As the number of hosts in a network increases, the routing and forwarding tables must establish and maintain more routes. As these tables become larger, the time routers require to look up particular routes so that packets can be forwarded becomes prohibitive. The solution to the problem of growing routing tables is to group (aggregate) the routers by subnetwork, as shown in Figure 48.
Figure 48: Route Aggregation
Figure 48 shows three different ASs. Each AS contains multiple subnetworks with thousands of host addresses. To allow traffic to be sent from any host to any host, the routing tables for each host must include a route for each destination. For the routing tables to include every combination of hosts, the flooding of route advertisements for each possible route becomes prohibitive. In a network of hosts numbering in the thousands or even millions, simple route advertisement is not only impractical but impossible.
By employing route aggregation, instead of advertising a route for each host in AS 3, the gateway router advertises only a single route that includes all the routes to all the hosts within the AS. For example, instead of advertising the particular route 170.16.124.17, the AS 3 gateway router advertises only 170.16/16. This single route advertisement encompasses all the hosts within the 170.16/16 subnetwork, which reduces the number of routes in the routing table from 216 (one for every possible IP address within the subnetwork) to 1. Any traffic destined for a host within the AS is forwarded to the gateway router, which is then responsible for forwarding the packet to the appropriate host.
Similarly, in this example, the gateway router is responsible for maintaining 216 routes within the AS (in addition to any external routes). The division of this AS into subnetworks allows for further route aggregation to reduce this number. In the subnetwork in the example, the subnetwork gateway router advertises only a single route (170.16.124/24), which reduces the number of routes from 28 to 1.