On IPv6 linux router, autoconf and accept router advertisements for single interface

Apparently right now if you have
/proc/sys/net/ipv6/conf/all/forwarding set to a value of
1 that completely disables the auto configuration of Interfaces
and routes, but I have a system with one interface that I want to dynamically configure
the address.

I have a Linux box with multiple
interfaces acting as a router with href="http://www.lartc.org/lartc.html#LARTC.RPDB.MULTIPLE-LINKS" rel="nofollow
noreferrer">multiple wan connections. On the IPv4 side I am using multiple
route tables and ip rules to direct traffic to separate
uplinks.

My primary wan connection has static
IPv6 address that are permanently assigned to my connection. The backup connection is
basically a cheap broadband connection, and I have no static addresses IPv6, or IPv4. I
can see via radvdump that my provider of my cheap broadband
backup link that they are now sending out IPv6 router advertisements. on that link.
Since my box is a router and has forwarding enabled, how do I dynamically configured the
address on this link? Is there any way to have my system accept router advertisements
configure its address, and then add the route with a metric higher than my routes from
my primary connection?

src="https://i.stack.imgur.com/UuOM8.png" alt="dual wan
diagram">

My firewall is also running squid,
and the vast bulk of my communication goes through the proxy. So in the case of a
failure of the primary link I am not going to have to do any weird IPv6 NAT or anything
to get my internal hosts to work with the dynamic network assigned on my broadband
interface. The bulk of the communication will be nicely handled by the application level
proxy.

So how do I get that interface on my
Linux system connected to broadband network configured for IPv6? In the slight chance it
matters, the Linux box is running Debian wheezy, with the 3.14-0.bpo.2-amd64
kernel.