H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 117)

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2. If Host A receives an NA message from Host B, Host A decides that Host B is reachable. Otherwise,
Host B is unreachable.
Duplicate address detection
After Host A acquires an IPv6 address, it performs Duplicate Address Detection (DAD) to check whether
the address is being used by any other node (similar to the gratuitous ARP function in IPv4). DAD is
accomplished through NS and NA message exchanges. Figure 47 shows the DAD process.
Figure 47 Duplicate address detection
1. Host A sends an NS message whose source address is the unspecified address and whose
destination address is the corresponding solicited-node multicast address of the IPv6 address to be
detected. The NS message contains the IPv6 address.
2. If Host B uses this IPv6 address, Host B returns an NA message. The NA message contains the IPv6
address of Host B.
3. Host A learns that the IPv6 address is being used by Host B after receiving the NA message from
Host B. If receiving no NA message, Host A decides that the IPv6 address is not in use and uses this
address.
Router/prefix discovery and address autoconfiguration
Router/prefix discovery enables a node to locate the neighboring routers and to learn from the received
RA message configuration parameters such as the prefix of the network where the node is located.
Stateless address autoconfiguration enables a node to generate an IPv6 address automatically
according to the information obtained through router/prefix discovery.
Router/prefix discovery is implemented through RS and RA messages in the following steps.
1. At startup, a node sends an RS message to request the address prefix and other configuration
information for autoconfiguration.
2. A router returns an RA message containing information such as Prefix Information options. (The
router also periodically sends an RA message.)
3. The node automatically generates an IPv6 address and other configuration information according
to the address prefix and other configuration parameters in the RA message.
NOTE:
•
In addition to an address prefix, the Prefix Information option also contains the preferred lifetime and
valid lifetime of the address prefix. Nodes update the preferred lifetime and valid lifetime accordingly
through periodic RA messages.
• An automatically generated address is applicable within the valid lifetime and is removed when the
valid lifetime expires.