<s>
A	O
router	O
is	O
a	O
networking	B-Device
device	I-Device
that	O
forwards	O
data	B-Protocol
packets	I-Protocol
between	O
computer	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
Data	O
sent	O
through	O
a	O
network	O
,	O
such	O
as	O
a	O
web	O
page	O
or	O
email	O
,	O
is	O
in	O
the	O
form	O
of	O
data	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
A	O
packet	B-Protocol
is	O
typically	O
forwarded	B-Protocol
from	O
one	O
router	O
to	O
another	O
router	O
through	O
the	O
networks	O
that	O
constitute	O
an	O
internetwork	B-Architecture
(	O
e.g.	O
</s>
<s>
the	O
Internet	O
)	O
until	O
it	O
reaches	O
its	O
destination	O
node	B-Protocol
.	O
</s>
<s>
A	O
router	O
is	O
connected	O
to	O
two	O
or	O
more	O
data	O
lines	O
from	O
different	O
IP	B-Protocol
networks	I-Protocol
.	O
</s>
<s>
When	O
a	O
data	B-Protocol
packet	I-Protocol
comes	O
in	O
on	O
one	O
of	O
the	O
lines	O
,	O
the	O
router	O
reads	O
the	O
network	B-Protocol
address	I-Protocol
information	O
in	O
the	O
packet	B-General_Concept
header	I-General_Concept
to	O
determine	O
the	O
ultimate	O
destination	O
.	O
</s>
<s>
Then	O
,	O
using	O
information	O
in	O
its	O
routing	B-Protocol
table	I-Protocol
or	O
routing	B-Protocol
policy	I-Protocol
,	O
it	O
directs	O
the	O
packet	B-Protocol
to	O
the	O
next	O
network	O
on	O
its	O
journey	O
.	O
</s>
<s>
The	O
most	O
familiar	O
type	O
of	O
IP	B-Protocol
routers	O
are	O
home	B-Application
and	I-Application
small	I-Application
office	I-Application
routers	I-Application
that	O
simply	O
forward	O
IP	B-Protocol
packets	B-Protocol
between	O
the	O
home	O
computers	O
and	O
the	O
Internet	O
.	O
</s>
<s>
More	O
sophisticated	O
routers	O
,	O
such	O
as	O
enterprise	O
routers	O
,	O
connect	O
large	O
business	O
or	O
ISP	O
networks	O
up	O
to	O
the	O
powerful	O
core	B-Protocol
routers	I-Protocol
that	O
forward	O
data	O
at	O
high	O
speed	O
along	O
the	O
optical	B-Architecture
fiber	I-Architecture
lines	O
of	O
the	O
Internet	B-General_Concept
backbone	I-General_Concept
.	O
</s>
<s>
Routers	O
can	O
be	O
built	O
from	O
standard	O
computer	O
parts	O
but	O
are	O
mostly	O
specialized	B-Architecture
purpose-built	I-Architecture
computers	I-Architecture
.	O
</s>
<s>
Early	O
routers	O
used	O
software-based	O
forwarding	O
,	O
running	O
on	O
a	O
CPU	B-General_Concept
.	O
</s>
<s>
More	O
sophisticated	O
devices	O
use	O
application-specific	O
integrated	O
circuits	O
(	O
ASICs	O
)	O
to	O
increase	O
performance	O
or	O
add	O
advanced	O
filtering	B-Application
and	O
firewall	B-Application
functionality	O
.	O
</s>
<s>
When	O
multiple	O
routers	O
are	O
used	O
in	O
interconnected	O
networks	O
,	O
the	O
routers	O
can	O
exchange	O
information	O
about	O
destination	O
addresses	O
using	O
a	O
routing	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Each	O
