OSPF Router Types
When an OSPF network grows beyond a single area, you need to be aware of the role played by each OSPF router in a topology. Specifically, four OSPF router types exist:
■ Internal router: All the networks directly connected to an internal router belong to the same OSPF area. Therefore, an internal router has a single link-state database.
■ Area border router (ABR): An ABR connects to more than one OSPF area and therefore maintains multiple link-state databases (one for each connected area). A primary responsibility of an ABR is to exchange topological information between the backbone area and other connected areas.
■ Backbone router: A backbone router has at least one of its connected networks participating in OSPF area 0 (that is, the backbone area). If all the connected networks are participating in the backbone area, the router is also considered to be an internal router. However, if a backbone router has one or more connected networks participating in another area, the backbone router is also considered to be an ABR.
■ Autonomous system boundary router (ASBR): An ASBR has at least one connected route participating in an OSPF area and at least one connected route participating in a different autonomous system. The primary role of an ASBR is to exchange information between an OSPF autonomous system and one or more external autonomous systems.
As an example, Figure 7-2 illustrates these various OSPF router types.
Table 7-3 lists the OSPF router type or types for each OSPF router in the topology (that is,
■ Internal router: All the networks directly connected to an internal router belong to the same OSPF area. Therefore, an internal router has a single link-state database.
■ Area border router (ABR): An ABR connects to more than one OSPF area and therefore maintains multiple link-state databases (one for each connected area). A primary responsibility of an ABR is to exchange topological information between the backbone area and other connected areas.
■ Backbone router: A backbone router has at least one of its connected networks participating in OSPF area 0 (that is, the backbone area). If all the connected networks are participating in the backbone area, the router is also considered to be an internal router. However, if a backbone router has one or more connected networks participating in another area, the backbone router is also considered to be an ABR.
■ Autonomous system boundary router (ASBR): An ASBR has at least one connected route participating in an OSPF area and at least one connected route participating in a different autonomous system. The primary role of an ASBR is to exchange information between an OSPF autonomous system and one or more external autonomous systems.
As an example, Figure 7-2 illustrates these various OSPF router types.
Table 7-3 lists the OSPF router type or types for each OSPF router in the topology (that is,
routers R1, R2, R3, and R4).
