<s>
The	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
STP	O
)	O
is	O
a	O
network	O
protocol	O
that	O
builds	O
a	O
loop-free	O
logical	B-Architecture
topology	I-Architecture
for	O
Ethernet	O
networks	O
.	O
</s>
<s>
The	O
basic	O
function	O
of	O
STP	O
is	O
to	O
prevent	O
bridge	B-Architecture
loops	I-Architecture
and	O
the	O
broadcast	B-General_Concept
radiation	I-General_Concept
that	O
results	O
from	O
them	O
.	O
</s>
<s>
Spanning	O
tree	O
also	O
allows	O
a	O
network	B-Architecture
design	I-Architecture
to	O
include	O
backup	O
links	O
providing	O
fault	B-General_Concept
tolerance	I-General_Concept
if	O
an	O
active	O
link	O
fails	O
.	O
</s>
<s>
As	O
the	O
name	O
suggests	O
,	O
STP	O
creates	O
a	O
spanning	O
tree	O
that	O
characterizes	O
the	O
relationship	O
of	O
nodes	O
within	O
a	O
network	O
of	O
connected	O
layer-2	O
bridges	B-Protocol
,	O
and	O
disables	O
those	O
links	O
that	O
are	O
not	O
part	O
of	O
the	O
spanning	O
tree	O
,	O
leaving	O
a	O
single	O
active	O
path	O
between	O
any	O
two	O
network	O
nodes	O
.	O
</s>
<s>
In	O
2001	O
,	O
the	O
IEEE	O
introduced	O
Rapid	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
RSTP	O
)	O
as	O
802.1w	O
.	O
</s>
<s>
RSTP	O
provides	O
significantly	O
faster	O
recovery	O
in	O
response	O
to	O
network	O
changes	O
or	O
failures	O
,	O
introducing	O
new	O
convergence	O
behaviors	O
and	O
bridge	B-Protocol
port	O
roles	O
to	O
do	O
this	O
.	O
</s>
<s>
STP	O
was	O
originally	O
standardized	O
as	O
IEEE	O
802.1D	O
but	O
the	O
functionality	O
of	O
spanning	O
tree	O
(	O
802.1D	O
)	O
,	O
rapid	O
spanning	O
tree	O
(	O
802.1w	O
)	O
,	O
and	O
multiple	B-Protocol
spanning	I-Protocol
tree	I-Protocol
(	O
802.1s	O
)	O
has	O
since	O
been	O
incorporated	O
into	O
IEEE	O
802.1Q-2014	O
.	O
</s>
<s>
While	O
STP	O
is	O
still	O
in	O
use	O
today	O
,	O
in	O
most	O
modern	O
networks	O
its	O
primary	O
use	O
is	O
as	O
a	O
loop-protection	O
mechanism	O
rather	O
than	O
a	O
fault	B-General_Concept
tolerance	I-General_Concept
mechanism	O
.	O
</s>
<s>
Link	B-Architecture
aggregation	I-Architecture
protocols	O
such	O
as	O
LACP	O
will	O
bond	O
two	O
or	O
more	O
links	O
to	O
provide	O
fault	B-General_Concept
tolerance	I-General_Concept
while	O
simultaneously	O
increasing	O
overall	O
link	O
capacity	O
.	O
</s>
<s>
The	O
need	O
for	O
the	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
STP	O
)	O
arose	O
because	O
switches	B-Protocol
in	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
(	O
LANs	B-General_Concept
)	O
are	O
often	O
interconnected	O
using	O
redundant	O
links	O
to	O
improve	O
resilience	O
should	O
one	O
connection	O
fail	O
.	O
</s>
<s>
However	O
,	O
this	O
connection	O
configuration	O
creates	O
a	O
switching	B-Architecture
loop	I-Architecture
resulting	O
in	O
broadcast	B-General_Concept
radiations	I-General_Concept
and	O
MAC	B-Protocol
table	I-Protocol
instability	O
.	O
</s>
<s>
If	O
redundant	O
links	O
are	O
used	O
to	O
connect	O
switches	B-Protocol
,	O
then	O
switching	B-Architecture
loops	I-Architecture
need	O
to	O
be	O
avoided	O
.	O
</s>
<s>
To	O
avoid	O
the	O
problems	O
associated	O
with	O
redundant	O
links	O
in	O
a	O
switched	B-Protocol
LAN	B-General_Concept
,	O
STP	O
is	O
implemented	O
on	O
switches	B-Protocol
to	O
monitor	O
the	O
network	B-Architecture
topology	I-Architecture
.	O
</s>
<s>
Every	O
link	O
between	O
switches	B-Protocol
,	O
and	O
in	O
particular	O
redundant	O
links	O
,	O
are	O
catalogued	O
.	O
</s>
<s>
The	O
spanning-tree	B-Protocol
algorithm	O
then	O
blocks	O
forwarding	O
on	O
redundant	O
links	O
by	O
setting	O
up	O
one	O
preferred	O
link	O
between	O
switches	B-Protocol
in	O
the	O
LAN	B-General_Concept
.	O
</s>
<s>
When	O
implemented	O
in	O
a	O
network	O
,	O
STP	O
designates	O
one	O
layer-2	O
switch	B-Protocol
as	O
root	O
bridge	B-Protocol
.	O
</s>
<s>
All	O
switches	B-Protocol
then	O
select	O
their	O
best	O
connection	O
towards	O
the	O
root	O
bridge	B-Protocol
for	O
forwarding	O
and	O
block	O
other	O
redundant	O
links	O
.	O
</s>
<s>
All	O
switches	B-Protocol
constantly	O
communicate	O
with	O
their	O
neighbors	O
in	O
the	O
LAN	B-General_Concept
using	O
bridge	B-Protocol
protocol	O
data	O
units	O
(	O
BPDUs	O
)	O
.	O
</s>
<s>
Provided	O
there	O
is	O
more	O
than	O
one	O
link	O
between	O
