Which one of these statements is true of OSPF type 5 LSAs?
A.
They are used to summarize area routes to other areas.
B.
They are used in not-so-stubby areas to propagate external routes.
C.
They are used to notify areas of the ASBR.
D.
They are flooded to all areas except stub areas (external route).
Explanation:
Type 5 external link LSAs are used to advertise external routes originated from an ASBR. They
are flooded through the whole OSPF domain.
Note: The dashed arrows show the directions of LSAs in this example
Below is a summary of OSPF Link-state advertisements (LSAs)
Router link LSA (Type 1) – Each router generates a Type 1 LSA that lists its neighbors and thecost to each.
LSA Type 1 is only flooded inside the router’s area, does not cross ABR. Network link LSA (Type
2) – is sent out by the designated router (DR) and lists all the routers on the segment it is adjacent
to. Types 2 are flooded within its area only; does not cross ABR. Type 1 & type 2 are the basis of
SPF path selection. Summary link LSA (Type 3) – ABRs generate this LSA to send between areas
(so type 3 is called inter-area link). It lists the networks inside other areas but still belonging to the
autonomous system and aggregates routes. Summary links are injected by the ABR from the
backbone into other areas and from other areas into the backbone. Summary LSA (Type 4) –
Generated by the ABR to describe routes to ASBRs. In the above example, the only ASBR
belongs to area 0 so the two ABRs send LSA Type 4 to area 1 & area 2 (not vice versa). This is an
indication of the existence of the ASBR in area 0. Note: Type 4 LSAs contain the router ID of the
ASBR.
External Link LSA (LSA 5) – Generated by ASBR to describe routes redistributed into the area
(which means networks from other autonomous systems). These routes appear as E1 or E2 in the
routing table. E2 (default) uses a static cost throughout the OSPF domain as it only takes the cost
into account that is reported at redistribution. E1 uses a cumulative cost of the cost reported into
the OSPF domain at redistribution plus the local cost to the ASBR. Type 5 LSAs flood throughout
the entire autonomous system but notice that Stubby Area and Totally Stubby Area do not accept
Type 5. Multicast LSA (Type 6) are specialized LSAs that are used in multicast OSPF applications.
NSSA External LSA (Type 7) – Generated by an ASBR inside a NSSA to describe routes
redistributed into the NSSA. LSA 7 is translated into LSA 5 as it leaves the NSSA. These routes
appear as N1 or N2 in the ip routing table inside the NSSA. Much like LSA 5, N2 is a static cost
while N1 is a cumulative cost that includes the cost up to the ASBRhttp://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml#appa
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