Duplicate Address Detection (Dad); Dad Operation - HP 2520-8 Configuration Manual

IPv6 Addressing Configuration

Duplicate Address Detection (DAD)

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For related information, refer to:
■ RFC 2461: "Neighbor Discovery for IP Version 6 (IPv6)"
Duplicate Address Detection (DAD)
Duplicate Address Detection verifies that a configured unicast IPv6 address
is unique before it is assigned to a VLAN interface on the switch. DAD is
enabled in the default IPv6 configuration, and can be reconfigured, disabled,
or re-enabled at the global config command level. DAD can be useful in helping
to troubleshoot erroneous replies to DAD requests, or where the neighbor
cache contains a large number of invalid entries due to an unauthorized station
sending false replies to the switch's neighbor discovery queries. If DAD
verifies that a unicast IPv6 address is a duplicate, the address is not used. If
the link-local address of the VLAN interface is found to be a duplicate of an
address for another device on the interface, then the interface stops
processing IPv6 traffic.

DAD Operation

On a given VLAN interface, when a new unicast address is configured, the
switch runs DAD for this address by sending a neighbor solicitation to the All-
Nodes multicast address (ff02::1). This operation discovers other devices on
the VLAN and verifies whether the proposed unicast address assignment is
unique on the VLAN. (During this time, the address being checked for unique-
ness is held in a tentative state, and cannot be used to receive traffic other
than neighbor solicitations and neighbor advertisements.) A device that
receives the neighbor solicitation responds with a Neighbor Advertisement
that includes its link-local address. If the newly configured address is from a
static or DHCPv6 source and is found to be a duplicate, it is labelled as
duplicate in the "Address Status" field of the show ipv6 command, and is not
used. If an autoconfigured address is found to be a duplicate, it is dropped and
the following message appears in the Event Log:
W < date > < time > 00019 ip: ip address < IPv6-address >
removed from vlan id < vid >
DAD does not perform periodic checks of existing addresses. However, when
a VLAN comes up with IPv6 unicast addresses configured (as can occur during
a reboot) the switch runs DAD for each address on the interface by sending
neighbor solicitations to the All-Nodes multicast address as described above.