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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.
T1
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.
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.
• Neighbor advertisement: A response from a node to announce its link-layer address.
• Router solicitation: A request from a host to locate a router that can act as the default router and
forward packets.
• Router advertisement: A response to a router solicitation or a periodical multicast advertisement from
a router to advertise its presence and other parameters.
Chapter 55 IPv6
T2
Renew
Renew
Renew
to S1
to S1
to S1
Renew
Renew
Renew
to S1
to S1
to S1
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Rebind
to S2
Rebind
to S2
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Troubleshooting
