<s>
A	O
forwarding	B-Protocol
information	I-Protocol
base	I-Protocol
(	O
FIB	O
)	O
,	O
also	O
known	O
as	O
a	O
forwarding	B-Protocol
table	I-Protocol
or	O
MAC	B-Protocol
table	I-Protocol
,	O
is	O
most	O
commonly	O
used	O
in	O
network	B-Protocol
bridging	I-Protocol
,	O
routing	B-Protocol
,	O
and	O
similar	O
functions	O
to	O
find	O
the	O
proper	O
output	O
network	B-Protocol
interface	I-Protocol
controller	I-Protocol
to	O
which	O
the	O
input	O
interface	O
should	O
forward	O
a	O
packet	O
.	O
</s>
<s>
It	O
is	O
the	O
essential	O
mechanism	O
that	O
separates	O
network	B-Protocol
switches	I-Protocol
from	O
Ethernet	B-Protocol
hubs	I-Protocol
.	O
</s>
<s>
Content-addressable	B-Data_Structure
memory	I-Data_Structure
(	O
CAM	B-Data_Structure
)	O
is	O
typically	O
used	O
to	O
efficiently	O
implement	O
the	O
FIB	O
,	O
thus	O
it	O
is	O
sometimes	O
called	O
a	O
CAM	B-Protocol
table	I-Protocol
.	O
</s>
<s>
At	O
the	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
,	O
a	O
FIB	O
is	O
most	O
notably	O
used	O
to	O
facilitate	O
Ethernet	O
bridging	B-Protocol
based	O
on	O
MAC	O
addresses	O
.	O
</s>
<s>
Other	O
data-link-layer	B-Protocol
technologies	O
using	O
FIBs	O
include	O
Frame	B-Protocol
Relay	I-Protocol
,	O
Asynchronous	O
Transfer	O
Mode	O
(	O
ATM	O
)	O
and	O
multiprotocol	B-Protocol
label	I-Protocol
switching	I-Protocol
(	O
MPLS	B-Protocol
)	O
.	O
</s>
<s>
The	O
role	O
of	O
an	O
Ethernet	B-Protocol
switch	I-Protocol
is	O
to	O
forward	O
Ethernet	B-Protocol
frames	I-Protocol
from	O
one	O
port	O
to	O
another	O
.	O
</s>
<s>
The	O
presence	O
of	O
a	O
FIB	O
is	O
one	O
attribute	O
that	O
separates	O
a	O
switch	O
from	O
a	O
hub	B-Protocol
.	O
</s>
<s>
Without	O
a	O
functional	O
FIB	O
,	O
all	O
frames	O
received	O
by	O
a	O
network	B-Protocol
switch	I-Protocol
would	O
be	O
echoed	O
back	O
out	O
to	O
all	O
other	O
ports	O
,	O
much	O
like	O
an	O
Ethernet	B-Protocol
hub	I-Protocol
.	O
</s>
<s>
In	O
bridging	B-Protocol
packets	O
between	O
ports	O
,	O
a	O
switch	O
should	O
only	O
emit	O
a	O
frame	O
on	O
the	O
port	O
where	O
the	O
destination	O
network	O
device	O
resides	O
(	O
unicast	B-Protocol
)	O
,	O
unless	O
the	O
frame	O
is	O
for	O
all	O
nodes	O
on	O
the	O
switch	O
(	O
broadcast	B-Operating_System
)	O
,	O
multiple	O
nodes	O
(	O
multicast	O
)	O
or	O
if	O
the	O
switch	O
does	O
n't	O
know	O
where	O
the	O
destination	O
device	O
resides	O
(	O
unicast	B-Protocol
flood	I-Protocol
)	O
.	O
</s>
<s>
The	O
FIB	O
is	O
a	O
memory	O
construct	O
used	O
by	O
Ethernet	B-Protocol
switch	I-Protocol
to	O
map	O
a	O
station	O
's	O
MAC	O
address	O
to	O
the	O
switch	B-Protocol
port	I-Protocol
the	O
station	O
is	O
connected	O
to	O
.	O
</s>
<s>
While	O
the	O
exact	O
mechanics	O
of	O
a	O
forwarding	B-Protocol
table	I-Protocol
is	O
implementation-specific	O
,	O
the	O
general	O
model	O
for	O
Frame	B-Protocol
Relay	I-Protocol
is	O
that	O
switches	O
have	O
statically	O
defined	O
forwarding	B-Protocol
tables	I-Protocol
,	O
one	O
per	O
interface	O
.	O
</s>
<s>
When	O
a	O
frame	O
with	O
a	O
given	O
data	B-Protocol
link	I-Protocol
connection	I-Protocol
identifier	I-Protocol
(	O
DLCI	B-Protocol
)	O
is	O
received	O
on	O
one	O
interface	O
,	O
the	O
table	O
associated	O
with	O
that	O
interface	O
gives	O
the	O
outgoing	O
interface	O
,	O
and	O
the	O
new	O
DLCI	B-Protocol
to	O
insert	O
into	O
the	O
frame	O
's	O
address	O
field	O
.	O
</s>
<s>
ATM	O
switches	O
have	O
link-level	O
forwarding	B-Protocol
tables	I-Protocol
much	O
like	O
those	O
used	O
in	O
Frame	B-Protocol
Relay	I-Protocol
.	O
</s>
<s>
Rather	O
than	O
a	O
DLCI	B-Protocol
,	O
however	O
,	O
interfaces	O
have	O
forwarding	B-Protocol
tables	I-Protocol
