<s>
A	O
multilayer	B-Application
switch	I-Application
(	O
MLS	O
)	O
is	O
a	O
computer	B-Device
networking	I-Device
device	I-Device
that	O
switches	O
on	O
OSI	B-Protocol
layer	I-Protocol
2	I-Protocol
like	O
an	O
ordinary	O
network	B-Protocol
switch	I-Protocol
and	O
provides	O
extra	O
functions	O
on	O
higher	O
OSI	O
layers	O
.	O
</s>
<s>
Switching	O
technologies	O
are	O
crucial	O
to	O
network	B-Architecture
design	I-Architecture
,	O
as	O
they	O
allow	O
traffic	O
to	O
be	O
sent	O
only	O
where	O
it	O
is	O
needed	O
in	O
most	O
cases	O
,	O
using	O
fast	O
,	O
hardware-based	O
methods	O
.	O
</s>
<s>
Switching	O
uses	O
different	O
kinds	O
of	O
network	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
A	O
standard	O
switch	O
is	O
known	O
as	O
a	O
layer	B-Protocol
2	I-Protocol
switch	O
and	O
is	O
commonly	O
found	O
in	O
nearly	O
any	O
LAN	O
.	O
</s>
<s>
Layer	B-Application
3	I-Application
or	O
layer	B-Application
4	I-Application
switches	I-Application
require	O
advanced	O
technology	O
(	O
see	O
managed	B-Protocol
switch	I-Protocol
)	O
and	O
are	O
more	O
expensive	O
and	O
thus	O
are	O
usually	O
only	O
found	O
in	O
larger	O
LANs	O
or	O
in	O
special	O
network	O
environments	O
.	O
</s>
<s>
Multi-layer	O
switching	O
combines	O
layer	B-Protocol
2	I-Protocol
,	O
3	O
and	O
4	O
switching	O
technologies	O
and	O
provides	O
high-speed	O
scalability	O
with	O
low	O
latency	O
.	O
</s>
<s>
Multi-layer	O
switching	O
can	O
move	O
traffic	O
at	O
wire	O
speed	O
and	O
also	O
provide	O
layer	B-Application
3	I-Application
routing	O
.	O
</s>
<s>
There	O
is	O
no	O
performance	O
difference	O
between	O
forwarding	O
at	O
different	O
layers	O
because	O
the	O
routing	O
and	O
switching	O
are	O
all	O
hardware-based	O
routing	O
decisions	O
are	O
made	O
by	O
specialized	O
application-specific	O
integrated	O
circuits	O
(	O
ASICs	O
)	O
with	O
the	O
help	O
of	O
content-addressable	B-Data_Structure
memory	I-Data_Structure
.	O
</s>
<s>
A	O
multilayer	B-Application
switch	I-Application
can	O
prioritize	O
packets	B-Protocol
by	O
the	O
6	O
bit	O
differentiated	O
services	O
code	O
point	O
(	O
DSCP	O
)	O
.	O
</s>
<s>
From	O
VLAN	B-General_Concept
IEEE	O
802.1p	O
to	O
port	O
egress	O
queue	O
.	O
</s>
<s>
MLSs	O
are	O
also	O
able	O
to	O
route	O
IP	O
traffic	O
between	O
VLANs	B-General_Concept
like	O
a	O
common	O
router	B-Protocol
.	O
</s>
<s>
Layer-2	B-Protocol
switching	O
uses	O
the	O
MAC	O
address	O
of	O
the	O
host	O
's	O
network	B-Protocol
interface	I-Protocol
cards	I-Protocol
(	O
NICs	O
)	O
to	O
decide	O
where	O
to	O
forward	O
frames	O
.	O
</s>
<s>
Layer	B-Protocol
2	I-Protocol
switching	O
is	O
hardware-based	O
,	O
which	O
means	O
switches	O
use	O
ASICs	O
to	O
build	O
and	O
maintain	O
the	O
Forwarding	B-Protocol
information	I-Protocol
base	I-Protocol
and	O
to	O
perform	O
packet	B-Protocol
forwarding	I-Protocol
at	O
wire	O
speed	O
.	O
</s>
<s>
One	O
way	O
to	O
think	O
of	O
a	O
layer-2	B-Protocol
switch	O
is	O
as	O
a	O
multiport	O
bridge	B-Protocol
.	O
</s>
<s>
Layer-2	B-Protocol
switching	O
is	O
highly	O
efficient	O
because	O
there	O
is	O
no	O
modification	O
to	O
the	O
frame	O
required	O
.	O
</s>
<s>
Encapsulation	O
of	O
the	O
packet	B-Protocol
changes	O
only	O
when	O
the	O
data	B-Protocol
packet	I-Protocol
