<s>
Virtual	B-Application
Private	I-Application
LAN	I-Application
Service	I-Application
(	O
VPLS	B-Application
)	O
is	O
a	O
way	O
to	O
provide	O
Ethernet-based	O
multipoint	O
to	O
multipoint	O
communication	O
over	O
IP	B-Protocol
or	O
MPLS	B-Protocol
networks	O
.	O
</s>
<s>
It	O
allows	O
geographically	O
dispersed	O
sites	O
to	O
share	O
an	O
Ethernet	O
broadcast	B-Architecture
domain	I-Architecture
by	O
connecting	O
sites	O
through	O
pseudowires	B-Protocol
.	O
</s>
<s>
The	O
technologies	O
that	O
can	O
be	O
used	O
as	O
pseudo-wire	B-Protocol
can	O
be	O
Ethernet	O
over	O
MPLS	B-Protocol
,	O
L2TPv3	B-Protocol
or	O
even	O
GRE	B-Protocol
.	O
</s>
<s>
There	O
are	O
two	O
IETF	O
standards	O
track	O
RFCs	O
(	O
RFC	O
4761	O
and	O
RFC	O
4762	O
)	O
describing	O
VPLS	B-Application
establishment	O
.	O
</s>
<s>
VPLS	B-Application
is	O
a	O
virtual	B-Application
private	I-Application
network	I-Application
(	O
VPN	B-Application
)	O
technology	O
.	O
</s>
<s>
In	O
contrast	O
to	O
L2TPv3	B-Protocol
,	O
which	O
allows	O
only	O
point-to-point	B-Architecture
layer	O
2	O
tunnels	O
,	O
VPLS	B-Application
allows	O
any-to-any	O
(	O
multipoint	O
)	O
connectivity	O
.	O
</s>
<s>
In	O
a	O
VPLS	B-Application
,	O
the	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
(	O
LAN	B-General_Concept
)	O
at	O
each	O
site	O
is	O
extended	O
to	O
the	O
edge	O
of	O
the	O
provider	O
network	O
.	O
</s>
<s>
The	O
provider	O
network	O
then	O
emulates	O
a	O
switch	B-Protocol
or	O
bridge	B-Protocol
to	O
connect	O
all	O
of	O
the	O
customer	O
LANs	B-General_Concept
to	O
create	O
a	O
single	O
bridged	O
LAN	B-General_Concept
.	O
</s>
<s>
VPLS	B-Application
is	O
designed	O
for	O
applications	O
that	O
require	O
multipoint	O
or	O
broadcast	O
access	O
.	O
</s>
<s>
Since	O
VPLS	B-Application
emulates	O
a	O
LAN	B-General_Concept
,	O
full	O
mesh	O
connectivity	O
is	O
required	O
.	O
</s>
<s>
There	O
are	O
two	O
methods	O
for	O
full	O
mesh	O
establishment	O
for	O
VPLS	B-Application
:	O
using	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
and	O
using	O
Label	B-Protocol
Distribution	I-Protocol
Protocol	I-Protocol
(	O
LDP	O
)	O
.	O
</s>
<s>
The	O
"	O
control	O
plane	O
"	O
is	O
the	O
means	O
by	O
which	O
provider	B-Protocol
edge	I-Protocol
(	O
PE	O
)	O
routers	O
communicate	O
for	O
auto-discovery	O
and	O
signalling	O
.	O
</s>
<s>
Auto-discovery	O
refers	O
to	O
the	O
process	O
of	O
finding	O
other	O
PE	B-Protocol
routers	I-Protocol
participating	O
in	O
the	O
same	O
VPN	B-Application
or	O
VPLS	B-Application
.	O
</s>
<s>
Signalling	O
is	O
the	O
process	O
of	O
establishing	O
pseudowires	B-Protocol
(	O
PW	O
)	O
.	O
</s>
<s>
The	O
PWs	O
constitute	O
the	O
"	O
data	O
plane	O
"	O
,	O
whereby	O
PEs	O
send	O
customer	O
VPN/VPLS	O
