<s>
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
(	O
SPB	O
)	O
,	O
specified	O
in	O
the	O
IEEE	B-Protocol
802.1aq	I-Protocol
standard	O
,	O
is	O
a	O
computer	B-Architecture
networking	I-Architecture
technology	O
intended	O
to	O
simplify	O
the	O
creation	O
and	O
configuration	O
of	O
Ethernet	O
networks	O
while	O
enabling	O
multipath	B-Protocol
routing	I-Protocol
.	O
</s>
<s>
It	O
is	O
the	O
replacement	O
for	O
the	O
older	O
spanning	B-Protocol
tree	I-Protocol
protocols	I-Protocol
:	O
IEEE	O
802.1D	O
,	O
IEEE	O
802.1w	O
,	O
IEEE	O
802.1s	O
.	O
</s>
<s>
These	O
blocked	O
any	O
redundant	O
paths	O
that	O
could	O
result	O
in	O
a	O
switching	B-Architecture
loop	I-Architecture
,	O
whereas	O
SPB	O
allows	O
all	O
paths	O
to	O
be	O
active	O
with	O
multiple	O
equal	O
cost	O
paths	O
,	O
provides	O
much	O
larger	O
topologies	B-Architecture
,	O
supports	O
faster	O
convergence	O
times	O
,	O
and	O
improves	O
the	O
efficiency	O
by	O
allowing	O
traffic	O
to	O
load	O
share	O
across	O
all	O
paths	O
of	O
a	O
mesh	B-Architecture
network	I-Architecture
.	O
</s>
<s>
The	O
technology	O
provides	O
VLANs	B-General_Concept
on	O
native	O
Ethernet	O
infrastructures	O
using	O
a	O
link-state	B-Protocol
protocol	I-Protocol
to	O
advertise	O
both	O
topology	B-Architecture
and	O
VLAN	B-General_Concept
membership	O
.	O
</s>
<s>
Packets	B-Protocol
are	O
encapsulated	O
at	O
the	O
edge	O
either	O
in	O
MAC-in-MAC	O
per	O
IEEE	O
802.1ah	O
or	O
tagged	O
per	O
IEEE	O
802.1Q	O
or	O
IEEE	O
802.1ad	O
and	O
transported	O
only	O
to	O
other	O
members	O
of	O
VLAN	B-General_Concept
.	O
</s>
<s>
Unicast	B-Protocol
,	O
multicast	B-Protocol
,	O
and	O
broadcast	O
are	O
supported	O
and	O
all	O
routing	O
is	O
on	O
symmetric	B-Algorithm
shortest	O
paths	O
.	O
</s>
<s>
The	O
control	O
plane	O
is	O
based	O
on	O
the	O
Intermediate	B-Protocol
System	I-Protocol
to	I-Protocol
Intermediate	I-Protocol
System	I-Protocol
(	O
IS-IS	B-Protocol
)	O
routing	B-Protocol
protocol	I-Protocol
,	O
leveraging	O
a	O
small	O
number	O
of	O
extensions	O
defined	O
in	O
.	O
</s>
<s>
On	O
4	O
March	O
2006	O
the	O
working	O
group	O
posted	O
802.1aq	B-Protocol
draft	O
0.1	O
.	O
</s>
<s>
In	O
December	O
2011	O
,	O
SPB	O
was	O
evaluated	O
by	O
the	O
Joint	O
Interoperability	O
Test	O
Command	O
(	O
JITC	O
)	O
and	O
approved	O
for	O
deployment	O
within	O
the	O
US	O
Department	O
of	O
Defense	O
(	O
DoD	O
)	O
because	O
of	O
the	O
ease	O
in	O
integrated	O
OA&M	B-Architecture
and	O
interoperability	O
with	O
current	O
protocols	O
.	O
</s>
<s>
In	O
March	O
2012	O
the	O
IEEE	O
approved	O
the	O
802.1aq	B-Protocol
standard	O
.	O
</s>
<s>
The	O
2014	O
Winter	O
Olympics	O
were	O
the	O
first	O
"	O
fabric-enabled	O
"	O
Games	O
using	O
SPB	O
"	O
IEEE	B-Protocol
802.1aq	I-Protocol
"	O
technology	O
.	O
</s>
<s>
IEEE	O
802.1Q-2014	O
-	O
Bridges	O
and	O
Bridged	O