router	O
builds	O
up	O
a	O
routing	B-Protocol
table	I-Protocol
,	O
a	O
list	O
of	O
routes	O
,	O
between	O
two	O
computer	O
systems	O
on	O
the	O
interconnected	O
networks	O
.	O
</s>
<s>
Control	B-Protocol
plane	I-Protocol
:	O
A	O
router	O
maintains	O
a	O
routing	B-Protocol
table	I-Protocol
that	O
lists	O
which	O
route	O
should	O
be	O
used	O
to	O
forward	O
a	O
data	B-Protocol
packet	I-Protocol
,	O
and	O
through	O
which	O
physical	O
interface	O
connection	O
.	O
</s>
<s>
It	O
does	O
this	O
using	O
internal	O
pre-configured	O
directives	O
,	O
called	O
static	B-Protocol
routes	I-Protocol
,	O
or	O
by	O
learning	O
routes	O
dynamically	B-Protocol
using	O
a	O
routing	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Static	O
and	O
dynamic	B-Protocol
routes	I-Protocol
are	O
stored	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
The	O
control-plane	O
logic	O
then	O
strips	O
non-essential	O
directives	O
from	O
the	O
table	O
and	O
builds	O
a	O
forwarding	B-Protocol
information	I-Protocol
base	I-Protocol
(	O
FIB	O
)	O
to	O
be	O
used	O
by	O
the	O
forwarding	B-Protocol
plane	I-Protocol
.	O
</s>
<s>
Forwarding	B-Protocol
plane	I-Protocol
:	O
This	O
unit	O
forwards	O
the	O
data	B-Protocol
packets	I-Protocol
between	O
incoming	O
and	O
outgoing	O
interface	O
connections	O
.	O
</s>
<s>
It	O
reads	O
the	O
header	B-General_Concept
of	O
each	O
packet	B-Protocol
as	O
it	O
comes	O
in	O
,	O
matches	O
the	O
destination	O
to	O
entries	O
in	O
the	O
FIB	O
supplied	O
by	O
the	O
control	B-Protocol
plane	I-Protocol
,	O
and	O
directs	O
the	O
packet	B-Protocol
to	O
the	O
outgoing	O
network	O
specified	O
in	O
the	O
FIB	O
.	O
</s>
<s>
A	O
router	O
may	O
have	O
interfaces	O
for	O
multiple	O
types	O
of	O
physical	B-Application
layer	I-Application
connections	O
,	O
such	O
as	O
copper	O
cables	O
,	O
fiber	B-Architecture
optic	I-Architecture
,	O
or	O
wireless	O
transmission	O
.	O
</s>
<s>
It	O
can	O
also	O
support	O
multiple	O
network	B-Application
layer	I-Application
transmission	O
standards	O
.	O
</s>
<s>
Each	O
network	O
interface	O
is	O
used	O
to	O
enable	O
data	B-Protocol
packets	I-Protocol
to	O
be	O
forwarded	B-Protocol
from	O
one	O
transmission	O
system	O
to	O
another	O
.	O
</s>
<s>
Routers	O
may	O
also	O
be	O
used	O
to	O
connect	O
two	O
or	O
more	O
logical	O
groups	O
of	O
computer	O
devices	O
known	O
as	O
subnets	B-Protocol
,	O
each	O
with	O
a	O
unique	O
network	O
prefix	O
.	O
</s>
<s>
The	O
largest	O
routers	O
(	O
such	O
as	O
the	O
Cisco	B-Protocol
CRS-1	I-Protocol
or	O
Juniper	O
PTX	O
)	O
interconnect	O
the	O
various	O
ISPs	O
,	O
or	O
may	O
be	O
used	O
in	O
large	O
enterprise	B-General_Concept
networks	I-General_Concept
.	O
</s>
<s>
Large	O
businesses	O
may	O
also	O
need	O
