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Configuring OSPF Services

4-21

Table 4-1 lists the OSPF area routers and describe the principal services they

provide.

All areas in the autonomous system must be physically contiguous with the

backbone or, if not contiguous, must have a virtual link to the backbone. An area

containing border routers that are conﬁgured to create a virtual link between

another area and the backbone is called a

transit area

OSPF supports interfaces to four types of network: point-to-point networks,

broadcast networks, nonbroadcast multicast networks, and point-to-multipoint

networks. OSPF also supports IP subnetting and supernetting, address ranges, and

special areas called

stubs

that rely on default routing.

There are three categories of OSPF routing: intra-area routing, interarea routing,

and external routing. Interarea routing occurs when source and destination reside

in the same area. Intra-area routing occurs when source and destination reside in

different areas within the same AS. External routing occurs when source and

destination reside in different ASs or when source or destination reside on a RIP

network within the AS.

Table 4-1. OSPF Area Routers

Router Type Description/Function

Internal

Router

The internal router resides within an area. All of its directly connected

networks belong to the same area. Routers with only backbone interfaces

also fall into this category. Each internal router runs a single copy of the

basic routing algorithm.

Area Border

Router

The area border router attaches to more than one area and runs multiple

copies of the basic routing algorithm — one copy for each area to which it

is attached. An area border router distributes topological information about

each of its attached areas to the backbone; then, the backbone distributes

that same information to other areas.

Backbone

Router

The backbone router is any router that has an interface to the backbone,

including all routers that have an interface to more than one area (area

border router). Backbone routers with all interfaces connected to the

backbone are considered to be internal routers.

AS Boundary

Router

The AS boundary router is the autonomous system’s link to other routing

domains. The AS boundary router exchanges router information with

routers belonging to other routing domains. Such a router has AS external

routes that are advertised throughout the autonomous system. The path to

each AS boundary router is known to every other router in the

autonomous system.

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