<s>
A	O
distance-vector	B-Protocol
routing	I-Protocol
protocol	I-Protocol
in	O
data	B-Architecture
networks	I-Architecture
determines	O
the	O
best	O
route	O
for	O
data	B-Protocol
packets	I-Protocol
based	O
on	O
distance	O
.	O
</s>
<s>
Distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
measure	O
the	O
distance	O
by	O
the	O
number	O
of	O
routers	B-Protocol
a	O
packet	B-Protocol
has	O
to	O
pass	O
;	O
one	O
router	B-Protocol
counts	O
as	O
one	O
hop	O
.	O
</s>
<s>
Some	O
distance-vector	B-Protocol
protocols	I-Protocol
also	O
take	O
into	O
account	O
network	B-Protocol
latency	I-Protocol
and	O
other	O
factors	O
that	O
influence	O
traffic	O
on	O
a	O
given	O
route	O
.	O
</s>
<s>
To	O
determine	O
the	O
best	O
route	O
across	O
a	O
network	O
,	O
routers	B-Protocol
using	O
a	O
distance-vector	B-Protocol
protocol	I-Protocol
exchange	O
information	O
with	O
one	O
another	O
,	O
usually	O
routing	B-Protocol
tables	I-Protocol
plus	O
hop	O
counts	O
for	O
destination	O
networks	O
and	O
possibly	O
other	O
traffic	O
information	O
.	O
</s>
<s>
Distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
also	O
require	O
that	O
a	O
router	B-Protocol
inform	O
its	O
neighbours	O
of	O
network	B-Architecture
topology	I-Architecture
changes	O
periodically	O
.	O
</s>
<s>
Distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
use	O
the	O
Bellman	B-Algorithm
–	I-Algorithm
Ford	I-Algorithm
algorithm	I-Algorithm
to	O
calculate	O
the	O
best	O
route	O
.	O
</s>
<s>
Another	O
way	O
of	O
calculating	O
the	O
best	O
route	O
across	O
a	O
network	O
is	O
based	O
on	O
link	O
cost	O
,	O
and	O
is	O
implemented	O
through	O
link-state	B-Protocol
routing	I-Protocol
protocols	I-Protocol
.	O
</s>
<s>
The	O
term	O
distance	B-Protocol
vector	I-Protocol
refers	O
to	O
the	O
fact	O
that	O
the	O
protocol	O
manipulates	O
vectors	O
(	O
arrays	B-Data_Structure
)	O
of	O
distances	O
to	O
other	O
nodes	O
in	O
the	O
network	O
.	O
</s>
<s>
The	O
distance	B-Protocol
vector	I-Protocol
algorithm	I-Protocol
was	O
the	O
original	O
ARPANET	O
routing	O
algorithm	O
and	O
was	O
implemented	O
more	O
widely	O
in	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
with	O
the	O
Routing	B-Protocol
Information	I-Protocol
Protocol	I-Protocol
(	O
RIP	B-Protocol
)	O
.	O
</s>
<s>
Distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
use	O
the	O
Bellman	B-Algorithm
–	I-Algorithm
Ford	I-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
In	O
these	O
protocols	O
,	O
each	O
router	B-Protocol
does	O
not	O
possess	O
information	O
about	O
the	O
full	O
network	B-Architecture
topology	I-Architecture
.	O
</s>
<s>
It	O
advertises	O
its	O
distance	O
value	O
(	O
DV	O
)	O
calculated	O
to	O
other	O
routers	B-Protocol
and	O
receives	O
similar	O
advertisements	O
from	O
other	O
routers	B-Protocol
unless	O
changes	O
are	O
done	O
in	O
the	O
local	B-General_Concept
network	I-General_Concept
or	O
by	O
neighbours	O
(	O
routers	B-Protocol
)	O
.	O
</s>
<s>
Using	O
these	O
routing	O
advertisements	O
each	O
router	B-Protocol
populates	O
its	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
In	O
the	O
next	O
advertisement	O
cycle	O
,	O
a	O
router	B-Protocol
advertises	O
updated	O
information	O
from	O
its	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
This	O
