<s>
In	O
network	B-Protocol
routing	I-Protocol
,	O
the	O
control	B-Protocol
plane	I-Protocol
is	O
the	O
part	O
of	O
the	O
router	B-Protocol
architecture	O
that	O
is	O
concerned	O
with	O
drawing	O
the	O
network	B-Architecture
topology	I-Architecture
,	O
or	O
the	O
information	O
in	O
a	O
routing	B-Protocol
table	I-Protocol
that	O
defines	O
what	O
to	O
do	O
with	O
incoming	O
packets	B-Protocol
.	O
</s>
<s>
Control	B-Protocol
plane	I-Protocol
functions	O
,	O
such	O
as	O
participating	O
in	O
routing	B-Protocol
protocols	I-Protocol
,	O
run	O
in	O
the	O
architectural	O
control	O
element	O
.	O
</s>
<s>
In	O
most	O
cases	O
,	O
the	O
routing	B-Protocol
table	I-Protocol
contains	O
a	O
list	O
of	O
destination	O
addresses	O
and	O
the	O
outgoing	O
interface(s )	O
associated	O
with	O
each	O
.	O
</s>
<s>
Control	B-Protocol
plane	I-Protocol
logic	O
also	O
can	O
identify	O
certain	O
packets	B-Protocol
to	O
be	O
discarded	O
,	O
as	O
well	O
as	O
preferential	O
treatment	O
of	O
certain	O
packets	B-Protocol
for	O
which	O
a	O
high	O
quality	O
of	O
service	O
is	O
defined	O
by	O
such	O
mechanisms	O
as	O
differentiated	O
services	O
.	O
</s>
<s>
Depending	O
on	O
the	O
specific	O
router	B-Protocol
implementation	O
,	O
there	O
may	O
be	O
a	O
separate	O
forwarding	B-Protocol
information	I-Protocol
base	I-Protocol
that	O
is	O
populated	O
by	O
the	O
control	B-Protocol
plane	I-Protocol
,	O
but	O
used	O
by	O
the	O
high-speed	O
forwarding	B-Protocol
plane	I-Protocol
to	O
look	O
up	O
packets	B-Protocol
and	O
decide	O
how	O
to	O
handle	O
them	O
.	O
</s>
<s>
In	O
computing	O
,	O
the	O
control	B-Protocol
plane	I-Protocol
is	O
the	O
part	O
of	O
the	O
software	O
that	O
configures	O
and	O
shuts	O
down	O
the	O
data	B-Protocol
plane	I-Protocol
.	O
</s>
<s>
By	O
contrast	O
,	O
the	O
data	B-Protocol
plane	I-Protocol
is	O
the	O
part	O
of	O
the	O
software	O
that	O
processes	O
the	O
data	O
requests	O
.	O
</s>
<s>
The	O
data	B-Protocol
plane	I-Protocol
is	O
also	O
sometimes	O
referred	O
to	O
as	O
the	O
forwarding	B-Protocol
plane	I-Protocol
.	O
</s>
<s>
The	O
distinction	O
has	O
proven	O
useful	O
in	O
the	O
networking	O
field	O
where	O
it	O
originated	O
,	O
as	O
it	O
separates	O
the	O
concerns	O
:	O
the	O
data	B-Protocol
plane	I-Protocol
is	O
optimized	O
for	O
speed	O
of	O
processing	O
,	O
and	O
for	O
simplicity	O
and	O
regularity	O
.	O
</s>
<s>
The	O
control	B-Protocol
plane	I-Protocol
is	O
optimized	O
for	O
customizability	O
,	O
handling	O
policies	O
,	O
handling	O
exceptional	O
situations	O
,	O
and	O
in	O
general	O
facilitating	O
and	O
simplifying	O
the	O
data	B-Protocol
plane	I-Protocol
processing	O
.	O
</s>
<s>
The	O
conceptual	O
separation	O
of	O
the	O
data	B-Protocol
plane	I-Protocol
from	O
the	O
control	B-Protocol
plane	I-Protocol
has	O
been	O
done	O
for	O
years	O
.	O
