This section discusses some of the details of route summarization and how CIDR and VLSMs use it to deal with the size of routing tables.
An Overview of Route Summarization
The use of CIDR and VLSM not only reduces address waste, but it also promotes route aggregation, or route summarization. Without route summarization, Internet backbone routing would likely have collapsed sometime before 1997.
Figure 2-11 shows how route summarization reduces the burden on upstream routers. This complex hierarchy of variable-sized networks and subnetworks is summarized at various points using a prefix address until the entire network is advertised as a single aggregate route of 192.168.48.0/20.
Figure 2-11 Route Summarization
Recall that this kind of route summarization, or supernetting, is possible only if the network routers run a classless routing protocol, such as OSPF or EIGRP. Classless routing protocols carry the prefix length and subnet mask with the 32-bit address in routing updates. In Figure 2-11, the summary route that eventually reaches the provider contains a 20-bit prefix common to all the addresses in the organization. This prefix is 192.168.48.0/20, or 11000000.10101000.00110000.00000000, with a subnet mask of 11111111.11111111. 11110000.00000000. For summarization to work properly, addresses must be carefully assigned in a hierarchical fashion so that summarized addresses share the same high-order bits.
Route flapping occurs when a router interface alternates rapidly between the up and down states. This can be caused by a number of factors, including a faulty interface or poorly terminated media.
Summarization can effectively insulate upstream routers from route-flapping problems. Consider RTC in Figure 2-12. If the RTC interface connected to the 18.104.22.168 network goes down, RTC removes that route from its table. If the routers were not configured to summarize, RTC would then send a triggered update to RTZ about the removal of the specific network, 22.214.171.124. In turn, RTZ would update the next router upstream, and so on. Every time these routers are updated with new information, their processors must go to work. It is possible, especially in the case of OSPF routing, that the processors can work hard enough to noticeably affect performance. Now, consider the impact on performance if the RTC interface to network 126.96.36.199 comes back up after only a few seconds. The routers update each other and recalculate. In addition, what happens when the RTC link goes back down seconds later? And then back up? This is called route flapping, and it can cripple a router with excessive updates and recalculations.
Figure 2-12 Routes Summarized to 188.8.131.52/20
However, the summarization configuration prevents the RTC route flapping from affecting any other routers. RTC updates RTZ about a supernet, 184.108.40.206/21, which includes eight networks, 220.127.116.11 through 18.104.22.168. The loss of one network does not invalidate the route to the supernet. While RTC might be kept busy dealing with its own route flap, RTZ, all upstream routers are unaware of any downstream problem. Summarization effectively insulates the other routers from the problem of route flapping.