Area-based routing considerations with protocol usage

Overview

When you configure a device, you configure the routing protocols used to determine how routers communicate with each other and the topology of packet handling between different devices in the network.

Networks can be grouped into areas for a number of reasons including scaling, administrative convenience, equipment compatibility, or business structure, For example, an area can be a collection of network segments within an AS that have been administratively assigned to the same group.

Area topology is concealed from the rest of the AS. This means a significant reduction in routing traffic. Routing in the AS takes place on three levels, depending on whether the source and destination of a packet reside in:

• the entire path is limited to a single area (known as intra-area routing)

• the path spans multiple areas all owned by the same carrier (known as inter-area routing)

• the path spans multiple carriers (known as inter-carrier routing)

In intra-area routing, the packet is routed based on information found within the area; no routing information from outside the area is used. This protects intra-area routing from the injection of bad routing information. Two devices, which are not area border routers, and belonging to the same area, have identical topological databases.

Devices that are aware of more than one area are called area border routers. In this case, all devices in an AS do not have an identical topological database. An area border router has a separate topological database for each area it is connected to. ASs share routing information, such as routes to each destination and information about the route or AS path, with other ASs using BGP.