Explanation/Reference:
Section: Addressing and Routing Protocols in an Existing Network Explanation Explanation:
Route summarization can reduce the amount of bandwidth used by a routing protocol. Summarization is the process of advertising a group of contiguous networks as a single route. When a router performs summarization, the router advertises a summary route rather than routes to each individual subnetwork.
Summarization can cause a routing protocol to converge faster and can reduce the consumption of network bandwidth, because only a single summary route will be advertised by the routing protocol. For example, summarizing routes from the distribution layer to the core layer of a hierarchical network enables the distribution layer devices to limit the number of routing advertisements that are sent to the core layer devices. Because fewer advertisements are sent, the routing tables of core layer devices are kept small and access layer topology changes are not advertised into the core layer.
You can configure a router to summarize its networks on either classful or classless network boundaries.
When combining routes to multiple subnetworks into a single summarized route, you must take bits away from the subnet mask. For example, consider a router that has interfaces connected to the 16 contiguous networks from 10.10.0.0/24 through 10.10.15.0/24. The routing table would contain a route to each of the
16 networks. The 16 contiguous networks can be summarized in 4 bits (24 = 16). Taking 4 bits away from the 24bit subnet mask yields a 20bit mask, which is 255.255.240.0. Thus the network and subnet mask combination of 10.10.0.0 255.255.240.0 encompasses all 16 networks. The process of taking bits away from the subnet mask to more broadly encompass multiple subnetworks is called supernetting. This is the opposite of subnetting, which divides a network into smaller subnetworks.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 11, Route Summarization, pp. 455-458 Cisco: IP Routing Frequently Asked Questions: What does route summarization mean?
Cisco: IP Addressing and Subnetting for New Users