more	O
powerful	O
routers	O
to	O
cope	O
with	O
ever-increasing	O
demands	O
of	O
intranet	B-General_Concept
data	O
traffic	O
.	O
</s>
<s>
A	O
hierarchical	B-Protocol
internetworking	I-Protocol
model	I-Protocol
for	O
interconnecting	O
routers	O
in	O
large	O
networks	O
is	O
in	O
common	O
use	O
.	O
</s>
<s>
Access	O
routers	O
,	O
including	O
small	O
office/home	O
office	O
(	O
SOHO	O
)	O
models	O
,	O
are	O
located	O
at	O
home	O
and	O
customer	O
sites	O
such	O
as	O
branch	O
offices	O
that	O
do	O
not	O
need	O
hierarchical	B-Protocol
routing	I-Protocol
of	O
their	O
own	O
.	O
</s>
<s>
Some	O
SOHO	O
routers	O
are	O
capable	O
of	O
running	O
alternative	O
free	O
Linux-based	O
firmware	O
like	O
Tomato	B-Protocol
,	O
OpenWrt	B-Application
,	O
or	O
DD-WRT	B-Protocol
.	O
</s>
<s>
In	O
enterprises	O
,	O
a	O
core	B-Protocol
router	I-Protocol
may	O
provide	O
a	O
collapsed	B-General_Concept
backbone	I-General_Concept
interconnecting	O
the	O
distribution	O
tier	O
routers	O
from	O
multiple	O
buildings	O
of	O
a	O
campus	O
,	O
or	O
large	O
enterprise	O
locations	O
.	O
</s>
<s>
External	O
networks	O
must	O
be	O
carefully	O
considered	O
as	O
part	O
of	O
the	O
overall	O
security	O
strategy	O
of	O
the	O
local	B-General_Concept
network	I-General_Concept
.	O
</s>
<s>
A	O
router	O
may	O
include	O
a	O
firewall	B-Application
,	O
VPN	B-Application
handling	O
,	O
and	O
other	O
security	O
functions	O
,	O
or	O
they	O
may	O
be	O
handled	O
by	O
separate	O
devices	O
.	O
</s>
<s>
Routers	O
also	O
commonly	O
perform	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
which	O
restricts	O
connections	O
initiated	O
from	O
external	O
connections	O
but	O
is	O
not	O
recognized	O
as	O
a	O
security	O
feature	O
by	O
all	O
experts	O
.	O
</s>
<s>
Some	O
experts	O
argue	O
that	O
open	B-Application
source	I-Application
routers	O
are	O
more	O
secure	O
and	O
reliable	O
than	O
closed	B-Application
source	I-Application
routers	O
because	O
open-source	B-Application
routers	O
allow	O
mistakes	O
to	O
be	O
quickly	O
found	O
and	O
corrected	O
.	O
</s>
<s>
A	O
router	O
in	O
a	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
of	O
a	O
single	O
organisation	O
is	O
called	O
an	O
interior	O
router	O
.	O
</s>
<s>
A	O
router	O
that	O
is	O
operated	O
in	O
the	O
Internet	B-General_Concept
backbone	I-General_Concept
is	O
described	O
as	O
exterior	O
router	O
.	O
</s>
<s>
While	O
a	O
router	O
that	O
connects	O
a	O
LAN	B-General_Concept
with	O
the	O
Internet	O
or	O
a	O
wide	O
area	O
network	O
(	O
WAN	O
)	O
is	O
called	O
a	O
border	O
router	O
,	O
or	O
gateway	B-Application
router	I-Application
.	O
</s>
<s>
Routers	O
intended	O
for	O
ISP	O
and	O
major	O
enterprise	O
connectivity	O
usually	O
exchange	O
routing	O
information	O
using	O
