Huawei Zhongguancun Laboratory USA NIST Tsinghua University This document proposes a mechanism that enables neighboring ASes (Source ASes) to actively advertise their locally observed Customer Cone and prefix information 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.