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Chapter 5
Interconnecting OSPF Areas
Consider the advantages of this hierarchical approach. First of all, routers
that are internal to a defined area need not worry about having a link state
database for the entire network, only their own areas, thus reducing memory
overhead. Second, routers that are internal to a defined area now only have
to recalculate their link state database when there is a topology change
within their particular area. Topology changes in one area will not cause glo-
bal OSPF recalculations, thus reducing processor overhead. Finally, since
routes can be summarized at area boundaries, the routing tables on each
router need not be as large as they would be in a single area environment.
Of course, as we start subdividing our OSPF topology into multiple areas,
we introduce some complexity into our configuration. Therefore, in this
chapter we will examine these various configuration subtleties, in addition to
strategies for effectively troubleshooting multi-area OSPF networks.
Categories of Multi-area Components
his section covers the various roles that routers play in an OSPF large
network. These include backbone routers, internal routers, area border rout-
ers, and autonomous system boundary routers. We'll also discuss the differ-
ent types of advertisements that are used in an OSPF network and the
different types of areas that can be configured.
OSPF Router Roles
As we alluded to earlier, routers within a multi-area OSPF network fall into
different categories. To gain an understanding of the various roles that our
routers can play, let's consider Figure 5.2.
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