I've flashed gnrc_networking on two samr21-xpro. I've noticed that they do not answer to router solicitation because "packet destination is not this host" when source address is ff02::2 (gnrc_ipv6.c). On native, there is no problem. By going deeper, I saw that the address ff02::2 is not added to the interface because samr21-xpro uses sixlowpan (compare to native), the function in question is 
So basically, when the first samr21-xpro send a router solicitation, nobody answers. Shouldn't the second samr21-xpro answer? What do you think?
 https://github.com/RIOT-OS/RIOT/blob/master/sys/net/gnrc/network_layer/ipv6/gnrc_ipv6.c#L905  https://github.com/RIOT-OS/RIOT/blob/master/sys/net/gnrc/network_layer/ipv6/netif/gnrc_ipv6_netif.c#L268