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Appendix G IPv6
NBG4615 User’s Guide
308
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 NBG4615 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 NBG4615 passes the IPv6 prefix information to its LAN hosts. The
hosts then can use the prefix to generate their IPv6 addresses.
ICMPv6
Internet Control Message Protocol for IPv6 (ICMPv6 or ICMP for IPv6) is defined in RFC 4443.
ICMPv6 has a preceding Next Header value of 58, which is different from the value used to identify
ICMP for IPv4. ICMPv6 is an integral part of IPv6. IPv6 nodes use ICMPv6 to report errors
encountered in packet processing and perform other diagnostic functions, such as "ping".
Neighbor Discovery Protocol (NDP)
The Neighbor Discovery Protocol (NDP) is a protocol used to discover other IPv6 devices and track
neighbor’s reachability in a network. An IPv6 device uses the following ICMPv6 messages types:
• Neighbor solicitation: A request from a host to determine a neighbor’s link-layer address (MAC
address) and detect if the neighbor is still reachable. A neighbor being “reachable” means it
responds to a neighbor solicitation message (from the host) with a neighbor advertisement
message.
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