BGP Unreachability Information SAFI

This document defines a new BGP Subsequent Address Family Identifier (SAFI) called "Unreachability Information" that allows the propagation of prefix unreachability information through BGP without affecting the installation or removal of routes in the Routing Information Base (RIB) or Forwarding Information Base (FIB). This mechanism enables network operators to share information about unreachable prefixes for monitoring, debugging, and coordination purposes while maintaining complete separation from the active routing plane.