAs provide reachability about the ASBR. Type 3 and Type 4 LSAs are interarea LSAs that have an area-flooding scope. ABRs exchange Type 3 and Type 4 LSAs. Type 5 LSAs advertise external destinations. Type 5 LSAs have a domain-flooding scope, which means they are flooded throughout all areas. Type 7 LSAs are originated by ASBRs in a not-so-stubby area (NSSA) and are similar to the Type 5 LSA and only flooded within the NSSA.

Table 4-4 Major LSA Types

Type Code	Type	Description
1	Router LSA	Produced by every router. Includes all the router’s links, interfaces, link states, and costs. This LSA type is flooded within a single area and does not travel into other areas.
2	Network LSA	Produced by every DR on every broadcast or NBMA network. It lists all the routers in the multiaccess network. This LSA type is contained within an area.
3	Summary LSA for ABRs	Produced by ABRs. It is sent into an area to advertise destinations outside the area.
4	Summary LSA for ASBRs	Originated by ABRs. Sent into an area by the ABR to advertise the IP addresses of the ASBRs. It does not advertise networks outside the OSPF network; only the ASBR does that.
5	Autonomous system external LSA	Originated by ASBRs. Advertises destinations external to the OSPF autonomous system, flooded throughout the whole OSPF autonomous system.
7	Not-so-stubby area (NSSA) external LSA	Originated by ASBRs in an NSSA. It is not flooded throughout the OSPF autonomous system but only to the NSSA. Similar to the Type 5 LSA.

Autonomous System External Path Types

The two types of autonomous system external paths are Type 1 (E1) and Type 2 (E2), and they are associated with Type 5 LSAs. ASBRs advertise external destinations whose cost can be just a redistribution metric (E2) or a redistribution metric plus the costs of each segment (E1) used to reach the ASBR.

By default, external routes are Type 2, which is the metric (cost) used in the redistribution. Type 1 external routes have a metric that is the sum of the redistribution cost plus the cost of the path to reach the ASBR.