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Chapter 3
IP Addressing
Route Summarization
In the "Design Considerations with VLSM" section, 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 indi-
vidual networks and subnetworks being advertised. All these routes can be
very taxing on a router's memory and processor.
In many cases, the router doesn't even need specific routes to each and
every subnet (e.g., It would be just as happy if it knew how
to get to the major network (e.g., and let another router take
it from there. A router's ability to take a group of subnetworks and summa-
rize them as one network (i.e., one advertisement) is called route summari-
, as shown in Figure 3.5.
In some of the literature, you may find route summarization referred to as
route aggregation.
F I G U R E 3 . 5
Route summarization
Besides reducing the number of routing entries that a router must keep
track of, route summarization can also help protect an external router from
making multiple changes to its routing table, due to instability within a par-
ticular subnet. For example, let's say that we were working on a router that
connected to As we were working on the router, we rebooted
it several times. If we were not summarizing our routes, an external router
would see each time went away and came back. Each time, it
would have to modify its own routing table. However, if our external router
I am the way to
get to network
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