Five commonly used types of link-state advertisements (LSAs) exist. The standard lists 11 types of LSA, although currently not all of these are in use. Six LSAs are listed here; this discussion focuses only on those advertisements sent between routers in the same area and on the same segment.
The five link-state advertisements are as follows:
The router link—This LSA is generated for each area to which the router belongs. This LSA gives the link states to all other routers within an area. This LSA is flooded into an area. This is identified as a Type 1 LSA.
The network link—This LSA is sent out by the designated router and lists all the routers on the segment for which it is the designated router and has a neighbor relationship. The LSA is flooded to the whole area. This is identified as a Type 2 LSA.
The network summary link—This LSA is sent between areas and summarizes the IP networks from one area to another. It is generated by an ABR. This is identified as a Type 3 LSA.
The AS external ASBR summary link—This LSA is sent to a router that connects to the outside world (ASBR). It is sent from the Area Border Router to the Autonomous System Boundary Router. The LSA contains the metric cost from the ABR to the ASBR. This is identified as a Type 4 LSA.
The external link—This LSA is originated by AS boundary routers and is flooded throughout the AS. Each external advertisement describes a route to a destination in another autonomous system. Default routes for the AS can also be described by AS external advertisements. This is identified as a Type 5 LSA.
The NSSA External LSA—Identified as Type 7, these LSAs are created by the ASBR residing in a not so stubby area (NSSA). This LSA is very similar to an autonomous system external LSA, except that this LSA is contained within the NSSA area and is not propagated into other areas.
Figure 6-4 clearly shows the relationships between the different LSAs. This section discusses the router and network LSAs. The LSAs concerned with communication outside an area are considered later.
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