Using a Compromised Router to Capture Network Traffic – This document details the approach, methodology and results of a recent experiment using GRE tunnels to sniff all traffic passing through a Cisco router.
You can download it from the following link: https://packetstormsecurity.com/files/download/26417/GRE_sniffing.doc
Source: https://packetstormsecurity.com/files/26417/GRE_sniffing.doc.html