There are four different types of OSPF routers:
• Backbone router: This is a router in the backbone area. It must be a contiguous area. Generally, the backbone area is set to area 0 or area 0.0.0.0 .
• Internal router: This is a router that has all of its interfaces in the same area. All internal routers in an area have identical LSDBs.
• Area Border Router (ABR): This is a router that has interfaces attached to multiple areas. An area border router is in the OSPF boundary between two areas.
The ABR maintains separate Link State Databases (LSDBs) for each area it is connected to, and can route between areas. In a multiarea network, an area can have one or more ABRs.
ABRs serves as an exit routes for the area to connected users, it then distribute the routing information into the backbone.
Autonomous System Boundary Router (ASBR): This is a router that has at least one interface attached to an external internetwork (another autonomous system), it advertises external destinations throughout the OSPF autonomous system such as a non-OSPF network.
An ASBR can redistributed, exchange and advertise messages from external routes or other protocol (EIGRP) into OSPF network. This process is known as routes redistribution.
Common LSAs Exchanged Between Multi-Area.
1. OSPF Router LSA
This is advertisements for every router in an area. It includes information on directly attached links. Each of the links is identified by an IP prefix and link type assigned it.
Router LSAs floods within its area only; does not cross ABR.
2. OSPF Network LSA
• Network LSA is sent by the DR meant for Broadcast and Non-Broadcast network types in an area. It Includes list of attached routers on the transit link other information in this list includes subnet mask of link. Network LSA floods within its area only; does not cross ABR.
3. OSPF Summary LSA
This is an inter-area link advertisement advertised by the ABR of an area. It’s used to send network information to areas outside the source area. Information on this list includes the network number and mask of a link.
4. OSPF Summary LSA
This is generated by the ABR of an area, its used to advertise an ASBR to all other areas in the AS. The information generated and sent includes the router ID of the ASBR.
5. OSPF External LSA
This is generated by the ASBR to describe external routes other autonomous systems that are redistributed into the area. The ASBR floods information to the entire Autonomous System (AS). By default, advertised routes are not summarized.