two	O
switches	B-Protocol
,	O
the	O
STP	O
root	O
bridge	B-Protocol
calculates	O
the	O
cost	O
of	O
each	O
path	O
based	O
on	O
bandwidth	O
.	O
</s>
<s>
STP	O
will	O
enable	O
this	O
preferred	O
link	O
as	O
the	O
only	O
path	O
to	O
be	O
used	O
for	O
Ethernet	O
frames	O
between	O
the	O
two	O
switches	B-Protocol
,	O
and	O
disable	O
all	O
other	O
possible	O
links	O
by	O
designating	O
the	O
switch	B-Protocol
ports	I-Protocol
that	O
connect	O
the	O
preferred	O
path	O
as	O
root	O
port	O
.	O
</s>
<s>
After	O
STP	O
enabled	O
switches	B-Protocol
in	O
a	O
LAN	B-General_Concept
have	O
elected	O
the	O
root	O
bridge	B-Protocol
,	O
all	O
non-root	O
bridges	B-Protocol
assign	O
one	O
of	O
their	O
ports	O
as	O
root	O
port	O
.	O
</s>
<s>
This	O
is	O
either	O
the	O
port	O
that	O
connects	O
the	O
switch	B-Protocol
to	O
the	O
root	O
bridge	B-Protocol
,	O
or	O
if	O
there	O
are	O
several	O
paths	O
,	O
the	O
port	O
with	O
the	O
preferred	O
path	O
as	O
calculated	O
by	O
the	O
root	O
bridge	B-Protocol
.	O
</s>
<s>
Because	O
not	O
all	O
switches	B-Protocol
are	O
directly	O
connected	O
to	O
the	O
root	O
bridge	B-Protocol
they	O
communicate	O
amongst	O
each	O
other	O
using	O
STP	O
BPDUs	O
.	O
</s>
<s>
Each	O
switch	B-Protocol
adds	O
the	O
cost	O
of	O
its	O
own	O
path	O
to	O
the	O
cost	O
received	O
from	O
the	O
neighboring	O
switches	B-Protocol
to	O
determine	O
the	O
total	O
cost	O
of	O
a	O
given	O
path	O
to	O
the	O
root	O
bridge	B-Protocol
.	O
</s>
<s>
Once	O
the	O
cost	O
of	O
all	O
possible	O
paths	O
to	O
the	O
root	O
bridge	B-Protocol
have	O
been	O
added	O
up	O
,	O
each	O
switch	B-Protocol
assigns	O
a	O
port	O
as	O
root	O
port	O
which	O
connects	O
to	O
the	O
path	O
with	O
the	O
lowest	O
cost	O
,	O
or	O
highest	O
bandwidth	O
,	O
that	O
will	O
eventually	O
lead	O
to	O
the	O
root	O
bridge	B-Protocol
.	O
</s>
<s>
All	O
switch	B-Protocol
ports	I-Protocol
in	O
the	O
LAN	B-General_Concept
where	O
STP	O
is	O
enabled	O
are	O
categorized	O
.	O
</s>
<s>
Blocking	O
A	O
port	O
that	O
would	O
cause	O
a	O
switching	B-Architecture
loop	I-Architecture
if	O
it	O
were	O
active	O
.	O
</s>
<s>
Listening	O
The	O
switch	B-Protocol
processes	O
BPDUs	O
and	O
awaits	O
possible	O
new	O
information	O
that	O
would	O
cause	O
it	O
to	O
return	O
to	O
the	O
blocking	O
state	O
.	O
</s>
<s>
It	O
does	O
not	O
populate	O
the	O
MAC	B-Protocol
table	I-Protocol
and	O
it	O
does	O
not	O
forward	O
frames	O
.	O
</s>
<s>
Learning	O
While	O
the	O
port	O
does	O
not	O
yet	O
forward	O
frames	O
,	O
it	O
does	O
learn	O
source	O
addresses	O
from	O
frames	O
received	O
and	O
adds	O
them	O
to	O
the	O
MAC	B-Protocol
table	I-Protocol
.	O
</s>
<s>
Disabled	O
A	O
network	O
administrator	O
has	O
manually	O
disabled	O
the	O
switch	B-Protocol
port	I-Protocol
.	O
</s>
<s>
When	O
a	O
device	O
is	O
first	O
attached	O
to	O
a	O
switch	B-Protocol
port	I-Protocol
,	O
it	O
will	O
not	O
immediately	O
start	O
to	O
forward	O
data	O
.	O
</s>
<s>
It	O
will	O
instead	O
go	O
through	O
a	O
number	O
of	O
states	O
while	O
it	O
processes	O
BPDUs	O
and	O
determines	O
the	O
topology	B-Architecture
of	O
the	O
network	O
.	O
</s>
<s>
The	O
port	O
attached	O
to	O
a	O
host	O
such	O
as	O
a	O
computer	O
,	O
printer	O
or	O
server	B-Application
always	O
goes	O
into	O
the	O
forwarding	O
state	O
,	O
albeit	O
after	O
a	O
delay	O
of	O
about	O
30	O
seconds	O
while	O
it	O
goes	O
through	O
the	O
listening	O
and	O
learning	O
states	O
.	O
</s>
<s>
The	O
time	O
spent	O
in	O
the	O
listening	O
and	O
learning	O
states	O
is	O
determined	O
by	O
a	O
value	O
known	O
as	O
the	O
forward	O
delay	O
(	O
default	O
15	O
seconds	O
and	O
set	O
by	O
the	O
root	O
bridge	B-Protocol
)	O
.	O
</s>
<s>
If	O
another	O
switch	B-Protocol
is	O
connected	O
,	O
the	O
port	O
may	O
remain	O
in	O
blocking	O
mode	O
if	O
it	O
is	O
determined	O
that	O
it	O
would	O
cause	O
a	O
loop	O
in	O
the	O
network	O
.	O
</s>
<s>
Topology	B-Architecture
change	O
notification	O
(	O
TCN	O
)	O
BPDUs	O
are	O
used	O
to	O
inform	O
other	O
switches	B-Protocol
of	O
port	O
changes	O
.	O
</s>
<s>
TCNs	O
are	O
injected	O
into	O
the	O
network	O
by	O
a	O
non-root	O
switch	B-Protocol
and	O
propagated	O
to	O
the	O
root	O
.	O
</s>
<s>
Upon	O
receipt	O
of	O
the	O
TCN	O
,	O
the	O
root	O
switch	B-Protocol
will	O
set	O
the	O
