When a large OSPF area is divided into smaller areas, this is called multiarea OSPF. Multiarea OSPF is useful in larger network deployments to reduce processing and memory overhead.

Multiarea OSPF requires a hierarchical network design. The main area is called the backbone area (area 0) and all other areas must connect to the backbone area.

Multiarea OSPF have these advantages:

• Smaller routing tables – There are fewer routing table entries as network addresses can be summarized between areas.
• 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.
• Reduced link-state update overhead – Minimizes processing and memory requirements, because there are fewer routers exchanging LSAs.
• Confines network instability to single areas of the network

two-layer area hierarchy:

• Backbone (Transit) area – An OSPF area whose primary function is the fast and efficient movement of IP packets. Backbone areas interconnect with other OSPF area types. Generally, end users are not found within a backbone area. The backbone area is also called OSPF area 0. Hierarchical networking defines area 0 as the core to which all other areas directly connect.

• Regular (Non-backbone) area – Connects users and resources. Regular areas are usually set up along functional or geographical groupings. By default, a regular area does not allow traffic from another area to use its links to reach other areas. All traffic from other areas must cross a transit area.

Inter-area traffic: All traffic moving from one area to another area must traverse the backbone area. This traffic is referred to as inter-area traffic.

Cisco recommends the following guidelines:

• An area should have no more than 50 routers.

• A router should not be in more than three areas.

• Any single router should not have more than 60 neighbors.

There are four different types of OSPF routers:

• Internal router – This is a router that has all of its interfaces in the same area. All internal routers in an area have identical LSDBs.
  

• Backbone router – This is a router in the backbone area. Generally, the backbone area is set to area 0.
  

• Area Border Router (ABR) – This is a router that has interfaces attached to multiple areas. It must maintain separate LSDBs for each area it is connected to, and can route between areas. ABRs are exit points for the area, which means that routing information destined for another area can get there only via the ABR of the local area. ABRs can be configured to summarize the routing information from the LSDBs of their attached areas. ABRs distribute the routing information into the backbone. The backbone routers then forward the information to the other ABRs. In a multiarea network, an area can have one or more ABRs.
  

• Autonomous System Boundary Router (ASBR) – This is a router that has at least one interface attached to an external internetwork (another autonomous system), such as a non-OSPF network. An ASBR can import non-OSPF network information to the OSPF network, and vice versa, using a process called route redistribution.
  

Redistribution in multiarea OSPF occurs when an ASBR connects different routing domains (e.g., EIGRP and OSPF) and configures them to exchange and advertise routing information between those routing domains.

A router can be classified as more than one router type. For example, if a router connects to area 0 and area 1, and in addition, maintains routing information for another, non-OSPF network, it falls under three different classifications: a backbone router, an ABR, and an ASBR.

LSA operation

LSAs are the building blocks of the OSPF LSDB. Individually, they act as database records and provide specific OSPF network details. In combination, they describe the entire topology of an OSPF network or area.

Type 1.

• All routers advertise their directly connected OSPF-enabled links in a type 1 LSA and forward their network information to OSPF neighbors. The LSA contains a list of the directly connected interfaces, link types, and link states.
• Type 1 LSAs are also referred to as router link entries.
• Type 1 LSAs are flooded only within the area in which they originated. ABRs subsequently advertise the networks learned from the type 1 LSAs to other areas as type 3 LSAs.
• The type 1 LSA link ID is identified by the router ID of the originating router.
  

Type 2