The Dynamic Host Configuration Protocol (DHCP) for IPv6 (RFC 3315) enables DHCP servers to pass configuration parameters such as IPv6 network addresses to IPv6 nodes. DHCP supports automatic allocation of reusable network addresses and of additional configuration parameters. This protocol is a stateful counterpart to stateless address autoconfiguration, and you can use it separately or concurrently with the latter to obtain configuration parameters. For more information about stateless address autoconfiguration, see Host autoconfiguration.
To request the assignment of one or more IPv6 addresses, a client first locates a DHCP server, and then requests the assignment of addresses and other configuration information from the server:
The client sends a solicit message to the All_DHCP_Relay_Agents_and_Servers (FF02::1:2) multicast address to find available DHCP servers.
Any server that can meet the requirements responds with an advertise message.
The client then chooses one of the servers and sends a request message to the server asking for confirmed assignment of addresses and other configuration information.
The server responds with a reply message that contains the confirmed addresses and configuration.
If a DHCP client does not need a DHCP server to assign it an IPv6 address, the client can obtain configuration information such as a list of available DNS servers or NTP servers through a single message and reply exchanged with a DHCP server.
IPv6 DHCP clients use link-local addresses to send and receive DHCP messages. To permit a DHCP client to send a message to a DHCP server that is not attached to the same link, you must configure a DHCP relay agent on the client link to relay messages between the client and server. The operation of the relay agent is transparent to the client.
A relay agent relays messages from clients and messages from other relay agents. The switch supports DHCP Relay for IPv6. Configure at least one relay agent when the client and server are in different networks.
You must configure the relay agent to use a list of destination addresses for available DHCP servers. The software does not support IPv6 multicast for site-local and global addresses.
The DHCP relay can be a Virtual Router Redundancy Protocol (VRRP) Address. The relay forwards the DHCP messages only if VRRP is in the Master state, otherwise the relay discards the messages.
Note
DHCP cannot work on the backup VRRP if the master fails. To achieve optimum results and to leverage redundancy, you must configure DHCP on the backup VRRP.
Clients listen for DHCP messages on UDP port 546. Servers and relay agents listen for DHCP messages on UDP port 547.
IPv6 DHCP Relay supports the remote ID parameter (RFC4649). After you enable remote ID on the switch, the relay agent adds information about the relay to DHCPv6 messages before relaying the messages to the DHCP server. The server can use the supplied information in the process of assigning the addresses, delegated prefixes, and configuration parameters that the client is to receive.
The remote ID option contains two fields:
vendor ID
MAC address of the client
The switch uses a vendor ID of 1584.
The following list identifies configuration limitations:
You can configure only one relay for a VLAN, regardless of how many addresses are configured on that VLAN. The default address is the smallest address configured. If the relay is a VRRP address, the default value is the first VRRP address configured.
The maximum number of servers to which a relay can send a message from one client, is 10.
You can configure the number of forwarding paths per system. For information on the maximum limit, see Fabric Engine Release Notes.
A DHCP Relay IPv6 is established only between agents within the context of each VRF and when no cross VRF interaction is present.
Note
All_DHCP_Servers multicast address option is not
implemented for IPv6, as there is no IPv6 MCAST support for site-local
and global address.