destination command specifies a destination address to which client messages are forwarded, and it enables DHCP for IPv6 relay service on the interface. When relay service is enabled on an interface, a DHCP for IPv6 message received on that interface will be forwarded to all configured relay destinations. The incoming DHCP for IPv6 message may have come from a client on that interface, or it may have been relayed by another relay agent.
The relay destination can be a unicast address of a server or another relay agent, or it may be a multicast address. There are two types of relay destination addresses:
A link-scoped unicast or multicast IPv6 address, for which a user must specify an output interface
A global or site-scoped unicast or multicast IPv6 address. A user can optionally specify an output interface for this kind of address.
If no output interface is configured for a destination, the output interface is determined by routing tables. In this case, it is recommended that a unicast or multicast routing protocol be running on the router.
Multiple destinations can be configured on one interface, and multiple output interfaces can be configured for one destination. When the relay agent relays messages to a multicast address, it sets the hop limit field in the IPv6 packet header to 32.
Unspecified, loopback, and node-local multicast addresses are not acceptable as the relay destination. If any one of them is configured, the message "Invalid destination address" is displayed.
Note that it is not necessary to enable the relay function on an interface for it to accept and forward an incoming relay reply message from servers. By default, the relay function is disabled, and there is no relay destination on an interface. The
no form of the command removes a relay destination on an interface or deletes an output interface for a destination. If all relay destinations are removed, the relay service is disabled on the interface.
The DHCP for IPv6 client, server, and relay functions are mutually exclusive on an interface. When one of these functions is already enabled and a user tries to configure a different function on the same interface, one of the following messages is displayed: "Interface is in DHCP client mode," "Interface is in DHCP server mode," or "Interface is in DHCP relay mode."
In Cisco CMTS, if you change one or more parameters of this command, you have to disable the command using the no form, and execute the command again with changed parameters.
The default behavior (when
no output interface commands are provisioned in the
ipv6 dhcp relay destination command) of the new functionality is to copy the Cisco IOS SAS-computed source address to the link-address of the DHCPv6 relay-forward message.