ABSTRACT
Link State Routing protocols require the storage of a full instance of the Link State Database at each router. This chapter introduces the main aspects related to the multi-area network structure and the inter-area routing protocol. It presents the distance vector and link state approaches to inter-area routing. The chapter discusses the consequences of restricting the routing information available at areas. It provides alternatives for advertising domain-external addressing information across areas. In a multi-area network, the network is partitioned in several areas that communicate through Area Border Routers (ABR). The set of ABRs and their interconnections form a routing overlay, i.e. a logical network over the physical network utilized for the exchange of inter-area routing information. The external address prefixes and the paths to reach them are learned through the inter-area routing protocol running on the ABR overlay. A convenient abstraction to study inter-area routing is the graph of the ABR overlay.
