What is AS-SET?

An AS-SET is a named Internet Routing Registry object grouping ASNs and other as-sets, which neighbours expand to build route filters.

The object is written in RPSL. Its name either starts with AS- (for example AS-EXAMPLE) or is hierarchical, prefixed with the owner's ASN, such as AS64500:AS-CUSTOMERS. The members: attribute lists ASNs and other as-sets, so sets nest: a transit provider's set includes each customer's set, which includes their customers' sets. A set can also accept members from the other side with mbrs-by-ref:, where an aut-num joins by naming the set in its member-of: attribute. To build a filter, a neighbour expands the set recursively to a flat list of ASNs, then collects every route object those ASNs originate, usually with a tool such as bgpq4.

The usual job of an as-set is to describe a network's customer cone: itself plus everyone it carries transit for. When you buy transit, your provider will ask which as-set to expand; most networks also publish the name in PeeringDB. Common problems:

• Stale members: a customer that left years ago still sits in the set, along with its routes.
• Huge or looping expansions: one careless member pulls in another network's entire cone.
• Name collisions: a flat name like AS-EXAMPLE can exist in several registries with different contents, which is why hierarchical names are preferred.

Because IRR data is not cryptographically signed, as-set filters work best alongside RPKI. The IRR, RPSL and AS-SETs lesson walks through as-set objects, their expansion and the pitfalls in detail.