DHCPv6 relay is only supported on 7210 SAS-Mxp.
DHCPv6 relay operation is similar to DHCP in that servers send configuration parameters, such as IPv6 network addresses, to IPv6 nodes; however, DHCPv6 relay is not based on the DHCP or BOOTP protocol. DHCPv6 can be used instead of, or in conjunction with, stateless autoconfiguration.
DHCPv6 uses IPv6 methods of addressing, especially the use of reserved, link-local scoped multicast addresses. DHCPv6 clients transmit messages to these reserved addresses, allowing messages to be sent without the client knowing the address of any DHCP server. This transmission allows efficient communication even before a client is assigned an IP address. When a client has an address and knows the identity of a server, the client can communicate with the server directly using unicast addressing.
The DHCPv6 protocol requires the client to transmit a request packet with a destination multicast address of ff02::1:2 (which addresses all DHCP servers and relay agents on the local network segment) that is processed by the DHCP server.
Similar to DHCP address allocation, if a client needs to obtain an IPv6 address and other configuration parameters, it sends a Solicit message to locate a DHCPv6 server, then requests an address assignment and other configuration information from the server. Servers that meet the requirements of the client respond with an Advertise message. The client chooses one of the servers and sends a Request message; the server sends back a Reply message with the confirmed IPv6 address and configuration information.
If the client already has an IPv6 address, either assigned manually or obtained in another way, the client only needs to obtain configuration information. In this case, exchanges are done using a two-message process. The client sends a Request message for only configuration information. A DHCPv6 server that has configuration information for the client sends back a Reply message with the information.
The 7210 SAS supports the DHCPv6 relay agent option in the same way that it supports the DHCP relay agent option: when the 7210 SAS is acting as a DHCPv6 relay agent, it relays messages between clients and servers that are not connected to the same link.
The DHCP relay agent uses one of its interfaces as an IP source address in the DHCP relay-forward message. The DHCPv6 server uses the same source IP address as the destination IP address in the DHCP relay-reply message. There are restrictions for the source IP address used by the DHCP relay agent, which depend on whether the relay agent is a few hops away or is directly connected.
In the case where the relay agent is a few hops away from the DHCPv6 server, the source address used by the relay agent must not fall under the subnet or prefix range configured on the IP interface on which the client is connected. For example, the loopback interface address of the DHCP relay agent can be used instead. To forward the DHCPv6 relay-reply message back to the relay agent, add a static route for the relay agent source IP address.
In the case where the relay agent is directly connected, there are two options. In the first option, the relay agent use the address of the directly connected interface as the relay-forward source address, and no additional configuration is required for the DHCP server to forward the relay-reply message back to the relay-agent. The other option is to use an interface address on the relay agent that does not fall under the subnet or prefix range configured on the IP interface on which the client is connected. Similar to the scenario where the relay-agent is a few hops away, a static route is required to forward the DHCP relay-reply message back to the relay agent.