The basic idea here is to test all eqiad hosts pinging all the static IPv6 addresses which exist in the same subnet as themselves.
First, the set of hosts with add_ip6_mapped defined in eqiad was gathered. Then on all of these hosts, "ip -6 addr" was filtered down to grab the static IPs configured on those hosts, and the total list was sorted and uniq'd. Then a cumin command was sent to all eqiad hosts on a given VLAN, asking it to ping6 once with timeout 3s to all of the IPs from the list that were in its own subnet.