</s>
<s>
An	O
early	O
example	O
is	O
Unix	B-Application
,	O
where	O
the	O
basic	O
file	O
operations	O
are	O
open	O
,	O
close	O
for	O
the	O
control	B-Protocol
plane	I-Protocol
and	O
read	O
write	O
for	O
the	O
data	B-Protocol
plane	I-Protocol
.	O
</s>
<s>
A	O
major	O
function	O
of	O
the	O
control	B-Protocol
plane	I-Protocol
is	O
deciding	O
which	O
routes	O
go	O
into	O
the	O
main	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
"	O
Main	O
"	O
refers	O
to	O
the	O
table	O
that	O
holds	O
the	O
unicast	B-Protocol
routes	O
that	O
are	O
active	O
.	O
</s>
<s>
Multicast	O
routing	B-Protocol
may	O
require	O
an	O
additional	O
routing	B-Protocol
table	I-Protocol
for	O
multicast	O
routes	O
.	O
</s>
<s>
Several	O
routing	B-Protocol
protocols	I-Protocol
e.g.	O
</s>
<s>
IS-IS	B-Protocol
,	O
OSPF	B-Protocol
and	O
BGP	B-Protocol
maintain	O
internal	O
databases	O
of	O
candidate	O
routes	O
which	O
are	O
promoted	O
when	O
a	O
route	O
fails	O
or	O
when	O
a	O
routing	B-Protocol
policy	I-Protocol
is	O
changed	O
.	O
</s>
<s>
Several	O
different	O
information	O
sources	O
may	O
provide	O
information	O
about	O
a	O
route	O
to	O
a	O
given	O
destination	O
,	O
but	O
the	O
router	B-Protocol
must	O
select	O
the	O
"	O
best	O
"	O
route	O
to	O
install	O
into	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
there	O
may	O
be	O
multiple	O
routes	O
of	O
equal	O
"	O
quality	O
"	O
,	O
and	O
the	O
router	B-Protocol
may	O
install	O
all	O
of	O
them	O
and	O
load-share	O
across	O
them	O
.	O
</s>
<s>
There	O
are	O
three	O
general	O
sources	O
of	O
routing	B-Protocol
information	I-Protocol
:	O
</s>
<s>
Routers	B-Protocol
forward	O
traffic	O
that	O
enters	O
on	O
an	O
input	O
interface	O
and	O
leaves	O
on	O
an	O
output	O
interface	O
,	O
subject	O
to	O
filtering	O
and	O
other	O
local	O
rules	O
.	O
</s>
<s>
While	O
routers	B-Protocol
usually	O
forward	O
from	O
one	O
physical	O
(	O
e.g.	O
,	O
Ethernet	O
,	O
serial	B-Protocol
)	O
to	O
another	O
physical	O
interface	O
,	O
it	O
is	O
also	O
possible	O
to	O
define	O
multiple	O
logical	O
interfaces	O
on	O
a	O
physical	O
interface	O
.	O
</s>
<s>
A	O
physical	O
Ethernet	O
interface	O
,	O
for	O
example	O
,	O
can	O
have	O
logical	O
interfaces	O
in	O
several	O
virtual	B-General_Concept
LANs	I-General_Concept
defined	O
by	O
IEEE	O
802.1Q	O
VLAN	B-General_Concept
headers	O
.	O
</s>
<s>
When	O
an	O
interface	O
has	O
an	O
address	O
configured	O
in	O
a	O
subnet	B-Protocol
,	O
such	O
as	O
192.0.2.1	O
in	O
the	O
192.0.2.0/24	O
(	O
i.e.	O
,	O
subnet	B-Protocol
mask	I-Protocol
255.255.255.0	O
)	O
subnet	B-Protocol
,	O
and	O
that	O
interface	O
is	O
considered	O
"	O
up	O
"	O
by	O
the	O
router	B-Protocol
,	O
the	O
router	B-Protocol
thus	O
has	O
a	O
directly	O
connected	O
route	O
to	O
192.0.2.0/24	O
.	O
</s>
<s>
If	O
a	O
routing	B-Protocol