the	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
.	O
</s>
<s>
defines	O
the	O
types	O
of	O
BGP	B-Protocol
routers	O
according	O
to	O
their	O
functions	O
:	O
</s>
<s>
The	O
router	O
uses	O
Exterior	B-Protocol
Border	I-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
EBGP	B-Protocol
)	O
to	O
routers	O
at	O
other	O
ISPs	O
or	O
large	O
enterprise	O
autonomous	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
Subscriber	O
edge	O
router	O
(	O
also	O
called	O
a	O
customer	O
edge	O
router	O
)	O
:	O
Located	O
at	O
the	O
edge	O
of	O
the	O
subscriber	O
's	O
network	O
,	O
it	O
also	O
uses	O
EBGP	B-Protocol
to	O
its	O
provider	O
's	O
autonomous	B-Architecture
system	I-Architecture
.	O
</s>
<s>
Inter-provider	O
border	O
router	O
:	O
A	O
BGP	B-Protocol
router	O
for	O
interconnecting	O
ISPs	O
that	O
maintains	O
BGP	B-Protocol
sessions	I-Protocol
with	O
other	O
BGP	B-Protocol
routers	O
in	O
ISP	O
Autonomous	B-Architecture
Systems	I-Architecture
.	O
</s>
<s>
Core	B-Protocol
router	I-Protocol
:	O
Resides	O
within	O
an	O
Autonomous	B-Architecture
System	I-Architecture
as	O
a	O
backbone	B-General_Concept
to	O
carry	O
traffic	O
between	O
edge	O
routers	O
.	O
</s>
<s>
Within	O
an	O
ISP	O
:	O
In	O
the	O
ISP	O
's	O
autonomous	B-Architecture
system	I-Architecture
,	O
a	O
router	O
uses	O
internal	B-Protocol
BGP	I-Protocol
to	O
communicate	O
with	O
other	O
ISP	O
edge	O
routers	O
,	O
other	O
intranet	B-General_Concept
core	B-Protocol
routers	I-Protocol
,	O
or	O
the	O
ISP	O
's	O
intranet	B-General_Concept
provider	O
border	O
routers	O
.	O
</s>
<s>
Internet	B-General_Concept
backbone	I-General_Concept
:	O
The	O
Internet	O
no	O
longer	O
has	O
a	O
clearly	O
identifiable	O
backbone	B-General_Concept
,	O
unlike	O
its	O
predecessor	O
networks	O
.	O
</s>
<s>
See	O
default-free	B-Protocol
zone	I-Protocol
(	O
DFZ	O
)	O
.	O
</s>
<s>
The	O
major	O
ISPs	O
 '	O
system	O
routers	O
make	O
up	O
what	O
could	O
be	O
considered	O
to	O
be	O
the	O
current	O
Internet	B-General_Concept
backbone	I-General_Concept
core	O
.	O
</s>
<s>
ISPs	O
operate	O
all	O
four	O
types	O
of	O
the	O
BGP	B-Protocol
routers	O
described	O
here	O
.	O
</s>
<s>
An	O
ISP	O
core	B-Protocol
router	I-Protocol
is	O
used	O
to	O
interconnect	O
its	O
edge	O
and	O
border	O
routers	O
.	O
</s>
<s>
Core	B-Protocol
routers	I-Protocol
may	O
also	O
have	O
specialized	O
functions	O
in	O
virtual	B-Application
private	I-Application
networks	I-Application
based	O
on	O
a	O
combination	O
of	O
BGP	B-Protocol
and	O
Multi-Protocol	B-Protocol
Label	I-Protocol
Switching	I-Protocol
protocols	O
.	O
</s>
<s>
Port	B-Protocol
forwarding	I-Protocol
:	O
Routers	O
are	O
also	O
used	O
for	O
port	B-Protocol
forwarding	I-Protocol
between	O
private	O
Internet-connected	O
servers	O
.	O
</s>
<s>
