<s>
The	O
Routing	B-Protocol
Information	I-Protocol
Protocol	I-Protocol
(	O
RIP	O
)	O
is	O
one	O
of	O
the	O
oldest	O
distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
which	O
employs	O
the	O
hop	B-Protocol
count	I-Protocol
as	O
a	O
routing	O
metric	O
.	O
</s>
<s>
RIP	O
prevents	O
routing	B-Protocol
loops	I-Protocol
by	O
implementing	O
a	O
limit	O
on	O
the	O
number	O
of	O
hops	B-Protocol
allowed	O
in	O
a	O
path	O
from	O
source	O
to	O
destination	O
.	O
</s>
<s>
The	O
largest	O
number	O
of	O
hops	B-Protocol
allowed	O
for	O
RIP	O
is	O
15	O
,	O
which	O
limits	O
the	O
size	O
of	O
networks	O
that	O
RIP	O
can	O
support	O
.	O
</s>
<s>
RIP	O
implements	O
the	O
split	B-Protocol
horizon	I-Protocol
,	O
route	B-Protocol
poisoning	I-Protocol
,	O
and	O
holddown	B-Protocol
mechanisms	O
to	O
prevent	O
incorrect	O
routing	O
information	O
from	O
being	O
propagated	O
.	O
</s>
<s>
In	O
RIPv1	B-Protocol
routers	O
broadcast	B-Operating_System
updates	O
with	O
their	O
routing	B-Protocol
table	I-Protocol
every	O
30	O
seconds	O
.	O
</s>
<s>
In	O
the	O
early	O
deployments	O
,	O
routing	B-Protocol
tables	I-Protocol
were	O
small	O
enough	O
that	O
the	O
traffic	O
was	O
not	O
significant	O
.	O
</s>
<s>
In	O
most	O
networking	O
environments	O
,	O
RIP	O
is	O
not	O
the	O
preferred	O
choice	O
of	O
routing	B-Protocol
protocol	I-Protocol
,	O
as	O
its	O
time	O
to	O
converge	O
and	O
scalability	B-Architecture
are	O
poor	O
compared	O
to	O
EIGRP	B-Protocol
,	O
OSPF	B-Protocol
,	O
or	O
IS-IS	B-Protocol
.	O
</s>
<s>
RIP	O
uses	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
as	O
its	O
transport	O
protocol	O
,	O
and	O
is	O
assigned	O
the	O
reserved	O
port	B-Protocol
number	I-Protocol
520	O
.	O
</s>
<s>
Based	O
on	O
the	O
Bellman	B-Algorithm
–	I-Algorithm
Ford	I-Algorithm
algorithm	I-Algorithm
and	O
the	O
Ford	B-Algorithm
–	I-Algorithm
Fulkerson	I-Algorithm
algorithm	I-Algorithm
,	O
distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
started	O
to	O
be	O
implemented	O
from	O
1969	O
onwards	O
in	O
data	B-Architecture
networks	I-Architecture
such	O
as	O
the	O
ARPANET	O
and	O
CYCLADES	O
.	O
</s>
<s>
As	O
part	O
of	O
the	O
Xerox	B-Protocol
Network	I-Protocol
Systems	I-Protocol
(	O
XNS	O
)	O
protocol	O
suite	O
GWINFO	O
transformed	O
into	O
the	O
XNS	O
Routing	B-Protocol
Information	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
This	O
XNS	O
RIP	O
in	O
turn	O
became	O
the	O
basis	O
for	O
early	O
routing	B-Protocol
protocols	I-Protocol
,	O
such	O
as	O
Novell	O
's	O
IPX	O
RIP	O
,	O
AppleTalk	B-Application
's	O
Routing	B-Protocol
Table	I-Protocol
Maintenance	O
Protocol	O
(	O
RTMP	O
)	O
,	O
and	O
the	O
IP	O
RIP	O
.	O
</s>
<s>
The	O
1982	O