process	O
continues	O
until	O
the	O
routing	B-Protocol
tables	I-Protocol
of	O
each	O
router	B-Protocol
converge	B-Protocol
to	O
stable	O
values	O
.	O
</s>
<s>
Some	O
of	O
these	O
protocols	O
have	O
the	O
disadvantage	O
of	O
slow	O
convergence	B-Protocol
.	O
</s>
<s>
Examples	O
of	O
distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
:	O
</s>
<s>
Routers	B-Protocol
that	O
use	O
distance-vector	B-Protocol
protocol	I-Protocol
determine	O
the	O
distance	O
between	O
themselves	O
and	O
a	O
destination	O
.	O
</s>
<s>
The	O
best	O
route	O
for	O
Internet	B-Protocol
Protocol	I-Protocol
packets	B-Protocol
that	O
carry	O
data	O
across	O
a	O
data	B-Architecture
network	I-Architecture
is	O
measured	O
in	O
terms	O
of	O
the	O
numbers	O
of	O
routers	B-Protocol
(	O
hops	O
)	O
a	O
packet	B-Protocol
has	O
to	O
pass	O
to	O
reach	O
its	O
destination	O
network	O
.	O
</s>
<s>
Additionally	O
some	O
distance-vector	B-Protocol
protocols	I-Protocol
take	O
into	O
account	O
other	O
traffic	O
information	O
,	O
such	O
as	O
network	B-Protocol
latency	I-Protocol
.	O
</s>
<s>
To	O
establish	O
the	O
best	O
route	O
,	O
routers	B-Protocol
regularly	O
exchange	O
information	O
with	O
neighbouring	O
routers	B-Protocol
,	O
usually	O
their	O
routing	B-Protocol
table	I-Protocol
,	O
hop	O
count	O
for	O
a	O
destination	O
network	O
and	O
possibly	O
other	O
traffic	O
related	O
information	O
.	O
</s>
<s>
Routers	B-Protocol
that	O
implement	O
distance-vector	B-Protocol
protocol	I-Protocol
rely	O
purely	O
on	O
the	O
information	O
provided	O
to	O
them	O
by	O
other	O
routers	B-Protocol
,	O
and	O
do	O
not	O
assess	O
the	O
network	B-Architecture
topology	I-Architecture
.	O
</s>
<s>
Distance-vector	B-Protocol
protocols	I-Protocol
update	O
the	O
routing	B-Protocol
tables	I-Protocol
of	O
routers	B-Protocol
and	O
determine	O
the	O
route	O
on	O
which	O
a	O
packet	B-Protocol
will	O
be	O
sent	O
by	O
the	O
next	O
hop	O
which	O
is	O
the	O
exit	O
interface	O
of	O
the	O
router	B-Protocol
and	O
the	O
IP	O
address	O
of	O
the	O
interface	O
of	O
the	O
receiving	O
router	B-Protocol
.	O
</s>
<s>
Updates	O
are	O
performed	O
periodically	O
in	O
a	O
distance-vector	B-Protocol
protocol	I-Protocol
where	O
all	O
or	O
part	O
of	O
a	O
router	B-Protocol
's	O
routing	B-Protocol
table	I-Protocol
is	O
sent	O
to	O
all	O
its	O
neighbours	O
that	O
are	O
configured	O
to	O
use	O
the	O
same	O
distance-vector	B-Protocol
routing	I-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Once	O
a	O
router	B-Protocol
has	O
this	O
information	O
it	O
is	O
able	O
to	O
amend	O
its	O
own	O
routing	B-Protocol
table	I-Protocol
to	O
reflect	O
the	O
changes	O
and	O
then	O
inform	O
its	O
neighbours	O
of	O
the	O
changes	O
.	O
</s>
<s>
This	O
process	O
has	O
been	O
described	O
as	O
‘	O
routing	O
by	O
rumour’	O
because	O
routers	B-Protocol
are	O
relying	O
on	O
the	O
information	O
they	O
receive	O
from	O
other	O
routers	B-Protocol
and	O
cannot	O
determine	O
if	O
the	O
information	O
is	O
actually	O
valid	O
and	O
true	O
.	O
</s>
<s>
The	O
oldest	O
routing	B-Protocol
protocol	I-Protocol
,	O
and	O
the	O
oldest	O
distance-vector	B-Protocol
protocol	I-Protocol
,	O
is	O
version	O
1	O
of	O
the	O
Routing	B-Protocol
Information	I-Protocol
Protocol	I-Protocol
(	O
RIPv1	B-Protocol
)	O
.	O
</s>
