I'm having a problem involving IPv6 with stateless autoconfig on my network. I have a Cisco 800 series router and we have several VLAN interfaces each configured with an IPv6 prefix. The problem I'm noticing is that whenever any host on the network tries to start up with stateless autoconfig they immediately detect a duplicate address for whatever address they are trying to use.

I performed a packet capture and what I'm seeing is that when the host selects an IPv6 address to use, it performs the Neighbor Solicitation to check if the address is in use. I immediately see the exact same Neighbor Solicitation message echo'd back with the source MAC being the Cisco router. This causes the host to reject the address as a duplicate since it is receiving a Neigbor Soliciation for the same address it is attempting to use.

This happens on all of the VLANs I have configured for IPv6. The basic VLAN config is like this:

interface Vlan109
 description Engineering VLAN
 ip address .....
 ip helper-address .....
 ip nat inside
 ip virtual-reassembly in
 ip tcp adjust-mss 1300
 ipv6 address HE-ENG ::/64 eui-64
 ipv6 enable
 ipv6 nd ra interval 60

Attached is a PCAP with some ICMPv6 traffic. You can see that the NS messages are duplicated and the source of the duplicates seems to be the Cisco router (70:ca:9b:e0:94:2e). Does anyone know what is going on here?