Berkley	B-Operating_System
Software	I-Operating_System
Distribution	I-Operating_System
of	O
the	O
UNIX	B-Application
operating	I-Application
system	I-Application
implemented	O
RIP	O
in	O
the	O
routed	B-Protocol
daemon	I-Protocol
.	O
</s>
<s>
The	O
4.2BSD	O
release	O
proved	O
popular	O
and	O
became	O
the	O
basis	O
for	O
subsequent	O
UNIX	B-Application
versions	O
,	O
which	O
implemented	O
RIP	O
in	O
the	O
routed	O
or	O
gated	O
daemon	B-Operating_System
.	O
</s>
<s>
Ultimately	O
,	O
RIP	O
had	O
been	O
extensively	O
deployed	O
before	O
the	O
standard	O
,	O
written	O
by	O
Charles	O
Hedrick	O
,	O
was	O
passed	O
as	O
RIPv1	B-Protocol
in	O
1988	O
.	O
</s>
<s>
The	O
hop	B-Protocol
count	I-Protocol
0	O
denotes	O
a	O
network	O
that	O
is	O
directly	O
connected	O
to	O
the	O
router	O
.	O
</s>
<s>
16	O
hops	B-Protocol
denote	O
a	O
network	O
that	O
is	O
unreachable	O
,	O
according	O
to	O
the	O
RIP	O
hop	B-Protocol
limit	O
.	O
</s>
<s>
There	O
are	O
three	O
standardized	O
versions	O
of	O
the	O
Routing	B-Protocol
Information	I-Protocol
Protocol	I-Protocol
:	O
RIPv1	B-Protocol
and	O
RIPv2	O
for	O
IPv4	B-Protocol
,	O
and	O
RIPng	O
for	O
IPv6	B-Protocol
.	O
</s>
<s>
When	O
starting	O
up	O
,	O
and	O
every	O
30	O
seconds	O
thereafter	O
,	O
a	O
router	O
with	O
RIPv1	B-Protocol
implementation	O
broadcasts	B-Operating_System
to	O
a	O
request	O
message	O
through	O
every	O
RIPv1	B-Protocol
enabled	O
interface	O
.	O
</s>
<s>
Neighbouring	O
routers	O
receiving	O
the	O
request	O
message	O
respond	O
with	O
a	O
RIPv1	B-Protocol
segment	O
,	O
containing	O
their	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
The	O
requesting	O
router	O
updates	O
its	O
own	O
routing	B-Protocol
table	I-Protocol
,	O
with	O
the	O
reachable	O
IP	O
network	O
address	O
,	O
hop	B-Protocol
count	I-Protocol
and	O
next	O
hop	B-Protocol
,	O
that	O
is	O
the	O
router	O
interface	O
IP	O
address	O
from	O
which	O
the	O
RIPv1	B-Protocol
response	O
was	O
sent	O
.	O
</s>
<s>
As	O
the	O
requesting	O
router	O
receives	O
updates	O
from	O
different	O
neighbouring	O
routers	O
it	O
will	O
only	O
update	O
the	O
reachable	O
networks	O
in	O
its	O
routing	B-Protocol
table	I-Protocol
,	O
if	O
it	O
receives	O
information	O
about	O
a	O
reachable	O
network	O
it	O
has	O
not	O
yet	O
in	O
its	O
routing	B-Protocol
table	I-Protocol
or	O
information	O
that	O
a	O
network	O
it	O
has	O
in	O
its	O
routing	B-Protocol
table	I-Protocol
is	O
reachable	O
with	O
a	O
lower	O
hop	B-Protocol
count	I-Protocol
.	O
</s>
<s>
Therefore	O
,	O
a	O
RIPv1	B-Protocol
router	O
will	O
in	O
most	O
cases	O
only	O
have	O
one	O
entry	O
for	O
a	O
reachable	O
network	O
,	O
the	O
one	O
with	O
the	O
lowest	O
hop	B-Protocol
count	I-Protocol
