<s>
Protocol-Independent	O
Multicast	O
(	O
PIM	O
)	O
is	O
a	O
family	O
of	O
multicast	O
routing	B-Protocol
protocols	I-Protocol
for	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
networks	O
that	O
provide	O
one-to-many	B-General_Concept
and	O
many-to-many	B-Architecture
distribution	O
of	O
data	O
over	O
a	O
LAN	B-General_Concept
,	O
WAN	O
or	O
the	O
Internet	O
.	O
</s>
<s>
It	O
is	O
termed	O
protocol-independent	O
because	O
PIM	O
does	O
not	O
include	O
its	O
own	O
topology	B-Architecture
discovery	I-Architecture
mechanism	O
,	O
but	O
instead	O
uses	O
routing	B-Protocol
information	I-Protocol
supplied	O
by	O
other	O
routing	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
PIM	O
is	O
not	O
dependent	O
on	O
a	O
specific	O
unicast	O
routing	B-Protocol
protocol	I-Protocol
;	O
it	O
can	O
make	O
use	O
of	O
any	O
unicast	O
routing	B-Protocol
protocol	I-Protocol
in	O
use	O
on	O
the	O
network	O
.	O
</s>
<s>
PIM	O
does	O
not	O
build	O
its	O
own	O
routing	B-Protocol
tables	I-Protocol
.	O
</s>
<s>
PIM	O
uses	O
the	O
unicast	O
routing	B-Protocol
table	I-Protocol
for	O
reverse-path	B-Protocol
forwarding	I-Protocol
.	O
</s>
<s>
PIM	B-Protocol
Dense	I-Protocol
Mode	I-Protocol
(	O
PIM-DM	O
)	O
uses	O
dense	O
multicast	O
routing	B-Protocol
.	O
</s>
<s>
It	O
implicitly	O
builds	O
shortest-path	O
trees	O
by	O
flooding	O
multicast	O
traffic	O
domain	O
wide	O
,	O
and	O
then	O
pruning	O
back	O
branches	O
of	O
the	O
tree	B-Application
where	O
no	O
receivers	O
are	O
present	O
.	O
</s>
<s>
The	O
first	O
multicast	O
routing	B-Protocol
protocol	I-Protocol
,	O
DVMRP	B-Protocol
used	O
dense-mode	O
multicast	O
routing	B-Protocol
.	O
</s>
<s>
It	O
never	O
builds	O
a	O
shortest	O
path	O
tree	B-Application
,	O
so	O
may	O
have	O
longer	O
end-to-end	O
delays	O
than	O
PIM-SM	O
,	O
but	O
scales	O
well	O
because	O
it	O
needs	O
no	O
source-specific	O
state	O
.	O
</s>
<s>
PIM	O
Source-Specific	O
Multicast	O
(	O
PIM-SSM	O
)	O
builds	O
trees	O
that	O
are	O
rooted	O
in	O
just	O
one	O
source	O
,	O
offering	O
a	O
more	O
secure	O
and	O
scalable	O
model	O
for	O
a	O
limited	O
number	O
of	O
applications	O
(	O
mostly	O
broadcasting	B-Operating_System
of	O
content	O
)	O
.	O
</s>
<s>
PIM-SM	O
is	O
commonly	O
used	O
in	O
IPTV	O
systems	O
for	O
routing	B-Protocol
multicast	O
streams	O
between	O
VLANs	B-General_Concept
,	O
Subnets	B-Protocol
or	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
.	O
</s>
<s>
RP	O
discovery	O
is	O
accomplished	O
by	O
a	O
Bootstrap	O
Router	B-Protocol
(	O
BSR	O
)	O
.	O
</s>
<s>
Groups	O
are	O
either	O
sparse	O
or	O
dense	O
mode	O
;	O
Interfaces	B-Application
can	O
be	O
either	O
.	O
</s>
<s>
Protocol	B-Protocol
Independent	I-Protocol
Multicast	I-Protocol
-	O
Sparse-Mode	O
(	O
PIM-SM	O
)	O
is	O
a	O
protocol	O
for	O
efficiently	O
routing	B-Protocol
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
packets	B-Protocol
to	O
multicast	O
groups	O
that	O
may	O
span	O
wide-area	O
and	O
inter-domain	B-Architecture