Voice	O
,	O
data	O
,	O
fax	O
,	O
and	O
video	O
processing	O
routers	O
:	O
Commonly	O
referred	O
to	O
as	O
access	O
servers	O
or	O
gateways	B-Application
,	O
these	O
devices	O
are	O
used	O
to	O
route	O
and	O
process	O
voice	O
,	O
data	O
,	O
video	O
and	O
fax	O
traffic	O
on	O
the	O
Internet	O
.	O
</s>
<s>
Since	O
2005	O
,	O
most	O
long-distance	O
phone	O
calls	O
have	O
been	O
processed	O
as	O
IP	B-Protocol
traffic	O
(	O
VOIP	B-Application
)	O
through	O
a	O
voice	O
gateway	B-Application
.	O
</s>
<s>
Larger	O
networks	O
commonly	O
use	O
multilayer	B-Application
switches	I-Application
,	O
with	O
layer-3	B-Application
devices	O
being	O
used	O
to	O
simply	O
interconnect	O
multiple	O
subnets	B-Protocol
within	O
the	O
same	O
security	O
zone	O
,	O
and	O
higher-layer	O
switches	O
when	O
filtering	B-Application
,	O
translation	B-Protocol
,	O
load	B-Application
balancing	I-Application
,	O
or	O
other	O
higher-level	O
functions	O
are	O
required	O
,	O
especially	O
between	O
zones	O
.	O
</s>
<s>
The	O
concept	O
of	O
an	O
Interface	O
computer	O
was	O
first	O
proposed	O
by	O
Donald	O
Davies	O
for	O
the	O
NPL	B-Protocol
network	I-Protocol
in	O
1966	O
.	O
</s>
<s>
Named	O
Interface	B-Protocol
Message	I-Protocol
Processors	I-Protocol
(	O
IMPs	O
)	O
,	O
these	O
computers	O
had	O
fundamentally	O
the	O
same	O
functionality	O
as	O
a	O
router	O
does	O
today	O
.	O
</s>
<s>
The	O
idea	O
for	O
a	O
router	O
(	O
called	O
gateway	B-Application
at	O
the	O
time	O
)	O
initially	O
came	O
about	O
through	O
an	O
international	O
group	O
of	O
computer	B-Architecture
networking	I-Architecture
researchers	O
called	O
the	O
International	B-Protocol
Networking	I-Protocol
Working	I-Protocol
Group	I-Protocol
(	O
INWG	O
)	O
.	O
</s>
<s>
These	O
gateway	B-Application
devices	O
were	O
different	O
from	O
most	O
previous	O
packet	B-Protocol
switching	I-Protocol
schemes	O
in	O
two	O
ways	O
.	O
</s>
<s>
First	O
,	O
they	O
connected	O
dissimilar	O
kinds	O
of	O
networks	O
,	O
such	O
as	O
serial	B-Protocol
lines	I-Protocol
and	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
.	O
</s>
<s>
Second	O
,	O
they	O
were	O
connectionless	B-Protocol
devices	O
,	O
which	O
had	O
no	O
role	O
in	O
assuring	O
that	O
traffic	O
was	O
delivered	O
reliably	O
,	O
leaving	O
that	O
function	O
entirely	O
to	O
the	O
hosts	B-Device
.	O
</s>
<s>
This	O
particular	O
idea	O
,	O
the	O
end-to-end	B-Application
principle	I-Application
,	O
had	O
been	O
previously	O
pioneered	O
in	O
the	O
CYCLADES	O
network	O
.	O
</s>
<s>
One	O
was	O
the	O
initial	O
DARPA-initiated	O
program	O
,	O
which	O
created	O
the	O
TCP/IP	B-Protocol
architecture	O
in	O
use	O
today	O
.	O
</s>
<s>
The	O
other	O
was	O
a	O
program	O
at	O
Xerox	O
PARC	O
to	O
explore	O
new	O
networking	O
technologies	O