traffic	O
to	O
other	O
PEs	O
.	O
</s>
<s>
BGP	B-Protocol
provides	O
both	O
auto-discovery	O
and	O
signalling	O
.	O
</s>
<s>
The	O
mechanisms	O
used	O
are	O
very	O
similar	O
to	O
those	O
used	O
in	O
establishing	O
Layer-3	O
MPLS	B-Protocol
VPNs	B-Application
.	O
</s>
<s>
Each	O
PE	O
is	O
configured	O
to	O
participate	O
in	O
a	O
given	O
VPLS	B-Application
.	O
</s>
<s>
The	O
PE	O
,	O
through	O
the	O
use	O
of	O
BGP	B-Protocol
,	O
simultaneously	O
discovers	O
all	O
other	O
PEs	O
in	O
the	O
same	O
VPLS	B-Application
,	O
and	O
establishes	O
a	O
full	O
mesh	O
of	O
pseudowires	B-Protocol
to	O
those	O
PEs	O
.	O
</s>
<s>
With	O
LDP	O
,	O
each	O
PE	B-Protocol
router	I-Protocol
must	O
be	O
configured	O
to	O
participate	O
in	O
a	O
given	O
VPLS	B-Application
,	O
and	O
,	O
in	O
addition	O
,	O
be	O
given	O
the	O
addresses	O
of	O
other	O
PEs	O
participating	O
in	O
the	O
same	O
VPLS	B-Application
.	O
</s>
<s>
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
(	O
STP	O
)	O
.	O
</s>
<s>
VPLS	B-Application
is	O
thus	O
a	O
more	O
reliable	O
solution	O
for	O
linking	O
together	O
Ethernet	O
networks	O
in	O
different	O
locations	O
than	O
simply	O
connecting	O
a	O
WAN	O
link	O
to	O
Ethernet	B-Protocol
switches	I-Protocol
in	O
both	O
locations	O
.	O
</s>
<s>
VPLS	B-Application
has	O
significant	O
advantages	O
for	O
both	O
service	O
providers	O
and	O
customers	O
.	O
</s>
<s>
VPLS	B-Application
is	O
also	O
simpler	O
and	O
more	O
cost-effective	O
to	O
operate	O
than	O
a	O
traditional	O
service	O
.	O
</s>
<s>
Customers	O
benefit	O
because	O
they	O
can	O
connect	O
all	O
of	O
their	O
sites	O
to	O
an	O
Ethernet	B-Protocol
VPN	I-Protocol
that	O
provides	O
a	O
secure	O
,	O
high	O
speed	O
and	O
homogenous	O
network	O
.	O
</s>
<s>
Moreover	O
,	O
VPLS	B-Application
provides	O
a	O
logical	O
next	O
step	O
in	O
the	O
continuing	O
evolution	O
of	O
Ethernet	O
from	O
a	O
10	O
Mbit/s	O
shared	O
LAN	B-General_Concept
protocol	O
to	O
a	O
multi-Gbps	O
global	O
service	O
.	O
</s>
<s>
VPLS	B-Application
MPLS	B-Protocol
packets	O
have	O
a	O
two-label	O
stack	O
.	O
</s>
<s>
The	O
outer	O
label	O
is	O
used	O
for	O
normal	O
MPLS	B-Protocol
forwarding	O
in	O
the	O
service	O
provider	O
's	O
network	O
.	O
</s>
<s>
If	O
BGP	B-Protocol
is	O
used	O
to	O
establish	O
the	O
VPLS	B-Application
,	O
the	O
inner	O
label	O
is	O
allocated	O
by	O
a	O
PE	O
as	O
part	O
of	O
a	O
label	O
block	O
.	O
</s>
<s>
Every	O
PE	O
keeps	O
track	O
of	O
assigned	O
inner	O
label	O
,	O
and	O
associates	O
these	O
with	O
the	O
VPLS	B-Application
instance	O
.	O
</s>
<s>
PEs	O
participating	O
in	O
a	O
VPLS-based	O
VPN	B-Application
must	O
appear	O
as	O
an	O
Ethernet	B-Protocol
bridge	I-Protocol
to	O
connected	O
customer	B-Protocol
edge	I-Protocol
(	O
CE	O
)	O
