<s>
A	O
switching	B-Architecture
loop	I-Architecture
or	O
bridge	B-Architecture
loop	I-Architecture
occurs	O
in	O
computer	B-Architecture
networks	I-Architecture
when	O
there	O
is	O
more	O
than	O
one	O
layer	B-Protocol
2	I-Protocol
path	O
between	O
two	O
endpoints	O
(	O
e.g.	O
</s>
<s>
multiple	O
connections	O
between	O
two	O
network	B-Protocol
switches	I-Protocol
or	O
two	O
ports	O
on	O
the	O
same	O
switch	O
connected	O
to	O
each	O
other	O
)	O
.	O
</s>
<s>
The	O
loop	O
creates	O
broadcast	B-General_Concept
storms	I-General_Concept
as	O
broadcasts	O
and	O
multicasts	O
are	O
forwarded	O
by	O
switches	O
out	O
every	O
port	B-Protocol
,	O
the	O
switch	O
or	O
switches	O
will	O
repeatedly	O
rebroadcast	O
the	O
broadcast	O
messages	O
flooding	O
the	O
network	O
.	O
</s>
<s>
Since	O
the	O
layer-2	B-Protocol
header	O
does	O
not	O
include	O
a	O
time	B-Protocol
to	I-Protocol
live	I-Protocol
(	O
TTL	O
)	O
field	O
,	O
if	O
a	O
frame	O
is	O
sent	O
into	O
a	O
looped	O
topology	O
,	O
it	O
can	O
loop	O
forever	O
.	O
</s>
<s>
A	O
physical	O
topology	O
that	O
contains	O
switching	O
or	O
bridge	B-Architecture
loops	I-Architecture
is	O
attractive	O
for	O
redundancy	O
reasons	O
,	O
yet	O
a	O
switched	O
network	O
must	O
not	O
have	O
loops	O
.	O
</s>
<s>
The	O
solution	O
is	O
to	O
allow	O
physical	O
loops	O
,	O
but	O
create	O
a	O
loop-free	O
logical	O
topology	O
using	O
link	B-Architecture
aggregation	I-Architecture
,	O
shortest	B-Protocol
path	I-Protocol
bridging	I-Protocol
,	O
spanning	B-Protocol
tree	I-Protocol
protocol	I-Protocol
or	O
TRILL	B-Protocol
on	O
the	O
network	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
In	O
the	O
case	O
of	O
broadcast	O
packets	O
over	O
a	O
switching	B-Architecture
loop	I-Architecture
,	O
the	O
situation	O
may	O
develop	O
into	O
a	O
broadcast	B-General_Concept
storm	I-General_Concept
.	O
</s>
<s>
In	O
a	O
very	O
simple	O
example	O
,	O
a	O
switch	O
with	O
three	O
ports	O
A	O
,	O
B	O
,	O
and	O
C	O
has	O
a	O
normal	O
node	B-Protocol
connected	O
to	O
port	B-Protocol
A	O
while	O
ports	O
B	O
and	O
C	O
are	O
connected	O
to	O
each	O
other	O
in	O
a	O
loop	O
.	O
</s>
<s>
Now	O
,	O
when	O
a	O
broadcast	O
frame	O
enters	O
the	O
switch	O
through	O
port	B-Protocol
A	O
,	O
this	O
frame	O
is	O
forwarded	O
to	O
all	O
ports	O
but	O
the	O
source	O
port	B-Protocol
,	O
i.e.	O
</s>
<s>
ports	O
B	O
and	O
C	O
.	O
Both	O
frames	B-General_Concept
exiting	O
ports	O
B	O
and	O
C	O
traverse	O
the	O
loop	O
in	O
opposite	O
directions	O
and	O
reenter	O
the	O
switch	O
through	O
their	O
counterpart	O
port	B-Protocol
.	O
</s>
<s>
The	O
frame	O
received	O
on	O
port	B-Protocol
B	O
is	O
then	O
forwarded	O
to	O
ports	O
A	O
and	O
C	O
,	O
the	O
frame	O
received	O
on	O
port	B-Protocol
C	O
to	O
ports	O
A	O
and	O
B	O
.	O
</s>
<s>
So	O
,	O
the	O
node	B-Protocol
on	O
port	B-Protocol
A	O
receives	O
two	O
copies	O
of	O
its	O
own	O
broadcast	O
frame	O
while	O
the	O
other	O
two	O
copies	O
produced	O
by	O
the	O
loop	O
continue	O
to	O
cycle	O
.	O
</s>
<s>
Eventually	O
,	O
the	O
accumulated	O
broadcasts	O
exhaust	O
the	O
egress	O
capacity	O
of	O
the	O
links	O
,	O
the	O
switch	O
begins	O
dropping	O
frames	B-General_Concept
,	O
and	O
communication	O
across	O
the	O
switch	O
becomes	O
unreliable	O
or	O
even	O
impossible	O
.	O
</s>
<s>
Switching	B-Architecture
loops	I-Architecture
can	O
cause	O
misleading	O
entries	O
in	O
a	O
switch	O
's	O
media	B-Protocol
access	I-Protocol
control	I-Protocol
(	O
MAC	O
)	O
database	O
and	O
can	O
cause	O
endless	O
unicast	O
frames	B-General_Concept
to	O
be	O
broadcast	O
throughout	O
the	O
network	O
.	O
</s>
<s>
A	O
loop	O
can	O
make	O
a	O
switch	O
receive	O
the	O
same	O
broadcast	O
frames	B-General_Concept
on	O
two	O
different	O
ports	O
,	O
and	O
alternatingly	O
associate	O
the	O
sending	O
MAC	O
address	O
with	O
the	O
one	O
or	O
the	O
other	O
port	B-Protocol
.	O
</s>
<s>
It	O
may	O
then	O
incorrectly	O
direct	O
traffic	O
for	O
that	O
MAC	O
address	O
to	O
the	O
wrong	O
port	B-Protocol
,	O
effectively	O
causing	O
this	O
traffic	O
to	O
be	O
lost	O
,	O
and	O
even	O
causing	O
other	O
switches	O
to	O
incorrectly	O
associate	O
the	O
sender	O
's	O
address	O
with	O
a	O
wrong	O
port	B-Protocol
as	O
well	O
.	O
</s>
<s>
Routing	B-Protocol
loops	I-Protocol
are	O
tempered	O
by	O
a	O
time	B-Protocol
to	I-Protocol
live	I-Protocol
(	O
TTL	O
)	O
field	O
in	O
layer-3	B-Application
packet	O
header	O
;	O
Packets	O
will	O
circulate	O
the	O
routing	B-Protocol
loop	I-Protocol
until	O
their	O
TTL	O
value	O
expires	O
.	O
</s>
<s>
No	O
TTL	O
concept	O
exists	O
at	O
layer	B-Protocol
2	I-Protocol
and	O
packets	O
in	O
a	O
switching	B-Architecture
loop	I-Architecture
will	O
circulate	O
until	O
dropped	O
,	O
e.g.	O
</s>