topology	B-Architecture
change	O
flag	O
in	O
its	O
normal	O
BPDUs	O
.	O
</s>
<s>
This	O
flag	O
is	O
propagated	O
to	O
all	O
other	O
switches	B-Protocol
and	O
instructs	O
them	O
to	O
rapidly	O
age	O
out	O
their	O
forwarding	B-Protocol
table	I-Protocol
entries	O
.	O
</s>
<s>
Before	O
configuring	O
STP	O
,	O
the	O
network	B-Architecture
topology	I-Architecture
should	O
be	O
carefully	O
planned	O
.	O
</s>
<s>
Basic	O
configuration	O
requires	O
that	O
STP	O
be	O
enabled	O
on	O
all	O
switches	B-Protocol
in	O
the	O
LAN	B-General_Concept
and	O
the	O
same	O
version	O
of	O
STP	O
chosen	O
on	O
each	O
.	O
</s>
<s>
The	O
administrator	O
may	O
determine	O
which	O
switch	B-Protocol
will	O
be	O
the	O
root	O
bridge	B-Protocol
and	O
configure	O
the	O
switches	B-Protocol
appropriately	O
.	O
</s>
<s>
If	O
the	O
root	O
bridge	B-Protocol
goes	O
down	O
,	O
the	O
protocol	O
will	O
automatically	O
assign	O
a	O
new	O
root	O
bridge	B-Protocol
based	O
on	O
bridge	B-Protocol
ID	I-Protocol
.	O
</s>
<s>
If	O
all	O
switches	B-Protocol
have	O
the	O
same	O
bridge	B-Protocol
ID	I-Protocol
,	O
such	O
as	O
the	O
default	O
ID	O
,	O
and	O
the	O
root	O
bridge	B-Protocol
goes	O
down	O
,	O
a	O
tie	O
situation	O
arises	O
and	O
the	O
protocol	O
will	O
assign	O
one	O
switch	B-Protocol
as	O
root	O
bridge	B-Protocol
based	O
on	O
the	O
switch	B-Protocol
MAC	O
addresses	O
.	O
</s>
<s>
Once	O
the	O
switches	B-Protocol
have	O
been	O
assigned	O
a	O
bridge	B-Protocol
ID	I-Protocol
and	O
the	O
protocol	O
has	O
chosen	O
the	O
root	O
bridge	B-Protocol
switch	B-Protocol
,	O
the	O
best	O
path	O
to	O
the	O
root	O
bridge	B-Protocol
is	O
calculated	O
based	O
on	O
port	O
cost	O
,	O
path	O
cost	O
and	O
port	O
priority	O
.	O
</s>
<s>
Ultimately	O
STP	O
calculates	O
the	O
path	O
cost	O
on	O
the	O
basis	O
of	O
the	O
bandwidth	O
of	O
a	O
link	O
,	O
however	O
links	O
between	O
switches	B-Protocol
may	O
have	O
the	O
same	O
bandwidth	O
.	O
</s>
<s>
The	O
selection	O
of	O
how	O
other	O
switches	B-Protocol
in	O
the	O
topology	B-Architecture
choose	O
their	O
root	O
port	O
,	O
or	O
the	O
least	O
cost	O
path	O
to	O
the	O
root	O
bridge	B-Protocol
,	O
can	O
be	O
influenced	O
by	O
the	O
port	O
priority	O
.	O
</s>
<s>
If	O
all	O
ports	O
of	O
a	O
switch	B-Protocol
have	O
the	O
same	O
priority	O
,	O
the	O
port	O
with	O
the	O
lowest	O
number	O
is	O
chosen	O
to	O
forward	O
frames	O
.	O
</s>
<s>
The	O
root	O
bridge	B-Protocol
of	O
the	O
spanning	O
tree	O
is	O
the	O
bridge	B-Protocol
with	O
the	O
smallest	O
(	O
lowest	O
)	O
bridge	B-Protocol
ID	I-Protocol
.	O
</s>
<s>
Each	O
bridge	B-Protocol
has	O
a	O
configurable	O
priority	O
number	O
and	O
a	O
MAC	O
address	O
;	O
the	O
bridge	B-Protocol
ID	I-Protocol
is	O
the	O
concatenation	O
of	O
the	O
bridge	B-Protocol
priority	O
and	O
the	O
MAC	O
address	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
ID	O
of	O
a	O
bridge	B-Protocol
with	O
priority	O
32,768	O
and	O
MAC	O
is	O
.	O
</s>
<s>
The	O
bridge	B-Protocol
priority	O
default	O
is	O
32,768	O
and	O
can	O
be	O
configured	O
only	O
in	O
multiples	O
of	O
4096	O
.	O
</s>
<s>
When	O
comparing	O
two	O
bridge	B-Protocol
IDs	I-Protocol
,	O
the	O
priority	O
portions	O
are	O
compared	O
first	O
and	O
the	O
MAC	O
addresses	O
are	O
compared	O
only	O
if	O
the	O
priorities	O
are	O
equal	O
.	O
</s>
<s>
The	O
switch	B-Protocol
with	O
the	O
lowest	O
priority	O
of	O
all	O
the	O
switches	B-Protocol
will	O
be	O
the	O
root	O
;	O
if	O
there	O
is	O
a	O
tie	O
,	O
then	O
the	O
switch	B-Protocol
with	O
the	O
lowest	O
priority	O
and	O
lowest	O
MAC	O
address	O
will	O
be	O
the	O
root	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
switches	B-Protocol
A	O
(	O
MAC	O
=	O
)	O
and	O
B	O
(	O
MAC	O
=	O
)	O
both	O
have	O
a	O
priority	O
of	O
32,768	O
then	O
switch	B-Protocol
A	O
will	O
be	O
selected	O
as	O
the	O
root	O
bridge	B-Protocol
.	O
</s>
<s>
If	O
the	O
network	O
administrators	O
would	O
like	O
switch	B-Protocol
B	O
to	O
become	O
the	O
root	O
bridge	B-Protocol
,	O
they	O
must	O
set	O
its	O
priority	O
to	O
be	O
less	O
than	O
32,768	O
.	O
</s>
<s>
Lowest	O
root	O
bridge	B-Protocol
ID	I-Protocol
(	O
BID	O
)	O
-	O
Determines	O
the	O
root	O
bridge	B-Protocol
.	O
</s>
<s>
Lowest	O
sender	O
port	O
ID	O
-	O
Serves	O
as	O
a	O
tiebreaker	O
if	O
a	O
