CIDR and Route Aggregation

As explained in Chapter 3, "IP Addressing," there is a shortage of IP addresses in the Internet. There is also a problem with the size of the routing tables, in terms of the memory, bandwidth, and CPU that they consume when updating the routing tables. Classless interdomain routing (CIDR) is one of the main solutions implemented in recent years. This is a method of consolidating addresses into a few summary addresses. Instead of a subnet having a subnet mask to identify the network portion of the address, it has a prefix mask, which is simply a number that indicates the number of bits that have been allocated to the network.

BGP-4 will propagate the prefix and the prefix mask together, allowing not only for the design of a truly hierarchical network, but also for the streamlining of the network resources. A router can pass on the aggregated routes, although it is capable of aggregating routes itself. Therefore, a router can send either aggregated routes, routes that have not been summarized, or a mixture of both.

The process of how BGP-4 aggregates routes in compliance to CIDR is shown in the Figure 8-6.

Several attributes are connected to the use of address aggregation in BGP-4. Table 8-2 covers some of these attributes.

Table 8-2 Route Aggregation Attributes


Type of Attribute

Description of Attribute

Atomic aggregate

Well known, discretionary

This is the default attribute. The originator of the aggregate route is stated. It is useful because it shows that some information has been lost due to the aggregation of routes. It states to the receiving router in another autonomous system that the originator of the route aggregated the routes.


Optional, transitive

This attribute gives the router ID and the autonomous system of the router that performed the aggregation.


Well known, mandatory

This attribute can include a list of all the autonomous systems that the aggregated routes have passed through.

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