Networks	O
-	O
This	O
standard	O
incorporates	O
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
(	O
IEEE	B-Protocol
802.1aq	I-Protocol
)	O
with	O
the	O
following	O
:	O
IEEE	O
Std	O
802.1Q-2011	O
,	O
IEEE	O
Std	O
802.1Qbe-2011	O
,	O
IEEE	O
Std	O
802.1Qbc-2011	O
,	O
IEEE	O
Std	O
802.1Qbb-2011	O
,	O
IEEE	O
Std	O
802.1Qaz-2011	O
,	O
IEEE	O
Std	O
802.1Qbf-2011	O
,	O
IEEE	O
Std	O
802.1Qbg-2012	O
,	O
IEEE	O
Std	O
802.1Q-2011/Cor	O
2	O
–	O
2012	O
,	O
and	O
IEEE	O
Std	O
802.1Qbp-2014	O
,	O
and	O
much	O
functionality	O
previously	O
specified	O
in	O
802.1D	O
.	O
</s>
<s>
The	O
Intermediate	B-Protocol
System	I-Protocol
to	I-Protocol
Intermediate	I-Protocol
System	I-Protocol
(	O
IS-IS	B-Protocol
)	O
protocol	O
,	O
as	O
defined	O
in	O
the	O
IETF	O
proposed	O
standard	O
,	O
is	O
used	O
as	O
the	O
control	O
plane	O
for	O
SPB	O
.	O
</s>
<s>
SPB	O
requires	O
no	O
state	O
machine	O
or	O
other	O
substantive	O
changes	O
to	O
IS-IS	B-Protocol
,	O
and	O
simply	O
requires	O
a	O
new	O
Network	O
Layer	O
Protocol	O
Identifier	O
(	O
NLPID	O
)	O
and	O
set	O
of	O
TLVs	B-Protocol
.	O
</s>
<s>
This	O
permits	O
SPB	O
to	O
support	O
large	O
Layer	O
2	O
topologies	B-Architecture
,	O
with	O
faster	O
convergence	O
,	O
and	O
improved	O
use	O
of	O
the	O
mesh	B-Architecture
topology	I-Architecture
when	O
compared	O
to	O
networks	O
configured	O
with	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
SPB	O
augments	O
IS-IS	B-Protocol
with	O
a	O
small	O
number	O
of	O
TLVs	B-Protocol
and	O
sub-TLVs	O
,	O
and	O
supports	O
two	O
Ethernet	O
encapsulating	O
data	O
paths	O
,	O
802.1ad	O
Provider	O
Bridges	O
(	O
PB	O
)	O
and	O
802.1ah	O
Provider	O
Backbone	O
Bridges	O
(	O
PBB	O
)	O
.	O
</s>
<s>
SPB	O
is	O
designed	O
to	O
run	O
in	O
parallel	O
with	O
other	O
network	O
layer	O
protocols	O
such	O
as	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
.	O
</s>
<s>
OSPF	B-Protocol
)	O
.	O
</s>
<s>
The	O
IS-IS	B-Protocol
extensions	O
defined	O
in	O
RFC	O
6329	O
to	O
deliver	O
standardized	O
support	O
for	O
802.1aq	B-Protocol
SPB	O
are	O
:	O
</s>
<s>
802.1aq	B-Protocol
has	O
been	O
designed	O
to	O
operate	O
in	O
parallel	O
with	O
other	O
network	O
layer	O
protocols	O
such	O
as	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
;	O
therefore	O
,	O
failure	O
of	O
two	O
nodes	O
to	O
establish	O
an	O
SPB	O
adjacency	O
will	O
not	O
cause	O
network	O
layer	O
protocols	O
to	O
also	O
reject	O
an	O
adjacency	O
.	O
</s>
<s>
802.1aq	B-Protocol
has	O
been	O
assigned	O
the	O
Network	O
Layer	O
Protocol	O
ID	O
(	O
NLPID	O
)	O
value	O
0xC1	O
,	O
as	O
per	O
RFC	O
6328	O
,	O
and	O
is	O
used	O
by	O
SPB	O
Bridges	O
to	O
indicate	O
their	O
ability	O
to	O
form	O
adjacencies	O
and	O
operate	O
as	O
part	O
of	O
a	O
802.1aq	B-Protocol
domain	O
.	O
</s>
<s>
802.1aq	B-Protocol
frames	O
flow	O
on	O
adjacencies	O
that	O
advertise	O
this	O