switch	B-Protocol
has	O
multiple	O
(	O
non-EtherChannel	O
)	O
links	O
to	O
a	O
single	O
upstream	O
switch	B-Protocol
,	O
where	O
:	O
</s>
<s>
When	O
multiple	O
paths	O
from	O
a	O
bridge	B-Protocol
are	O
least-cost	O
paths	O
,	O
the	O
chosen	O
path	O
uses	O
the	O
neighbor	O
bridge	B-Protocol
with	O
the	O
lower	O
bridge	B-Protocol
ID	I-Protocol
.	O
</s>
<s>
The	O
root	O
port	O
is	O
thus	O
the	O
one	O
connecting	O
to	O
the	O
bridge	B-Protocol
with	O
the	O
lowest	O
bridge	B-Protocol
ID	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
figures	O
,	O
if	O
switch	B-Protocol
4	O
were	O
connected	O
to	O
network	O
segment	O
d	O
instead	O
of	O
segment	O
f	O
,	O
there	O
would	O
be	O
two	O
paths	O
of	O
length	O
2	O
to	O
the	O
root	O
,	O
one	O
path	O
going	O
through	O
bridge	B-Protocol
24	O
and	O
the	O
other	O
through	O
bridge	B-Protocol
92	O
.	O
</s>
<s>
Because	O
there	O
are	O
two	O
least-cost	O
paths	O
,	O
the	O
lower	O
bridge	B-Protocol
ID	I-Protocol
(	O
24	O
)	O
would	O
be	O
used	O
as	O
the	O
tiebreaker	O
in	O
choosing	O
which	O
path	O
to	O
use	O
.	O
</s>
<s>
When	O
more	O
than	O
one	O
bridge	B-Protocol
on	O
a	O
segment	O
leads	O
to	O
a	O
least-cost	O
path	O
to	O
the	O
root	O
,	O
the	O
bridge	B-Protocol
with	O
the	O
lower	O
bridge	B-Protocol
ID	I-Protocol
is	O
used	O
to	O
forward	O
messages	O
to	O
the	O
root	O
.	O
</s>
<s>
The	O
port	O
attaching	O
that	O
bridge	B-Protocol
to	O
the	O
network	O
segment	O
is	O
the	O
designated	O
port	O
for	O
the	O
segment	O
.	O
</s>
<s>
In	O
the	O
figures	O
,	O
there	O
are	O
two	O
least-cost	O
paths	O
from	O
network	O
segment	O
d	O
to	O
the	O
root	O
,	O
one	O
going	O
through	O
bridge	B-Protocol
24	O
and	O
the	O
other	O
through	O
bridge	B-Protocol
92	O
.	O
</s>
<s>
The	O
lower	O
bridge	B-Protocol
ID	I-Protocol
is	O
24	O
,	O
so	O
the	O
tiebreaker	O
dictates	O
that	O
the	O
designated	O
port	O
is	O
the	O
port	O
through	O
which	O
network	O
segment	O
d	O
is	O
connected	O
to	O
bridge	B-Protocol
24	O
.	O
</s>
<s>
If	O
bridge	B-Protocol
IDs	I-Protocol
were	O
equal	O
,	O
then	O
the	O
bridge	B-Protocol
with	O
the	O
lowest	O
MAC	O
address	O
would	O
have	O
the	O
designated	O
port	O
.	O
</s>
<s>
When	O
the	O
root	O
bridge	B-Protocol
has	O
more	O
than	O
one	O
port	O
on	O
a	O
single	O
LAN	B-General_Concept
segment	O
,	O
the	O
bridge	B-Protocol
ID	I-Protocol
is	O
effectively	O
tied	O
,	O
as	O
are	O
all	O
root	O
path	O
costs	O
(	O
all	O
equal	O
zero	O
)	O
.	O
</s>
<s>
The	O
port	O
on	O
that	O
LAN	B-General_Concept
segment	O
with	O
the	O
lowest	O
port	O
ID	O
becomes	O
the	O
designated	O
port	O
.	O
</s>
<s>
It	O
is	O
put	O
into	O
forwarding	O
mode	O
while	O
all	O
other	O
ports	O
on	O
the	O
root	O
bridge	B-Protocol
on	O
that	O
same	O
LAN	B-General_Concept
segment	O
become	O
non-designated	O
ports	O
and	O
are	O
put	O
into	O
blocking	O
mode	O
.	O
</s>
<s>
Not	O
all	O
bridge	B-Protocol
manufacturers	O
follow	O
this	O
rule	O
,	O
instead	O
making	O
all	O
root	O
bridge	B-Protocol
ports	O
designated	O
ports	O
,	O
and	O
putting	O
them	O
all	O
in	O
forwarding	O
mode	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
there	O
may	O
still	O
be	O
a	O
tie	O
,	O
as	O
when	O
the	O
root	O
bridge	B-Protocol
has	O
multiple	O
active	O
ports	O
on	O
the	O
same	O
LAN	B-General_Concept
segment	O
(	O
see	O
above	O
)	O
with	O
equally	O
low	O
root	O
path	O
costs	O
and	O
bridge	B-Protocol
IDs	I-Protocol
,	O
or	O
,	O
in	O
other	O
cases	O
,	O
multiple	O
bridges	B-Protocol
are	O
connected	O
by	O
multiple	O
cables	O
and	O
multiple	O
ports	O
.	O
</s>
<s>
In	O
each	O
case	O
,	O
a	O
single	O
bridge	B-Protocol
may	O
have	O
multiple	O
candidates	O
for	O
its	O
root	O
port	O
.	O
</s>
<s>
the	O
"	O
best	O
"	O
)	O
bridge	B-Protocol
IDs	I-Protocol
,	O
and	O
the	O
final	O
tiebreaker	O
goes	O
to	O
the	O
port	O
that	O
received	O
the	O
lowest	O
(	O
i.e.	O
</s>
<s>
The	O
bridges	B-Protocol
have	O
to	O
determine	O
the	O
root	O
bridge	B-Protocol
and	O
compute	O
the	O
port	O
roles	O
(	O
root	O
,	O
designated	O
,	O
or	O
blocked	O
)	O
with	O
only	O
the	O
information	O
that	O
they	O
have	O
.	O
</s>
<s>
To	O
ensure	O
that	O
each	O
bridge	B-Protocol
has	O
enough	O
information	O
,	O
the	O
bridges	B-Protocol
use	O
special	O
data	O
frames	O
called	O
bridge	B-Protocol
protocol	O
data	O
