Appendix C IPv6
XS1930 Series User’s Guide
364
Each DHCP client and server has a unique DHCP Unique IDentifier (DUID), which is used for identification
when they are exchanging DHCPv6 messages. The DUID is generated from the MAC address, time,
vendor assigned ID and/or the vendor's private enterprise number registered with the IANA. It should not
change over time even after you reboot the device.
Identity Association
An Identity Association (IA) is a collection of addresses assigned to a DHCP client, through which the
server and client can manage a set of related IP addresses. Each IA must be associated with exactly
one interface. The DHCP client uses the IA assigned to an interface to obtain configuration from a DHCP
server for that interface. Each IA consists of a unique IAID and associated IP information.
The IA type is the type of address in the IA. Each IA holds one type of address. IA_NA means an identity
association for non-temporary addresses and IA_TA is an identity association for temporary addresses.
An IA_NA option contains the T1 and T2 fields, but an IA_TA option does not. The DHCPv6 server uses T1
and T2 to control the time at which the client contacts with the server to extend the lifetimes on any
addresses in the IA_NA before the lifetimes expire. After T1, the client sends the server (
S1
) (from which
the addresses in the IA_NA were obtained) a Renew message. If the time T2 is reached and the server
does not respond, the client sends a Rebind message to any available server (
S2
). For an IA_TA, the
client may send a Renew or Rebind message at the client's discretion.
DHCP Relay Agent
A DHCP relay agent is on the same network as the DHCP clients and helps forward messages between
the DHCP server and clients. When a client cannot use its link-local address and a well-known multicast
address to locate a DHCP server on its network, it then needs a DHCP relay agent to send a message to
a DHCP server that is not attached to the same network.
The DHCP relay agent can add the remote identification (remote-ID) option and the interface-ID option
to the Relay-Forward DHCPv6 messages. The remote-ID option carries a user-defined string, such as the
system name. The interface-ID option provides slot number, port information and the VLAN ID to the
DHCPv6 server. The remote-ID option (if any) is stripped from the Relay-Reply messages before the relay
agent sends the packets to the clients. The DHCP server copies the interface-ID option from the Relay-
Forward message into the Relay-Reply message and sends it to the relay agent. The interface-ID should
not change even after the relay agent restarts.
Prefix Delegation
Prefix delegation enables an IPv6 router to use the IPv6 prefix (network address) received from the ISP (or
a connected uplink router) for its LAN. The Switch uses the received IPv6 prefix (for example, 2001:db2::/
48) to generate its LAN IP address. Through sending Router Advertisements (RAs) regularly by multicast,
the Switch passes the IPv6 prefix information to its LAN hosts. The hosts then can use the prefix to
generate their IPv6 addresses.
T1
T2
Renew
Rebind
Rebind
to S1
Renew
to S1
Renew
to S1
Renew
to S1
Renew
to S1
Renew
to S1
to S2
to S2