NLPID	O
in	O
both	O
directions	O
,	O
and	O
nodes	O
regard	O
an	O
adjacency	O
that	O
has	O
not	O
advertised	O
in	O
both	O
directions	O
as	O
non-existent	O
(	O
infinite	O
link	O
metric	O
)	O
.	O
</s>
<s>
802.1aq	B-Protocol
augments	O
the	O
normal	O
IIH	O
PDU	O
with	O
three	O
new	O
TLVs	B-Protocol
,	O
which	O
like	O
all	O
other	O
SPB	O
TLVs	B-Protocol
,	O
travel	O
within	O
Multi-Topology	O
TLVs	B-Protocol
,	O
therefore	O
allowing	O
multiple	O
logical	O
instances	O
of	O
SPB	O
within	O
a	O
single	O
IS-IS	B-Protocol
protocol	O
instance	O
.	O
</s>
<s>
For	O
the	O
purposes	O
of	O
loop	O
prevention	O
SPB	O
neighbors	O
may	O
also	O
support	O
a	O
mechanism	O
to	O
verify	O
that	O
the	O
contents	O
of	O
their	O
topology	B-Architecture
databases	O
are	O
synchronized	O
.	O
</s>
<s>
Exchanging	O
digests	O
of	O
SPB	O
topology	B-Architecture
information	O
,	O
using	O
the	O
optional	O
SPB-Digest	O
sub-TLV	O
,	O
allows	O
nodes	O
to	O
compare	O
information	O
and	O
take	O
specific	O
action	O
where	O
a	O
mismatch	O
in	O
topology	B-Architecture
is	O
indicated	O
.	O
</s>
<s>
Finally	O
,	O
SPB	O
needs	O
to	O
know	O
which	O
Shortest	O
Path	O
Tree	O
(	O
SPT	O
)	O
sets	O
are	O
being	O
used	O
by	O
which	O
VIDs	O
,	O
and	O
this	O
is	O
carried	O
in	O
the	O
Base	O
VLAN	B-General_Concept
Identifiers	O
TLV	O
.	O
</s>
<s>
All	O
SPB	O
nodal	O
information	O
extensions	O
travel	O
within	O
a	O
new	O
Multi-Topology	O
(	O
MT	O
)	O
capability	O
TLV	O
.	O
</s>
<s>
There	O
can	O
be	O
one	O
or	O
many	O
MT-Capability	O
TLVs	B-Protocol
present	O
,	O
depending	O
on	O
the	O
amount	O
of	O
information	O
that	O
needs	O
to	O
be	O
carried	O
.	O
</s>
<s>
The	O
SPB	O
Instance	O
sub-TLV	O
gives	O
the	O
Shortest	O
Path	O
Source	O
ID	O
(	O
SPSourceID	O
)	O
for	O
this	O
node/topology	O
instance	O
.	O
</s>
<s>
This	O
used	O
in	O
the	O
formation	O
of	O
Multicast	B-Protocol
Destination	O
Addresses	O
(	O
DAs	O
)	O
for	O
frames	O
originating	O
from	O
this	O
node/instance	O
.	O
</s>
<s>
There	O
are	O
multiple	O
ECT	O
algorithms	O
defined	O
for	O
SPB	O
;	O
however	O
,	O
for	O
the	O
future	O
,	O
additional	O
algorithms	O
may	O
be	O
defined	O
,	O
including	O
but	O
not	O
limited	O
to	O
ECMP	B-Protocol
-	O
or	O
hash-based	O
behaviors	O
and	O
( *	O
,	O
G	O
)	O
Multicast	B-Protocol
trees	O
.	O
</s>
<s>
The	O
SPB	O
Link	O
Metric	O
sub-TLV	O
occurs	O
within	O
the	O
Multi-Topology	O
Intermediate	O
System	O
Neighbor	O
TLV	O
or	O
within	O
the	O
Extended	O
IS	O
Reachability	O
TLV	O
.	O
</s>
<s>
Where	O
this	O
sub-TLV	O
is	O
not	O
present	O
for	O
an	O
IS-IS	B-Protocol
adjacency	O
,	O
then	O
that	O
adjacency	O
will	O
not	O
carry	O
SPB	O
traffic	O
for	O
the	O
given	O
topology	B-Architecture
instance	O
.	O
</s>
<s>
There	O
are	O
multiple	O
ECT	O
algorithms	O
defined	O
for	O
SPB	O
;	O
however	O
,	O
for	O
the	O
future	O
,	O
additional	O
algorithms	O