units	O
(	O
BPDU	O
)	O
to	O
exchange	O
information	O
about	O
bridge	B-Protocol
IDs	I-Protocol
and	O
root	O
path	O
costs	O
.	O
</s>
<s>
A	O
bridge	B-Protocol
sends	O
a	O
BPDU	O
frame	O
using	O
the	O
unique	O
MAC	O
address	O
of	O
the	O
port	O
itself	O
as	O
a	O
source	O
address	O
,	O
and	O
a	O
destination	O
address	O
of	O
the	O
STP	O
multicast	B-Protocol
address	I-Protocol
.	O
</s>
<s>
BPDUs	O
are	O
exchanged	O
regularly	O
(	O
every	O
2	O
seconds	O
by	O
default	O
)	O
and	O
enable	O
switches	B-Protocol
to	O
keep	O
track	O
of	O
network	O
changes	O
and	O
to	O
start	O
and	O
stop	O
forwarding	O
at	O
ports	O
as	O
required	O
.	O
</s>
<s>
To	O
prevent	O
the	O
delay	O
when	O
connecting	O
hosts	O
to	O
a	O
switch	B-Protocol
and	O
during	O
some	O
topology	B-Architecture
changes	O
,	O
Rapid	O
STP	O
was	O
developed	O
,	O
which	O
allows	O
a	O
switch	B-Protocol
port	I-Protocol
to	O
rapidly	O
transition	O
into	O
the	O
forwarding	O
state	O
during	O
these	O
situations	O
.	O
</s>
<s>
IEEE	O
802.1D	O
and	O
IEEE	B-Protocol
802.1aq	I-Protocol
BPDUs	O
have	O
the	O
following	O
format	O
:	O
</s>
<s>
The	O
first	O
spanning	B-Protocol
tree	I-Protocol
protocol	I-Protocol
was	O
invented	O
in	O
1985	O
at	O
the	O
Digital	O
Equipment	O
Corporation	O
by	O
Radia	O
Perlman	O
.	O
</s>
<s>
The	O
original	O
Perlman-inspired	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
,	O
called	O
DEC	O
STP	O
,	O
is	O
not	O
a	O
standard	O
and	O
differs	O
from	O
the	O
IEEE	O
version	O
in	O
message	O
format	O
as	O
well	O
as	O
timer	O
settings	O
.	O
</s>
<s>
Some	O
bridges	B-Protocol
implement	O
both	O
the	O
IEEE	O
and	O
the	O
DEC	O
versions	O
of	O
the	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
,	O
but	O
their	O
interworking	O
can	O
create	O
issues	O
for	O
the	O
network	O
administrator	O
.	O
</s>
<s>
In	O
2001	O
,	O
the	O
IEEE	O
introduced	O
Rapid	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
RSTP	O
)	O
as	O
IEEE	O
802.1w	O
.	O
</s>
<s>
RSTP	O
provides	O
significantly	O
faster	O
spanning	O
tree	O
convergence	O
after	O
a	O
topology	B-Architecture
change	O
,	O
introducing	O
new	O
convergence	O
behaviors	O
and	O
bridge	B-Protocol
port	O
roles	O
to	O
accomplish	O
this	O
.	O
</s>
<s>
While	O
STP	O
can	O
take	O
30	O
to	O
50	O
seconds	O
to	O
respond	O
to	O
a	O
topology	B-Architecture
change	O
,	O
RSTP	O
is	O
typically	O
able	O
to	O
respond	O
to	O
changes	O
within	O
3×hello	O
times	O
(	O
default	O
:	O
3	O
2	O
seconds	O
)	O
or	O
within	O
a	O
few	O
milliseconds	O
of	O
a	O
physical	O
link	O
failure	O
.	O
</s>
<s>
RSTP	O
adds	O
new	O
bridge	B-Protocol
port	O
roles	O
in	O
order	O
to	O
speed	O
convergence	O
following	O
a	O
link	O
failure	O
:	O
</s>
<s>
Alternate	O
-	O
An	O
alternate	O
path	O
to	O
the	O
root	O
bridge	B-Protocol
.	O
</s>
<s>
The	O
number	O
of	O
switch	B-Protocol
port	I-Protocol
states	O
a	O
port	O
can	O
be	O
in	O
has	O
been	O
reduced	O
to	O
three	O
instead	O
of	O
STP	O
's	O
original	O
five	O
:	O
</s>
<s>
Detection	O
of	O
root	O
switch	B-Protocol
failure	O
is	O
done	O
in	O
3	O
hello	O
times	O
,	O
which	O
is	O
6	O
seconds	O
if	O
the	O
default	O
hello	O
times	O
have	O
not	O
been	O
changed	O
.	O
</s>
<s>
Ports	O
may	O
be	O
configured	O
as	O
edge	O
ports	O
if	O
they	O
are	O
attached	O
to	O
a	O
LAN	B-General_Concept
that	O
has	O
no	O
other	O
bridges	B-Protocol
attached	O
.	O
</s>
<s>
RSTP	O
still	O
continues	O
to	O
monitor	O
the	O
port	O
for	O
BPDUs	O
in	O
case	O
a	O
bridge	B-Protocol
is	O
connected	O
.	O
</s>
<s>
As	O
soon	O
as	O
the	O
bridge	B-Protocol
detects	O
a	O
BPDU	O
coming	O
to	O
an	O
edge	O
port	O
,	O
the	O
port	O
becomes	O
a	O
non-edge	O
port	O
.	O
</s>
<s>
RSTP	O
calls	O
the	O
connection	O
between	O
two	O
or	O
more	O
switches	B-Protocol
as	O
a	O
"	O
link-type	O
"	O
connection	O
.	O
</s>
<s>
RSTP	O
improves	O
convergence	O
on	O
point-to-point	O
links	O
by	O
reducing	O
the	O
Max-Age	O
time	O
to	O
3	O
times	O
Hello	O
interval	O
,	O
removing	O
the	O
STP	O
listening	O
state	O
,	O
and	O
exchanging	O
a	O
handshake	O
between	O
two	O
switches	B-Protocol
to	O
quickly	O
transition	O
the	O
port	O
to	O
forwarding	O
state	O
.	O
</s>
<s>
Unlike	O
in	O
STP	O
,	O
RSTP	O
will	O
respond	O
to	O
BPDUs	O
sent	O
from	O
the	O
direction	O
of	O
the	O
root	O
bridge	B-Protocol
.	O