,	O
which	O
produced	O
the	O
PARC	B-Protocol
Universal	I-Protocol
Packet	I-Protocol
system	O
;	O
due	O
to	O
corporate	O
intellectual	O
property	O
concerns	O
it	O
received	O
little	O
attention	O
outside	O
Xerox	O
for	O
years	O
.	O
</s>
<s>
The	O
first	O
true	O
IP	B-Protocol
router	O
was	O
developed	O
by	O
Ginny	O
Strazisar	O
at	O
BBN	O
,	O
as	O
part	O
of	O
that	O
DARPA-initiated	O
effort	O
,	O
during	O
1975	O
–	O
1976	O
.	O
</s>
<s>
By	O
the	O
end	O
of	O
1976	O
,	O
three	O
PDP-11-based	O
routers	O
were	O
in	O
service	O
in	O
the	O
experimental	O
prototype	O
Internet	O
.	O
</s>
<s>
The	O
first	O
multiprotocol	O
routers	O
were	O
independently	O
created	O
by	O
staff	O
researchers	O
at	O
MIT	O
and	O
Stanford	O
in	O
1981	O
and	O
both	O
were	O
also	O
based	O
on	O
PDP-11s	B-Device
.	O
</s>
<s>
Virtually	O
all	O
networking	O
now	O
uses	O
TCP/IP	B-Protocol
,	O
but	O
multiprotocol	O
routers	O
are	O
still	O
manufactured	O
.	O
</s>
<s>
They	O
were	O
important	O
in	O
the	O
early	O
stages	O
of	O
the	O
growth	O
of	O
computer	B-Architecture
networking	I-Architecture
when	O
protocols	O
other	O
than	O
TCP/IP	B-Protocol
were	O
in	O
use	O
.	O
</s>
<s>
From	O
the	O
mid-1970s	O
and	O
in	O
the	O
1980s	O
,	O
general-purpose	O
minicomputers	B-Architecture
served	O
as	O
routers	O
.	O
</s>
<s>
Modern	O
high-speed	O
routers	O
are	O
network	B-General_Concept
processors	I-General_Concept
or	O
highly	O
specialized	O
computers	O
with	O
extra	O
hardware	B-General_Concept
acceleration	I-General_Concept
added	O
to	O
speed	O
both	O
common	O
routing	O
functions	O
,	O
such	O
as	O
packet	B-Protocol
forwarding	I-Protocol
,	O
and	O
specialized	O
functions	O
such	O
as	O
IPsec	B-Protocol
encryption	O
.	O
</s>
<s>
There	O
is	O
substantial	O
use	O
of	O
Linux	B-Application
and	O
Unix	B-Application
software-based	O
machines	O
,	O
running	O
open	B-Application
source	I-Application
routing	O
code	O
,	O
for	O
research	O
and	O
other	O
applications	O
.	O
</s>
<s>
The	O
Cisco	B-Protocol
IOS	I-Protocol
operating	O
system	O
was	O
independently	O
designed	O
.	O
</s>
<s>
Major	O
router	O
operating	O
systems	O
,	O
such	O
as	O
Junos	B-Application
and	O
NX-OS	B-Operating_System
,	O
are	O
extensively	O
modified	O
versions	O
of	O
Unix	B-Application
software	O
.	O
</s>
<s>
The	O
main	O
purpose	O
of	O
a	O
router	O
is	O
to	O
connect	O
multiple	O
networks	O
and	O
forward	O
packets	B-Protocol
destined	O
either	O
for	O
directly	O
attached	O
networks	O
or	O
more	O
remote	O
networks	O
.	O
</s>
<s>
A	O
router	O
is	O
considered	O
a	O
layer-3	B-Application
device	O
because	O
its	O
primary	O
forwarding	O
decision	O
is	O
based	O
on	O
the	O
information	O
in	O
the	O
layer-3	B-Application
IP	B-Protocol
packet	B-Protocol
,	O
specifically	O
the	O
destination	O
