7340: How to disable internal DHCHv6

[DualIP]

My setup:
7340 , freetz trunk 7170, running ipv6 native mode. Using AVMs radv.
Linux router on LAN for: radvd, DNS, routing to extra internal subnet (got /48 from ISP)

problem:
When host on internal LAN receives RA from linux-router, it gets ipv6 address fine. After that, the host queries for dhcpv6 server, to autoconfigure its DNS settings.
Both 7340 and my linux router send back dhcpv6 reply.However, 7340 is faster, and I end up using fritzbox fd00::EUI64 address for DNS, instead of linux router/dns server.
This way, DNS lookup for internal stuff fails.

How to disable internal DHCHv6 on FB7340? AR7.cfg doesn't give me any clue.

 

[DualIP]

Ugly work-around:
Blocked DHCPv6 multicast on FB port on Linksys SPS224G4 switch , so the DHCPv6 request never reaches the fritzbox.