Avaya Configuring IP Services Manuel d'utilisateur Page 145

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Configuring OSPF Services
4-7
Dividing an AS into Areas
OSPF routers reduce and restrict the amount of internal and external routing
information that is flooded through the AS by dividing the AS into
areas
.
Figure 4-2 shows an OSPF autonomous system divided into three areas and a
required central area called a
backbone
that is used to distribute routing
information among areas. Each area has a unique ID number in the AS (the
backbone always has ID 0.0.0.0).
Figure 4-2. OSPF Areas
An AS divided into areas requires three kinds of OSPF router: internal routers,
boundary routers, and border routers.
Internal routers
flood each area with complete routing information about
changes that occur within the area. In Area A, for example, R4, R5, and R6
are internal routers.
Boundary routers
receive information about routes outside the AS, generate
AS external (ASE) routing advertisements, and inject the ASEs into the AS.
RI, R2, R7, and R14 are boundary routers.
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