that	O
specify	O
the	O
outgoing	O
interface	O
by	O
virtual	O
path	O
identifier	O
(	O
VPI	O
)	O
and	O
virtual	O
circuit	O
identifier	O
(	O
VCI	O
)	O
.	O
</s>
<s>
When	O
PNNI	O
is	O
in	O
use	O
,	O
the	O
ATM	O
switches	O
at	O
the	O
edges	O
of	O
the	O
network	O
map	O
one	O
of	O
the	O
standard	O
ATM	O
end-to-end	O
identifiers	O
,	O
such	O
as	O
an	O
NSAP	B-Protocol
address	I-Protocol
,	O
to	O
the	O
next-hop	O
VPI/VCI	O
.	O
</s>
<s>
MPLS	B-Protocol
has	O
many	O
similarities	O
,	O
at	O
the	O
forwarding	O
level	O
,	O
to	O
ATM	O
.	O
</s>
<s>
The	O
label	B-Protocol
edge	I-Protocol
routers	I-Protocol
at	O
the	O
edges	O
of	O
an	O
MPLS	B-Protocol
cloud	O
map	O
between	O
the	O
end-to-end	O
identifier	O
,	O
such	O
as	O
an	O
IP	B-Protocol
address	I-Protocol
,	O
and	O
a	O
link-local	O
label	O
.	O
</s>
<s>
At	O
each	O
MPLS	B-Protocol
hop	O
,	O
there	O
is	O
a	O
forwarding	B-Protocol
table	I-Protocol
that	O
tells	O
the	O
label	O
switched	O
router	O
which	O
outgoing	O
interface	O
is	O
to	O
receive	O
the	O
MPLS	B-Protocol
packet	O
,	O
and	O
what	O
label	O
to	O
use	O
when	O
sending	O
the	O
packet	O
out	O
that	O
interface	O
.	O
</s>
<s>
Network	B-Application
layer	I-Application
addresses	O
,	O
such	O
as	O
IP	B-Protocol
addresses	I-Protocol
,	O
are	O
used	O
on	O
different	O
types	O
of	O
media	O
and	O
can	O
be	O
handled	O
similarly	O
in	O
all	O
cases	O
.	O
</s>
<s>
FIBs	O
are	O
optimized	O
for	O
fast	O
lookup	O
of	O
destination	O
addresses	O
and	O
can	O
improve	O
performance	O
of	O
forwarding	O
compared	O
to	O
using	O
the	O
routing	B-Protocol
information	I-Protocol
base	I-Protocol
(	O
RIB	O
)	O
directly	O
.	O
</s>
<s>
The	O
RIB	O
is	O
optimized	O
for	O
efficient	O
updating	O
by	O
routing	B-Protocol
protocols	I-Protocol
and	O
other	O
control	B-Protocol
plane	I-Protocol
methods	O
,	O
and	O
contain	O
the	O
full	O
set	O
of	O
routes	O
learned	O
by	O
the	O
router	O
.	O
</s>
<s>
While	O
the	O
IETF	O
document	O
BCP	O
38	O
on	O
ingress	O
filtering	O
does	O
not	O
specify	O
a	O
method	O
of	O
implementing	O
source	O
address	O
filtering	O
,	O
some	O
router	O
vendors	O
have	O
implemented	O
a	O
mechanism	O
that	O
employs	O
reverse-path	B-Protocol
forwarding	I-Protocol
lookups	O
in	O
the	O
router	O
's	O
tables	O
to	O
perform	O
this	O
check	O
.	O
</s>
<s>
When	O
the	O
router	O
is	O
multihomed	B-Protocol
,	O
ingress	O
filtering	O
becomes	O
more	O
complex	O
.	O
</s>
<s>
For	O
the	O
routers	O
near	O
the	O
edge	O
of	O
the	O
Internet	O
,	O
packet	O
filters	O
can	O
provide	O
a	O
simpler	O
and	O
more	O
effective	O
solution	O
than	O
methods	O
that	O
employ	O
routing	B-Protocol
information	I-Protocol
lookup	O
,	O
though	O
this	O
approach	O
can	O
be	O
challenging	O
when	O
managing	O
routers	O
that	O
are	O
reconfigured	O
often	O
.	O
</s>
<s>
Ingress	O
filtering	O
for	O
multihomed	B-Protocol
routers	O
will	O
accept	O
the	O
packet	O
if	O
there	O
is	O
a	O
route	O
back	O
to	O
its	O
source	O
address	O
from	O
any	O
interface	O
on	O
the	O
router	O
.	O
</s>
<s>
CAM	B-Protocol
tables	I-Protocol
can	O
be	O
targeted	O
for	O
setting	O
up	O
a	O
man-in-the-middle	O
attack	O
.	O
</s>
<s>
A	O
threat	O
agent	O
which	O
has	O
control	O
of	O
a	O
device	O
connected	O
to	O
an	O
Ethernet	B-Protocol
switch	I-Protocol
can	O
use	O
MAC	B-Protocol
flooding	I-Protocol
to	O
attack	O
the	O
switch	O
's	O
CAM	B-Protocol
table	I-Protocol
.	O
</s>
<s>
If	O
the	O
table	O
fills	O
up	O
,	O
other	O
traffic	O
is	O
treated	O
as	O
broadcast	B-Protocol
,	I-Protocol
unknown-unicast	I-Protocol
and	I-Protocol
multicast	I-Protocol
traffic	I-Protocol
and	O
is	O
forwarded	O
to	O
all	O
ports	O
making	O
it	O
available	O
to	O
the	O
attacker	O
.	O
</s>