</s>
<s>
An	O
RSTP	O
bridge	B-Protocol
will	O
propose	O
its	O
spanning	O
tree	O
information	O
to	O
its	O
designated	O
ports	O
.	O
</s>
<s>
If	O
another	O
RSTP	O
bridge	B-Protocol
receives	O
this	O
information	O
and	O
determines	O
this	O
is	O
the	O
superior	O
root	O
information	O
,	O
it	O
sets	O
all	O
its	O
other	O
ports	O
to	O
discarding	O
.	O
</s>
<s>
The	O
bridge	B-Protocol
may	O
send	O
an	O
agreement	O
to	O
the	O
first	O
bridge	B-Protocol
confirming	O
its	O
superior	O
spanning	O
tree	O
information	O
.	O
</s>
<s>
The	O
first	O
bridge	B-Protocol
,	O
upon	O
receiving	O
this	O
agreement	O
,	O
knows	O
it	O
can	O
rapidly	O
transition	O
that	O
port	O
to	O
the	O
forwarding	O
state	O
bypassing	O
the	O
listening/learning	O
state	O
transition	O
.	O
</s>
<s>
This	O
essentially	O
creates	O
a	O
cascading	O
effect	O
away	O
from	O
the	O
root	O
bridge	B-Protocol
where	O
each	O
designated	O
bridge	B-Protocol
proposes	O
to	O
its	O
neighbors	O
to	O
determine	O
if	O
it	O
can	O
make	O
a	O
rapid	O
transition	O
.	O
</s>
<s>
STP	O
and	O
RSTP	O
do	O
not	O
segregate	O
switch	B-Protocol
ports	I-Protocol
by	O
VLAN	B-General_Concept
.	O
</s>
<s>
However	O
,	O
in	O
Ethernet	O
switched	B-Protocol
environments	O
where	O
multiple	O
VLANs	B-General_Concept
exist	O
,	O
it	O
is	O
often	O
desirable	O
to	O
create	O
multiple	O
spanning	O
trees	O
so	O
that	O
traffic	O
on	O
different	O
VLANs	B-General_Concept
uses	O
different	O
links	O
.	O
</s>
<s>
Before	O
the	O
IEEE	O
published	O
a	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
standard	O
for	O
VLANs	B-General_Concept
,	O
a	O
number	O
of	O
vendors	O
who	O
sold	O
VLAN	B-General_Concept
capable	O
switches	B-Protocol
developed	O
their	O
own	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
versions	O
that	O
were	O
VLAN	B-General_Concept
capable	O
.	O
</s>
<s>
Cisco	O
developed	O
,	O
implemented	O
and	O
published	O
the	O
Per-VLAN	O
Spanning	O
Tree	O
(	O
PVST	O
)	O
proprietary	O
protocol	O
using	O
its	O
own	O
proprietary	O
Inter-Switch	B-Protocol
Link	I-Protocol
(	O
ISL	O
)	O
for	O
VLAN	B-General_Concept
encapsulation	B-Protocol
,	O
and	O
PVST+	O
which	O
uses	O
802.1Q	O
VLAN	B-General_Concept
encapsulation	B-Protocol
.	O
</s>
<s>
Both	O
standards	O
implement	O
a	O
separate	O
spanning	O
tree	O
for	O
every	O
VLAN	B-General_Concept
.	O
</s>
<s>
Cisco	O
switches	B-Protocol
now	O
commonly	O
implement	O
PVST+	O
and	O
can	O
only	O
implement	O
Spanning	O
Trees	O
for	O
VLANs	B-General_Concept
if	O
the	O
other	O
switches	B-Protocol
in	O
the	O
LAN	B-General_Concept
implement	O
the	O
same	O
VLAN	B-General_Concept
STP	O
protocol	O
.	O
</s>
<s>
HP	O
provides	O
PVST	O
and	O
PVST+	O
compatibility	O
in	O
some	O
of	O
its	O
network	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
Extreme	O
Networks	O
does	O
so	O
with	O
two	O
limitations	O
:	O
Lack	O
of	O
support	O
on	O
ports	O
where	O
the	O
VLAN	B-General_Concept
is	O
untagged/native	O
,	O
and	O
also	O
on	O
the	O
VLAN	B-General_Concept
with	O
ID	O
1	O
.	O
</s>
<s>
PVST+	O
can	O
tunnel	O
across	O
an	O
MSTP	B-Protocol
Region	O
.	O
</s>
<s>
The	O
switch	B-Protocol
vendor	O
Juniper	O
Networks	O
in	O
turn	O
developed	O
and	O
implemented	O
its	O
VLAN	B-General_Concept
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
VSTP	O
)	O
to	O
provide	O
compatibility	O
with	O
Cisco	O
's	O
PVST	O
,	O
so	O
that	O
the	O
switches	B-Protocol
from	O
both	O
vendors	O
can	O
be	O
included	O
in	O
one	O
LAN	B-General_Concept
.	O
</s>
<s>
VSTP	O
supports	O
only	O
253	O
different	O
spanning-tree	B-Protocol
topologies	B-Architecture
.	O
</s>
<s>
If	O
there	O
are	O
more	O
than	O
253	O
VLANs	B-General_Concept
,	O
it	O
is	O
recommended	O
to	O
configure	O
RSTP	O
in	O
addition	O
to	O
VSTP	O
,	O
and	O
VLANs	B-General_Concept
beyond	O
253	O
will	O
be	O
handled	O
by	O
RSTP	O
.	O
</s>
<s>
If	O
this	O
protocol	O
is	O
in	O
use	O
,	O
VLAN	B-General_Concept
membership	O
for	O
trunk	O
interfaces	O
must	O
be	O
statically	O
configured	O
.	O
</s>
<s>
By	O
default	O
,	O
VSTP	O
uses	O
the	O
RSTP	O
protocol	O
as	O
its	O
core	O
spanning-tree	B-Protocol
protocol	I-Protocol
,	O
but	O
usage	O
of	O
STP	O
can	O
be	O
forced	O
if	O
the	O
network	O
includes	O
old	O
bridges	B-Protocol
.	O
</s>
<s>
More	O
information	O
about	O
