Monitoring traffic between the ISATAP hosts at a site is difficult. All hosts using the same ISATAP router are on the same virtual link, so the packets do not really pass through any other routers (of course the packets might pass through IPv4 routers on the layer below but there they are hardly distinguishable from the normal IPv4 traffic). Monitoring of non-link-local traffic can thus really only be done on the ISATAP router but itself. Note that ISATAP clients within the site can send packets to each other directly using IPv6-in-IPv4 encapsulation and their link-local ISATAP addresses. This traffic does not go through the ISATAP server and can only be monitored at the sending and receiving nodes which is hardly feasible for all hosts of the site.