.	O
</s>
<s>
If	O
a	O
router	O
receives	O
information	O
from	O
two	O
different	O
neighbouring	O
router	O
that	O
the	O
same	O
network	O
is	O
reachable	O
with	O
the	O
same	O
hop	B-Protocol
count	I-Protocol
but	O
via	O
two	O
different	O
routes	O
,	O
the	O
network	O
will	O
be	O
entered	O
into	O
the	O
routing	B-Protocol
table	I-Protocol
two	O
times	O
with	O
different	O
next	O
hop	B-Protocol
routers	O
.	O
</s>
<s>
The	O
RIPv1	B-Protocol
enabled	O
router	O
will	O
then	O
perform	O
what	O
is	O
known	O
as	O
equal-cost	O
load	B-Operating_System
balancing	O
for	O
IP	O
packets	O
.	O
</s>
<s>
RIPv1	B-Protocol
enabled	O
routers	O
not	O
only	O
request	O
the	O
routing	B-Protocol
tables	I-Protocol
of	O
other	O
routers	O
every	O
30	O
seconds	O
,	O
they	O
also	O
listen	O
to	O
incoming	O
requests	O
from	O
neighbouring	O
routers	O
and	O
send	O
their	O
own	O
routing	B-Protocol
table	I-Protocol
in	O
turn	O
.	O
</s>
<s>
RIPv1	B-Protocol
routing	B-Protocol
tables	I-Protocol
are	O
therefore	O
updated	O
every	O
25	O
to	O
35	O
seconds	O
.	O
</s>
<s>
The	O
RIPv1	B-Protocol
protocol	O
adds	O
a	O
small	O
random	O
time	O
variable	O
to	O
the	O
update	O
time	O
,	O
to	O
avoid	O
routing	B-Protocol
tables	I-Protocol
synchronizing	O
across	O
a	O
LAN	O
.	O
</s>
<s>
RIPv1	B-Protocol
can	O
be	O
configured	O
into	O
silent	O
mode	O
,	O
so	O
that	O
a	O
router	O
requests	O
and	O
processes	O
neighbouring	O
routing	B-Protocol
tables	I-Protocol
,	O
and	O
keeps	O
its	O
routing	B-Protocol
table	I-Protocol
and	O
hop	B-Protocol
count	I-Protocol
for	O
reachable	O
networks	O
up	O
to	O
date	O
,	O
but	O
does	O
not	O
needlessly	O
send	O
its	O
own	O
routing	B-Protocol
table	I-Protocol
into	O
the	O
network	O
.	O
</s>
<s>
RIPv1	B-Protocol
uses	O
classful	B-Protocol
routing	I-Protocol
.	O
</s>
<s>
The	O
periodic	O
routing	O
updates	O
do	O
not	O
carry	O
subnet	B-Protocol
information	O
,	O
lacking	O
support	O
for	O
variable	B-Protocol
length	I-Protocol
subnet	I-Protocol
masks	I-Protocol
(	O
VLSM	O
)	O
.	O
</s>
<s>
This	O
limitation	O
makes	O
it	O
impossible	O
to	O
have	O
different-sized	O
subnets	B-Protocol
inside	O
of	O
the	O
same	O
network	B-Protocol
class	I-Protocol
.	O
</s>
<s>
In	O
other	O
words	O
,	O
all	O
subnets	B-Protocol
in	O
a	O
network	B-Protocol
class	I-Protocol
must	O
have	O
the	O
same	O
size	O
.	O
</s>
<s>
It	O
included	O
the	O
ability	O
to	O
carry	O
subnet	B-Protocol
information	O
,	O
thus	O
supporting	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
.	O
</s>
<s>
To	O
maintain	O
backward	O
compatibility	O
,	O
the	O
hop	B-Protocol
count	I-Protocol
limit	O
of	O
15	O
remained	O
.	O
</s>
<s>
RIPv2	O
has	O
facilities	O
to	O
fully	O
interoperate	O
with	O
the	O
