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Link state routing protocols such as OSPF

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allow for the division of large Autonomous System into smaller groupings called areas.

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In this example, we have a RIP router

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connected to a router running both RIP and OSPF

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so RIP is connected to an OSPF Autonomous System.

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The routers within OSPF are under common administrative domain

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in other words, they are part of the Autonomous System.

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The OSPF Autonomous System however

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can be broken up into multiple areas to allow for smaller routing tables

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reduction of flooding of link state advertisements

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rank summarization and more efficient use of resources.

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If you have many routers in a single area, the routing tables of those routers

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can get very large, so this allows for the reduction of routing tables on certain routers

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by breaking the Autonomous System into a hierarchical design with multiple areas.

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In OSPF, these routers are known as Area Border Routers or ABRs

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they have interfaces in multiple areas.

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These routers are known as internal routers as they are only part of a single area

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they have no interfaces in any other areas.

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This router is known as an Autonomous System border router as it connects OSPF

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to an external Autonomous System or routing process which in this case is RIP.

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These routers are known as backbone routers

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and they also called internal routers.

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They are internal to the backbone area.

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one of the advantages of creating multiple areas is that

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link changes within 1 area are hidden from other routers in other areas

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so within OSPF if we had network 10.1.1.0 in area 1

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and that network went down that information will be hidden from routers in other areas.

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They wouldn’t have to rerun the SPF algorithm

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because they're unaware of that network going down.

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so this router in area 2 and this router in are 3 will not be aware of that fact

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that network 10.1.1.0 has gone down

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you can do this by configuring area 2

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and area 3 as what are called stub and totally stubby areas.

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You can also use route summarization to reduce

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the need to rerun the SPF algorithm when a network goes down.

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The idea here is to use your resources more efficiently

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by reducing the flooding of link state advertisements.

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Links state advertisements within area 1 are contained within that area

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or limited to that area and summary LSAs propagated to other areas.

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These summary LSAs are blocked by Area Border of Routers or ABRs

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so they don’t go into other areas such as area 2.

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So routers in area 2 are unaware of specific routes in area 1.

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To enable connectivity a summary route

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or default route can be propagated into area 2.