internets	O
.	O
</s>
<s>
The	O
protocol	O
is	O
named	O
protocol-independent	O
because	O
it	O
is	O
not	O
dependent	O
on	O
any	O
particular	O
unicast	O
routing	B-Protocol
protocol	I-Protocol
for	O
topology	B-Architecture
discovery	I-Architecture
,	O
and	O
sparse-mode	O
because	O
it	O
is	O
suitable	O
for	O
groups	O
where	O
a	O
very	O
low	O
percentage	O
of	O
the	O
nodes	O
(	O
and	O
their	O
routers	B-Protocol
)	O
will	O
subscribe	O
to	O
the	O
multicast	O
session	O
.	O
</s>
<s>
Unlike	O
earlier	O
dense-mode	O
multicast	O
routing	B-Protocol
protocols	I-Protocol
such	O
as	O
DVMRP	B-Protocol
and	O
dense	O
multicast	O
routing	B-Protocol
which	O
flooded	O
packets	B-Protocol
across	O
the	O
network	O
and	O
then	O
pruned	O
off	O
branches	O
where	O
there	O
were	O
no	O
receivers	O
,	O
PIM-SM	O
explicitly	O
constructs	O
a	O
tree	B-Application
from	O
each	O
sender	O
to	O
the	O
receivers	O
in	O
the	O
multicast	O
group	O
.	O
</s>
<s>
A	O
router	B-Protocol
receives	O
explicit	O
Join/Prune	O
messages	O
from	O
those	O
neighboring	O
routers	B-Protocol
that	O
have	O
downstream	O
group	O
members	O
.	O
</s>
<s>
In	O
order	O
to	O
join	O
a	O
multicast	O
group	O
,	O
G	O
,	O
a	O
host	O
conveys	O
its	O
membership	O
information	O
through	O
the	O
Internet	B-Protocol
Group	I-Protocol
Management	I-Protocol
Protocol	I-Protocol
(	O
IGMP	B-Protocol
)	O
.	O
</s>
<s>
The	O
router	B-Protocol
then	O
forwards	O
data	B-Protocol
packets	I-Protocol
addressed	O
to	O
a	O
multicast	O
group	O
G	O
to	O
only	O
those	O
interfaces	B-Application
on	O
which	O
explicit	O
joins	O
have	O
been	O
received	O
.	O
</s>
<s>
A	O
Designated	O
Router	B-Protocol
(	O
DR	O
)	O
sends	O
periodic	O
Join/Prune	O
messages	O
toward	O
a	O
group-specific	O
Rendezvous	O
Point	O
(	O
RP	O
)	O
for	O
each	O
group	O
for	O
which	O
it	O
has	O
active	O
members	O
.	O
</s>
<s>
Note	O
that	O
one	O
router	B-Protocol
will	O
be	O
automatically	O
or	O
statically	O
designated	O
as	O
the	O
rendezvous	O
point	O
(	O
RP	O
)	O
,	O
and	O
all	O
routers	B-Protocol
must	O
explicitly	O
join	O
through	O
the	O
RP	O
.	O
</s>
<s>
Each	O
router	B-Protocol
along	O
the	O
path	O
toward	O
the	O
RP	O
builds	O
a	O
wild	O
card	O
(	O
any-source	O
)	O
state	O
for	O
the	O
group	O
and	O
sends	O
Join/Prune	O
messages	O
on	O
toward	O
the	O
RP	O
.	O
</s>
<s>
The	O
term	O
route	O
entry	O
is	O
used	O
to	O
refer	O
to	O
the	O
state	O
maintained	O
in	O
a	O
router	B-Protocol
to	O
represent	O
the	O
distribution	O
tree	B-Application
.	O
</s>
<s>
The	O
outgoing	O
interfaces	B-Application
point	O
to	O
the	O
neighboring	O
downstream	O
routers	B-Protocol
that	O
have	O
sent	O
Join/Prune	O
messages	O
toward	O
the	O
RP	O
as	O
well	O
as	O
the	O
directly	O
connected	O
hosts	O
which	O
have	O
requested	O
membership	O
to	O
group	O
G	O
.	O
</s>
<s>
This	O
state	O
creates	O
a	O
shared	O
,	O
RP-centered	O
,	O
distribution	O
tree	B-Application
that	O
reaches	O
all	O
group	O
members	O
.	O
</s>
<s>
When	O
a	O
data	O
source	O
first	O
sends	O
to	O
a	O
group	O