earlier	O
specification	O
if	O
all	O
Must	O
Be	O
Zero	O
protocol	O
fields	O
in	O
the	O
RIPv1	B-Protocol
messages	O
are	O
properly	O
specified	O
.	O
</s>
<s>
In	O
an	O
effort	O
to	O
avoid	O
unnecessary	O
load	B-Operating_System
on	O
hosts	O
that	O
do	O
not	O
participate	O
in	O
routing	O
,	O
RIPv2	O
multicasts	O
the	O
entire	O
routing	B-Protocol
table	I-Protocol
to	O
all	O
adjacent	O
routers	O
at	O
the	O
address	O
,	O
as	O
opposed	O
to	O
RIPv1	B-Protocol
which	O
uses	O
broadcast	B-Operating_System
.	O
</s>
<s>
Unicast	B-Protocol
addressing	O
is	O
still	O
allowed	O
for	O
special	O
applications	O
.	O
</s>
<s>
(	O
MD5	B-Algorithm
)	O
authentication	O
for	O
RIP	O
was	O
introduced	O
in	O
1997	O
.	O
</s>
<s>
RIPng	O
(	O
RIP	O
next	O
generation	O
)	O
is	O
an	O
extension	O
of	O
RIPv2	O
for	O
support	O
of	O
IPv6	B-Protocol
,	O
the	O
next	O
generation	O
Internet	O
Protocol	O
.	O
</s>
<s>
Support	O
of	O
IPv6	B-Protocol
networking	O
.	O
</s>
<s>
While	O
RIPv2	O
supports	O
RIPv1	B-Protocol
updates	O
authentication	O
,	O
RIPng	O
does	O
not	O
.	O
</s>
<s>
IPv6	B-Protocol
routers	O
were	O
,	O
at	O
the	O
time	O
,	O
supposed	O
to	O
use	O
IPsec	B-Protocol
for	O
authentication	O
.	O
</s>
<s>
RIPv2	O
encodes	O
the	O
next-hop	O
into	O
each	O
route	O
entry	O
,	O
RIPng	O
requires	O
specific	O
encoding	O
of	O
the	O
next	O
hop	B-Protocol
for	O
a	O
set	O
of	O
route	O
entries	O
.	O
</s>
<s>
RIP	O
messages	O
use	O
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
on	O
port	O
520	O
and	O
all	O
RIP	O
messages	O
exchanged	O
between	O
routers	O
are	O
encapsulated	O
in	O
a	O
UDP	O
segment	O
.	O
</s>
<s>
Asking	O
a	O
neighbouring	O
RIPv1	B-Protocol
enabled	O
router	O
to	O
send	O
its	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
Carries	O
the	O
routing	B-Protocol
table	I-Protocol
of	O
a	O
router	O
.	O
</s>
<s>
The	O
routing	B-Protocol
information	I-Protocol
protocol	I-Protocol
uses	O
the	O
following	O
timers	O
as	O
part	O
of	O
its	O
operation	O
:	O
</s>
<s>
The	O
response	O
message	O
is	O
broadcast	B-Operating_System
to	O
all	O
its	O
RIP	O
enabled	O
interface	O
.	O
</s>
<s>
The	O
invalid	O
timer	O
specifies	O
how	O
long	O
a	O
routing	O
entry	O
can	O
be	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
without	O
being	O
updated	O
.	O
</s>
<s>
After	O
the	O
timer	O
expires	O
the	O
hop	B-Protocol
count	I-Protocol
of	O
the	O
routing	O
entry	O
will	O
be	O
set	O
to	O
16	O
,	O
marking	O
the	O
destination	O
as	O
unreachable	O
.	O
</s>
<s>
The	O
flush	O
timer	O
controls	O
the	O
time	O
between	O
the	O
route	O
is	O
invalidated	O
or	O
marked	O
as	O
unreachable	O
and	O
removal	O
of	O
entry	O
from	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
The	O
