5.1 Creating Multiple OSPF Areas
5.1.4 Types of OSPF routers
To control the traffic types that go in and out of the various types of areas, you need certain types of OSPF routers, as shown in the main figure.

The router types are as follows:

  • Internal router --- As discussed previously, routers that have all their interfaces within the same area are called internal routers. Internal routers in the same area have identical link-state databases and run a single copy of the routing algorithm.
  • Backbone router --- Routers that are attached to the backbone area of the OSPF network are called backbone routers. They have at least one interface connected to area 0 (the backbone area). These routers maintain OSPF routing information using the same procedures and algorithms as internal routers.
  • Area Border Router (ABR) --- Routers with interfaces attached to multiple areas. These routers maintain separate link-state databases for each area to which they are connected, and route traffic destined to or arriving from other areas. ABRs are exit points for the area, which means routing information destined for another area can only travel there via the local area's ABR. ABRs summarize information about the attached areas from their link-state databases and distribute the information into the backbone. The backbone ABRs then forward the information to all other connected areas. An area can have one or more ABR.
  • Autonomous System Boundary Router (ASBR) --- Routers that have at least one interface connected to an external internetwork (another autonomous system), such as a non-OSPF network. These routers can import non-OSPF network information to the OSPF network, and visa versa (this is referred to as redistribution).

A router can be more than one router type. For example, if a router interconnects to area 0 and area 1, as well as to a non-OSPF network, it would be both an ABR and ASBR.