<s>
A	O
UDP	B-Protocol
Helper	I-Protocol
Address	I-Protocol
is	O
a	O
special	O
router	B-Protocol
configuration	O
used	O
to	O
forward	O
broadcast	B-Protocol
network	O
traffic	O
from	O
a	O
client	B-Protocol
machine	I-Protocol
on	O
one	O
subnet	B-Protocol
to	O
a	O
server	B-Application
in	O
another	O
subnet	B-Protocol
.	O
</s>
<s>
The	O
Internet	B-Protocol
Protocol	I-Protocol
requires	O
every	O
network	B-Protocol
interface	I-Protocol
controller	I-Protocol
to	O
be	O
assigned	O
at	O
least	O
one	O
unique	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Groups	O
of	O
machines	O
with	O
similar	O
addresses	O
are	O
considered	O
to	O
be	O
part	O
of	O
the	O
same	O
logical	O
subnet	B-Protocol
.	O
</s>
<s>
One	O
method	O
of	O
assigning	O
IP	B-Protocol
addresses	I-Protocol
is	O
DHCP	B-Protocol
in	O
which	O
addresses	O
typically	O
are	O
issued	O
by	O
a	O
DHCP	B-Protocol
server	I-Protocol
running	O
on	O
one	O
or	O
more	O
hosts	O
.	O
</s>
<s>
If	O
one	O
of	O
these	O
machines	O
is	O
on	O
the	O
same	O
subnet	B-Protocol
as	O
its	O
clients	O
,	O
the	O
DHCP	B-Protocol
server	I-Protocol
can	O
respond	O
to	O
their	O
broadcast	B-Protocol
DHCP	B-Protocol
requests	O
and	O
issue	O
an	O
address	O
.	O
</s>
<s>
But	O
the	O
DHCP	B-Protocol
servers	I-Protocol
may	O
be	O
hosted	O
on	O
a	O
different	O
subnet	B-Protocol
and	O
,	O
by	O
default	O
,	O
most	O
routers	B-Protocol
do	O
not	O
pass	O
broadcast	B-Protocol
messages	O
to	O
nodes	O
outside	O
their	O
own	O
subnet	B-Protocol
.	O
</s>
<s>
To	O
resolve	O
this	O
,	O
a	O
UDP	B-Protocol
helper	I-Protocol
address	I-Protocol
is	O
established	O
in	O
the	O
router	B-Protocol
configuration	O
to	O
forward	O
broadcast	B-Protocol
network	O
traffic	O
outside	O
the	O
local	O
subnet	B-Protocol
.	O
</s>
<s>
If	O
a	O
DHCP	B-Protocol
client	B-Protocol
outside	O
the	O
DHCP	B-Protocol
server	I-Protocol
's	O
subnet	B-Protocol
broadcasts	O
an	O
address	O
request	O
,	O
it	O
is	O
the	O
helper	O
that	O
forwards	O
the	O
message	O
to	O
the	O
DHCP	B-Protocol
server	I-Protocol
.	O
</s>
<s>
The	O
server	B-Application
then	O
chooses	O
an	O
address	O
and	O
sends	O
the	O
client	B-Protocol
a	O
unicast	B-Protocol
message	O
,	O
using	O
the	O
helper	O
to	O
send	O
the	O
message	O
back	O
to	O
the	O
client	B-Protocol
's	O
subnet	B-Protocol
.	O
</s>
<s>
The	O
address	O
is	O
reserved	O
for	O
a	O
limited	O
time	O
while	O
the	O
DHCP	B-Protocol
server	I-Protocol
waits	O
for	O
a	O
response	O
.	O
</s>
<s>
If	O
the	O
client	B-Protocol
responds	O
with	O
another	O
broadcast	B-Protocol
message	O
,	O
the	O
DHCP	B-Protocol
server	I-Protocol
distributes	O
the	O
address	O
.	O
</s>
<s>
Helper	O
addresses	O
also	O
can	O
be	O
used	O
to	O
forward	O
other	O
UDP	B-Protocol
traffic	O
(	O
for	O
example	O
,	O
BOOTP	B-Device
)	O
.	O
</s>
<s>
Cisco	O
's	O
first	O
implementation	O
of	O
this	O
protocol	O
was	O
introduced	O
in	O
version	O
10	O
of	O
their	O
router	B-Protocol
software	O
.	O
</s>
<s>
It	O
is	O
implemented	O
through	O
the	O
use	O
of	O
the	O
router	B-Protocol
configuration	O
commands	O
ip	O
helper-address	O
and	O
ip	O
forward-protocol	O
.	O
</s>
<s>
To	O
enable	O
the	O
forwarding	O
of	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	B-Protocol
)	O
broadcasts	O
,	O
including	O
BOOTP	B-Device
,	O
received	O
on	O
an	O
interface	O
,	O
use	O
the	O
ip	O
helper-address	O
command	O
in	O
interface	O
configuration	O
mode	O
.	O
</s>
<s>
To	O
disable	O
the	O
forwarding	O
of	O
broadcast	B-Protocol
packets	O
to	O
specific	O
addresses	O
,	O
use	O
the	O
no	O
form	O
of	O
this	O
command	O
.	O
</s>
<s>
(	O
Optional	O
)	O
Enables	O
VPN	B-Application
routing	O
and	O
forwarding	O
(	O
VRF	O
)	O
instance	O
and	O
VRF	O
name	O
.	O
</s>
<s>
Destination	O
broadcast	B-Protocol
or	O
host	O
address	O
to	O
be	O
used	O
when	O
forwarding	O
UDP	B-Protocol
broadcasts	O
.	O
</s>
<s>
The	O
use	O
of	O
UDP	B-Protocol
helper	O
addresses	O
can	O
cause	O
issues	O
with	O
some	O
Windows-based	O
network	O
configurations	O
..	O
</s>
<s>
According	O
to	O
Microsoft	O
these	O
issues	O
stem	O
from	O
the	O
fact	O
that	O
ports	O
137,138	O
are	O
forwarded	O
by	O
default	O
on	O
Cisco	O
routers	B-Protocol
.	O
</s>
<s>
Since	O
these	O
ports	O
are	O
used	O
by	O
NetBIOS	B-Protocol
to	O
help	O
determine	O
network	O
configuration	O
the	O
added	O
broadcasts	O
can	O
confuse	O
the	O
system	O
.	O
</s>
