Cisco Systems, Inc. Cisco Systems, Inc. Cisco Systems, Inc. HPE Network operators managing overlay networks require visibility into underlay network hops during traceroute operations from overlay endpoints. This document defines an ICMP extension object, the Underlay Information Object (UIO), which allows underlay head-end nodes to encapsulate underlay error information within ICMP error messages. This mechanism provides overlay operators with crucial visibility into underlay network paths for troubleshooting.