<s>
RIPv1	B-Protocol
was	O
formally	O
standardised	O
in	O
1988	O
.	O
</s>
<s>
It	O
establishes	O
the	O
shortest	O
path	O
across	O
a	O
network	O
purely	O
on	O
the	O
basis	O
of	O
the	O
hops	O
,	O
that	O
is	O
numbers	O
of	O
routers	B-Protocol
that	O
need	O
to	O
be	O
passed	O
to	O
reach	O
the	O
destination	O
network	O
.	O
</s>
<s>
RIP	B-Protocol
is	O
an	O
interior	B-Protocol
gateway	I-Protocol
protocol	I-Protocol
,	O
so	O
it	O
can	O
be	O
used	O
in	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
(	O
LANs	B-General_Concept
)	O
on	O
interior	O
or	O
border	O
routers	B-Protocol
.	O
</s>
<s>
Routers	B-Protocol
with	O
RIPv1	B-Protocol
implementation	O
exchange	O
their	O
routing	B-Protocol
tables	I-Protocol
with	O
neighbouring	O
routers	B-Protocol
by	O
broadcasting	B-Operating_System
a	O
RIPv1	B-Protocol
packet	B-Protocol
every	O
30	O
second	O
into	O
all	O
connected	O
networks	O
.	O
</s>
<s>
RIPv1	B-Protocol
is	O
not	O
suitable	O
for	O
large	O
networks	O
as	O
it	O
limits	O
the	O
number	O
of	O
hops	O
to	O
15	O
.	O
</s>
<s>
This	O
hop	O
limit	O
was	O
introduced	O
to	O
avoid	O
routing	B-Protocol
loops	I-Protocol
,	O
but	O
also	O
means	O
that	O
networks	O
that	O
are	O
connected	O
through	O
more	O
than	O
15	O
routers	B-Protocol
are	O
unreachable	O
.	O
</s>
<s>
The	O
distance-vector	B-Protocol
protocol	I-Protocol
designed	O
for	O
use	O
in	O
wide	O
area	O
networks	O
(	O
WANs	O
)	O
is	O
the	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
.	O
</s>
<s>
BGP	B-Protocol
is	O
an	O
exterior	B-Protocol
gateway	I-Protocol
protocol	I-Protocol
and	O
therefore	O
implemented	O
on	O
border	O
and	O
exterior	O
routers	B-Protocol
on	O
the	O
Internet	O
.	O
</s>
<s>
It	O
exchanges	O
information	O
between	O
routers	B-Protocol
through	O
a	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
session	O
.	O
</s>
<s>
Routers	B-Protocol
with	O
BGP	B-Protocol
implementation	O
determine	O
the	O
shortest	O
path	O
across	O
a	O
network	O
based	O
on	O
a	O
range	O
of	O
factors	O
other	O
than	O
hops	O
.	O
</s>
<s>
BGP	B-Protocol
can	O
also	O
be	O
configured	O
by	O
administrators	O
so	O
that	O
certain	O
routes	O
are	O
preferred	O
or	O
avoided	O
.	O
</s>
<s>
BGP	B-Protocol
is	O
used	O
by	O
internet	O
service	O
providers	O
(	O
ISPs	O
)	O
and	O
telecommunication	O
companies	O
.	O
</s>
<s>
Among	O
the	O
distance-vector	B-Protocol
protocols	I-Protocol
that	O
have	O
been	O
described	O
as	O
a	O
hybrid	O
,	O
because	O
it	O
uses	O
routing	O
methods	O
associated	O
with	O
link-state	B-Protocol
routing	I-Protocol
protocols	I-Protocol
,	O
is	O
the	O
proprietary	O
Enhanced	B-Protocol
Interior	I-Protocol
Gateway	I-Protocol
Routing	I-Protocol
Protocol	I-Protocol
(	O
EIGRP	B-Protocol
)	O
.	O
</s>
<s>
It	O
was	O
developed	O
by	O
Cisco	O
in	O
the	O
1980s	O
and	O
was	O
designed	O
to	O
offer	O
better	O
convergence	B-Protocol
and	O
cause	O
less	O
network	O
traffic	O
between	O
routers	B-Protocol
than	O
the	O
link-state	B-Protocol
routing	I-Protocol
protocol	I-Protocol
Open	B-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
(	O
OSPF	B-Protocol
)	O
.	O
</s>
<s>
Another	O
example	O
of	O
a	O
distance-vector	B-Protocol
routing	I-Protocol
protocol	I-Protocol
is	O
Babel	B-Protocol
.	O
</s>
<s>
The	O
Bellman	B-Algorithm
–	I-Algorithm
Ford	I-Algorithm