may	O
be	O
defined	O
;	O
similarly	O
the	O
SPB	O
Adjacency	O
Opaque	O
Equal	O
Cost	O
Tree	O
Algorithm	O
TLV	O
also	O
occurs	O
within	O
the	O
Multi-Topology	O
Intermediate	O
System	O
TLV	O
or	O
the	O
Extended	O
IS	O
Reachability	O
TLV	O
.	O
</s>
<s>
The	O
SPBM	O
Service	O
Identifier	O
and	O
Unicast	B-Protocol
Address	I-Protocol
TLV	O
is	O
used	O
to	O
introduce	O
service	O
group	O
membership	O
on	O
the	O
originating	O
node	O
and/or	O
to	O
advertise	O
an	O
additional	O
B-MAC	O
Unicast	B-Protocol
Address	I-Protocol
present	O
on	O
,	O
or	O
reachable	O
by	O
the	O
node	O
.	O
</s>
<s>
The	O
SPBV	O
MAC	O
Address	O
TLV	O
is	O
the	O
IS-IS	B-Protocol
sub-TLV	O
used	O
for	O
advertisement	O
of	O
Group	O
MAC	O
Addresses	O
in	O
SPBV	O
mode	O
.	O
</s>
<s>
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
-	O
VID	O
(	O
SPBV	O
)	O
and	O
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
-	O
MAC	O
(	O
SPBM	O
)	O
are	O
two	O
operating	O
modes	O
of	O
802.1aq	B-Protocol
,	O
and	O
are	O
described	O
in	O
more	O
detail	O
below	O
.	O
</s>
<s>
Both	O
inherit	O
key	O
benefits	O
of	O
link	B-Protocol
state	I-Protocol
routing	I-Protocol
:	O
</s>
<s>
A	O
key	O
component	O
of	O
how	O
it	O
does	O
this	O
is	O
implementing	O
VLANs	B-General_Concept
with	O
scoped	O
multicast	B-Protocol
trees	O
,	O
which	O
means	O
no	O
egress	O
discard	O
of	O
broadcast/unknown	O
traffic	O
,	O
a	O
feature	O
common	O
to	O
approaches	O
that	O
use	O
a	O
small	O
number	O
of	O
shared	O
trees	O
,	O
hence	O
the	O
network	O
does	O
not	O
simply	O
degrade	O
with	O
size	O
as	O
the	O
percentage	O
of	O
frames	O
discarded	O
goes	O
up	O
.	O
</s>
<s>
It	O
also	O
supports	O
"	O
single	O
touch	O
"	O
provisioning	O
,	O
so	O
that	O
configuration	O
is	O
simple	O
and	O
robust	O
;	O
the	O
port	O
of	O
a	O
virtual	O
server	O
must	O
simply	O
be	O
bound	O
locally	O
to	O
the	O
SPBM	O
I-SID	O
identifying	O
the	O
LAN	O
segment	O
,	O
after	O
which	O
IS-IS	B-Protocol
for	O
SPB	O
floods	O
this	O
binding	O
,	O
and	O
all	O
nodes	O
that	O
need	O
to	O
install	O
forwarding	O
state	O
to	O
implement	O
the	O
LAN	O
segment	O
do	O
so	O
automatically	O
.	O
</s>
<s>
The	O
multiple	O
virtual	B-General_Concept
LAN	I-General_Concept
segment	O
model	O
provides	O
this	O
,	O
and	O
the	O
single-touch	O
provisioning	O
model	O
eases	O
carrier	O
operations	O
.	O
</s>
<s>
This	O
latter	O
is	O
clearly	O
very	O
relevant	O
to	O
enterprise	O
customers	O
of	O
carrier	O
VPN/MPLS	O
services	O
which	O
have	O
this	O
network	O
structure	O
internally	O
.	O
</s>
<s>
MEF	O
E-TREE	O
does	O
just	O
this	O
,	O
and	O
further	O
provides	O
an	O
efficient	O
multicast	B-Protocol
fabric	O
for	O
the	O
distribution	O
of	O
IPTV	O
.	O
</s>
<s>
SPBM	O
offers	O
both	O
the	O
ideal	O
multicast	B-Protocol
replication	O
model	O
,	O
where	O
packets	B-Protocol
are	O
replicated	O
only	O
at	O
fork	O
