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Chapter 5
Interconnecting OSPF Areas
partially in Area 10. There is yet another term to define routers that have
interfaces in more than one area:
Area border router An area border router is any router that is connected
to multiple OSPF areas.
Recall that the topology of an OSPF area is contained in a link state data-
base. Therefore, if a router is connected to multiple areas, it will contain mul-
tiple link state databases. This should be a design consideration when sizing
a router that will function as an area border router.
Also notice that RouterB is connected to an EIGRP network. Whether an
OSPF network is connected to an EIGRP network, a BGP network, an OSPF
network with a different Process ID, or a network running any other such
external routing process, this external network may be referred to as an
autonomous system. The scenario of an OSPF router sitting at the boundary
of an external routing process leads us to a fourth category of OSPF router:
Autonomous system boundary router An autonomous system bound-
ary router is any OSPF router that is connected to an external routing process.
The ability of an ASBR to exchange routing information between its
OSPF routing process and the external routing process to which the router
is connected is not an automatic process. Such routes are exchanged through
a process called route redistribution, which is the focus of Chapter 10,
"Route Optimization."
Link State Advertisements
Recall that a router's link state database is made up of Link State Advertise-
ments (LSAs). However, just as we had multiple OSPF router categories to
consider, we also have multiple types of LSAs to consider. Specifically, there
are five types of LSAs that we need to be concerned with. While the impor-
tance of LSA classification may not be immediately apparent, we will see its
application when we examine the various types of OSPF areas. Let's examine
the function of the various LSA types:
Type 1 LSA Referred to as a Router Link Advertisement (RLA), the
Type 1 LSA is an advertisement sent by a router to other routers in its
area. The advertisement contains the status of a router's link to the area
it is connected to. If a router is connected to multiple areas, then it will
send a Type 1 LSA for each of the areas it is connected to.
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