configuring	O
VSTP	O
on	O
Juniper	O
Networks	O
switches	B-Protocol
was	O
published	O
in	O
the	O
official	O
documentation	O
.	O
</s>
<s>
Cisco	O
also	O
published	O
a	O
proprietary	O
version	O
of	O
Rapid	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
It	O
creates	O
a	O
spanning	O
tree	O
for	O
each	O
VLAN	B-General_Concept
,	O
just	O
like	O
PVST	O
.	O
</s>
<s>
Cisco	O
refers	O
to	O
this	O
as	O
Rapid	O
Per-VLAN	O
Spanning	O
Tree	O
(	O
RPVST	O
)	O
.	O
</s>
<s>
The	O
Multiple	B-Protocol
Spanning	I-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
MSTP	B-Protocol
)	O
,	O
originally	O
defined	O
in	O
IEEE	O
802.1s-2002	O
and	O
later	O
merged	O
into	O
IEEE	O
802.1Q-2005	O
,	O
defines	O
an	O
extension	O
to	O
RSTP	O
to	O
further	O
develop	O
the	O
usefulness	O
of	O
VLANs	B-General_Concept
.	O
</s>
<s>
In	O
the	O
standard	O
,	O
a	O
spanning	O
tree	O
that	O
maps	O
one	O
or	O
more	O
VLANs	B-General_Concept
is	O
called	O
a	O
multiple	B-Protocol
spanning	I-Protocol
tree	I-Protocol
(	O
MST	O
)	O
.	O
</s>
<s>
Under	O
MSTP	B-Protocol
,	O
a	O
spanning	O
tree	O
can	O
be	O
defined	O
for	O
individual	O
VLANs	B-General_Concept
or	O
for	O
groups	O
of	O
VLANs	B-General_Concept
.	O
</s>
<s>
Switches	B-Protocol
are	O
first	O
assigned	O
to	O
an	O
MST	O
region	O
,	O
then	O
VLANs	B-General_Concept
are	O
mapped	O
against	O
or	O
assigned	O
to	O
this	O
MST	O
.	O
</s>
<s>
A	O
common	B-Protocol
spanning	I-Protocol
tree	I-Protocol
(	O
CST	O
)	O
is	O
an	O
MST	O
to	O
which	O
several	O
VLANs	B-General_Concept
are	O
mapped	O
,	O
this	O
group	O
of	O
VLANs	B-General_Concept
is	O
called	O
MST	O
instance	O
(	O
MSTI	O
)	O
.	O
</s>
<s>
A	O
MST	O
that	O
has	O
only	O
one	O
VLAN	B-General_Concept
assigned	O
to	O
it	O
is	O
a	O
internal	O
spanning	O
tree	O
(	O
IST	O
)	O
.	O
</s>
<s>
Unlike	O
some	O
proprietary	O
per-VLAN	O
spanning	O
tree	O
implementations	O
,	O
MSTP	B-Protocol
includes	O
all	O
of	O
its	O
spanning	O
tree	O
information	O
in	O
a	O
single	O
BPDU	O
format	O
.	O
</s>
<s>
Not	O
only	O
does	O
this	O
reduce	O
the	O
number	O
of	O
BPDUs	O
required	O
to	O
communicate	O
spanning	O
tree	O
information	O
for	O
each	O
VLAN	B-General_Concept
,	O
but	O
it	O
also	O
ensures	O
backward	O
compatibility	O
with	O
RSTP	O
and	O
,	O
in	O
effect	O
,	O
classic	O
STP	O
too	O
.	O
</s>
<s>
MSTP	B-Protocol
does	O
this	O
by	O
encoding	O
an	O
additional	O
region	O
of	O
information	O
after	O
the	O
standard	O
RSTP	O
BPDU	O
as	O
well	O
as	O
a	O
number	O
of	O
MSTI	O
messages	O
(	O
from	O
0	O
to	O
64	O
instances	O
,	O
although	O
in	O
practice	O
many	O
bridges	B-Protocol
support	O
fewer	O
)	O
.	O
</s>
<s>
Each	O
instance	O
can	O
be	O
assigned	O
a	O
number	O
of	O
configured	O
VLANs	B-General_Concept
and	O
frames	O
assigned	O
to	O
these	O
VLANs	B-General_Concept
operate	O
in	O
this	O
spanning	O
tree	O
instance	O
whenever	O
they	O
are	O
inside	O
the	O
MST	O
region	O
.	O
</s>
<s>
In	O
order	O
to	O
avoid	O
conveying	O
their	O
entire	O
VLAN	B-General_Concept
to	O
spanning	O
tree	O
mapping	O
in	O
each	O
BPDU	O
,	O
bridges	B-Protocol
encode	O
an	O
MD5	O
digest	O
of	O
their	O
VLAN	B-General_Concept
to	O
instance	O
table	O
in	O
the	O
MSTP	B-Protocol
BPDU	O
.	O
</s>
<s>
This	O
digest	O
is	O
then	O
used	O
by	O
other	O
MSTP	B-Protocol
bridges	B-Protocol
,	O
along	O
with	O
other	O
administratively	O
configured	O
values	O
,	O
to	O
determine	O
if	O
the	O
neighboring	O
bridge	B-Protocol
is	O
in	O
the	O
same	O
MST	O
region	O
as	O
itself	O
.	O
</s>
<s>
MSTP	B-Protocol
is	O
fully	O
compatible	O
with	O
RSTP	O
bridges	B-Protocol
in	O
that	O
an	O
MSTP	B-Protocol
BPDU	O
can	O
be	O
interpreted	O
by	O
an	O
RSTP	O
bridge	B-Protocol
as	O
an	O
RSTP	O
BPDU	O
.	O
</s>
<s>
This	O
not	O
only	O
allows	O
compatibility	O
with	O
RSTP	O
bridges	B-Protocol
without	O
configuration	O
changes	O
but	O
also	O
causes	O
any	O
RSTP	O
bridges	B-Protocol
outside	O
of	O
an	O
MSTP	B-Protocol
region	O
to	O
see	O
the	O
region	O
as	O
a	O
single	O
RSTP	O
bridge	B-Protocol
regardless	O
of	O
the	O
number	O
of	O
MSTP	B-Protocol
bridges	B-Protocol
inside	O
the	O
region	O
itself	O
.	O
</s>
<s>
In	O
order	O
to	O
further	O
facilitate	O
this	O
view	O
of	O
an	O
MSTP	B-Protocol
region	O
as	O
a	O
single	O
RSTP	O
bridge	B-Protocol
,	O
the	O
MSTP	B-Protocol
protocol	O
