Differentiate single-area OSPF from multi-area OSPF?

Differentiate single-area OSPF from multi-area OSPF?
Marilyn NavarroAsked:
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Predrag JovicConnect With a Mentor Network EngineerCommented:
Beside all other differences you can identify it easy by issuing:

# sh ip ospf database

Single-area OSPF will have only LSAs from Area 0 (and maybe external)
Router Link States (Area 0)
Net Link States (Area 0)
Type-5 AS External Link States
The first two you will find in every area

Multi-area OSPF will have more LSA types (but at least will have more area numbers) :)
Router Link States (Area 0)
Net Link States (Area 0)
Summary Net Link States (Area 0)
Summary ASB Link States (Area 0)
Router Link States (Area X)
Net Link States (Area X)
Summary Net Link States (Area X)
Summary ASB Link States (Area X)
Type-5 AS External Link States

OSPF Link state advertisements
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RantiConnect With a Mentor Network EngineerCommented:
The reason why people create multiple areas in OSPF is to divide the network map before it gets too big. OSPF uses Link State routing protocol which use Open Shortest Path first. So each router has the whole map of the network.
Now if the network grows too big it gets slower and slower. So in many organisation, the architect's usually create new areas when the network grows more than 60-70 routers.
Basically in ospf network, every area need to have one router thats an Area Boarder Router(ABR) which is connected to the area0. Now there are exception to this rule, when people create virtual links. In my opinion, that only a work around and should not be implemented unless really necessary.

So in Summery, single area you only have one area, which is area 0, every router has a route map. When area grows bigger and bigger network architects create multiple areas and summarise the ip address from one area and advertise to another area.
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RantiNetwork EngineerCommented:
Covered the base differences.
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