OSPF uses a hierarchical structure of areas to limit and control the flooding of LSAs and is important for the scalability of the routing protocol. In smaller networks, it may not be necessary to employ any areas beyond Area 0, but in larger networks, it may be required.
An OSPF two-level hierarchy consists of –
- Backbone (Area 0): Must connect to all other non-backbone areas, and must be contiguous (it cannot be split).
- Non-backbone area: Connects users and resources. Setup according to geographical or functional groupings. Traffic between these areas must pass through the backbone area.
Smaller areas reduce the time it takes for LSDBs to converge.
Note: inter-area design does not inherently make OSPF scalable. However, the use of multiple areas allows an administrator the ability to summarize at the ABR to limit the flooding of LSAs, thereby providing scalability.
OSPF Routers support single-area configuration only.
A backbone router has at least one interface in Area 0.
OSPF ABR (Area Border Routers) support multi-area configuration. This is where summarization can be implemented.
OSPF ASBR (Autonomous System Border Routers) provide redistribution from an external routing domain, like EIGRP, into OSPF. Summarization is also possible on an ASBR.
Stuck-in-Active: Journal of an IT-Network Administrator 