IP	B-Protocol
address	O
.	O
</s>
<s>
When	O
a	O
router	O
receives	O
a	O
packet	B-Protocol
,	O
it	O
searches	O
its	O
routing	B-Protocol
table	I-Protocol
to	O
find	O
the	O
best	O
match	O
between	O
the	O
destination	O
IP	B-Protocol
address	O
of	O
the	O
packet	B-Protocol
and	O
one	O
of	O
the	O
addresses	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
Once	O
a	O
match	O
is	O
found	O
,	O
the	O
packet	B-Protocol
is	O
encapsulated	O
in	O
the	O
layer-2	B-Protocol
data	O
link	O
frame	O
for	O
the	O
outgoing	O
interface	O
indicated	O
in	O
the	O
table	O
entry	O
.	O
</s>
<s>
A	O
router	O
typically	O
does	O
not	O
look	O
into	O
the	O
packet	B-Protocol
payload	O
,	O
but	O
only	O
at	O
the	O
layer-3	B-Application
addresses	O
to	O
make	O
a	O
forwarding	O
decision	O
,	O
plus	O
optionally	O
other	O
information	O
in	O
the	O
header	B-General_Concept
for	O
hints	O
on	O
,	O
for	O
example	O
,	O
quality	O
of	O
service	O
(	O
QoS	O
)	O
.	O
</s>
<s>
For	O
pure	O
IP	B-Protocol
forwarding	O
,	O
a	O
router	O
is	O
designed	O
to	O
minimize	O
the	O
state	B-Application
information	O
associated	O
with	O
individual	O
packets	B-Protocol
.	O
</s>
<s>
Once	O
a	O
packet	B-Protocol
is	O
forwarded	B-Protocol
,	O
the	O
router	O
does	O
not	O
retain	O
any	O
historical	O
information	O
about	O
the	O
packet	B-Protocol
.	O
</s>
<s>
The	O
routing	B-Protocol
table	I-Protocol
itself	O
can	O
contain	O
information	O
derived	O
from	O
a	O
variety	O
of	O
sources	O
,	O
such	O
as	O
a	O
default	B-Protocol
or	O
static	B-Protocol
routes	I-Protocol
that	O
are	O
configured	O
manually	O
,	O
or	O
dynamic	O
entries	O
from	O
routing	B-Protocol
protocols	I-Protocol
where	O
the	O
router	O
learns	O
routes	O
from	O
other	O
routers	O
.	O
</s>
<s>
A	O
default	B-Protocol
route	I-Protocol
is	O
one	O
that	O
is	O
used	O
to	O
route	O
all	O
traffic	O
whose	O
destination	O
does	O
not	O
otherwise	O
appear	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
;	O
it	O
is	O
common	O
–	O
even	O
necessary	O
–	O
in	O
small	O
networks	O
,	O
such	O
as	O
a	O
home	O
or	O
small	O
business	O
where	O
the	O
default	B-Protocol
route	I-Protocol
simply	O
sends	O
all	O
non-local	O
traffic	O
to	O
the	O
Internet	O
service	O
provider	O
.	O
</s>
<s>
The	O
default	B-Protocol
route	I-Protocol
can	O
be	O
manually	O
configured	O
(	O
as	O
a	O
static	B-Protocol
route	I-Protocol
)	O
;	O
learned	O
by	O
dynamic	B-Protocol
routing	I-Protocol
protocols	O
;	O
or	O
be	O
obtained	O
by	O
DHCP	B-Protocol
.	O
</s>
<s>
A	O
router	O
can	O
run	O
more	O
than	O
one	O
routing	B-Protocol
protocol	I-Protocol
at	O
a	O
time	O
,	O
particularly	O
if	O
it	O
serves	O
as	O
an	O
autonomous	B-Architecture
system	I-Architecture
border	O
router	O
between	O
parts	O
of	O
a	O
network	O
that	O
run	O
different	O
