In reality, OSPF is a link-state routing protocol only within an area (intra-area); but almost a distance-vector routing protocol between areas (inter-area).
One of the advantages of link state protocols is that the link state database provides a “view” of the entire network but only within the area. Within the same area every OSPF router floods information about itself, its links, and its neighbors to every other router. From this flooded information each router builds an identical link state database. Each router then independently runs a shortest-path-first calculation on its database and calculates the best path to each destination.
When an OSPF domain grows large, the flooding and the resulting size of the link state database becomes a scaling problem. The problem is remedied by breaking the routing domain into areas: That first concept is modified so that flooding occurs only within the boundaries of an area in order to reduce the routing table and the LSDB sizes, therefore to protect the memory resources and CPU processing power. When a changes occurs in an area in the topology only the routers in this area trigger the SPF algorithm through the LSA Type 1 and LSA Type 2.
When an ABR receive the LSA Type 1 and LSA Type 2 within the area, it will only send the reachability information through the LSA Type 3to another area. ABR hides the topology information and only reachability information sends between the areas.
When ABR sends a summary type 3 LSAs into another area, it says I can reach for example network 5.5.5.0 and this my metric to reach it, and you can reach this network through me. Which mean is ABR will hide topology information.
To prevent routing loops, areas must be connected to the backbone area 0. All LSAs Type 3 must therefore pass into or out of area 0 when multiple areas are in use, whereas type 1 and 2 LSAs are confined to the local area. In other when we have multiple ABRs, an ABR ignores an LSA Type 3 learned through a non-backbone, this called as split-horizon inter-area loop prevention.
RFC 3509 explains OSPF ABR Behavior and split-horizon mechanism in the following section:
1.2 Motivation
In OSPF domains the area topology is restricted so that there must be
a backbone area (area 0) and all other areas must have either
physical or virtual connections to the backbone. The reason for this
star-like topology is that OSPF inter-area routing uses the
distance-vector approach and a strict area hierarchy permits
avoidance of the "counting to infinity" problem. OSPF prevents
inter-area routing loops by implementing a split-horizon mechanism,
allowing ABRs to inject into the backbone only Summary-LSAs derived
from the
intra-area routes, and limiting ABRs' SPF calculation to consider
only Summary-LSAs in the backbone area's link-state database.
The last restriction leads to a problem when an ABR has no backbone
connection (in OSPF, an ABR does not need to be attached to the
backbone).