uses	O
a	O
variable	O
known	O
as	O
remaining	O
hops	O
as	O
a	O
time	O
to	O
live	O
counter	O
instead	O
of	O
the	O
message	O
age	O
timer	O
used	O
by	O
RSTP	O
.	O
</s>
<s>
The	O
message	O
age	O
time	O
is	O
only	O
incremented	O
once	O
when	O
spanning-tree	B-Protocol
information	O
enters	O
an	O
MST	O
region	O
,	O
and	O
therefore	O
RSTP	O
bridges	B-Protocol
will	O
see	O
a	O
region	O
as	O
only	O
one	O
hop	O
in	O
the	O
spanning	O
tree	O
.	O
</s>
<s>
Ports	O
at	O
the	O
edge	O
of	O
an	O
MSTP	B-Protocol
region	O
connected	O
to	O
either	O
an	O
RSTP	O
or	O
STP	O
bridge	B-Protocol
or	O
an	O
endpoint	O
are	O
known	O
as	O
boundary	O
ports	O
.	O
</s>
<s>
IEEE	B-Protocol
802.1aq	I-Protocol
,	O
also	O
known	O
as	O
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
(	O
SPB	O
)	O
,	O
allows	O
redundant	O
links	O
between	O
switches	B-Protocol
to	O
be	O
active	O
through	O
multiple	O
equal	O
cost	O
paths	O
,	O
and	O
provides	O
much	O
larger	O
layer-2	O
topologies	B-Architecture
,	O
faster	O
convergence	O
,	O
and	O
improves	O
the	O
use	O
of	O
the	O
mesh	O
topologies	B-Architecture
through	O
increased	O
bandwidth	O
between	O
all	O
devices	O
by	O
allowing	O
traffic	O
to	O
load	O
share	O
across	O
all	O
paths	O
on	O
a	O
mesh	O
network	O
.	O
</s>
<s>
SPB	O
consolidates	O
multiple	O
existing	O
functionalities	O
,	O
including	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
STP	O
)	O
,	O
Multiple	B-Protocol
Spanning	I-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
MSTP	B-Protocol
)	O
,	O
Rapid	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
RSTP	O
)	O
,	O
Link	B-Architecture
aggregation	I-Architecture
,	O
and	O
Multiple	O
MAC	O
Registration	O
Protocol	O
(	O
MMRP	O
)	O
into	O
a	O
one	O
link	O
state	O
protocol	O
.	O
</s>
<s>
The	O
bridge	B-Protocol
ID	I-Protocol
(	O
BID	O
)	O
is	O
a	O
field	O
inside	O
a	O
BPDU	O
packet	O
.	O
</s>
<s>
The	O
first	O
two	O
bytes	O
are	O
the	O
bridge	B-Protocol
priority	O
,	O
an	O
unsigned	O
integer	O
of	O
0	O
–	O
65,535	O
.	O
</s>
<s>
The	O
last	O
six	O
bytes	O
are	O
a	O
MAC	O
address	O
supplied	O
by	O
the	O
bridge	B-Protocol
.	O
</s>
<s>
Prior	O
to	O
IEEE	O
802.1D-2004	O
,	O
the	O
first	O
two	O
bytes	O
gave	O
a	O
16-bit	O
bridge	B-Protocol
priority	O
.	O
</s>
<s>
Since	O
IEEE	O
802.1D-2004	O
,	O
the	O
first	O
four	O
bits	O
are	O
a	O
configurable	O
priority	O
,	O
and	O
the	O
last	O
twelve	O
bits	O
carry	O
the	O
bridge	B-Protocol
system	O
ID	O
extension	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
MST	O
,	O
the	O
bridge	B-Protocol
system	O
ID	O
extension	O
carries	O
the	O
MSTP	B-Protocol
instance	O
number	O
.	O
</s>
<s>
Some	O
vendors	O
set	O
the	O
bridge	B-Protocol
system	O
ID	O
extension	O
to	O
carry	O
a	O
VLAN	B-General_Concept
ID	O
allowing	O
a	O
different	O
spanning	O
tree	O
per	O
VLAN	B-General_Concept
,	O
such	O
as	O
Cisco	O
's	O
PVST	O
.	O
</s>
<s>
Modern	O
networks	O
can	O
make	O
use	O
of	O
all	O
connected	O
links	O
by	O
use	O
of	O
protocols	O
that	O
inhibit	O
,	O
control	O
or	O
suppress	O
the	O
natural	O
behavior	O
of	O
logical	O
or	O
physical	B-Architecture
topology	I-Architecture
loops	O
.	O
</s>
<s>
Newer	O
,	O
more	O
robust	O
protocols	O
include	O
the	O
TRILL	B-Protocol
(	O
Transparent	B-Protocol
Interconnection	I-Protocol
of	I-Protocol
Lots	I-Protocol
of	I-Protocol
Links	I-Protocol
)	O
protocol	O
,	O
also	O
created	O
by	O
Perlman	O
,	O
and	O
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
from	O
the	O
IEEE	O
.	O
</s>
<s>
Configuring	O
connections	O
between	O
network	O
equipment	O
as	O
layer-3	B-Application
IP	O
links	O
and	O
relying	O
on	O
IP	B-Protocol
routing	I-Protocol
for	O
resiliency	O
and	O
to	O
prevent	O
loops	O
is	O
a	O
popular	O
alternative	O
.	O
</s>
<s>
Switch	B-Protocol
virtualization	O
techniques	O
like	O
Cisco	O
Virtual	O
Switching	O
System	O
and	O
Virtual	O
PortChannel	O
and	O
HP	O
Intelligent	B-Protocol
Resilient	I-Protocol
Framework	I-Protocol
combine	O
multiple	O
switches	B-Protocol
into	O
a	O
single	O
logical	O
entity	O
.	O
</s>
<s>
Such	O
a	O
multi-chassis	B-Protocol
link	I-Protocol
aggregation	I-Protocol
group	I-Protocol
works	O
like	O
a	O
normal	O
port	B-Architecture
trunk	I-Architecture
,	O
only	O
distributed	O
through	O
multiple	O
switches	B-Protocol
.	O
</s>
