<s>
Network	B-Protocol
load	I-Protocol
balancing	I-Protocol
is	O
the	O
ability	O
to	O
balance	O
traffic	O
across	O
two	O
or	O
more	O
WAN	O
links	O
without	O
using	O
complex	O
routing	B-Protocol
protocols	I-Protocol
like	O
BGP	B-Protocol
.	O
</s>
<s>
over	O
multiple	O
connections	O
in	O
order	O
to	O
spread	O
out	O
the	O
amount	O
of	O
bandwidth	O
used	O
by	O
each	O
LAN	B-General_Concept
user	O
,	O
thus	O
increasing	O
the	O
total	O
amount	O
of	O
bandwidth	O
available	O
.	O
</s>
<s>
An	O
FTP	B-Protocol
application	O
only	O
uses	O
a	O
single	O
session	O
so	O
it	O
is	O
not	O
balanced	O
;	O
however	O
if	O
a	O
secondary	O
FTP	B-Protocol
connection	O
is	O
made	O
,	O
then	O
it	O
may	O
be	O
balanced	O
so	O
that	O
the	O
traffic	O
is	O
distributed	O
across	O
two	O
of	O
the	O
various	O
connections	O
and	O
thus	O
provides	O
an	O
overall	O
increase	O
in	O
throughput	O
.	O
</s>
<s>
Additionally	O
,	O
network	B-Protocol
load	I-Protocol
balancing	I-Protocol
is	O
commonly	O
used	O
to	O
provide	O
network	O
redundancy	O
so	O
that	O
in	O
the	O
event	O
of	O
a	O
WAN	O
link	O
outage	O
,	O
access	O
to	O
network	O
resources	O
is	O
still	O
available	O
via	O
the	O
secondary	O
link(s )	O
.	O
</s>
<s>
Redundancy	O
is	O
a	O
key	O
requirement	O
for	O
business	O
continuity	O
plans	O
and	O
generally	O
used	O
in	O
conjunction	O
with	O
critical	O
applications	O
like	O
VPNs	B-Application
and	O
VoIP	B-Application
.	O
</s>
<s>
Finally	O
,	O
most	O
network	B-Protocol
load	I-Protocol
balancing	I-Protocol
systems	O
also	O
incorporate	O
the	O
ability	O
to	O
balance	O
both	O
outbound	O
and	O
inbound	O
traffic	O
.	O
</s>
<s>
Inbound	O
load	B-Application
balancing	I-Application
is	O
generally	O
performed	O
via	O
dynamic	B-Protocol
DNS	I-Protocol
which	O
can	O
either	O
be	O
built	O
into	O
the	O
system	O
,	O
or	O
provided	O
by	O
an	O
external	O
service	O
or	O
system	O
.	O
</s>
<s>
Having	O
the	O
dynamic	B-Protocol
DNS	I-Protocol
service	O
within	O
the	O
system	O
is	O
generally	O
thought	O
to	O
be	O
better	O
from	O
a	O
cost	O
savings	O
and	O
overall	O
control	O
point	O
of	O
view	O
.	O
</s>
<s>
Microsoft	O
has	O
also	O
purchased	O
a	O
technology	O
that	O
it	O
renamed	O
Network	B-Protocol
Load	I-Protocol
Balancing	I-Protocol
(	O
NLB	O
)	O
that	O
allows	O
for	O
efficient	O
utilization	O
of	O
multiple	O
network	B-Protocol
cards	I-Protocol
.	O
</s>
<s>
MS	O
NLB	O
can	O
be	O
configured	O
in	O
unicast	B-Protocol
or	O
in	O
multicast	O
mode	O
where	O
in	O
multicast	O
mode	O
you	O
can	O
enable	O
IGMP	B-Protocol
snooping	I-Protocol
.	O
</s>
<s>
MS	O
NLB	O
was	O
introduced	O
for	O
the	O
first	O
time	O
in	O
Windows	B-Device
NT	I-Device
server	I-Device
to	O
spread	O
traffic	O
over	O
multiple	O
hosts	O
without	O
the	O
need	O
for	O
a	O
hardware	O
based	O
load	B-Application
balancer	I-Application
,	O
e.g.	O
</s>
<s>
In	O
unicast	B-Protocol
mode	O
MS	O
NLB	O
reassigns	O
the	O
stations	O
MAC	O
address	O
(	O
which	O
applies	O
to	O
the	O
clusters	O
IP	B-Protocol
address	I-Protocol
)	O
to	O
a	O
virtual	O
MAC	O
address	O
and	O
all	O
NIC	B-Protocol
's	O
in	O
the	O
NLB	O
cluster	B-Architecture
use	O
this	O
same	O
MAC	O
address	O
.	O
</s>
<s>
This	O
setup	O
will	O
cause	O
all	O
incoming	O
traffic	O
for	O
the	O
cluster	B-Architecture
to	O
be	O
flooded	O
to	O
all	O
ports	O
of	O
the	O
switch	B-Protocol
as	O
unknown	O
unicast	B-Protocol
frames	O
:	O
even	O
to	O
hosts	O
that	O
are	O
not	O
joining	O
in	O
the	O
cluster	B-Architecture
.	O
</s>
<s>
To	O
keep	O
flooding	O
minimal	O
you	O
would	O
need	O
to	O
use	O
a	O
dedicated	O
VLAN	B-General_Concept
for	O
the	O
cluster	B-Architecture
.	O
</s>
<s>
The	O