algorithm	I-Algorithm
does	O
not	O
prevent	O
routing	B-Protocol
loops	I-Protocol
from	O
happening	O
and	O
suffers	O
from	O
the	O
count	O
to	O
infinity	O
problem	O
.	O
</s>
<s>
To	O
see	O
the	O
problem	O
,	O
imagine	O
a	O
subnet	O
connected	O
like	O
A	O
–	O
B	O
–	O
C	O
–	O
D	O
–	O
E	O
–	O
F	O
,	O
and	O
let	O
the	O
metric	O
between	O
the	O
routers	B-Protocol
be	O
"	O
number	O
of	O
jumps	O
"	O
.	O
</s>
<s>
This	O
slowly	O
propagates	O
through	O
the	O
network	O
until	O
it	O
becomes	O
infinity	O
(	O
in	O
which	O
case	O
the	O
algorithm	O
corrects	O
itself	O
,	O
due	O
to	O
the	O
relaxation	O
property	O
of	O
Bellman-Ford	B-Algorithm
)	O
.	O
</s>
<s>
RIP	B-Protocol
uses	O
the	O
split	B-Protocol
horizon	I-Protocol
with	O
poison	O
reverse	O
technique	O
to	O
reduce	O
the	O
chance	O
of	O
forming	O
loops	O
and	O
uses	O
a	O
maximum	O
number	O
of	O
hops	O
to	O
counter	O
the	O
'	O
count	O
to	O
infinity	O
 '	O
problem	O
.	O
</s>
<s>
These	O
measures	O
avoid	O
the	O
formation	O
of	O
routing	B-Protocol
loops	I-Protocol
in	O
some	O
,	O
but	O
not	O
all	O
,	O
cases	O
.	O
</s>
<s>
The	O
addition	O
of	O
a	O
hold	O
time	O
(	O
refusing	O
route	O
updates	O
for	O
a	O
few	O
minutes	O
after	O
a	O
route	O
retraction	O
)	O
avoids	O
loop	O
formation	O
in	O
virtually	O
all	O
cases	O
,	O
but	O
causes	O
a	O
significant	O
increase	O
in	O
convergence	B-Protocol
times	O
.	O
</s>
<s>
More	O
recently	O
,	O
a	O
number	O
of	O
loop-free	O
distance	B-Protocol
vector	I-Protocol
protocols	I-Protocol
have	O
been	O
developed	O
—	O
notable	O
examples	O
are	O
EIGRP	B-Protocol
,	O
DSDV	B-Protocol
and	O
Babel	B-Protocol
.	O
</s>
<s>
These	O
avoid	O
loop	O
formation	O
in	O
all	O
cases	O
,	O
but	O
suffer	O
from	O
increased	O
complexity	O
,	O
and	O
their	O
deployment	O
has	O
been	O
slowed	O
down	O
by	O
the	O
success	O
of	O
link	B-Protocol
state	I-Protocol
routing	I-Protocol
protocols	I-Protocol
such	O
as	O
OSPF	B-Protocol
.	O
</s>
<s>
In	O
this	O
network	O
we	O
have	O
4	O
routers	B-Protocol
A	O
,	O
B	O
,	O
C	O
and	O
D	O
:	O
</s>
<s>
We	O
mark	O
the	O
current	O
time	O
(	O
or	O
iteration	O
)	O
in	O
the	O
algorithm	O
with	O
T	O
,	O
and	O
begin	O
(	O
at	O
time	O
0	O
,	O
or	O
T	O
=	O
0	O
)	O
by	O
creating	O
distance	O
matrices	O
for	O
each	O
router	B-Protocol
to	O
its	O
immediate	O
neighbours	O
.	O
</s>
<s>
As	O
we	O
build	O
the	O
routing	B-Protocol
tables	I-Protocol
below	O
,	O
the	O
shortest	O
path	O
is	O
highlighted	O
in	O
green	O
,	O
and	O
a	O
new	O
shortest	O
path	O
is	O
highlighted	O
in	O
yellow	O
.	O
</s>
<s>
T	O
=	O
0	O
from	O
Avia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
Ato	O
B3to	O
C23to	O
D	O
from	O
Bvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A3to	O
Bto	O
C2to	O
D	O
from	O
Cvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A23to	O
B2to	O
Cto	O
D5	O
from	O
Dvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
Ato	O
Bto	O
C5to	O
D	O
At	O
this	O
point	O
,	O
all	O
the	O
routers	B-Protocol
(	O
A	O
,	O
B	O
,	O
C	O
,	O
D	O
)	O
have	O
new	O
"	O
shortest-paths	O
"	O
for	O
their	O
DV	O
(	O
the	O
list	O
of	O
distances	O
that	O
are	O
from	O
them	O
to	O
another	O
router	B-Protocol
via	O
a	O
neighbor	O
)	O
.	O
</s>
<s>
They	O
each	O
broadcast	B-Operating_System
this	O
new	O
DV	O
to	O
all	O
their	O
neighbors	O
