IPv6 routing problem

Hi,

I'm having a strange problem with IPv6. I have the following setup:

Cisco 877 <--> m0n0wall <--> LAN

If I ping6 a host on the LAN from the Cisco 877, the ICMPv6 echo-request 
is routed through m0n0wall to the host, the host then sends an ICMPv6 
echo reply back to the Cisco 877. However, m0n0wall doesn't forward the 
echo-reply, it sends an ICMPv6 neighbour solicitation message to 
ff02::1:ff00:1. The cisco appears to be either ignoring or not seeing 
these solicitations so the echo-reply never gets forwarded.

Has anybody seen this before? I suspect the problem lies with the Cisco 
and the key is getting it to respond to the solicitations.

Thanks in advance,

Mike