passes	O
through	O
dissimilar	O
media	O
(	O
such	O
as	O
from	O
Ethernet	O
to	O
FDDI	O
)	O
.	O
</s>
<s>
Layer-2	B-Protocol
switching	O
is	O
used	O
for	O
workgroup	O
connectivity	O
and	O
network	O
segmentation	O
(	O
breaking	O
up	O
collision	B-Protocol
domains	I-Protocol
)	O
.	O
</s>
<s>
This	O
allows	O
a	O
flatter	O
network	B-Architecture
design	I-Architecture
with	O
more	O
network	O
segments	O
than	O
traditional	O
networks	O
joined	O
by	O
repeater	B-Protocol
hubs	I-Protocol
and	O
routers	B-Protocol
.	O
</s>
<s>
Layer-2	B-Protocol
switches	O
have	O
the	O
same	O
limitations	O
as	O
bridges	O
.	O
</s>
<s>
Bridges	O
break	O
up	O
collision	B-Protocol
domains	I-Protocol
,	O
but	O
the	O
network	O
remains	O
one	O
large	O
broadcast	B-Architecture
domain	I-Architecture
which	O
can	O
cause	O
performance	O
issues	O
and	O
limits	O
the	O
size	O
of	O
a	O
network	O
.	O
</s>
<s>
Because	O
of	O
these	O
problems	O
,	O
layer-2	B-Protocol
switches	O
cannot	O
completely	O
replace	O
routers	B-Protocol
.	O
</s>
<s>
A	O
layer-3	B-Application
switch	I-Application
can	O
perform	O
some	O
or	O
all	O
of	O
the	O
functions	O
normally	O
performed	O
by	O
a	O
router	B-Protocol
.	O
</s>
<s>
Most	O
network	B-Protocol
switches	I-Protocol
,	O
however	O
,	O
are	O
limited	O
to	O
supporting	O
a	O
single	O
type	O
of	O
physical	O
network	O
,	O
typically	O
Ethernet	O
,	O
whereas	O
a	O
router	B-Protocol
may	O
support	O
different	O
kinds	O
of	O
physical	O
networks	O
on	O
different	O
ports	O
.	O
</s>
<s>
Layer-3	B-Application
switching	O
is	O
solely	O
based	O
on	O
(	O
destination	O
)	O
IP	B-Protocol
address	I-Protocol
stored	O
in	O
the	O
header	O
of	O
IP	B-Protocol
datagram	I-Protocol
(	O
layer-4	O
switching	O
may	O
use	O
other	O
information	O
in	O
the	O
header	O
)	O
.	O
</s>
<s>
The	O
difference	O
between	O
a	O
layer-3	B-Application
switch	I-Application
and	O
a	O
router	B-Protocol
is	O
the	O
way	O
the	O
device	O
is	O
making	O
the	O
routing	O
decision	O
.	O
</s>
<s>
Traditionally	O
,	O
routers	B-Protocol
use	O
microprocessors	O
to	O
make	O
forwarding	O
decisions	O
in	O
software	O
,	O
while	O
the	O
switch	O
performs	O
only	O
hardware-based	O
packet	B-Protocol
switching	O
(	O
by	O
specialized	O
ASICs	O
with	O
the	O
help	O
of	O
content-addressable	B-Data_Structure
memory	I-Data_Structure
)	O
.	O
</s>
<s>
However	O
,	O
many	O
routers	B-Protocol
now	O
also	O
have	O
advanced	O
hardware	O
functions	O
to	O
assist	O
with	O
forwarding	O
.	O
</s>
<s>
The	O
main	O
advantage	O
of	O
layer-3	B-Application
switches	I-Application
is	O
the	O
potential	O
for	O
lower	O
network	O
latency	O
as	O
a	O
packet	B-Protocol
can	O
be	O
routed	O
without	O
making	O
extra	O
network	O
hops	O
to	O
a	O
router	B-Protocol
.	O
</s>
<s>
VLANs	B-General_Concept
)	O
with	O
a	O
router	B-Protocol
to	O
a	O
standard	O
layer-2	B-Protocol
switch	O
requires	O
passing	O
the	O
frame	O
to	O
the	O
switch	O
(	O
first	O
L2	O
hop	O
)	O
,	O
then	O
to	O
the	O
router	B-Protocol
(	O
second	O
L2	O
hop	O
)	O
where	O
the	O
packet	B-Protocol
inside	O
the	O
frame	O
is	O
routed	O
(	O
L3	O
hop	O
)	O
and	O
then	O
passed	O
back	O
to	O
the	O
switch	O
(	O
third	O
L2	O
hop	O
)	O
.	O
</s>
<s>
A	O
layer-3	B-Application
switch	I-Application
accomplishes	O
the	O
same	O
task	O
without	O
