1.1.5 Route aggregation with VLSM

When VLSM is used, it is important to keep the subnetwork numbers grouped together in the network to allow for aggregation. For example, networks like 172.16.14.0 and 172.16.15.0 should be near one another so that the routers only carry a route for 172.16.14.0/23.

The use of classless interdomain routing (CIDR) and VLSM prevents address waste and promotes route aggregation, or summarization. Without route summarization, Internet backbone routing would likely have collapsed sometime before 1997.

Figure illustrates how route summarization reduces the burden on upstream routers. This complex hierarchy of variable-sized networks and subnetworks is summarized at various points with a prefix address, until the entire network is advertised as a single aggregate route of 200.199.48.0/20. Route summarization, or supernetting, is only possible if the routers of a network use a classless routing protocol, such as OSPF or EIGRP. Classless routing protocols carry a prefix that consists of a 32-bit IP address and bit mask in the routing updates. In Figure , the summary route that eventually reaches the provider contains a 20-bit prefix common to all of the addresses in the organization. That address is 200.199.48.0/22 or 11001000.11000111.0011. For summarization to work, addresses should be carefully assigned in a hierarchical fashion so that summarized addresses will share the same high-order bits.

The following are important rules to remember:

• A router must know in detail the subnet numbers attached to it.
• A router does not need to inform other routers about each subnet if the router can send one aggregate route for a set of routes.
• A router that uses aggregate routes has fewer entries in its routing table.

VLSM increases route summarization flexibility because it uses the higher-order bits shared on the left, even if the networks are not contiguous.

Figure shows that the addresses share the first 20 bits. These bits are colored red. The 21^st bit is not the same for all the routes. Therefore the prefix for the summary route will be 20 bits long. This is used to calculate the network number of the summary route.

Figure shows that the addresses share the first 21 bits. These bits are colored red. The 22^nd bit is not the same for all the routes. Therefore the prefix for the summary route will be 21 bits long. This is used to calculate the network number of the summary route.

The next page will teach students how to configure VLSM