Hey all, now that we are familiar with OSPF areas in my last post, let us go ahead. As we know, OSPF divides the entire topology into areas. Areas are nothing but the groups of routers. And routers within these areas are classified based on their functionality and positioning in areas. Let us find out the router types.
The above image is quite self-explanatory and you might have guessed the router types đ
Internal routers : Routers whose all the interfaces fall in one area are area routers of that area. These routers have the same copy of LSDB and are aware of only their area topology.
Area border router (ABR): A router  whose interfaces fall in more that one area is area border router (ABR). ABR basically connects two areas. ABR will have multiple copies of LSDB, each representing the area it is connected to. Summarization and aggregation is done on these routers to contract the size of the routing table.
Backbone routers (BR): A router whose at least one interface fall in the backbone area is backbone router (BR). Thus, you can say that all ABRs are backbone routers, really? Yes, you guessed it right. All the areas are only connected to backbone area, so an inter-area connection is always area 0 to other area connection. However, all backbone routers are not ABRs.
Autonomous System boundary router (ASBR): A router which connects Autonomous Systems ie. whose interfaces fall in different Autonomous Systems is ASBR.  This router forwards external routing information throughout the AS, so route redistribution is applied on ASBR. The routes to each ASBR are known to all the routers in the AS. ASBR may be internal or area border routers.
Thus, based on the areas the we have seen how routers are categorized. To know more about OSPF, stay tuned..!!
Till then, happy networking with OSPF..!!!