protocol	I-Protocol
offered	O
another	O
router	B-Protocol
's	O
route	O
to	O
that	O
same	O
subnet	B-Protocol
,	O
the	O
routing	B-Protocol
table	I-Protocol
installation	O
software	O
will	O
normally	O
ignore	O
the	O
dynamic	O
route	O
and	O
prefer	O
the	O
directly	O
connected	O
route	O
.	O
</s>
<s>
There	O
also	O
may	O
be	O
software-only	O
interfaces	O
on	O
the	O
router	B-Protocol
,	O
which	O
it	O
treats	O
as	O
if	O
they	O
were	O
locally	O
connected	O
.	O
</s>
<s>
Packets	B-Protocol
having	O
this	O
interface	O
as	O
a	O
next	O
hop	O
will	O
be	O
discarded	O
,	O
which	O
can	O
be	O
a	O
very	O
efficient	O
way	O
to	O
filter	O
traffic	O
.	O
</s>
<s>
Routers	B-Protocol
usually	O
can	O
route	O
traffic	O
faster	O
than	O
they	O
can	O
examine	O
it	O
and	O
compare	O
it	O
to	O
filters	O
,	O
so	O
,	O
if	O
the	O
criterion	O
for	O
discarding	O
is	O
the	O
packet	B-Protocol
's	O
destination	O
address	O
,	O
"	O
blackholing	O
"	O
the	O
traffic	O
will	O
be	O
more	O
efficient	O
than	O
explicit	O
filters	O
.	O
</s>
<s>
Other	O
software	O
defined	O
interfaces	O
that	O
are	O
treated	O
as	O
directly	O
connected	O
,	O
as	O
long	O
as	O
they	O
are	O
active	O
,	O
are	O
interfaces	O
associated	O
with	O
tunneling	B-Protocol
protocols	I-Protocol
such	O
as	O
Generic	B-Protocol
Routing	I-Protocol
Encapsulation	I-Protocol
(	O
GRE	O
)	O
or	O
Multi-Protocol	B-Protocol
Label	I-Protocol
Switching	I-Protocol
(	O
MPLS	B-Protocol
)	O
.	O
</s>
<s>
Router	B-Protocol
configuration	O
rules	O
may	O
contain	O
static	O
routes	O
.	O
</s>
<s>
A	O
static	O
route	O
minimally	O
has	O
a	O
destination	O
address	O
,	O
a	O
prefix	O
length	O
or	O
subnet	B-Protocol
mask	I-Protocol
,	O
and	O
a	O
definition	O
where	O
to	O
send	O
packets	B-Protocol
for	O
the	O
route	O
.	O
</s>
<s>
That	O
definition	O
can	O
refer	O
to	O
a	O
local	O
interface	O
on	O
the	O
router	B-Protocol
,	O
or	O
a	O
next-hop	O
address	O
that	O
could	O
be	O
on	O
the	O
far	O
end	O
of	O
a	O
subnet	B-Protocol
to	O
which	O
the	O
router	B-Protocol
is	O
connected	O
.	O
</s>
<s>
The	O
next-hop	O
address	O
could	O
also	O
be	O
on	O
a	O
subnet	B-Protocol
that	O
is	O
directly	O
connected	O
,	O
and	O
,	O
before	O
the	O
router	B-Protocol
can	O
determine	O
if	O
the	O
static	O
route	O
is	O
usable	O
,	O
it	O
must	O
do	O
a	O
recursive	O
lookup	O
of	O
the	O
next	O
hop	O
address	O
in	O
the	O
local	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
One	O
application	O
is	O
called	O
a	O
floating	O
static	O
route	O
,	O
where	O
the	O
static	O
route	O
is	O
less	O
preferred	O
than	O
a	O
route	O
from	O
any	O
routing	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
See	O
routing	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
The	O
routing	B-Protocol
table	I-Protocol
manager	O
,	O
according	O
to	O
implementation	O
and	O
configuration	O
rules	O
,	O