devices	O
.	O
</s>
<s>
Ethernet	O
does	O
not	O
have	O
a	O
time	B-Protocol
to	I-Protocol
live	I-Protocol
(	O
TTL	O
)	O
field	O
in	O
its	O
frame	O
header	O
,	O
so	O
loop	O
avoidance	O
must	O
be	O
arranged	O
by	O
other	O
means	O
.	O
</s>
<s>
In	O
regular	O
Ethernet	O
deployments	O
,	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
is	O
used	O
for	O
this	O
.	O
</s>
<s>
In	O
VPLS	B-Application
,	O
loop	O
avoidance	O
is	O
arranged	O
by	O
the	O
following	O
rule	O
:	O
A	O
PE	O
never	O
forwards	O
a	O
frame	O
received	O
from	O
a	O
PE	O
to	O
another	O
PE	O
.	O
</s>
<s>
The	O
use	O
of	O
a	O
full	O
mesh	O
combined	O
with	O
split	B-Protocol
horizon	I-Protocol
forwarding	O
guarantees	O
a	O
loop-free	O
broadcast	B-Architecture
domain	I-Architecture
.	O
</s>
<s>
VPLS	B-Application
is	O
typically	O
used	O
to	O
link	O
a	O
large	O
number	O
of	O
sites	O
together	O
.	O
</s>
<s>
VPLS	B-Application
requires	O
a	O
full	O
mesh	O
in	O
both	O
the	O
control	O
and	O
data	O
planes	O
;	O
this	O
can	O
be	O
difficult	O
to	O
scale	O
.	O
</s>
<s>
For	O
BGP	B-Protocol
,	O
the	O
control	O
plane	O
scaling	O
issue	O
has	O
long	O
been	O
addressed	O
,	O
through	O
the	O
use	O
of	O
route	O
reflectors	O
(	O
RRs	O
)	O
.	O
</s>
<s>
RRs	O
are	O
extensively	O
used	O
in	O
the	O
context	O
of	O
Internet	O
routing	O
,	O
as	O
well	O
as	O
for	O
several	O
types	O
of	O
VPNs	B-Application
.	O
</s>
<s>
To	O
scale	O
the	O
data	O
plane	O
for	O
multicast	O
and	O
broadcast	O
traffic	O
,	O
there	O
is	O
work	O
in	O
progress	O
to	O
use	O
point-to-multipoint	B-General_Concept
LSPs	O
as	O
the	O
underlying	O
transport	O
.	O
</s>
<s>
For	O
LDP	O
,	O
a	O
method	O
of	O
subdividing	O
a	O
VPLS	B-Application
VPN	B-Application
into	O
two	O
or	O
three	O
tiered	O
hierarchical	O
networks	O
was	O
developed	O
.	O
</s>
<s>
Called	O
hierarchical	O
VPLS	B-Application
(	O
HVPLS	O
)	O
,	O
it	O
introduces	O
a	O
new	O
type	O
of	O
MPLS	B-Protocol
device	O
:	O
the	O
multi-tenant	O
unit	O
(	O
MTU	O
)	O
switch	B-Protocol
.	O
</s>
<s>
This	O
switch	B-Protocol
aggregates	O
multiple	O
customers	O
into	O
a	O
single	O
PE	O
,	O
which	O
in	O
turn	O
needs	O
only	O
one	O
control	O
and	O
data	O
plane	O
connection	O
into	O
the	O
mesh	O
.	O
</s>
<s>
HVPLS	O
(	O
LDP	O
)	O
may	O
also	O
be	O
used	O
to	O
join	O
two	O
VPLS	B-Application
mesh	O
structures	O
together	O
.	O
</s>
<s>
Without	O
using	O
HVPLS	O
,	O
every	O
node	O
in	O
each	O
VPLS	B-Application
mesh	O
must	O
become	O
meshed	O
with	O
all	O
nodes	O
in	O
the	O
other	O
VPLS	B-Application
mesh	O
.	O
</s>
<s>
Techniques	O
such	O
as	O
redundant	O
pseudowires	B-Protocol
can	O
provide	O
resiliency	O
in	O
case	O
of	O
failures	O
at	O
the	O
interconnection	O
points	O
.	O
</s>
<s>
Since	O
VPLS	B-Application
links	O
multiple	O
