OSPF's capability to separate a large
internetwork into multiple areas is referred to as hierarchical
routing. Hierarchical routing enables you to separate large
internetworks (autonomous system) into smaller internetworks that are
called areas. With this technique, routing still occurs between
the areas (called inter-area routing), but many of the smaller
internal routing operations, such as recalculating the database, are
restricted within an area. For example, if area 1 shown in the main figure
is having problems with a
link going up and down, routers in other areas do not need to run their
Shortest Path First (SPF) calculation, since they are isolated from the
problems in area 1.
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Lab
Activity |
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In this lab,
you will learn how to enable OSPF routing protocol for
Multiple areas. |
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The hierarchical topology possibilities of OSPF have several
important advantages:
- Reduced frequency of SPF calculations
--- Because detailed
route information is kept within each area, it is not necessary to
flood all link-state changes to all other areas. Thus, only those
routers affected by a change need to run the SPF calculation.
Smaller routing tables --- When using multiple areas, detailed
route entries for specific networks within an area are kept inside
the area. Rather than advertising these explicit routes outside the
area, you can have the routes summarized into one or more summary
routes. Advertising these summaries reduces the amount of LSAs
propagated between areas, but allows all networks to remain
reachable.
Reduced Link State Update (LSU) overhead --- LSUs can contain
a variety of LSA types, including link-state information and summary
information. Rather than send an LSU about each network to every
area, you can advertise a single route or a few summarized routes
between areas to reduce the overhead associated with link-state
updates that cross multiple areas.
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Lab
Activity |
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In this lab,
you will learn how to configure OSPF to calculate the
correct cost of an interface. |
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