An AS-SET is a special object that represents a group of ASNs and forms the basis for IRR-based route filtering. However, many AS-SETs in circulation today have grown so big that they effectively whitelist much of the routing table, rendering them ineffective. According to recent analysis, there are currently 2,192 AS-SETs which expand to over 1,000 ASNs each! In this blog post, we’ll describe what an AS-SET is, its role in route filtering, and how to deal with excessively large AS-SETs.