routing	B-Protocol
protocols	I-Protocol
;	O
if	O
it	O
does	O
so	O
,	O
then	O
redistribution	O
may	O
be	O
used	O
(	O
usually	O
selectively	O
)	O
to	O
share	O
information	O
between	O
the	O
different	O
protocols	O
running	O
on	O
the	O
same	O
router	O
.	O
</s>
<s>
Besides	O
deciding	O
to	O
which	O
interface	O
a	O
packet	B-Protocol
is	O
forwarded	B-Protocol
,	O
which	O
is	O
handled	O
primarily	O
via	O
the	O
routing	B-Protocol
table	I-Protocol
,	O
a	O
router	O
also	O
has	O
to	O
manage	O
congestion	O
when	O
packets	B-Protocol
arrive	O
at	O
a	O
rate	O
higher	O
than	O
the	O
router	O
can	O
process	O
.	O
</s>
<s>
Three	O
policies	O
commonly	O
used	O
are	O
tail	B-Protocol
drop	I-Protocol
,	O
random	B-Algorithm
early	I-Algorithm
detection	I-Algorithm
(	O
RED	O
)	O
,	O
and	O
weighted	O
random	B-Algorithm
early	I-Algorithm
detection	I-Algorithm
(	O
WRED	O
)	O
.	O
</s>
<s>
Tail	B-Protocol
drop	I-Protocol
is	O
the	O
simplest	O
and	O
most	O
easily	O
implemented	O
:	O
the	O
router	O
simply	O
drops	O
new	O
incoming	O
packets	B-Protocol
once	O
buffer	O
space	O
in	O
the	O
router	O
is	O
exhausted	O
.	O
</s>
<s>
RED	O
probabilistically	O
drops	O
datagrams	O
early	O
when	O
the	O
queue	O
exceeds	O
a	O
pre-configured	O
portion	O
of	O
the	O
buffer	O
,	O
until	O
reaching	O
a	O
pre-determined	O
maximum	O
,	O
when	O
it	O
drops	O
all	O
incoming	O
packets	B-Protocol
,	O
thus	O
reverting	O
to	O
tail	B-Protocol
drop	I-Protocol
.	O
</s>
<s>
WRED	O
can	O
be	O
configured	O
to	O
drop	O
packets	B-Protocol
more	O
readily	O
dependent	O
on	O
the	O
type	O
of	O
traffic	O
.	O
</s>
<s>
Another	O
function	O
a	O
router	O
performs	O
is	O
traffic	O
classification	O
and	O
deciding	O
which	O
packet	B-Protocol
should	O
be	O
processed	O
first	O
.	O
</s>
<s>
This	O
is	O
managed	O
through	O
QoS	O
,	O
which	O
is	O
critical	O
when	O
Voice	B-Application
over	I-Application
IP	I-Application
is	O
deployed	O
,	O
so	O
as	O
not	O
to	O
introduce	O
excessive	O
latency	O
.	O
</s>
<s>
Yet	O
another	O
function	O
a	O
router	O
performs	O
is	O
called	O
policy-based	B-Protocol
routing	I-Protocol
where	O
special	O
rules	O
are	O
constructed	O
to	O
override	O
the	O
rules	O
derived	O
from	O
the	O
routing	B-Protocol
table	I-Protocol
when	O
a	O
packet	B-Protocol
forwarding	I-Protocol
decision	O
is	O
made	O
.	O
</s>
<s>
Some	O
of	O
the	O
functions	O
may	O
be	O
performed	O
through	O
an	O
application-specific	O
integrated	O
circuit	O
(	O
ASIC	O
)	O
to	O
avoid	O
overhead	O
of	O
scheduling	O
CPU	B-General_Concept
time	O
to	O
process	O
the	O
packets	B-Protocol
.	O
</s>
<s>
Others	O
may	O
have	O
to	O
be	O
performed	O
through	O
the	O
CPU	B-General_Concept
as	O
these	O
packets	B-Protocol
need	O
special	O
attention	O
that	O
cannot	O
be	O
handled	O
by	O
an	O
ASIC	O
.	O
</s>