,	O
its	O
Designated	O
Router	B-Protocol
(	O
DR	O
)	O
unicasts	O
Register	O
messages	O
to	O
the	O
Rendezvous	O
Point	O
(	O
RP	O
)	O
with	O
the	O
source	O
's	O
data	B-Protocol
packets	I-Protocol
encapsulated	O
within	O
.	O
</s>
<s>
If	O
the	O
data	O
rate	O
is	O
high	O
,	O
the	O
RP	O
can	O
send	O
source-specific	O
Join/Prune	O
messages	O
back	O
towards	O
the	O
source	O
and	O
the	O
source	O
's	O
data	B-Protocol
packets	I-Protocol
will	O
follow	O
the	O
resulting	O
forwarding	O
state	O
and	O
travel	O
un-encapsulated	O
to	O
the	O
RP	O
.	O
</s>
<s>
Whether	O
they	O
arrive	O
encapsulated	O
or	O
natively	O
,	O
the	O
RP	O
forwards	O
the	O
source	O
's	O
de-capsulated	O
data	B-Protocol
packets	I-Protocol
down	O
the	O
RP-centered	O
distribution	O
tree	B-Application
toward	O
group	O
members	O
.	O
</s>
<s>
If	O
the	O
data	O
rate	O
warrants	O
it	O
,	O
routers	B-Protocol
with	O
local	O
receivers	O
can	O
join	O
a	O
source-specific	O
,	O
shortest	O
path	O
,	O
distribution	O
tree	B-Application
,	O
and	O
prune	O
this	O
source	O
's	O
packets	B-Protocol
off	O
the	O
shared	O
RP-centered	O
tree	B-Application
.	O
</s>
<s>
For	O
low	O
data	O
rate	O
sources	O
,	O
neither	O
the	O
RP	O
,	O
nor	O
last-hop	O
routers	B-Protocol
need	O
join	O
a	O
source-specific	O
shortest	O
path	O
tree	B-Application
and	O
data	B-Protocol
packets	I-Protocol
can	O
be	O
delivered	O
via	O
the	O
shared	O
RP-tree	O
.	O
</s>
<s>
Once	O
the	O
other	O
routers	B-Protocol
which	O
need	O
to	O
receive	O
those	O
group	O
packets	B-Protocol
have	O
subscribed	O
,	O
the	O
RP	O
will	O
unsubscribe	O
to	O
that	O
multicast	O
group	O
,	O
unless	O
it	O
also	O
needs	O
to	O
forward	O
packets	B-Protocol
to	O
another	O
router	B-Protocol
or	O
node	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
routers	B-Protocol
will	O
use	O
reverse-path	B-Protocol
forwarding	I-Protocol
to	O
ensure	O
that	O
there	O
are	O
no	O
loops	O
for	O
packet	B-Protocol
forwarding	O
among	O
routers	B-Protocol
that	O
wish	O
to	O
receive	O
multicast	O
packets	B-Protocol
.	O
</s>
<s>
Dense	O
mode	O
multicast	O
is	O
one	O
mode	O
that	O
multicast	O
can	O
use	O
to	O
construct	O
a	O
tree	B-Application
for	O
sending	O
packets	B-Protocol
to	O
the	O
multicast	O
subscribers	O
.	O
</s>
<s>
The	O
basic	O
assumption	O
behind	O
dense	O
mode	O
is	O
that	O
the	O
multicast	O
packet	B-Protocol
stream	O
has	O
receivers	O
at	O
most	O
locations	O
.	O
</s>
<s>
Dense	O
mode	O
is	O
ideal	O
for	O
groups	O
where	O
many	O
of	O
the	O
nodes	O
will	O
subscribe	O
to	O
receive	O
the	O
multicast	O
packets	B-Protocol
,	O
so	O
that	O
most	O
of	O
the	O
routers	B-Protocol
must	O
receive	O
and	O
forward	O
these	O
packets	B-Protocol
(	O
groups	O
of	O
a	O
high	O
density	O
)	O
.	O
</s>
<s>
Dense	O
Mode	O
uses	O
a	O
fairly	O
simple	O
approach	O
to	O
handle	O
IP	B-Protocol
multicast	I-Protocol
routing	B-Protocol
.	O
</s>
<s>
The	O
source	O
initially	O
broadcasts	B-Operating_System
to	O
every	O
router	B-Protocol
directly	O
connected	O
to	O
it	O
