<s>
Holddown	B-Protocol
works	O
by	O
having	O
each	O
router	B-Protocol
start	O
a	O
timer	O
when	O
they	O
first	O
receive	O
information	O
about	O
a	O
network	O
that	O
is	O
unreachable	O
.	O
</s>
<s>
Until	O
the	O
timer	O
expires	O
,	O
the	O
router	B-Protocol
will	O
discard	O
any	O
subsequent	O
route	O
messages	O
that	O
indicate	O
the	O
route	O
is	O
in	O
fact	O
reachable	O
.	O
</s>
<s>
It	O
can	O
solve	O
the	O
case	O
where	O
multiple	O
routers	B-Protocol
are	O
connected	O
indirectly	O
.	O
</s>
<s>
There	O
are	O
realistic	O
scenarios	O
where	O
split	B-Protocol
horizon	I-Protocol
and	O
split	B-Protocol
horizon	I-Protocol
with	O
poisoned	O
reverse	O
can	O
do	O
nothing	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
a	O
holddown	B-Protocol
keeps	O
a	O
router	B-Protocol
from	O
receiving	O
route	O
updates	O
until	O
the	O
network	O
appears	O
to	O
be	O
stable	O
—	O
until	O
either	O
an	O
interface	B-Application
stops	O
changing	O
state	O
(	O
flapping	O
)	O
or	O
a	O
better	O
route	O
is	O
learned	O
.	O
</s>
<s>
Holddowns	B-Protocol
are	O
usually	O
implemented	O
with	O
timers	O
.	O
</s>
<s>
If	O
the	O
router	B-Protocol
detects	O
that	O
a	O
network	O
is	O
unreachable	O
,	O
the	O
timer	O
is	O
started	O
.	O
</s>
<s>
The	O
router	B-Protocol
will	O
then	O
wait	O
a	O
preset	O
number	O
of	O
seconds	O
until	O
the	O
network	O
is	O
stable	O
.	O
</s>
<s>
When	O
the	O
timer	O
expires	O
,	O
the	O
router	B-Protocol
will	O
then	O
receive	O
its	O
routing	O
updates	O
from	O
other	O
routers	B-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
in	O
RIP	B-Protocol
the	O
default	O
holddown	B-Protocol
timer	I-Protocol
is	O
set	O
to	O
180	O
seconds	O
.	O
</s>
