<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
split-horizon	O
route	O
advertisement	O
is	O
a	O
method	O
of	O
preventing	O
routing	B-Protocol
loops	I-Protocol
in	O
distance-vector	B-Protocol
routing	I-Protocol
protocols	I-Protocol
by	O
prohibiting	O
a	O
router	O
from	O
advertising	O
a	O
route	O
back	O
onto	O
the	O
interface	O
from	O
which	O
it	O
was	O
learned	O
.	O
</s>
<s>
Route	B-Protocol
poisoning	I-Protocol
:	O
if	O
a	O
node	O
N	O
learns	O
that	O
its	O
route	O
to	O
a	O
destination	O
D	O
is	O
unreachable	O
,	O
inform	O
that	O
to	O
all	O
nodes	O
in	O
the	O
network	O
by	O
sending	O
them	O
a	O
message	O
stating	O
that	O
the	O
distance	O
from	O
N	O
to	O
D	O
,	O
as	O
perceived	O
by	O
N	O
,	O
is	O
infinite	O
.	O
</s>
<s>
Split	B-Protocol
horizon	I-Protocol
rule	O
:	O
if	O
a	O
node	O
N	O
uses	O
interface	O
I	O
to	O
transmit	O
to	O
a	O
given	O
destination	O
D	O
,	O
N	O
should	O
not	O
send	O
through	O
I	O
new	O
information	O
about	O
D	O
.	O
</s>
<s>
Whereas	O
under	O
split	B-Protocol
horizon	I-Protocol
N	O
does	O
not	O
send	O
any	O
information	O
through	O
I	O
,	O
under	O
poison	O
reverse	O
node	O
N	O
tells	O
a	O
white-lie	O
.	O
</s>
<s>
With	O
the	O
split-horizon	O
rule	O
in	O
place	O
,	O
this	O
particular	O
loop	O
scenario	O
cannot	O
happen	O
,	O
improving	O
convergence	B-Protocol
time	I-Protocol
in	O
complex	O
,	O
highly-redundant	O
environments	O
.	O
</s>
<s>
Split-horizon	O
routing	O
with	O
poison	O
reverse	O
is	O
a	O
variant	O
of	O
split-horizon	O
route	O
advertising	O
in	O
which	O
a	O
router	O
actively	O
advertises	O
routes	O
as	O
unreachable	O
over	O
the	O
interface	O
over	O
which	O
they	O
were	O
learned	O
by	O
setting	O
the	O
route	O
metric	O
to	O
infinite	O
(	O
16	O
for	O
RIP	B-Protocol
)	O
.	O
</s>
<s>
Split	B-Protocol
horizon	I-Protocol
states	O
that	O
if	O
a	O
neighboring	O
router	O
sends	O
a	O
route	O
to	O
a	O
router	O
,	O
the	O
receiving	O
router	O
will	O
not	O
propagate	O
this	O
route	O
back	O
to	O
the	O
advertising	O
router	O
on	O
the	O
same	O
interface	O
.	O
</s>
<s>
With	O
route	B-Protocol
poisoning	I-Protocol
,	O
when	O
a	O
router	O
detects	O
that	O
one	O
of	O
its	O
connected	O
routes	O
has	O
failed	O
,	O
the	O
router	O
will	O
poison	O
the	O
route	O
by	O
assigning	O
an	O
infinite	O
metric	O
to	O
it	O
and	O
advertising	O
it	O
to	O
neighbors	O
.	O
</s>
<s>
When	O
a	O
router	O
advertises	O
a	O
poisoned	O
route	O
to	O
its	O
neighbors	O
,	O
its	O
neighbors	O
break	O
the	O
rule	O
of	O
split	B-Protocol
horizon	I-Protocol
and	O
send	O
back	O
to	O
the	O
originator	O
the	O
same	O
poisoned	O
route	O
,	O
called	O
a	O
poison	O
reverse	O
.	O
</s>
<s>
In	O
order	O
to	O
give	O
the	O
router	O
enough	O
time	O
to	O
propagate	O
the	O
poisoned	O
route	O
and	O
to	O
ensure	O
that	O
no	O
routing	B-Protocol
loops	I-Protocol
occur	O
while	O
propagation	O
occurs	O
,	O
the	O
routers	O
implement	O
a	O
hold-down	O
mechanism	O
.	O
</s>
<s>
Poison	O
Reverse	O
is	O
often	O
used	O
within	O
distance-vector	B-Protocol
routing	I-Protocol
to	O
solve	O
the	O
count-to-infinity	O
problem	O
.	O
</s>
<s>
Practically	O
,	O
poison	O
reverse	O
can	O
be	O
thought	O
of	O
as	O
an	O
alternative	O
to	O
split	B-Protocol
horizon	I-Protocol
.	O
</s>
<s>
With	O
poison	O
reverse	O
,	O
route	O
advertisements	O
that	O
would	O
be	O
suppressed	O
by	O
split	B-Protocol
horizon	I-Protocol
are	O
instead	O
advertised	O
with	O
a	O
distance	O
of	O
infinity	O
.	O
</s>
<s>
Following	O
this	O
topology	O
,	O
we	O
build	O
the	O
distance	B-Protocol
vectors	I-Protocol
of	O
all	O
nodes	O
in	O
the	O
network	O
:	O
</s>
<s>
The	O
first	O
,	O
second	O
and	O
third	O
lines	O
correspond	O
to	O
node	O
Z	O
,	O
node	O
Y	O
and	O
node	O
X	O
distance	B-Protocol
vectors	I-Protocol
,	O
respectively	O
.	O
</s>
<s>
The	O
poison	O
reverse	O
kicks	O
in	O
when	O
a	O
node	O
broadcasts	O
its	O
distance	B-Protocol
vector	I-Protocol
to	O
its	O
neighbors	O
.	O
</s>
<s>
The	O
distance	B-Protocol
vectors	I-Protocol
broadcast	O
by	O
Z	O
are	O
:	O
</s>
<s>
As	O
we	O
see	O
in	O
the	O
distance	B-Protocol
vector	I-Protocol
that	O
is	O
broadcast	O
to	O
node	O
Y	O
the	O
end	O
destination	O
X	O
has	O
an	O
infinity	O
value	O
.	O
</s>
<s>
This	O
can	O
though	O
be	O
completed	O
with	O
an	O
implementation	O
of	O
a	O
distance	B-Protocol
vector	I-Protocol
protocol	I-Protocol
called	O
RIP	B-Protocol
.	O
</s>
<s>
The	O
split-horizon	O
method	O
is	O
effective	O
and	O
simple	O
to	O
implement	O
,	O
and	O
is	O
therefore	O
used	O
by	O
most	O
distance-vector	B-Protocol
protocols	I-Protocol
.	O
</s>
