Explanation/Reference:
Explanation:
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.
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 particular subnet.
For example, let's say that we were working on a router that connected to 172.16.2.0/24. 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 172.16.2.0/24 went away and came back. Each time, it would have to modify its own routing table. However, if our external router were receiving only a summary route (i.e.,
172.16.0.0/16), then it wouldn't have to be concerned with our work on one particular subnet. This is especially a problem for EIGRP, which can create stuck in active (SIA) routes that can lead to a network melt-down.
Summarization Example We have the following networks that we want to advertise as a single summary route:
* 172.16.100.0/24 * 172.16.101.0/24 * 172.16.102.0/24 * 172.16.103.0/24 * 172.16.104.0/24 *
172.16.105.0/24 * 172.16.106.0/24