What is a backbone router (BR)?
A backbone router is a part of the OSPF backbone area. Area Border Routers (ABRs) comes in the category of backbone routers because those routers pass routing information between areas. However, a backbone router can also be a router that connects only to other backbone routers. Therefore, sometimes a backbone router is not a part of any area. In short, an ABR is always a backbone router, but a backbone router is not necessarily an ABR.
