Because of the hierarchical nature of a multiple area OSPF network, routers have different responsibilities,
depending on their position and functionality within the hierarchical design. These routers have different
designations such as internal routers, backbone routers, area border routers (ABR), and autonomous system
boundary routers (ASBR).
• The Internal Router exists within an area. It is responsible for maintaining a current and accurate
database of every subnet within the area. It is also responsible for forwarding data to other networks by
the shortest path. Flooding of routing updates is confined to the area. All interfaces on this router are
within the same area.
• The Backbone Router exists within the backbone area, which is also called Area 0. The design rules for
OSPF require that all the areas be connected through a single area, known as Area 0. Area 0 is also
known as Area 0.0.0.0 on other routers. A router within this area is referred to as a backbone router. It
may also be an internal router or an Area Border Router.
• The Area Border Router (ABR) is responsible for connecting two or more areas. It holds a full
topological database for each area to which it is connected and sends LSA updates between the areas.
These LSA updates are summary updates of the subnets within an area. It is at the area border that
summarization should be configured for OSPF because this is where the LSAs make use of the reduced
routing updates to minimize the routing overhead on both the network and the routers.
• The Autonomous System Boundary Router (ASBR) is used to connect to a network or routing
protocol outside the OSPF domain. OSPF is an interior routing protocol or Interior Gateway Protocol
(IGP); gateway is an older term for a router. If there is any redistribution between other protocols to
OSPF on a router, it will be an ASBR. This router should reside in the backbone area but you can place it
anywhere in the OSPF hierarchical design.