Because of hierarchical design, the OSPF is more efficient and scalable. The OSPF segments network into different areas. An area is a group of routers sharing the same link-state information in their Link-State Databases (LSDBs). We can implement the OSPF in the following two ways.
- Single Area OSPF
- Multi-Area OSPF
Single Area OSPF (Area 0)
Area 0 is also known as the backbone area for OSPF which links all other smaller areas within the hierarchy. The single area OSPF is useful in smaller networks where only a few routers are working and the web of router links is not complex, and paths to individual destinations are easy. If an area becomes too big, the following issues happen with the network.
- Large routing table
- Large link-state database (LSDB)
- Frequent SPF algorithm calculations
OSPF supports hierarchical routing using areas which make OSPF more efficient and scalable. An area is a group of routers that share the same link-state information in their link-state databases. The figure below illustrates the single area OSPF network.
When a large OSPF area is separated into several smaller areas, this is called multiarea OSPF. It is useful in larger network deployments to decrease processing and memory overhead. The network contains OSPF areas in a hierarchal design. All areas have connected to area 0, known as the backbone area. The interconnecting routers between the backbone area and other area known as area border router (ABR).
As shown in the figure the multi-area OSPF segments one large network into several networks. One autonomous system ( AS) has divided into multiple areas to support hierarchical routing. The routing occurs between the multiple areas. The more operation required system resources’ such as re-calculating the database, has done only within a relevant area.
For example, when there are changes occurs in the topology like addition, deletion or modification of the link, the router must rerun the SPD algorithm to create new SPF tree and update the routing table in the same area where changes occur. But changes have shared to routers in other areas in a distance-vector format to update their routing tables and these router and areas do not need to rerun the SPF algorithm. As illustrated in Figure 1, the hierarchical-topology possibilities of multiarea OSPF have these advantages:
- Smaller routing tables– There are fewer routing table entries as network addresses can be summarized between areas. For example, R1 summarizes the routes from area 1 to area 0 and R2 summarizes the routes from area 51 to area 0. R1 and R2 also propagate a default static route to area 1 and area 51.
- Reduced link-state update overhead– Minimizes processing and memory requirements, because there are fewer routers exchanging LSAs.
- Reduced frequency of SPF calculations– Localizes impact of a topology change within an area. For instance, it minimizes routing update impact, because LSA flooding stops at the area boundary.
- Small Link-State Data Base. Multi-areas OSPF efficiently partitions a potentially large database into smaller and more manageable databases.
OSPF Two-Layer Area Hierarchy
Multiarea OSPF is implemented in a two-layer area hierarchy:
- Backbone (Transit) area– An OSPF area transfer IP packet fast and efficient. Backbone areas interconnect multiple OSPF area with one another. usually, end users are not found within a backbone area.
- Regular (Non-backbone) area –This area connects users and other resources. The areas usually set up along functional or geographical groupings. By default, a regular area does not allow traffic from another area. Traffic from other areas must forward through a transit area. A regular can contains a number of subtypes, including a standard area, stub area, totally stubby area, and not-so-stubby area (NSSA). The Cisco recommends the following guidelines for areas:
- A maximum number of router per area is 50.
- One router can join the maximum three areas.
- A maximum number of neighbours for a single router is 60.