:	O
A	O
to	O
B	O
and	O
C	O
,	O
B	O
to	O
C	O
and	O
A	O
,	O
C	O
to	O
A	O
,	O
B	O
,	O
and	O
D	O
,	O
and	O
D	O
to	O
C	O
.	O
As	O
each	O
of	O
these	O
neighbors	O
receives	O
this	O
information	O
,	O
they	O
now	O
recalculate	O
the	O
shortest	O
path	O
using	O
it	O
.	O
</s>
<s>
As	O
there	O
are	O
no	O
other	O
shorter	O
paths	O
that	O
A	O
knows	O
about	O
,	O
it	O
puts	O
this	O
as	O
its	O
current	O
estimate	O
for	O
the	O
shortest-path	O
from	O
itself	O
(	O
A	O
)	O
to	O
D	O
,	O
via	O
C	O
.	O
T	O
=	O
1	O
from	O
Avia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
Ato	O
B325to	O
C523to	O
D28	O
from	O
Bvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A325to	O
Bto	O
C262to	O
D7	O
from	O
Cvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A235to	O
B262to	O
Cto	O
D5	O
from	O
Dvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A28to	O
B7to	O
C5to	O
D	O
Again	O
,	O
all	O
the	O
routers	B-Protocol
have	O
gained	O
in	O
the	O
last	O
iteration	O
(	O
at	O
T	O
=	O
1	O
)	O
new	O
"	O
shortest-paths	O
"	O
,	O
so	O
they	O
all	O
broadcast	B-Operating_System
their	O
DVs	O
to	O
their	O
neighbors	O
;	O
This	O
prompts	O
each	O
neighbor	O
to	O
re-calculate	O
their	O
shortest	O
distances	O
again	O
.	O
</s>
<s>
This	O
path	O
to	O
D	O
of	O
length	O
10	O
(	O
via	O
B	O
)	O
is	O
shorter	O
than	O
the	O
existing	O
"	O
shortest-path	O
"	O
to	O
D	O
of	O
length	O
28	O
(	O
via	O
C	O
)	O
,	O
so	O
it	O
becomes	O
the	O
new	O
"	O
shortest-path	O
"	O
to	O
D	O
.	O
T	O
=	O
2	O
from	O
Avia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
Ato	O
B325to	O
C523to	O
D1028	O
from	O
Bvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A37to	O
Bto	O
C82to	O
D317	O
from	O
Cvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A23533to	O
B26212to	O
Cto	O
D5195	O
from	O
Dvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A10to	O
B7to	O
C5to	O
D	O
This	O
time	O
,	O
only	O
routers	B-Protocol
A	O
and	O
D	O
have	O
new	O
shortest-paths	O
for	O
their	O
DVs	O
.	O
</s>
<s>
So	O
they	O
broadcast	B-Operating_System
their	O
new	O
DVs	O
to	O
their	O
neighbors	O
:	O
A	O
broadcasts	B-Operating_System
to	O
B	O
and	O
C	O
,	O
and	O
D	O
broadcasts	B-Operating_System
to	O
C	O
.	O
This	O
causes	O
each	O
of	O
the	O
neighbors	O
receiving	O
the	O
new	O
DVs	O
to	O
re-calculate	O
their	O
shortest	O
paths	O
.	O
</s>
<s>
However	O
,	O
since	O
the	O
information	O
from	O
the	O
DVs	O
does	O
n't	O
yield	O
any	O
shorter	O
paths	O
than	O
they	O
already	O
have	O
in	O
their	O
routing	B-Protocol
tables	I-Protocol
,	O
then	O
there	O
are	O
no	O
changes	O
to	O
the	O
routing	B-Protocol
tables	I-Protocol
.	O
</s>
<s>
T	O
=3	O
from	O
Avia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
Ato	O
B325to	O
C523to	O
D1028	O
from	O
Bvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A37to	O
Bto	O
C82to	O
D137	O
from	O
Cvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A23515to	O
B26212to	O
Cto	O
D3395	O
from	O
Dvia	O
Avia	O
Bvia	O
Cvia	O
Dto	O
A10to	O
B7to	O
C5to	O
D	O
None	O
of	O
the	O
routers	B-Protocol
have	O
any	O
new	O
shortest-paths	O
to	O
broadcast	B-Operating_System
.	O
</s>
<s>
Therefore	O
,	O
none	O
of	O
the	O
routers	B-Protocol
receive	O
any	O
new	O
information	O
that	O
might	O
change	O
their	O
routing	B-Protocol
tables	I-Protocol
.	O
</s>
