Huawei Zhongguancun Laboratory USA NIST Tsinghua University This document proposes a mechanism that enables a Source AS to actively advertise their locally observed Customer Cone and prefix information to the adjacent Validating AS via a new inter-domain message called Source Prefix Advertisement (SPA). The Validating AS then combines this SPA-carried information with local source address validation-related information to construct more accurate prefix allowlists for interfaces connected to Source ASes.