OSPF router types

OSPF defines the following router types:

• Area border router (ABR)


• Autonomous system border router (ASBR)


• Internal router (IR)


• Backbone router (BR)

The router types are attributes of an OSPF process. A given physical router may have one or more OSPF processes. For example, a router that is connected to more than one area, and which receives routes from a BGP process connected to another AS, is both an ABR and an ASBR.
Each router has an identifier, customarily written in the dotted decimal format (e.g.: 1.2.3.4) of an IP address. This ID must be configured to an OSPF instance in order to be considered the ID. If not explicitly configured, the highest logical IP address will assume the router ID role. The router ID does not have to be a part of any routable subnet in the network and often isn't to avoid confusion.
Note: Do not confuse router types with designated router (DR), or backup designated router (BDR), which is an attribute of a router interface, not the router itself.
Area border router

An ABR is a router that connects one or more OSPF areas to the main backbone network. It is considered a member of all areas it is connected to. An ABR keeps multiple copies of the link-state database in memory, one for each area to which that router is connected.
Autonomous system boundary router

An ASBR is a router that is connected to more than one AS and that exchanges routing information with routers in other ASs. ASBRs typically also run a non-IGP routing protocol (e.g., BGP), or use static routes, or both. An ASBR is used to distribute routes received from other ASs throughout its own AS.
Internal router

An IR is a router that only has OSPF neighbor relationships with interfaces in the same area.
Backbone router

Backbone Routers: These are routers that are part of the OSPF backbone. By definition, this includes all area border routers, since those routers pass routing information between areas. However, a backbone router may also be a router that connects only to other backbone (or area border) routers, and is therefore not part of any area (other than Area 0).
Note that: an area border router is always a backbone router, but a backbone router is not necessarily an area border router.

source:http://en.wikipedia.org/wiki/Open_Shortest_Path_First