the	O
need	O
for	O
a	O
router	B-Protocol
(	O
and	O
therefore	O
additional	O
hops	O
)	O
by	O
making	O
the	O
routing	O
decision	O
itself	O
,	O
i.e.	O
</s>
<s>
the	O
packet	B-Protocol
is	O
routed	O
to	O
another	O
subnet	O
and	O
switched	O
to	O
the	O
destination	O
network	B-Protocol
port	I-Protocol
simultaneously	O
.	O
</s>
<s>
Because	O
many	O
layer-3	B-Application
switches	I-Application
offer	O
the	O
same	O
functionality	O
as	O
traditional	O
routers	B-Protocol
they	O
can	O
be	O
used	O
as	O
cheaper	O
,	O
lower	O
latency	O
replacements	O
in	O
some	O
networks	O
.	O
</s>
<s>
Layer	B-Application
3	I-Application
switches	O
can	O
perform	O
the	O
following	O
actions	O
that	O
can	O
also	O
be	O
performed	O
by	O
routers	B-Protocol
:	O
</s>
<s>
The	O
benefits	O
of	O
layer	B-Application
3	I-Application
switching	O
include	O
the	O
following	O
:	O
</s>
<s>
Network	B-Device
devices	I-Device
without	O
the	O
capability	O
to	O
forward	O
packets	B-Protocol
between	O
subnetworks	O
are	O
called	O
end	O
systems	O
(	O
ESs	O
,	O
singular	O
ES	O
)	O
,	O
whereas	O
network	B-Device
devices	I-Device
with	O
these	O
capabilities	O
are	O
called	O
intermediate	O
systems	O
(	O
ISs	O
)	O
.	O
</s>
<s>
A	O
common	O
layer-3	B-Application
capability	O
is	O
an	O
awareness	O
of	O
IP	B-Protocol
multicast	I-Protocol
through	O
IGMP	B-Protocol
snooping	I-Protocol
.	O
</s>
<s>
With	O
this	O
awareness	O
,	O
a	O
layer-3	B-Application
switch	I-Application
can	O
increase	O
efficiency	O
by	O
delivering	O
the	O
traffic	O
of	O
a	O
multicast	O
group	O
only	O
to	O
ports	O
where	O
the	O
attached	O
device	O
has	O
signaled	O
that	O
it	O
wants	O
to	O
listen	O
to	O
that	O
group	O
.	O
</s>
<s>
Layer-3	B-Application
switches	I-Application
typically	O
support	O
IP	B-Protocol
routing	I-Protocol
between	O
VLANs	B-General_Concept
configured	O
on	O
the	O
switch	O
.	O
</s>
<s>
Some	O
layer-3	B-Application
switches	I-Application
support	O
the	O
routing	B-Protocol
protocols	I-Protocol
that	O
routers	B-Protocol
use	O
to	O
exchange	O
information	O
about	O
routes	O
between	O
networks	O
.	O
</s>
<s>
Layer	B-Architecture
4	I-Architecture
switching	O
means	O
hardware-based	O
layer	B-Application
3	I-Application
switching	O
technology	O
that	O
can	O
also	O
consider	O
the	O
type	O
of	O
network	O
traffic	O
(	O
for	O
example	O
,	O
distinguishing	O
between	O
UDP	B-Protocol
and	O
TCP	B-Protocol
)	O
.	O
</s>
<s>
Layer	B-Architecture
4	I-Architecture
switching	O
provides	O
additional	O
datagram	B-Protocol
inspection	O
by	O
reading	O
the	O
port	B-Protocol
numbers	I-Protocol
found	O
in	O
the	O
transport	B-Architecture
layer	I-Architecture
header	O
to	O
make	O
routing	O
decisions	O
(	O
i.e.	O
</s>
<s>
ports	O
used	O
by	O
HTTP	B-Protocol
,	O
FTP	B-Protocol
and	O
VoIP	B-Application
)	O
.	O
</s>
<s>
These	O
port	B-Protocol
numbers	I-Protocol
are	O
found	O
in	O
RFC	O
1700	O
and	O
reference	O
the	O
upper-layer	O
protocol	O
,	O
program	O
,	O
or	O
application	O
.	O
</s>
<s>
For	O
example	O
,	O
extended	O
access	O
lists	O
can	O
filter	O
packets	B-Protocol
based	O
on	O
layer-4	O
port	B-Protocol
numbers	I-Protocol
.	O
</s>
<s>
A	O
layer-4	O
switch	O
can	O
use	O
information	O
in	O
the	O
transport-layer	B-Architecture
protocols	I-Architecture
to	O
make	O
