Address Aggregation

A second task in allocating infrastructure routes for subnets is to make sure your address assignment will support either a protocol-driven aggregation scheme or a manual route aggregation scheme. This allows control of the size of the routing table inside the network and yields efficiency when advertising subnets outside the local domain.


Aggregation may not be necessary at the start of a small network deployment because there just aren’t enough subnets to aggregate. However, it does come into play in larger networks and quite commonly in service overlays.

What Is Aggregation?

Aggregation is an address allocation goal for any network requiring high availability. Aggregation, or supernetting as it is described in Cisco and Microsoft textbooks, is a less specific way to refer to a collection of more specific routes. The ability to aggregate a collection of more specific routes into fewer, less specific routes is based on binary bit patterns in the subnets themselves. Table 12-2 shows a collection of multiaccess subnets that can be aggregated into a single route advertisement, thereby reducing the number of routes that would need to be advertised.

Table 12-2. Route aggregation example

IP address

Binary breakout

1100 0000. 1010 0100. 0000 0000. 0000 0000

1100 0000. 1010 0100. 0000 0001. 0000 0000

1100 0000. 1010 0100. 0000 0010. 0000 0000

1100 0000. 1010 0100. 0000 0011. 0000 0000

1100 0000. 1010 0100. ...

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