What is Route Aggregation?

Understanding Route Aggregation

Route aggregation is the technique of combining multiple contiguous IP network prefixes into a single, less specific route, also known as supernetting or route summarization. By advertising one summary prefix instead of many individual routes, it reduces the size of routing tables and the volume of routing updates exchanged between routers.

The mechanism identifies the shared high-order bits across a block of adjacent networks and represents them with one shorter prefix. For instance, 203.0.112.0/24 through 203.0.115.0/24 can be aggregated into 203.0.112.0/22. This is the foundation of CIDR (Classless Inter-Domain Routing), which replaced rigid classful addressing and lets providers advertise large customer blocks as a single entry. Aggregation depends on contiguous, well-planned address allocation to produce clean summaries without including unintended networks.

Route aggregation matters because it is essential to the scalability and stability of large networks and the global internet. Without it, the BGP routing table would balloon with hundreds of thousands of extra entries, straining router memory and CPU. Aggregation also dampens instability: a single flapping subnet hidden inside a summary does not trigger updates across the wider network, isolating churn. From a security standpoint, however, overly broad or misconfigured aggregates can contribute to route leaks or mask hijacked sub-prefixes.

For example, an ISP assigns a customer four contiguous /24 networks. Rather than advertising four separate routes to its BGP peers, the ISP aggregates them into one /22 announcement. Upstream routers across the internet store a single prefix for that customer instead of four, conserving table space globally, and brief outages within one of the /24s no longer propagate update storms to distant peers.