<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
a	O
unicast	B-Protocol
flood	I-Protocol
is	O
when	O
a	O
switch	O
receives	O
a	O
unicast	B-Protocol
frame	O
and	O
treats	O
it	O
as	O
a	O
broadcast	O
frame	O
,	O
flooding	O
the	O
frame	O
to	O
all	O
other	O
ports	O
on	O
the	O
switch	O
.	O
</s>
<s>
The	O
term	O
unicast	B-Protocol
refers	O
to	O
a	O
one-to-one	O
transmission	O
from	O
one	O
point	O
in	O
the	O
network	O
to	O
another	O
point	O
.	O
</s>
<s>
Conventionally	O
,	O
unicast	B-Protocol
is	O
considered	O
more	O
secure	O
because	O
the	O
frame	O
is	O
delivered	O
solely	O
to	O
the	O
intended	O
recipient	O
and	O
not	O
to	O
multiple	O
hosts	O
.	O
</s>
<s>
This	O
diagram	O
illustrates	O
the	O
unicast	B-Protocol
transmission	O
of	O
a	O
frame	O
from	O
one	O
network	O
host	O
to	O
another	O
:	O
</s>
<s>
When	O
a	O
switch	O
receives	O
a	O
unicast	B-Protocol
frame	O
with	O
a	O
destination	O
address	O
not	O
in	O
the	O
switch	O
’s	O
forwarding	B-Protocol
table	I-Protocol
,	O
the	O
frame	O
is	O
treated	O
like	O
a	O
broadcast	O
frame	O
and	O
sent	O
to	O
all	O
hosts	O
on	O
a	O
network	O
:	O
</s>
<s>
The	O
learning	O
process	O
of	O
transparent	O
bridging	O
requires	O
that	O
the	O
switch	O
receive	O
a	O
frame	O
from	O
a	O
device	O
before	O
unicast	B-Protocol
frames	O
can	O
be	O
forwarded	O
to	O
it	O
.	O
</s>
<s>
Before	O
any	O
such	O
transmission	O
is	O
received	O
,	O
unicast	B-Protocol
flooding	O
is	O
used	O
to	O
assure	O
transmissions	O
reach	O
their	O
intended	O
destination	O
.	O
</s>
<s>
The	O
process	O
occurs	O
when	O
a	O
device	O
is	O
initially	O
connected	O
to	O
a	O
network	O
,	O
or	O
is	O
purged	O
from	O
the	O
forwarding	B-Protocol
information	I-Protocol
base	I-Protocol
.	O
</s>
<s>
An	O
entry	O
is	O
purged	O
when	O
the	O
device	O
is	O
moved	O
from	O
one	O
port	O
to	O
another	O
(	O
causing	O
the	O
link	O
status	O
to	O
change	O
on	O
the	O
original	O
port	O
)	O
or	O
after	O
a	O
MAC	B-Protocol
table	I-Protocol
entry	O
expires	O
due	O
to	O
inactivity	O
(	O
5	O
minutes	O
is	O
the	O
default	O
on	O
Cisco	O
switches	O
)	O
.	O
</s>
<s>
Less	O
common	O
is	O
the	O
artificial	O
flooding	O
of	O
address	O
tables	O
—	O
this	O
is	O
termed	O
MAC	B-Protocol
flooding	I-Protocol
.	O
</s>
<s>
Devices	O
other	O
than	O
switches	O
may	O
create	O
unicast	B-Protocol
floods	I-Protocol
as	O
well	O
.	O
</s>
<s>
Misconfigured	O
features	O
of	O
the	O
networks	O
may	O
lead	O
to	O
unicast	B-Protocol
flooding	O
as	O
well	O
.	O
</s>
<s>
A	O
final	O
cause	O
of	O
unicast	B-Protocol
floods	I-Protocol
are	O
topology	O
changes	O
.	O
</s>
<s>
Blocking	O
unicast	B-Protocol
floods	I-Protocol
on	O
a	O
Cisco	O
switch	O
is	O
easy	O
to	O
do	O
,	O
but	O
it	O
is	O
not	O
enabled	O
by	O
default	O
.	O
</s>
<s>
After	O
ensuring	O
that	O
timeouts	O
and/or	O
security	O
features	O
have	O
been	O
configured	O
to	O
maintain	O
table	O
entries	O
on	O
client	O
access	O
ports	O
longer	O
than	O
typical	O
host	O
ARP	B-Protocol
cache	I-Protocol
timeouts	O
,	O
this	O
command	O
is	O
used	O
to	O
quiet	O
down	O
the	O
unicast	B-Protocol
floods	I-Protocol
on	O
those	O
ports	O
:	O
</s>
<s>
A	O
more	O
robust	O
,	O
cross-switch	O
solution	O
than	O
'	O
switchport	O
protected	O
 '	O
is	O
the	O
use	O
of	O
Private	B-General_Concept
VLANs	I-General_Concept
.	O
</s>
<s>
To	O
block	O
flooding	O
on	O
a	O
Linux	O
machine	O
modern	O
enough	O
to	O
have	O
iproute2	B-Protocol
installed	O
,	O
you	O
can	O
control	O
the	O
flooding	O
in	O
the	O
devices	O
bridge	O
by	O
running	O
bridge	O
link	O
set	O
dev	O
phy6	O
flood	O
off	O
.	O
</s>
<s>
When	O
a	O
network	O
is	O
experiencing	O
unicast	B-Protocol
flooding	O
,	O
network	O
performance	O
is	O
degraded	O
.	O
</s>
<s>
The	O
security	O
considerations	O
are	O
discussed	O
in	O
the	O
MAC	B-Protocol
flooding	I-Protocol
—	O
one	O
of	O
several	O
causes	O
of	O
unicast	B-Protocol
floods	I-Protocol
.	O
</s>
<s>
If	O
an	O
end	O
user	O
is	O
running	O
a	O
packet	B-Protocol
sniffer	I-Protocol
,	O
the	O
flooded	O
frames	O
could	O
be	O
captured	O
and	O
viewed	O
.	O
</s>
