<s>
Unidirectional	B-Protocol
Link	I-Protocol
Detection	I-Protocol
(	O
UDLD	B-Protocol
)	O
is	O
a	O
data	B-Protocol
link	I-Protocol
layer	I-Protocol
protocol	O
from	O
Cisco	O
Systems	O
to	O
monitor	O
the	O
physical	O
configuration	O
of	O
the	O
cables	O
and	O
detect	O
unidirectional	O
links	O
.	O
</s>
<s>
UDLD	B-Protocol
complements	O
the	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
which	O
is	O
used	O
to	O
eliminate	O
switching	B-Architecture
loops	I-Architecture
.	O
</s>
<s>
Unidirectional	B-Protocol
Link	I-Protocol
Detection	I-Protocol
(	O
UDLD	B-Protocol
)	O
is	O
one	O
of	O
two	O
major	O
features	O
(	O
UDLD	B-Protocol
and	O
loop	O
guard	O
)	O
in	O
Cisco	O
Switches	O
to	O
prevent	O
Layer	B-Protocol
2	I-Protocol
loops	O
.	O
</s>
<s>
Spanning-Tree	B-Protocol
Protocol	I-Protocol
(	O
STP	O
)	O
resolves	O
redundant	O
physical	O
topology	O
into	O
a	O
loop-free	O
,	O
tree-like	O
forwarding	O
topology	O
via	O
blocking	O
one	O
or	O
more	O
ports	B-Protocol
.	O
</s>
<s>
However	O
,	O
Unidirectional	O
Link	O
failure	O
can	O
cause	O
"	O
traffic	B-Protocol
blackholing	I-Protocol
"	O
and	O
loops	O
in	O
the	O
Switch	O
topology	O
.	O
</s>
<s>
In	O
order	O
to	O
detect	O
the	O
unidirectional	O
links	O
before	O
the	O
forwarding	O
loop	O
is	O
created	O
,	O
UDLD	B-Protocol
works	O
by	O
exchanging	O
protocol	O
packets	B-Protocol
between	O
the	O
neighboring	O
devices	O
.	O
</s>
<s>
In	O
order	O
for	O
UDLD	B-Protocol
to	O
work	O
,	O
both	O
switch	O
devices	O
on	O
the	O
link	O
must	O
support	O
UDLD	B-Protocol
and	O
have	O
it	O
enabled	O
on	O
respective	O
ports	B-Protocol
.	O
</s>
<s>
If	O
two	O
devices	O
,	O
A	O
and	O
B	O
,	O
are	O
connected	O
via	O
a	O
pair	O
of	O
optical	B-Architecture
fibers	I-Architecture
,	O
one	O
used	O
for	O
sending	O
from	O
A	O
to	O
B	O
and	O
other	O
for	O
sending	O
from	O
B	O
to	O
A	O
,	O
the	O
link	O
is	O
bidirectional	O
(	O
two-way	O
)	O
.	O
</s>
<s>
The	O
goal	O
of	O
the	O
UDLD	B-Protocol
protocol	O
is	O
to	O
detect	O
a	O
broken	O
bidirectional	O
link	O
(	O
e.g.	O
</s>
<s>
transmitted	O
packets	B-Protocol
do	O
not	O
arrive	O
at	O
the	O
receiver	O
,	O
or	O
the	O
fibers	O
are	O
connected	O
to	O
different	O
ports	B-Protocol
)	O
.	O
</s>
<s>
For	O
each	O
device	O
and	O
for	O
each	O
port	O
,	O
a	O
UDLD	B-Protocol
packet	B-Protocol
is	O
sent	O
to	O
the	O
port	O
it	O
links	O
to	O
.	O
</s>
<s>
The	O
packet	B-Protocol
contains	O
sender	O
identity	O
information	O
(	O
device	O
and	O
port	O
)	O
,	O
and	O
expected	O
receiver	O
identity	O
information	O
(	O
device	O
and	O
port	O
)	O
.	O
</s>
<s>
Each	O
port	O
checks	O
that	O
the	O
UDLD	B-Protocol
packets	B-Protocol
it	O
receives	O
contain	O
the	O
identifiers	O
of	O
his	O
own	O
device	O
and	O
port	O
.	O
</s>
<s>
UDLD	B-Protocol
is	O
a	O
Cisco-proprietary	O
protocol	O
but	O
HP	B-Device
,	O
Extreme	O
Networks	O
,	O
and	O
AVAYA	O
all	O
have	O
a	O
similar	O
feature	O
calling	O
it	O
by	O
a	O
different	O
name	O
.	O
</s>
<s>
HP	B-Device
calls	O
theirs	O
Device	O
Link	O
Detection	O
Protocol	O
(	O
DLDP	O
)	O
.	O
</s>
<s>
Brocade/Ruckus	O
Networks	O
ICX	O
Switches	O
offer	O
this	O
feature	O
as	O
Uni-Directional	O
Link	O
Detection(UDLD )	O
.	O
</s>
<s>
Similar	O
functionality	O
in	O
a	O
standardized	O
form	O
is	O
provided	O
as	O
part	O
of	O
the	O
Ethernet	O
OAM	B-Architecture
protocol	O
that	O
is	O
defined	O
as	O
part	O
of	O
the	O
Ethernet	B-Protocol
in	I-Protocol
the	I-Protocol
First	I-Protocol
Mile	I-Protocol
changes	O
to	O
802.3	O
(	O
previously	O
802.3ah	B-Protocol
)	O
.	O
</s>
<s>
D-Link	B-General_Concept
has	O
their	O
DULD	O
feature	O
built	O
on	O
top	O
of	O
Ethernet	O
OAM	B-Architecture
function	O
.	O
</s>
<s>
Brocade	O
devices	O
running	O
Ironware	O
support	O
a	O
proprietary	O
form	O
of	O
UDLD	B-Protocol
.	O
</s>
<s>
The	O
use	O
of	O
UDLD	B-Protocol
over	O
10GbE	O
is	O
augmented	O
,	O
as	O
per	O
802.3ae/D3.2	O
standard	O
,	O
when	O
a	O
fault	O
is	O
detected	O
in	O
the	O
physical	O
link	O
:	O
</s>
