Configuring gated

Configuring the OSPF Protocol

it is not required that an AS boundary router be a backbone router. An AS boundary router learns about routes other than its attached AS through exchanges with other routing protocols or through configuration information. Each AS boundary router calculates paths to destinations outside of its attached AS. It then advertises these paths to all routers in its AS.

Following are the two levels of routing in an AS:

•Intra-area routing, where the source and destination of a packet both reside in the same area. Routing is handled by internal routers.

•Inter-area routing, where the source and destination of a packet reside in different areas. Packets travel through an intra-area route from the source to an area border router, then travel an inter-area route on a backbone path between areas. Finally, they travel another intra-area route to the destination.

Planning Your OSPF Configuration

Following is a suggested sequence of steps in planning the OSPF routing in your autonomous system:

1.If your AS exchanges routing information with other autonomous systems, you need to obtain a unique AS number from the Internet Assigned Numbers Authority.

2.Partition the AS into areas. You can partition any interconnected networks into lists of address ranges, with each address range represented as an address-mask pair. The area border routers summarize the area content for each address range and distribute the summaries to the backbone. See “The networks Statement” on page 66 for more information on specifying address ranges.

3.Identify the internal routers for each area. An internal router configuration contains only one area definition.

4.Identify the area border routers and the areas to which they interface. The configuration for each area border router contains multiple area definitions.

5.For each router, determine the interface type for each area. Router interfaces can be multicast, non-broadcast multi-access (NBMA), or point-to-point. See “The interface Statement” on page 67 for more information on router interfaces.

Chapter 3

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