.	O
</s>
<s>
These	O
neighboring	O
routers	B-Protocol
further	O
forward	O
the	O
data	O
to	O
their	O
neighbors	O
.	O
</s>
<s>
When	O
a	O
router	B-Protocol
does	O
not	O
wish	O
to	O
receive	O
this	O
group	O
's	O
data	O
(	O
if	O
no	O
other	O
neighboring	O
PIM	O
routers	B-Protocol
are	O
present	O
and	O
no	O
host	O
is	O
interested	O
in	O
the	O
group	O
)	O
,	O
it	O
sends	O
a	O
Prune	O
message	O
to	O
indicate	O
its	O
lack	O
of	O
interest	O
.	O
</s>
<s>
Upon	O
receiving	O
a	O
Prune	O
message	O
,	O
the	O
router	B-Protocol
will	O
modify	O
its	O
state	O
so	O
that	O
it	O
will	O
not	O
forward	O
those	O
packets	B-Protocol
out	O
that	O
interface	B-Application
.	O
</s>
<s>
If	O
every	O
interface	B-Application
on	O
a	O
router	B-Protocol
is	O
pruned	O
,	O
the	O
router	B-Protocol
will	O
also	O
be	O
pruned	O
.	O
</s>
<s>
This	O
is	O
fine	O
for	O
low	O
volume	O
multicast	O
,	O
but	O
not	O
higher	O
bandwidth	O
multicast	O
packet	B-Protocol
streams	O
.	O
</s>
<s>
More	O
recent	O
Cisco	O
IOS	O
versions	O
support	O
a	O
new	O
feature	O
called	O
PIM	B-Protocol
Dense	I-Protocol
Mode	I-Protocol
State	O
Refresh	O
,	O
since	O
12.1(5 )	O
T	O
.	O
This	O
feature	O
uses	O
a	O
PIM	O
state	O
refresh	O
messages	O
to	O
refresh	O
the	O
Prune	O
state	O
on	O
outgoing	O
interfaces	B-Application
.	O
</s>
<s>
Another	O
benefit	O
is	O
that	O
topology	B-Architecture
changes	O
are	O
recognized	O
more	O
quickly	O
.	O
</s>
<s>
Additionally	O
,	O
the	O
routers	B-Protocol
will	O
use	O
reverse-path	B-Protocol
forwarding	I-Protocol
to	O
ensure	O
that	O
there	O
are	O
no	O
loops	O
for	O
packet	B-Protocol
forwarding	O
among	O
routers	B-Protocol
that	O
wish	O
to	O
receive	O
multicast	O
packets	B-Protocol
.	O
</s>
<s>
When	O
a	O
data	B-Protocol
packet	I-Protocol
is	O
received	O
on	O
a	O
non-RPF	O
interface	B-Application
,	O
a	O
mechanism	O
is	O
required	O
to	O
prevent	O
loops	O
.	O
</s>
<s>
If	O
the	O
non-RPF	O
interface	B-Application
is	O
a	O
LAN	B-General_Concept
,	O
an	O
Assert	O
message	O
is	O
sent	O
.	O
</s>
<s>
Non-Forwarder	O
routers	B-Protocol
then	O
send	O
a	O
Prune	O
on	O
their	O
RPF	O
interface	B-Application
if	O
they	O
do	O
n't	O
need	O
the	O
multicast	O
stream	O
.	O
</s>
<s>
Only	O
one	O
such	O
Prune	O
is	O
sent	O
,	O
at	O
the	O
time	O
of	O
the	O
transition	O
to	O
having	O
no	O
interfaces	B-Application
in	O
the	O
Outgoing	O
Interface	B-Application
List	O
(	O
OILIST	O
)	O
.	O
</s>
<s>
The	O
LAN	B-General_Concept
Prune	O
receiver	O
delays	O
acting	O
on	O
it	O
for	O
3	O
seconds	O
,	O
so	O
that	O
if	O
another	O
LAN	B-General_Concept
router	B-Protocol
still	O
needs	O
the	O
multicast	O
stream	O
,	O
it	O
can	O
send	O
a	O
PIM	O
Join	O
message	O
to	O
counteract	O
(	O
cancel	O
)	O
the	O
Prune	O
.	O
</s>
<s>
Suppose	O
a	O
router	B-Protocol
has	O
Pruned	O
,	O
and	O
some	O
time	O
later	O
a	O
receiver	O
requests	O
the	O
multicast	O
stream	O
with	O
an	O
IGMP	B-Protocol
message	O
.	O
</s>
<s>
The	O
router	B-Protocol
then	O
sends	O
a	O
Graft	O
message	O
.	O
</s>
