What is an AS-SET?

An AS-SET is a collection of Autonomous System Numbers grouped under a single name. Rather than listing each ASN individually in your routing policy, you just refer to the set, and the routing registry handles the details. You can think of it like a mailing list for networks. When you apply policy rules to the list name, everyone on the list is automatically covered. This article covers what an AS-SET is at a high level and why it matters.

Why AS-SETs exist
BGP policies can grow quickly. You might peer with one network today, but tomorrow they could add customers, and next month they might acquire another ASN. If you manually enter every ASN, your filters can become difficult to manage. AS-SETs help solve this issue. They allow a network to publish an up-to-date list of all ASNs it owns or represents. Peers can use this list to build prefix filters without needing to keep up with every change. This approach keeps routing organized and saves time. It also lowers the chances of missing important routes or accepting routes you should not.

What an AS-SET actually contains
An AS-SET only contains ASNs. It does not include prefixes or IP addresses.
The name of an AS-SET typically looks like this:
AS-EXAMPLE
AS-EXAMPLE:AS-CUSTOMERS

I like to do AS-Sets in the form of: AS<yourAS>:AS-ALL
Example: AS123:AS-ALL

Inside the AS-SET, you’ll find a list of AS numbers. Some of these can also refer to other AS-SETs, creating a tree structure that reflects real business relationships. This nesting feature is powerful because a single reference can include hundreds of downstream networks without extra effort.

How AS-SETs get used in real life
Most networks rely on AS-SETs to create prefix filters. Here’s how it usually works: A peer provides their AS-SET name, and your router or automation system queries the routing registry. The system expands the AS-SET into a list of ASNs, then gathers the prefixes those ASNs are allowed to announce. This process creates a prefix list that updates automatically when the registry changes. There’s no need for support tickets, emails, or late-night fixes. That’s why Internet exchanges and large peers place so much importance on keeping routing registries accurate and up to date.

AS-SETs and routing safety
AS-SETs are not a magic solution. They do not secure BGP on their own and only work well when they are kept accurate and up to date. An outdated AS-SET can be more harmful than not having one at all. It may lead to dropped routes or accidental leaks. That’s why responsible operators update their registry entries before adding new customers. When used correctly, AS-SETs help prevent route leaks and make mistakes easier to find. If used poorly, they can cause hidden problems that are difficult to diagnose.

Why you should care, even on a small network
AS-SETs are useful even for small networks. As soon as you connect to more than one upstream provider or join an Internet exchange, managing ASN lists by hand becomes unmanageable. Using an AS-SET gives your peers a straightforward way to filter your routes. It also shows that you take routing seriously, which can improve your reputation more than you might expect. Over time, AS-SETs help build trust by providing structure. People can define their intentions once, and systems can enforce those rules everywhere. BGP can be unpredictable, but AS-SETs are one of the tools that help keep things under control and prevent late-night troubleshooting stories.

PeeringDB also plays a role in how AS-SETs get shared. Networks can publish their AS-SET directly in their PeeringDB profile. This gives peers a single, trusted place to find the correct AS-SET name without having to dig through emails or tickets. Many automation systems now pull AS-SET values straight from PeeringDB and use them to build filters. When the PeeringDB entry is accurate, turn-ups go faster, and mistakes drop. Centralize Management of your peers happens.

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