points	O
in	O
the	O
shortest	O
path	O
tree	O
that	O
connects	O
members	O
,	O
and	O
also	O
the	O
less	O
state	O
intensive	O
head	O
end	O
replication	O
model	O
where	O
in	O
essence	O
serial	O
unicast	B-Protocol
packets	B-Protocol
are	O
sent	O
to	O
all	O
other	O
members	O
along	O
the	O
same	O
shortest	B-Algorithm
path	I-Algorithm
first	I-Algorithm
tree	O
.	O
</s>
<s>
These	O
two	O
models	O
are	O
selected	O
by	O
specifying	O
properties	O
of	O
the	O
service	O
at	O
the	O
edge	O
which	O
affect	O
the	O
transit	O
node	O
decisions	O
on	O
multicast	B-Protocol
state	O
installation	O
.	O
</s>
<s>
Head	O
end	O
replication	O
is	O
not	O
shown	O
as	O
it	O
is	O
trivial	O
and	O
simply	O
uses	O
the	O
existing	O
unicast	B-Protocol
FIBs	O
to	O
forward	O
copies	O
serially	O
to	O
the	O
known	O
other	O
receivers	O
.	O
</s>
<s>
802.1aq	B-Protocol
builds	O
on	O
all	O
existing	O
Ethernet	O
operations	B-Architecture
,	I-Architecture
administration	I-Architecture
and	I-Architecture
management	I-Architecture
(	O
OA&M	B-Architecture
)	O
.	O
</s>
<s>
Since	O
802.1aq	B-Protocol
ensures	O
that	O
its	O
unicast	B-Protocol
and	O
multicast	B-Protocol
packets	B-Protocol
for	O
a	O
given	O
virtual	B-General_Concept
LAN	I-General_Concept
(	O
VLAN	B-General_Concept
)	O
follow	O
the	O
same	O
forward	O
and	O
reverse	O
path	O
and	O
use	O
completely	O
standard	O
802	O
encapsulations	O
,	O
all	O
of	O
the	O
methods	O
of	O
802.1ag	O
and	O
Y.1731	O
operate	O
unchanged	O
on	O
an	O
802.1aq	B-Protocol
network	O
.	O
</s>
<s>
802.1aq	B-Protocol
is	O
the	O
Institute	O
of	O
Electrical	O
and	O
Electronics	O
Engineers	O
(	O
IEEE	O
)	O
sanctioned	O
link	B-Protocol
state	I-Protocol
Ethernet	O
control	O
plane	O
for	O
all	O
IEEE	O
VLANs	B-General_Concept
covered	O
in	O
IEEE	O
802.1Q	O
.	O
</s>
<s>
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
virtual	B-General_Concept
local	I-General_Concept
area	I-General_Concept
network	I-General_Concept
identifier	O
(	O
VLAN	B-General_Concept
ID	O
)	O
or	O
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
VID	O
(	O
SPBV	O
)	O
provides	O
capability	O
that	O
is	O
backwards	O
compatible	O
with	O
spanning	O
tree	O
technologies	O
.	O
</s>
<s>
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
Media	O
Access	O
Control	O
(	O
MAC	O
)	O
or	O
(	O
SPBM	O
)	O
,	O
(	O
previously	O
known	O
as	O
Provider	O
Backbone	O
Bridge	O
PBB	O
)	O
provides	O
additional	O
values	O
which	O
capitalize	O
on	O
Provider	O
Backbone	O
Bridge	O
(	O
PBB	O
)	O
capabilities	O
.	O
</s>
<s>
SPB	O
(	O
the	O
generic	O
term	O
for	O
both	O
)	O
combines	O
an	O
Ethernet	O
data	O
path	O
(	O
either	O
IEEE	O
802.1Q	O
in	O
the	O
case	O
of	O
SPBV	O
,	O
or	O
Provider	O
Backbone	O
Bridges	O
(	O
PBBs	O
)	O
IEEE	O
802.1ah	O
in	O
the	O
case	O
of	O
SPBM	O
)	O
with	O
an	O
IS-IS	B-Protocol
link	B-Protocol
state	I-Protocol
control	O
protocol	O
