In the OSPF (Open Shortest Path First) protocol, the device that is used to connect different areas is called Area Border Router (ABR).

The ABR plays a crucial role in OSPF hierarchy and efficiency, as it helps organize and reduce network load in large OSPF deployments by segmenting the network into multiple smaller areas.

Below, we explain more about the functions and importance of ABRs in OSPF.

ABR Functions

1. Connect areas: The ABR connects at least two OSPF areas, usually including the backbone area (Area 0) and one or more non-backbone areas. This router maintains a separate routing database for each area it is connected to.
2. Propagate LSAs: ABRs are responsible for propagating Link-State Advertisements (LSAs) between areas. ABRs take type 1 and 2 LSAs (area internal LSAs) from an area and summarize them into type 3 LSAs (summary LSAs) to advertise to other areas. This helps reduce the size of the routing table and the routing overhead in each area.
3. Route summary: ABRs can also be configured to perform route summarization between areas. This further reduces the number of routes that must be processed by other routers on the network, improving the performance and scalability of the OSPF network.
4. Route filtering: In some cases, ABRs can also be configured to perform route filtering, thereby controlling what routing information is allowed to pass between areas. This can be useful for security policies or to handle routing overhead more efficiently.

Importance of ABR

• Routing efficiency: By segmenting the network into areas, ABRs help OSPF scale to very large networks, reducing the amount of routing information each router must process and maintain.
• Reducing OSPF overheadBy summarizing and possibly filtering routes as they pass between areas, ABRs help reduce the overhead associated with OSPF, especially in networks with a large number of routes.
• Stability improvement: ABRs help contain faults within a specific area, preventing link state changes in one area from affecting other areas, thus increasing the stability of the entire OSPF network.

ABRs are essential for the effective operation of OSPF networks that span multiple areas, allowing OSPF to be used efficiently in medium to very large environments.