unicast	B-Protocol
IPv4	B-Protocol
address	O
of	O
the	O
cluster	B-Architecture
is	O
linked	O
to	O
a	O
multicast	O
MAC	O
address	O
.	O
</s>
<s>
The	O
hosts	O
is	O
in	O
the	O
cluster	B-Architecture
will	O
never	O
send	O
traffic	O
to	O
the	O
switch	B-Protocol
using	O
this	O
MAC	O
address	O
with	O
the	O
cluster	B-Architecture
IPv4	B-Protocol
address	O
so	O
one	O
would	O
need	O
to	O
create	O
a	O
static	O
ARP	O
entry	O
on	O
the	O
router	O
(	O
layer	O
3	O
)	O
in	O
the	O
attached	O
network	O
.	O
</s>
<s>
Not	O
all	O
vendors	O
will	O
allow	O
you	O
to	O
create	O
an	O
ARP	O
entry	O
where	O
you	O
use	O
a	O
unicast	B-Protocol
IP	B-Protocol
address	I-Protocol
and	O
a	O
multicast	O
MAC	O
address	O
.	O
</s>
<s>
Cisco	O
publishes	O
some	O
examples	O
how	O
to	O
set	O
up	O
MS	O
NLB	O
on	O
Catalyst	O
switches	O
running	O
IOS	B-Application
and	O
these	O
same	O
examples	O
can	O
be	O
used	O
for	O
switches	O
from	O
many	O
other	O
vendors	O
.	O
</s>
<s>
As	O
with	O
NLB	O
in	O
unicast	B-Protocol
mode	O
:	O
incoming	O
traffic	O
towards	O
the	O
cluster	B-Architecture
will	O
be	O
flooded	O
to	O
all	O
ports	O
in	O
the	O
switch/VLAN	O
and	O
not	O
all	O
vendors	O
support	O
this	O
setup	O
.	O
</s>
<s>
To	O
limit	O
the	O
flooding	O
,	O
MS	O
NLB	O
now	O
supports	O
IGMP	O
which	O
should	O
lead	O
to	O
the	O
switches	O
learning	O
which	O
ports	O
are	O
actually	O
using	O
the	O
multicast	B-Protocol
address	I-Protocol
,	O
but	O
it	O
does	O
n't	O
always	O
lead	O
to	O
the	O
desired	O
result	O
.	O
</s>
<s>
For	O
example	O
,	O
Dell	B-Operating_System
PowerConnect	I-Operating_System
multi-layer	O
switches	O
officially	O
do	O
n't	O
support	O
MS	O
NLB	O
in	O
multicast	O
.	O
</s>
<s>
Even	O
though	O
it	O
does	O
work	O
,	O
it	O
will	O
lead	O
to	O
high	O
CPU	O
utilization	O
-	O
affecting	O
(	O
other	O
)	O
traffic	O
in	O
the	O
switch	B-Protocol
and	O
on	O
other	O
switches	O
one	O
might	O
have	O
other	O
limitations	O
such	O
that	O
the	O
switch	B-Protocol
to	O
which	O
the	O
NLB	O
NICs	O
are	O
connected	O
ca	O
n't	O
be	O
the	O
same	O
switch	B-Protocol
that	O
does	O
the	O
IP	O
routing	O
.	O
</s>
<s>
When	O
multiple	O
servers	O
are	O
joined	O
to	O
create	O
a	O
cluster	B-Architecture
.	O
</s>
<s>
Clusters	O
can	O
use	O
network	B-Protocol
load	I-Protocol
balancing	I-Protocol
whereby	O
simultaneous	O
cluster	B-Architecture
request	O
are	O
distributed	O
between	O
cluster	B-Architecture
servers	I-Architecture
.	O
</s>
<s>
Round-robin	B-Protocol
DNS	I-Protocol
records	O
is	O
one	O
form	O
of	O
cluster	B-Architecture
load	B-Application
balancing	I-Application
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
before	O
mentioned	O
,	O
to	O
configure	O
a	O
terminal	B-Device
server	I-Device
cluster	B-Architecture
,	O
one	O
needs	O
a	O
load-balancing	O
technology	O
such	O
as	O
Network	B-Protocol
Load	I-Protocol
Balancing	I-Protocol
(	O
NLB	O
)	O
or	O
DNS	B-Protocol
round	I-Protocol
robin	I-Protocol
.	O
</s>
<s>
A	O
load	B-Application
balancing	I-Application
solution	O
will	O
distribute	O
client	O
connections	O
to	O
each	O
of	O
the	O
terminal	B-Device
servers	I-Device
.	O
</s>
<s>
Terminal	B-Device
Server	I-Device
Session	O
Directory	O
is	O
a	O
feature	O
that	O
allows	O
users	O
to	O
easily	O
and	O
automatically	O
reconnect	O
to	O
a	O
disconnected	O
session	O
in	O
a	O
load	O
balanced	O
Terminal	B-Device
Server	I-Device
farm	O
.	O
</s>
<s>
This	O
enables	O
a	O
user	O
,	O
after	O
disconnecting	O
a	O
session	O
,	O
to	O
reconnect	O
to	O
the	O
correct	O
Terminal	B-Device
Server	I-Device
where	O
the	O
disconnected	O
session	O
resides	O
in	O
order	O
to	O
resume	O
working	O
in	O
that	O
session	O
.	O
</s>
