OSPF uses four different types of routers with a unique role and set of defining characteristics within the hierarchy. The routers control the traffic that goes in and out of areas. The OSPF routers are categorized based on the function they perform in the routing domain. The four types of routers are following:-
- Internal router– All interfaces of internal routers (IRs) are directly connected to the same area. An internal router has a single link-state database because they belong to only one area.
- Backbone router– This is a router in the backbone area (area 0). The interfaces of the backbone routers are connected with the backbone area. The backbone routers do not have an interface connected to the other OSPF areas, because if they connected to other areas, they would be considered Area Border Router (ABRs).
- Area Border Router(ABR) – Routers placed on the border of one or more OSPF areas and that connect those areas to the backbone network are known as ABRs. The ABRs are considered the members of both the OSPF backbone and the attached areas. It gathers topology information from their connected areas and shares it to the backbone area. The ABR maintain separate LSDBs for each area it is connected to. It summarized the LSDBs and then presented to the backbone for distribution to other areas.
- Autonomous System Boundary Router (ASBR)– The Autonomous System Boundary Router (ASBR) has at least one interface connected to an external internetwork, for example, a non-OSPF network. ASBRs run both OSPF and another routing protocol, such as RIP. ASBRs must reside in a non-stub OSPF area.
The redistribution command is used to bring two routing protocols together when using Cisco routers. BGP also support multiple autonomous systems together. If a router connects to area 0 and area 1, and in addition, maintains routing information for another, non-OSPF network, it falls under three different classifications: a backbone router, an ABR, and an ASBR.