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3-15
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
OL-20002-02
Chapter 3 Configuring IPv6
Information About IPv6
Note A neighbor advertisement message that has the solicited flag set to a value of 0 is not considered as a
positive acknowledgment that the forward path is still working.
Neighbor solicitation messages are also used in the stateless auto-configuration process to verify the
uniqueness of unicast IPv6 addresses before the addresses are assigned to an interface. Duplicate address
detection is performed first on a new, link-local IPv6 address before the address is assigned to an
interface (the new address remains in a tentative state while duplicate address detection is performed).
A node sends a neighbor solicitation message with an unspecified source address and a tentative
link-local address in the body of the message. If another node is already using that address, the node
returns a neighbor advertisement message that contains the tentative link-local address. If another node
is simultaneously verifying the uniqueness of the same address, that node also returns a neighbor
solicitation message. If no neighbor advertisement messages are received in response to the neighbor
solicitation message and no neighbor solicitation messages are received from other nodes that are
attempting to verify the same tentative address, the node that sent the original neighbor solicitation
message considers the tentative link-local address to be unique and assigns the address to the interface.
IPv6 Router Advertisement Message
Router advertisement (RA) messages, which have a value of 134 in the Type field of the ICMP packet
header, are periodically sent out to each configured interface of an IPv6 router. For stateless
auto-configuration to work properly, the advertised prefix length in RA messages must always be 64 bits.
The RA messages are sent to the all-nodes multicast address (see Figure 3-13).
Figure 3-13 IPv6 Neighbor Discovery—RA Message
RA messages typically include the following information:
• One or more onlink IPv6 prefixes that nodes on the local link can use to automatically configure
their IPv6 addresses
• Life-time information for each prefix included in the advertisement
• Sets of flags that indicate the type of auto-configuration (stateless or stateful) that can be completed
• Default router information (whether the router sending the advertisement should be used as a default
router and, if so, the amount of time in seconds that the router should be used as a default router)
• Additional information for hosts, such as the hop limit and MTU that a host should use in packets
that it originates
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Router advertisement packet definitions:
ICMPv6 Type = 134
Src = router link-local address
Dst = all-nodes multicast address
Data = options, prefix, lifetime, autoconfig flag
Router
advertisement
Router
advertisement
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