A typical interface configuration on a Cisco router

interface Ethernet1/0
 ip address
 ipv6 address 2001:db8:21F0:117::1/64
 ipv6 address 2001:db8:21F0:117::/64 eui-64
 ipv6 enable
 ipv6 nd prefix 2001:db8:21F0:117::/64
 ipv6 nd managed-config-flag
 ipv6 nd other-config-flag

If the DHCP server is on a different network segment than the client one has to add to following 2 lines for IPv4 and IPv6

 ip helper-address
 ipv6 dhcp relay destination 2001:db8:21F0:116::2 Ethernet1/7

See also

IPv6 with GNU/Linux

DHCP with IPv6 and GNU/Linux