Link state routing protocols such as OSPF
allow for the division of large Autonomous System into smaller groupings called areas.
In this example, we have a RIP router
connected to a router running both RIP and OSPF
so RIP is connected to an OSPF Autonomous System.
The routers within OSPF are under common administrative domain
in other words, they are part of the Autonomous System.
The OSPF Autonomous System however
can be broken up into multiple areas to allow for smaller routing tables
reduction of flooding of link state advertisements
rank summarization and more efficient use of resources.
If you have many routers in a single area, the routing tables of those routers
can get very large, so this allows for the reduction of routing tables on certain routers
by breaking the Autonomous System into a hierarchical design with multiple areas.
In OSPF, these routers are known as Area Border Routers or ABRs
they have interfaces in multiple areas.
These routers are known as internal routers as they are only part of a single area
they have no interfaces in any other areas.
This router is known as an Autonomous System border router as it connects OSPF
to an external Autonomous System or routing process which in this case is RIP.
These routers are known as backbone routers
and they also called internal routers.
They are internal to the backbone area.
one of the advantages of creating multiple areas is that
link changes within 1 area are hidden from other routers in other areas
so within OSPF if we had network 10.1.1.0 in area 1
and that network went down that information will be hidden from routers in other areas.
They wouldn’t have to rerun the SPF algorithm
because they're unaware of that network going down.
so this router in area 2 and this router in are 3 will not be aware of that fact
that network 10.1.1.0 has gone down
you can do this by configuring area 2
and area 3 as what are called stub and totally stubby areas.
You can also use route summarization to reduce
the need to rerun the SPF algorithm when a network goes down.
The idea here is to use your resources more efficiently
by reducing the flooding of link state advertisements.
Links state advertisements within area 1 are contained within that area
or limited to that area and summary LSAs propagated to other areas.
These summary LSAs are blocked by Area Border of Routers or ABRs
so they don’t go into other areas such as area 2.
So routers in area 2 are unaware of specific routes in area 1.
To enable connectivity a summary route
or default route can be propagated into area 2.