This notification-only value carried in packet-mirroring traps is the internal UID the E-Series router uses to identify the specific user or interface associated with the mirroring event. An admin uses this internal identifier together with the router's own management tools to look up further detail about the exact user or interface object referenced by a trap. For example, looking up juniPacketMirrorRouterId in the router's internal object tables resolves it to the specific subscriber session or interface the mirroring trap concerns.