This specifies the IPv6 ifindex of a interface to reach a DHCPv6 relay server, to which DHCPv6 messages received on this interface will be relayed. This parameter is only meaningful when the interface is in DHCPv6 relay mode. A value of zero signifies that the relay address is a global IPv6 address and the router can evaluate the relay interface from the global Ipv6 address.
Sources for this description
NETGEAR official product and EOL MIB downloads