Cisco Systems Cisco Systems Swisscom AG Nvidia Arrcus, Inc. Summarization is often used in multi-domain networks to improve network efficiency and scalability. With summarization in place, there is a need to signal loss of reachability to an individual prefix covered by the summary. This enables fast convergence by steering traffic away from the node which owns the prefix and is no longer reachable. This document specifies the mechanism, referred to as Unreachable Prefix Announcement (UPA), for networks where BGP is used to carry summary routes. It is also equally beneficial for operators to share the unreachable prefixes.