hi, im trying to do that, using an eth0:1 but radvd complains about link local address for the /48 block interface. adding manually does not work because the link local is assigned to eth0 rather than eth0:1. is this possible or am i messing up?
thanks in advance.
As far as I know eth0:whatever was never anything more than a hack to allow assigning multiple IPv4 addresses to a single physical interface. I think all incoming packets are treated as arriving on eth0 regardless of what the IPs are anyway.
With IPv6 you can do it much cleaner since there is built in support for assigning multiple address to an interface. This leaves little reason to ever use addresses of the form eth0:whatever for IPv6.
I don't know what you are trying to achieve. Though it is possible to assign multiple /64 prefixes to a single segment, I don't think there is much point in doing so with /64s from a single /48. (Are you just trying it for the experiment in order to be prepared for the day where you want two /64 blocks from different ISPs on the same segment?)
If you are trying to configure multiple VLANs, then the format of addresses to use is different. You could for example have eth0.4000 in order to address VLAN tag 4000 on eth0.