In the “Design Considerations with VLSM” section earlier in this chapter, we briefly mentioned the
concept of route summarization. So what is it, and why do we need it? On very large networks, there
may be hundreds or even thousands of individual networks and subnetworks being advertised. All
these routes can be very taxing on a router’s memory and processor. For example, routers on the
Internet were starting to be overwhelmed with a couple of hundred thousand routes. After summarizing
routes and using CIDR, the number of routes has been dramatically reduced.
In many cases, the router doesn’t even need specific routes to each and every subnet (for
example, 172.16.1.0/24). It would be just as happy if it knew how to get to the major network
(for example, 172.16.0.0/16) and let another router take it from there. In our telephone network
example, the local telephone switch should only need to know to route a phone call to the
switch for the called area code. Similarly, a router’s ability to take a group of subnetworks and
summarize them as one network (in other words, one advertisement) is called route summarization,
as illustrated in Figure 2.5.
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