BGP-4 Aggregation

A route aggregate is a route advertisement that has consolidated contiguous networks.

For instance;

Networks: (192.168.1.0/24, 192.168.2.0/24, …192.168.254.0/24, 192.168.255.0/24)

Aggregate: 192.168.0.0/16

Rather then processing 256 networks, a single aggregate network address that covers all the networks can be advertised. Aggregates vastly reduce the Internet Routing Table size.

The following posts are taken from the book “Internet Routing Architectures”. These examples demonstrate different methods of aggregation that are seen on the Internet. The way aggregates are formed and advertised and whether they carry with them more specific routes will influence traffic patterns and sizes of BGP routing tables. Remember that aggregation applies to routes that exist in the BGP routing table. An aggregate can be sent if at least one more-specific route of that aggregate exists in the BGP table.

Aggregate only, suppressing more specific routes:

Aggregate plus more specific routes:

Aggregate with a subset of more specific routes:

Loss of information inside aggregates:

Changing the aggregates attributes:

Forming the Aggregate Based on a subset of specific routes