hold-down	O
timer	O
is	O
started	O
per	O
route	O
entry	O
,	O
when	O
the	O
hop	B-Protocol
count	I-Protocol
is	O
changing	O
from	O
lower	O
value	O
to	O
higher	O
value	O
.	O
</s>
<s>
The	O
hop	B-Protocol
count	I-Protocol
cannot	O
exceed	O
15	O
,	O
or	O
routes	O
will	O
be	O
dropped	O
.	O
</s>
<s>
Variable	B-Protocol
Length	I-Protocol
Subnet	I-Protocol
Masks	I-Protocol
are	O
not	O
supported	O
by	O
RIP	O
version	O
1	O
(	O
which	O
is	O
obsolete	O
)	O
.	O
</s>
<s>
RIP	O
has	O
slow	O
convergence	B-Protocol
and	O
count	O
to	O
infinity	O
problems	O
.	O
</s>
<s>
A	O
RIP	O
implementation	O
first	O
introduced	O
in	O
4.2BSD	O
,	O
routed	O
,	O
survives	O
in	O
several	O
of	O
its	O
descendants	O
,	O
including	O
FreeBSD	B-Operating_System
and	O
NetBSD	B-Device
.	O
</s>
<s>
OpenBSD	B-Operating_System
introduced	O
a	O
new	O
implementation	O
,	O
ripd	O
,	O
in	O
version	O
4.1	O
and	O
retired	O
routed	O
in	O
version	O
4.4	O
.	O
</s>
<s>
RIP_2M	O
is	O
the	O
standard	O
RIPv2	O
implementation	O
using	O
multicasting	O
-	O
which	O
requires	O
all	O
routers	O
on	O
the	O
network	O
to	O
support	O
RIPv2	O
and	O
multicasting	O
,	O
whereas	O
RIP_2B	O
sends	O
RIPv2	O
packets	O
using	O
subnet	B-Protocol
broadcasting	B-Operating_System
-	O
making	O
it	O
more	O
compatible	O
with	O
routers	O
that	O
do	O
not	O
support	O
multicasting	O
,	O
including	O
RIPv1	B-Protocol
routers	O
.	O
</s>
<s>
Cisco	O
's	O
proprietary	O
Interior	B-Protocol
Gateway	I-Protocol
Routing	I-Protocol
Protocol	I-Protocol
(	O
IGRP	B-Protocol
)	O
was	O
a	O
somewhat	O
more	O
capable	O
protocol	O
than	O
RIP	O
.	O
</s>
<s>
It	O
belongs	O
to	O
the	O
same	O
basic	O
family	O
of	O
distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
.	O
</s>
<s>
Cisco	O
has	O
ceased	O
support	O
and	O
distribution	O
of	O
IGRP	B-Protocol
in	O
their	O
router	O
software	O
.	O
</s>
<s>
It	O
was	O
replaced	O
by	O
the	O
Enhanced	B-Protocol
Interior	I-Protocol
Gateway	I-Protocol
Routing	I-Protocol
Protocol	I-Protocol
(	O
EIGRP	B-Protocol
)	O
which	O
is	O
a	O
completely	O
new	O
design	O
.	O
</s>
<s>
While	O
EIGRP	B-Protocol
still	O
uses	O
a	O
distance-vector	B-Protocol
model	O
,	O
it	O
relates	O
to	O
IGRP	B-Protocol
only	O
in	O
using	O
the	O
same	O
composite	O
routing	O
metric	O
.	O
</s>
<s>
Both	O
IGRP	B-Protocol
and	O
EIGRP	B-Protocol
calculated	O
a	O
single	O
composite	O
metric	O
for	O
each	O
route	O
,	O
from	O
a	O
formula	O
of	O
five	O
variables	O
:	O
bandwidth	O
,	O
delay	B-Protocol
,	O
reliability	B-Protocol
,	O
load	B-Operating_System
,	O
and	O
MTU	B-Protocol
;	O
though	O
on	O
Cisco	O
routers	O
,	O
by	O
default	O
,	O
only	O
bandwidth	O
and	O
delay	B-Protocol
are	O
used	O
in	O
this	O
calculation	O
.	O
</s>
