to discard any received packet whose link-layer source address is the
same as the receiving interface's. Unfortunately, use of that
criteria also results in the discarding of all packets sent by
another node using the same link-layer address. Duplicate Address
Detection will fail on interfaces that filter received packets in
this manner:
o If a node performing Duplicate Address Detection discards received
packets having the same source link-layer address as the receiving
interface, it will also discard packets from other nodes also using
the same link-layer address, including Neighbor Advertisement and
Neighbor Solicitation messages required to make Duplicate Address
Detection work correctly. This particular problem can be avoided
by temporarily disabling the software suppression of loopbacks
while a node performs Duplicate Address Detection.
o If a node that is already using a particular IP address discards
received packets having the same link-layer source address as the
interface, it will also discard Duplicate Address Detection-related
Neighbor Solicitation messages sent by another node also using the
same link-layer address. Consequently, Duplicate Address Detection
will fail, and the other node will configure a non-unique address.
Since it is generally impossible to know when another node is
performing Duplicate Address Detection, this scenario can be
avoided only if software suppression of loopback is permanently
disabled.
Thus, to perform Duplicate Address Detection correctly in the case
where two interfaces are using the same link-layer address, an
implementation must have a good understanding of the interface's
multicast loopback semantics, and the interface cannot discard
received packets simply because the source link-layer address is the
same as the interfaces.