may	O
select	O
a	O
particular	O
route	O
or	O
routes	O
from	O
those	O
advertised	O
by	O
various	O
routing	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Different	O
implementations	O
have	O
different	O
sets	O
of	O
preferences	O
for	O
routing	B-Protocol
information	I-Protocol
,	O
and	O
these	O
are	O
not	O
standardized	O
among	O
IP	B-Protocol
routers	I-Protocol
.	O
</s>
<s>
It	O
is	O
fair	O
to	O
say	O
that	O
subnets	B-Protocol
on	O
directly	O
connected	O
active	O
interfaces	O
are	O
always	O
preferred	O
.	O
</s>
<s>
Cisco	O
's	O
IOS	O
implementation	O
makes	O
exterior	O
BGP	B-Protocol
the	O
most	O
preferred	O
source	O
of	O
dynamic	O
routing	B-Protocol
information	I-Protocol
,	O
while	O
Nortel	O
RS	O
makes	O
intra-area	O
OSPF	B-Protocol
most	O
preferred	O
.	O
</s>
<s>
If	O
the	O
route	O
is	O
not	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
,	O
install	O
it	O
.	O
</s>
<s>
A	O
/28	O
route	O
,	O
with	O
a	O
subnet	B-Protocol
mask	I-Protocol
of	O
255.255.255.240	O
,	O
is	O
more	O
specific	O
than	O
a	O
/24	O
route	O
,	O
with	O
a	O
subnet	B-Protocol
mask	I-Protocol
of	O
255.255.255.0	O
.	O
</s>
<s>
If	O
the	O
route	O
is	O
of	O
equal	O
specificity	O
to	O
a	O
route	O
already	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
,	O
but	O
comes	O
from	O
a	O
more	O
preferred	O
source	O
of	O
routing	B-Protocol
information	I-Protocol
,	O
replace	O
the	O
route	O
in	O
the	O
table	O
.	O
</s>
<s>
If	O
the	O
route	O
is	O
of	O
equal	O
specificity	O
to	O
a	O
route	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
,	O
yet	O
comes	O
from	O
a	O
source	O
of	O
the	O
same	O
preference	O
,	O
</s>
<s>
If	O
the	O
routes	O
are	O
of	O
equal	O
metric	O
and	O
the	O
router	B-Protocol
supports	O
load-sharing	O
,	O
add	O
the	O
new	O
route	O
and	O
designate	O
it	O
as	O
part	O
of	O
a	O
load-sharing	O
group	O
.	O
</s>
<s>
See	O
forwarding	B-Protocol
plane	I-Protocol
for	O
more	O
detail	O
,	O
but	O
each	O
implementation	O
has	O
its	O
own	O
means	O
of	O
updating	O
the	O
forwarding	B-Protocol
information	I-Protocol
base	I-Protocol
(	O
FIB	O
)	O
with	O
new	O
routes	O
installed	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
Multicast	O
routing	B-Protocol
builds	O
on	O
unicast	B-Protocol
routing	B-Protocol
.	O
</s>
<s>
Each	O
multicast	O
group	O
to	O
which	O
the	O
local	O
router	B-Protocol
can	O
route	O
has	O
a	O
multicast	O
routing	B-Protocol
table	I-Protocol
entry	O
with	O
a	O
next	O
hop	O
for	O
the	O
group	O
,	O
rather	O
than	O
for	O
a	O
specific	O
destination	O
as	O
in	O
unicast	B-Protocol
routing	B-Protocol
.	O
</s>
<s>
There	O
can	O
be	O
multicast	O
static	O
routes	O
as	O
well	O
as	O
learning	O
dynamic	O
multicast	O
routes	O
from	O
a	O
protocol	O
such	O
as	O
Protocol	B-Protocol
Independent	I-Protocol
Multicast	I-Protocol
(	O
PIM	O
)	O
.	O
</s>
