<s>
IGMP	B-Protocol
snooping	I-Protocol
is	O
the	O
process	O
of	O
listening	O
to	O
Internet	B-Protocol
Group	I-Protocol
Management	I-Protocol
Protocol	I-Protocol
(	O
IGMP	B-Protocol
)	O
network	O
traffic	O
to	O
control	O
delivery	O
of	O
IP	B-Protocol
multicasts	I-Protocol
.	O
</s>
<s>
Network	B-Protocol
switches	I-Protocol
with	O
IGMP	B-Protocol
snooping	I-Protocol
listen	O
in	O
on	O
the	O
IGMP	B-Protocol
conversation	O
between	O
hosts	B-Device
and	O
routers	B-Protocol
and	O
maintain	O
a	O
map	O
of	O
which	O
links	O
need	O
which	O
IP	B-Protocol
multicast	I-Protocol
transmission	O
.	O
</s>
<s>
IGMP	B-Protocol
snooping	I-Protocol
is	O
described	O
in	O
an	O
informational	O
IETF	O
RFC	O
but	O
affects	O
bridging	O
operations	O
,	O
the	O
purview	O
of	O
the	O
IEEE	O
.	O
</s>
<s>
A	O
switch	O
will	O
,	O
by	O
default	O
,	O
flood	B-Protocol
multicast	O
traffic	O
to	O
all	O
the	O
ports	O
in	O
a	O
broadcast	B-Architecture
domain	I-Architecture
(	O
or	O
the	O
VLAN	B-General_Concept
equivalent	O
)	O
.	O
</s>
<s>
IGMP	B-Protocol
snooping	I-Protocol
is	O
designed	O
to	O
prevent	O
hosts	B-Device
on	O
a	O
local	O
network	O
from	O
receiving	O
traffic	O
for	O
a	O
multicast	O
group	O
they	O
have	O
not	O
explicitly	O
joined	O
.	O
</s>
<s>
It	O
provides	O
switches	O
with	O
a	O
mechanism	O
to	O
prune	O
multicast	O
traffic	O
from	O
links	O
that	O
do	O
not	O
contain	O
a	O
multicast	O
listener	O
(	O
an	O
IGMP	B-Protocol
client	O
)	O
.	O
</s>
<s>
Essentially	O
,	O
IGMP	B-Protocol
snooping	I-Protocol
is	O
a	O
layer	O
2	O
optimization	O
for	O
the	O
layer	O
3	O
IGMP	B-Protocol
.	O
</s>
<s>
IGMP	B-Protocol
snooping	I-Protocol
takes	O
place	O
internally	O
on	O
switches	O
and	O
is	O
not	O
a	O
protocol	O
feature	O
.	O
</s>
<s>
IGMP	B-Protocol
snooping	I-Protocol
allows	O
a	O
switch	O
to	O
only	O
forward	O
multicast	O
traffic	O
to	O
the	O
links	O
that	O
have	O
solicited	O
them	O
.	O
</s>
<s>
Snooping	O
is	O
therefore	O
especially	O
useful	O
for	O
bandwidth-intensive	O
IP	B-Protocol
multicast	I-Protocol
applications	O
such	O
as	O
IPTV	O
.	O
</s>
<s>
IGMP	B-Protocol
snooping	I-Protocol
,	O
although	O
an	O
important	O
technique	O
,	O
overlaps	O
two	O
standards	O
organizations	O
,	O
namely	O
IEEE	O
which	O
standardizes	O
Ethernet	B-Protocol
switches	I-Protocol
,	O
and	O
IETF	O
which	O
standardizes	O
IP	B-Protocol
multicast	I-Protocol
.	O
</s>
<s>
This	O
is	O
why	O
on	O
IGMP	B-Protocol
snooping	I-Protocol
carries	O
only	O
an	O
informational	O
status	O
,	O
despite	O
actually	O
being	O
referred	O
to	O
in	O
other	O
standards	O
work	O
,	O
such	O
as	O
,	O
as	O
normative	O
.	O
</s>
<s>
In	O
order	O
for	O
IGMP	B-Protocol
,	O
and	O
thus	O
IGMP	B-Protocol
snooping	I-Protocol
,	O
to	O
function	O
,	O
a	O
multicast	O
router	B-Protocol
must	O
exist	O
on	O
the	O
network	O
and	O
generate	O
IGMP	B-Protocol
queries	O
.	O
</s>
<s>
Without	O
a	O
querier	O
IGMP	B-Protocol
membership	O
reporting	O
may	O
be	O
incomplete	O
and	O
the	O
tables	O
associating	O
member	O
ports	O
and	O
multicast	O
groups	O
are	O
potentially	O
incomplete	O
and	O
snooping	O
will	O
not	O
work	O
reliably	O
.	O
</s>
<s>
Some	O
IGMP	B-Protocol
snooping	I-Protocol
implementations	O
include	O
full	O
querier	O
capability	O
.	O
</s>
<s>
IGMPv2	B-Protocol
and	O
IGMPv3	B-Protocol
contain	O
provision	O
for	O
selecting	O
a	O
querier	O
when	O
multiple	O
are	O
available	O
.	O
</s>
<s>
IGMP	B-Protocol
general	O
queries	O
from	O
the	O
querier	O
must	O
be	O
unconditionally	O
forwarded	O
by	O
all	O
switches	O
involved	O
in	O
IGMP	B-Protocol
snooping	I-Protocol
.	O
</s>
<s>
IGMP	B-Protocol
snooping	I-Protocol
with	O
proxy	O
reporting	O
or	O
report	O
suppression	O
actively	O
filters	O
IGMP	B-Protocol
packets	O
in	O
order	O
to	O
reduce	O
load	O
on	O
the	O
multicast	O
router	B-Protocol
.	O
</s>
<s>
Joins	O
and	O
leaves	O
heading	O
upstream	O
to	O
the	O
router	B-Protocol
are	O
filtered	O
so	O
that	O
only	O
the	O
minimal	O
quantity	O
of	O
information	O
is	O
sent	O
.	O
</s>
<s>
The	O
switch	O
is	O
trying	O
to	O
ensure	O
the	O
router	B-Protocol
only	O
has	O
a	O
single	O
report	O
for	O
the	O
group	O
,	O
regardless	O
of	O
how	O
many	O
active	O
listeners	O
there	O
are	O
.	O
</s>
<s>
If	O
there	O
are	O
two	O
active	O
listeners	O
in	O
a	O
group	O
and	O
the	O
first	O
one	O
leaves	O
,	O
then	O
the	O
switch	O
determines	O
that	O
the	O
router	B-Protocol
does	O
not	O
need	O
this	O
information	O
since	O
it	O
does	O
not	O
affect	O
the	O
status	O
of	O
the	O
group	O
from	O
the	O
router	B-Protocol
's	O
point	O
of	O
view	O
.	O
</s>
<s>
The	O
next	O
time	O
there	O
is	O
a	O
routine	O
query	O
from	O
the	O
router	B-Protocol
the	O
switch	O
will	O
forward	O
the	O
reply	O
from	O
the	O
remaining	O
host	O
.	O
</s>
<s>
In	O
the	O
presence	O
of	O
proxy	O
reporting	O
,	O
the	O
router	B-Protocol
will	O
generally	O
only	O
know	O
about	O
the	O
most	O
recently	O
joined	O
member	O
of	O
the	O
group	O
.	O
</s>
