<s>
A	O
route	B-General_Concept
distinguisher	I-General_Concept
is	O
an	O
address	O
qualifier	O
used	O
only	O
within	O
a	O
single	O
internet	O
service	O
provider	O
's	O
Multiprotocol	B-Protocol
Label	I-Protocol
Switching	I-Protocol
(	O
MPLS	B-Protocol
)	O
network	O
.	O
</s>
<s>
It	O
is	O
used	O
to	O
distinguish	O
the	O
distinct	O
virtual	B-Application
private	I-Application
network	I-Application
(	O
VPN	B-Application
)	O
routes	O
of	O
separate	O
customers	O
who	O
connect	O
to	O
the	O
provider	O
.	O
</s>
<s>
The	O
route	B-General_Concept
distinguisher	I-General_Concept
is	O
an	O
8-octet	O
field	O
prefixed	O
to	O
the	O
customer	O
's	O
Internet	B-Protocol
Protocol	I-Protocol
address	I-Protocol
(	O
IPv4	B-Protocol
)	O
.	O
</s>
<s>
The	O
resulting	O
12-octet	O
field	O
is	O
a	O
unique	O
"	O
VPN-IPv4	O
"	O
address	O
.	O
</s>
<s>
At	O
the	O
edge	O
of	O
an	O
MPLS	B-Protocol
provider	O
's	O
network	O
,	O
a	O
router	O
which	O
connects	O
to	O
a	O
customer	O
's	O
network	O
is	O
called	O
a	O
Provider	O
Edge	O
(	O
PE	O
)	O
router	O
.	O
</s>
<s>
Within	O
an	O
MPLS	B-Protocol
network	O
,	O
a	O
PE	O
router	O
needs	O
to	O
be	O
configured	O
to	O
associate	O
each	O
route	B-General_Concept
distinguisher	I-General_Concept
with	O
routes	O
which	O
lead	O
to	O
a	O
particular	O
CE	O
router	O
.	O
</s>
<s>
The	O
PE	O
router	O
may	O
be	O
configured	O
to	O
associate	O
all	O
routes	O
leading	O
to	O
the	O
same	O
CE	O
router	O
with	O
the	O
same	O
route	B-General_Concept
distinguisher	I-General_Concept
,	O
or	O
it	O
may	O
be	O
configured	O
to	O
associate	O
different	O
routes	O
with	O
different	O
route	B-General_Concept
distinguishers	I-General_Concept
,	O
even	O
if	O
they	O
lead	O
to	O
the	O
same	O
CE	O
router	O
.	O
</s>
<s>
The	O
route	B-General_Concept
distinguisher	I-General_Concept
has	O
only	O
one	O
purpose	O
:	O
to	O
make	O
IPv4	B-Protocol
prefixes	O
globally	O
unique	O
.	O
</s>
<s>
It	O
is	O
not	O
used	O
for	O
IP	O
forwarding	O
by	O
the	O
provider	O
's	O
core	O
(	O
non-edge	O
)	O
routers	O
(	O
within	O
the	O
MPLS	B-Protocol
cloud	O
)	O
,	O
but	O
it	O
is	O
used	O
by	O
the	O
edge	O
routers	O
to	O
identify	O
which	O
VPN	B-Application
a	O
packet	O
belongs	O
to	O
.	O
</s>
<s>
For	O
example	O
,	O
for	O
a	O
PE	O
router	O
to	O
be	O
able	O
to	O
distinguish	O
between	O
the	O
IP	B-Protocol
address	I-Protocol
10.0.0.0	O
of	O
one	O
customer	O
from	O
the	O
10.0.0.0	O
of	O
another	O
customer	O
,	O
the	O
network	O
administrator	O
must	O
configure	O
the	O
PE	O
to	O
add	O
a	O
unique	O
route	B-General_Concept
distinguisher	I-General_Concept
to	O
each	O
packet	O
arriving	O
from	O
the	O
CEs	O
.	O
</s>
<s>
The	O
route	B-General_Concept
distinguisher	I-General_Concept
(	O
RD	O
)	O
is	O
an	O
8-octet	O
value	O
consisting	O
of	O
two	O
major	O
fields	O
,	O
the	O
Type	O
Field	O
(	O
2	O
octets	O
)	O
and	O
Value	O
Field	O
(	O
6	O
octets	O
)	O
.	O
</s>
<s>
The	O
administrator	O
field	O
must	O
contain	O
an	O
IP	B-Protocol
address	I-Protocol
(	O
using	O
private	O
IP	B-Protocol
address	I-Protocol
space	O
is	O
discouraged	O
)	O
.	O
</s>
<s>
Normally	O
the	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
used	O
by	O
the	O
provider	O
's	O
routers	O
only	O
looks	O
at	O
the	O
four-octet	O
IP	B-Protocol
address	I-Protocol
,	O
but	O
the	O
BGP	B-Protocol
Multiprotocol	O
Extensions	O
allow	O
BGP	B-Protocol
to	O
view	O
the	O
entire	O
12-octet	O
VPN-IPv4	O
address	O
,	O
and	O
carry	O
routes	O
from	O
multiple	O
"	O
address	O
families	O
"	O
.	O
</s>
<s>
If	O
the	O
route	B-General_Concept
distinguisher	I-General_Concept
Administrator	O
subfield	O
and	O
the	O
Assigned	O
Number	O
subfield	O
of	O
a	O
VPN-IPv4	O
address	O
are	O
both	O
set	O
to	O
all	O
zeroes	O
,	O
the	O
VPN-IPv4	O
address	O
is	O
considered	O
to	O
have	O
exactly	O
the	O
same	O
meaning	O
as	O
the	O
corresponding	O
globally	O
unique	O
IPv4	B-Protocol
address	O
.	O
</s>
<s>
In	O
particular	O
,	O
this	O
VPN-IPv4	O
address	O
and	O
the	O
corresponding	O
globally	O
unique	O
IPv4	B-Protocol
address	O
will	O
be	O
considered	O
comparable	O
by	O
BGP	B-Protocol
.	O
</s>
<s>
In	O
all	O
other	O
cases	O
,	O
a	O
VPN-IPv4	O
address	O
and	O
its	O
corresponding	O
globally	O
unique	O
IPv4	B-Protocol
address	O
will	O
be	O
considered	O
noncomparable	O
by	O
BGP	B-Protocol
.	O
</s>
<s>
A	O
given	O
per-site	O
forwarding	O
table	O
will	O
only	O
have	O
one	O
VPN-IPv4	O
route	O
for	O
any	O
given	O
IPv4	B-Protocol
address	O
prefix	O
.	O
</s>
<s>
When	O
a	O
packet	O
's	O
destination	O
address	O
is	O
matched	O
against	O
a	O
VPN-IPv4	O
route	O
,	O
only	O
the	O
IPv4	B-Protocol
part	O
is	O
actually	O
matched	O
.	O
</s>