forwarding	O
decisions	O
.	O
</s>
<s>
Principally	O
this	O
refers	O
to	O
an	O
ability	O
to	O
use	O
source	O
and	O
destination	O
port	B-Protocol
numbers	I-Protocol
in	O
TCP	B-Protocol
and	O
UDP	B-Protocol
communications	O
to	O
allow	O
,	O
block	O
and	O
prioritize	O
communications	O
.	O
</s>
<s>
Some	O
switches	O
can	O
use	O
packet	B-Protocol
information	O
up	O
to	O
OSI	O
layer	O
7	O
;	O
these	O
may	O
be	O
called	O
layer	B-Architecture
4	I-Architecture
–	O
7	O
switches	O
,	O
,	O
,	O
web	O
switches	O
or	O
application	O
switches	O
.	O
</s>
<s>
Content	O
switches	O
are	O
typically	O
used	O
for	O
load	B-Application
balancing	I-Application
among	O
groups	O
of	O
servers	O
.	O
</s>
<s>
Load	B-Application
balancing	I-Application
can	O
be	O
performed	O
on	O
HTTP	B-Protocol
,	O
HTTPS	B-Protocol
,	O
VPN	B-Application
,	O
or	O
any	O
TCP/IP	O
traffic	O
using	O
a	O
specific	O
port	O
.	O
</s>
<s>
Load	B-Application
balancing	I-Application
often	O
involves	O
destination	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
so	O
that	O
the	O
client	O
of	O
the	O
load-balanced	B-Application
service	O
is	O
not	O
fully	O
aware	O
of	O
which	O
server	O
is	O
handling	O
its	O
requests	O
.	O
</s>
<s>
Some	O
layer	B-Architecture
4	I-Architecture
–	O
7	O
switches	O
can	O
perform	O
Network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
at	O
wire	O
speed	O
.	O
</s>
<s>
Content	O
switches	O
can	O
often	O
be	O
used	O
to	O
perform	O
standard	O
operations	O
such	O
as	O
SSL	B-Protocol
encryption	I-Protocol
and	O
decryption	O
to	O
reduce	O
the	O
load	O
on	O
the	O
servers	O
receiving	O
the	O
traffic	O
,	O
or	O
to	O
centralize	O
the	O
management	O
of	O
digital	O
certificates	O
.	O
</s>
<s>
Layer	O
7	O
switching	O
is	O
a	O
technology	O
used	O
in	O
a	O
content	B-Operating_System
delivery	I-Operating_System
network	I-Operating_System
(	O
CDN	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
requests	O
from	O
the	O
same	O
source	O
IP	B-Protocol
address	I-Protocol
are	O
directed	O
to	O
the	O
same	O
application	O
server	O
each	O
time	O
.	O
</s>
<s>
Stickiness	O
can	O
also	O
be	O
based	O
on	O
SSL	B-Protocol
IDs	O
,	O
and	O
some	O
content	O
switches	O
can	O
use	O
cookies	B-Application
to	O
provide	O
this	O
functionality	O
.	O
</s>
<s>
The	O
router	B-Protocol
operates	O
on	O
the	O
transport	B-Architecture
layer	I-Architecture
and	O
makes	O
decisions	O
on	O
where	O
to	O
send	O
the	O
packets	B-Protocol
.	O
</s>
<s>
Modern	O
load	B-Application
balancing	I-Application
routers	B-Protocol
can	O
use	O
different	O
rules	O
to	O
make	O
decisions	O
on	O
where	O
to	O
route	O
traffic	O
.	O
</s>
<s>
This	O
is	O
also	O
a	O
redundancy	O
method	O
,	O
so	O
if	O
one	O
machine	O
is	O
not	O
up	O
,	O
the	O
router	B-Protocol
will	O
not	O
send	O
traffic	O
to	O
it	O
.	O
</s>
<s>
The	O
router	B-Protocol
may	O
also	O
have	O
NAT	O
capability	O
with	O
port	O
and	O
transaction	O
awareness	O
and	O
performs	O
a	O
form	O
of	O
port	O
translation	O
for	O
sending	O
incoming	O
packets	B-Protocol
to	O
one	O
or	O
more	O
machines	O
that	O
are	O
hidden	O
behind	O
a	O
single	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
A	O
layer-7	O
switch	O
may	O
include	O
a	O
web	B-Protocol
cache	I-Protocol
and	O
participate	O
in	O
a	O
CDN	O
.	O
</s>
