<s>
A	O
routing	B-Protocol
loop	I-Protocol
is	O
a	O
common	O
problem	O
with	O
various	O
types	O
of	O
networks	O
,	O
particularly	O
computer	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
They	O
are	O
formed	O
when	O
an	O
error	O
occurs	O
in	O
the	O
operation	O
of	O
the	O
routing	B-Protocol
algorithm	I-Protocol
,	O
and	O
as	O
a	O
result	O
,	O
in	O
a	O
group	O
of	O
nodes	O
,	O
the	O
path	O
to	O
a	O
particular	O
destination	O
forms	O
a	O
loop	O
.	O
</s>
<s>
In	O
the	O
simplest	O
version	O
,	O
a	O
routing	B-Protocol
loop	I-Protocol
of	O
size	O
two	O
,	O
node	B-Data_Structure
A	O
thinks	O
that	O
the	O
path	O
to	O
some	O
destination	O
(	O
call	O
it	O
C	O
)	O
is	O
through	O
its	O
neighbouring	O
node	B-Data_Structure
,	O
node	B-Data_Structure
B	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
node	B-Data_Structure
B	O
thinks	O
that	O
the	O
path	O
to	O
C	O
starts	O
at	O
node	B-Data_Structure
A	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
this	O
illustration	O
,	O
node	B-Data_Structure
A	O
is	O
transmitting	O
data	O
to	O
node	B-Data_Structure
C	O
via	O
node	B-Data_Structure
B	O
.	O
</s>
<s>
If	O
the	O
link	O
between	O
nodes	O
B	O
and	O
C	O
goes	O
down	O
and	O
B	O
has	O
not	O
yet	O
informed	O
node	B-Data_Structure
A	O
about	O
the	O
breakage	O
,	O
node	B-Data_Structure
A	O
transmits	O
the	O
data	O
to	O
node	B-Data_Structure
B	O
assuming	O
that	O
the	O
link	O
A-B-C	O
is	O
operational	O
and	O
of	O
lowest	O
cost	O
.	O
</s>
<s>
Node	B-Data_Structure
B	O
knows	O
of	O
the	O
broken	O
link	O
and	O
tries	O
to	O
reach	O
node	B-Data_Structure
C	O
via	O
node	B-Data_Structure
A	O
,	O
thus	O
sending	O
the	O
original	O
data	O
back	O
to	O
node	B-Data_Structure
A	O
.	O
</s>
<s>
Furthermore	O
,	O
node	B-Data_Structure
A	O
receives	O
the	O
data	O
that	O
it	O
originated	O
back	O
from	O
node	B-Data_Structure
B	O
and	O
consults	O
its	O
routing	B-Protocol
table	O
.	O
</s>
<s>
Node	B-Data_Structure
A	O
's	O
routing	B-Protocol
table	O
will	O
say	O
that	O
it	O
can	O
reach	O
node	B-Data_Structure
C	O
via	O
node	B-Data_Structure
B	O
(	O
because	O
it	O
still	O
has	O
not	O
been	O
informed	O
of	O
the	O
break	O
)	O
thus	O
sending	O
its	O
data	O
back	O
to	O
node	B-Data_Structure
B	O
creating	O
an	O
infinite	O
loop	O
.	O
</s>
<s>
This	O
routing	B-Protocol
loop	I-Protocol
problem	I-Protocol
is	O
also	O
called	O
a	O
two-node	O
loop	O
.	O
</s>
<s>
Consider	O
now	O
what	O
happens	O
if	O
both	O
the	O
link	O
from	O
A	O
to	O
C	O
and	O
the	O
link	O
from	O
B	O
to	O
C	O
vanish	O
at	O
the	O
same	O
time	O
(	O
this	O
can	O
happen	O
if	O
node	B-Data_Structure
C	O
has	O
crashed	O
)	O
.	O
</s>
<s>
In	O
a	O
simple	O
reachability	O
protocol	O
,	O
such	O
as	O
EGP	B-Protocol
,	O
the	O
routing	B-Protocol
loop	I-Protocol
will	O
persist	O
forever	O
.	O
</s>
<s>
In	O
a	O
naive	O
distance-vector	B-Protocol
protocol	I-Protocol
,	O
such	O
as	O
the	O
routing	B-Protocol
information	I-Protocol
protocol	I-Protocol
,	O
the	O
loop	O
will	O
persist	O
until	O
the	O
metrics	O
for	O
C	O
reach	O
infinity	O
(	O
the	O
maximum	O
number	O
of	O
routers	O
that	O
a	O
packet	O
can	O
traverse	O
in	O
RIP	B-Protocol
is	O
15	O
.	O
</s>
<s>
In	O
a	O
link-state	B-Protocol
routing	I-Protocol
protocol	I-Protocol
,	O
such	O
as	O
OSPF	B-Protocol
or	O
IS-IS	B-Protocol
,	O
a	O
routing	B-Protocol
loop	I-Protocol
disappears	O
as	O
soon	O
as	O
the	O
new	O
network	O
topology	O
is	O
flooded	O
to	O
all	O
the	O
routers	O
within	O
the	O
routing	B-Protocol
area	O
.	O
</s>
<s>
Newer	O
distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
like	O
EIGRP	B-Protocol
,	O
DSDV	B-Protocol
,	O
and	O
Babel	B-Protocol
have	O
built-in	O
loop	O
prevention	O
:	O
they	O
use	O
algorithms	O
that	O
assure	O
that	O
routing	B-Protocol
loops	I-Protocol
can	O
never	O
happen	O
,	O
not	O
even	O
transiently	O
.	O
</s>
<s>
Older	O
routing	B-Protocol
protocols	O
like	O
RIP	B-Protocol
and	O
IGRP	B-Protocol
do	O
not	O
implement	O
the	O
newest	O
forms	O
of	O
loop	O
prevention	O
and	O
only	O
implement	O
mitigations	O
such	O
as	O
split	B-Protocol
horizon	I-Protocol
,	O
route	B-Protocol
poisoning	I-Protocol
,	O
and	O
holddown	B-Protocol
timers	I-Protocol
.	O
</s>
