Avaya Configuring IP Services Manual do Utilizador descarregar pdf (Página 250)

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101

Configuring IP Services

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303528-A Rev 00

Each border router in an AS does the following:

• Receives routing information from its attached areas, creates summaries of

this information, and forwards the summaries to the backbone and to any

other attached area. In Figure 7-6

, for example, border router 2 floods

summaries from area B to the backbone and area C. Through the backbone,

the summaries are forwarded to all other areas in the AS.

• Receives (via the backbone) summaries from other border routers, uses this

information to create new routing summaries (which add in the cost of the

backbone routes), and forwards the new summaries to its attached areas.

By definition, a border router has an interface to the backbone and interfaces to

one or more other areas. To configure OSPF as a border router:

Assign the backbone ID (0.0.0.0) to an OSPF interface.

Assign an area ID to another OSPF interface.

Configuring a Virtual Backbone Link through a Transit Area

Every border router must have a connection to the backbone. This connection can

be physical or virtual.

If the border router has an interface to a backbone network, that router is

considered to be physically connected to the backbone. In Figure 7-6

, border

router 1 and border router 2 are both physically connected to the backbone.

In some cases, it may not be possible to configure a border router with an interface

to a backbone network. If the router has an OSPF neighbor that is physically

connected to the backbone, the router can use that neighbor to establish a virtual

link to the backbone.

In Figure 7-7

, for example, border router 1 has lost its interface to the backbone

network. In its place, the network administrator has configured an interface to a

network in area B. Through this network, border router 1 now has a neighbor --

border router 2 -- that is connected physically to the backbone. The network

administrator can use border router 2 to configure a virtual link between border

router 1 and the backbone.

An area that provides a virtual link between a border router and the backbone is

considered to be a transit area. In Figure 7-7

, area B functions as a transit area.

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