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Categories of Multi-area Components
Type 2 LSA Referred to as a Network Link Advertisement (NLA), the
Type 2 LSA is generated by designated routers (DRs). Recall that a des-
ignated 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. Note that the Type 2 LSA
is only sent to routers that are in the area containing the specific network.
Type 3 and Type 4 LSAs Referred to as Summary Link Advertisements
, 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 an area. These
LSAs advertise intra-area routes to the backbone area (Area 0) and both
intra-area and inter-area routes to non-backbone areas.
Type 5 LSA Referred to as AS External Link Advertisements, Type 5
LSAs are sent by autonomous system boundary routers (ASBRs). These
ASBRs use Type 5 LSAs to advertise routes that are external to the OSPF
autonomous system.
OSPF Area Types
One of our main motivations for subdividing a single OSPF area into multiple
areas was to reduce router overhead. We decided that all routers didn't need
to have the entire network topology in their link state databases. Let's
now examine the types of non-standard areas we can use to reduce router
Stub area Routers in a stub area do not receive Type 5 LSAs. Instead,
they receive a default route that is used to reach external networks. There-
fore, stub area routers have reduced overhead since they do not have to
process Type 5 LSAs.
Totally stubby area To further reduce the number of LSAs that an inter-
nal router will need to process, the router can be configured as a totally
stubby area. In addition to not receiving Type 5 LSAs, a totally stubby
area does not receive summary LSAs. The function of a totally stubby area
is Cisco-specific, which is an important concept to remember when
designing an OSPF network in a multi-vendor routing environment.
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