LSA Types

TABLE 6 . 1
LSA Types
Type Code Description
1 Router LSA
2 Network LSA
3 Network Summary LSA
4 ASBR Summary LSA
5 AS External LSA
6 Group Membership LSA
7 NSSA External LSA
8 External Attributes LSA
9 Opaque LSA (link-local scope)
10 Opaque LSA (area-local scope)
11 Opaque LSA (AS scope)

There are a total of 11 LSA types in existence. For the BSCI exam, you should be concerned
with only the first five types and Type 7. We will now drill down into each of these LSA types
to give you a better understanding of each. Keep in mind that it is very important to understand
the different LSA types, not only for the exam but also for the real world.
Type 1 LSA
Referred to as a
router link advertisement (RLA)
, the Type 1 LSA is an advertisement
sent by a router to all other routers in its area. The Type 1 LSA contains information
about all of the router’s links in the area, the status of each link, and the cost for each
link. A router, which has connections to multiple areas, will send a Type 1 LSA to each of
the areas the router is connected to, but only describing links it has in each respective area,
not sharing this information across area boundaries.
Type 2 LSA
Referred to as a
network link advertisement (NLA)
, the Type 2 LSA is generated
only on multi-access networks, and then only by designated routers (DRs) when they are fully
adjacent with at least one other router on the network. Recall that a designated router is elected
to represent other routers in its network, and it has established adjacencies with each of the routers
within its network. The DR uses the Type 2 LSA to send out information about the state of other
routers that are part of the same network, information it has learned through receipt of Type 1
LSAs from these routers. The Type 2 LSA describes each of the routers on the DR’s network,
including the DR. Note that the Type 2 LSA is sent only to routers that are in the area containing
the network for which the advertising router is the DR.
Type 3 and Type 4 LSAs
Referred to as
summary link advertisements (SLAs)
, the Type 3 and
Type 4 LSAs are generated by area border routers. These ABRs send Type 3 and Type 4 LSAs
to all routers within a single area only. These LSAs advertise non-backbone intra-area routes, routes
within an area, into the backbone area (Area 0) and both intra-area and inter-area routes,
routes to other areas, to non-backbone areas. The only real difference between Type 3 and Type 4
LSAs is that a Type 3 will advertise networks outside of an area into an area and a Type 4 will
advertise routes to ASBRs into an area. A special example of a Type 3 LSA is created by the
default information originate
command.
Type 5 LSA
Referred to as
AS external link advertisements
, Type 5 LSAs are sent by
autonomous system boundary routers. These ASBRs use Type 5 LSAs to advertise routes
that are external to the OSPF autonomous system or a default route external to the OSPF
autonomous system that is reachable through them.
Type 7 LSA
To overcome the limitations of an ASBR not being able to belong to a stub area,
LSA Type 7,
NSSA external LSA
, was created. Type 7 LSAs are generated only by an ASBR in
9 Opaque LSA (link-local scope)
10 Opaque LSA (area-local scope)
11 Opaque LSA (AS scope)
TABLE 6 . 1
LSA Types
(continued)
Type Code Description
Categories of Multi-Area Components
181
a not-so-stubby area (NSSA). The Type 7 LSA will propagate across the area to the NSSA ABR.
Once the Type 7 LSA reaches the ABR, the ABR will convert the Type 7 LSA into a Type 5 LSA
and propagate it to the backbone. The Type 7 LSA advertises routes that are external to the
OSPF autonomous system.