NDPMon is an equivalent of ArpWatch for IPv6.

NDPMon, Neighbor Discovery Protocol Monitor, is a tool working with
ICMPv6 packets. NDPMon observes the local network to see if nodes
using neighbor discovery messages behave properly. When it detects
a suspicious Neighbor Discovery message, it notifies the administrator
by writing in the syslog and in some cases by sending an email
report.

WWW: http://ndpmon.sourceforge.net

Janos Mohacsi <janos.mohacsi@bsd.hu>