Ethernet	O
broadcast	B-Architecture
domains	I-Architecture
together	O
,	O
it	O
effectively	O
creates	O
a	O
much	O
larger	O
broadcast	B-Architecture
domain	I-Architecture
.	O
</s>
<s>
PE	O
devices	O
may	O
also	O
be	O
equipped	O
with	O
content-addressable	B-Data_Structure
memory	I-Data_Structure
(	O
CAM	B-Data_Structure
)	O
,	O
similar	O
to	O
high-end	O
Ethernet	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
In	O
a	O
VPLS-based	O
VPN	B-Application
with	O
a	O
large	O
number	O
of	O
sites	O
,	O
manually	O
configuring	O
every	O
participating	O
PE	O
does	O
not	O
scale	O
well	O
.	O
</s>
<s>
The	O
LDP	O
method	O
of	O
PE	O
auto-discovery	O
is	O
based	O
on	O
that	O
used	O
by	O
the	O
Label	B-Protocol
Distribution	I-Protocol
Protocol	I-Protocol
to	O
distribute	O
labels	O
across	O
P	O
and	O
PE	B-Protocol
routers	I-Protocol
within	O
a	O
single	O
autonomous	O
system	O
.	O
</s>
<s>
The	O
BGP	B-Protocol
method	O
of	O
PE	O
auto-discovery	O
is	O
based	O
on	O
that	O
used	O
by	O
Layer-3	O
MPLS	B-Protocol
VPNs	B-Application
to	O
distribute	O
VPN	B-Application
routes	O
among	O
PEs	O
participating	O
in	O
a	O
VPN	B-Application
.	O
</s>
<s>
The	O
BGP4	B-Protocol
Multi-Protocol	O
(	O
BGP-MP	O
)	O
extensions	O
are	O
used	O
to	O
distribute	O
VPN	B-Application
IDs	O
and	O
VPN-specific	O
reachability	O
information	O
.	O
</s>
<s>
Since	O
IBGP	B-Protocol
requires	O
either	O
a	O
full	O
mesh	O
of	O
BGP	B-Protocol
sessions	I-Protocol
or	O
the	O
use	O
of	O
a	O
route	O
reflector	O
,	O
enabling	O
the	O
VPN	B-Application
ID	O
in	O
a	O
participating	O
PEs	O
existing	O
BGP	B-Protocol
configuration	O
provides	O
it	O
with	O
a	O
list	O
of	O
all	O
PEs	O
in	O
that	O
VPN	B-Application
.	O
</s>
<s>
The	O
method	O
of	O
establishing	O
VPLS	B-Application
with	O
BGP	B-Protocol
described	O
above	O
accomplishes	O
both	O
auto-discovery	O
and	O
signalling	O
.	O
</s>
<s>
This	O
method	O
requires	O
ALL	O
PEs	O
to	O
be	O
configured	O
with	O
one	O
or	O
more	O
RADIUS	B-Protocol
servers	I-Protocol
to	O
use	O
.	O
</s>
<s>
When	O
the	O
first	O
CE	B-Protocol
router	I-Protocol
in	O
a	O
particular	O
VPLS	B-Application
VPN	B-Application
connects	O
to	O
the	O
PE	O
,	O
it	O
uses	O
the	O
CE	O
's	O
identification	O
to	O
request	O
authentication	O
from	O
the	O
RADIUS	B-Protocol
server	I-Protocol
.	O
</s>
<s>
In	O
addition	O
to	O
a	O
username	O
and	O
password	O
,	O
the	O
identification	O
string	O
also	O
contains	O
a	O
VPN	B-Application
name	O
and	O
an	O
optional	O
provider	O
name	O
.	O
</s>
<s>
The	O
RADIUS	B-Protocol
server	I-Protocol
keeps	O
track	O
of	O
all	O
PEs	O
that	O
requested	O
authentication	O
for	O
a	O
particular	O
VPN	B-Application
and	O
returns	O
a	O
list	O
of	O
them	O
to	O
the	O
PE	O
requesting	O
authentication	O
.	O
</s>