running	O
between	O
Shortest	O
Path	O
bridges	O
(	O
network-to-network	B-Protocol
interface	I-Protocol
(	O
NNI	O
)	O
links	O
)	O
.	O
</s>
<s>
The	O
link	B-Protocol
state	I-Protocol
protocol	I-Protocol
is	O
used	O
to	O
discover	O
and	O
advertise	O
the	O
network	B-Architecture
topology	I-Architecture
and	O
compute	O
shortest	O
path	O
trees	O
(	O
SPT	O
)	O
from	O
all	O
bridges	O
in	O
the	O
SPT	O
Region	O
.	O
</s>
<s>
Topology	B-Architecture
data	O
is	O
then	O
input	O
to	O
a	O
calculation	O
engine	O
which	O
computes	O
symmetric	B-Algorithm
shortest	O
path	O
trees	O
based	O
on	O
minimum	O
cost	O
from	O
each	O
participating	O
node	O
to	O
all	O
other	O
participating	O
nodes	O
.	O
</s>
<s>
In	O
SPBM	O
the	O
shortest	O
path	O
trees	O
are	O
then	O
used	O
to	O
populate	O
forwarding	O
tables	O
for	O
each	O
participating	O
node	O
's	O
individual	O
B-MAC	O
addresses	O
and	O
for	O
Group	O
addresses	O
;	O
Group	O
multicast	B-Protocol
trees	O
are	O
sub	O
trees	O
of	O
the	O
default	O
shortest	O
path	O
tree	O
formed	O
by	O
(	O
Source	O
,	O
Group	O
)	O
pairing	O
.	O
</s>
<s>
Depending	O
on	O
the	O
topology	B-Architecture
several	O
different	O
equal	O
cost	O
multi	O
path	O
trees	O
are	O
possible	O
and	O
SPB	O
supports	O
multiple	O
algorithms	O
per	O
IS-IS	B-Protocol
instance	O
.	O
</s>
<s>
In	O
SPB	O
as	O
with	O
other	O
link	B-Protocol
state	I-Protocol
based	O
protocols	O
,	O
the	O
computations	O
are	O
done	O
in	O
a	O
distributed	O
fashion	O
.	O
</s>
<s>
The	O
other	O
(	O
SPBV	O
)	O
is	O
intended	O
where	O
such	O
isolation	O
of	O
client	O
device	O
MAC	O
addresses	O
is	O
not	O
necessary	O
,	O
and	O
it	O
reuses	O
only	O
the	O
existing	O
VLAN	B-General_Concept
tag	O
a.k.a.	O
</s>
<s>
IEEE	O
802.1Q	O
on	O
participating	O
network-to-network	B-Protocol
interface	I-Protocol
(	O
NNI	O
)	O
links	O
.	O
</s>
<s>
At	O
the	O
time	O
the	O
specification	O
of	O
Provider	O
Backbone	O
bridging	O
was	O
progressing	O
and	O
it	O
became	O
apparent	O
that	O
leveraging	O
both	O
the	O
PBB	O
data	O
plane	O
and	O
a	O
link	B-Protocol
state	I-Protocol
control	O
plane	O
would	O
significantly	O
extend	O
Ethernet	O
's	O
capabilities	O
and	O
applications	O
.	O
</s>
<s>
Provider	O
Link	B-Protocol
State	I-Protocol
Bridging	O
(	O
PLSB	O
)	O
was	O
a	O
strawman	O
proposal	O
brought	O
to	O
the	O
IEEE	B-Protocol
802.1aq	I-Protocol
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
Working	O
Group	O
,	O
in	O
order	O
to	O
provide	O
a	O
concrete	O
example	O
of	O
such	O
a	O
system	O
.	O
</s>
<s>
As	O
IEEE	B-Protocol
802.1aq	I-Protocol
standardization	O
has	O
progressed	O
,	O
some	O
of	O
the	O
detailed	O
mechanisms	O
proposed	O
by	O
PLSB	O
have	O
been	O
replaced	O
by	O
functional	O
equivalents	O
,	O
but	O
all	O
of	O
the	O
key	O
concepts	O
embodied	O
in	O
PLSB	O
are	O
being	O
carried	O
forward	O
into	O
the	O
standard	O
.	O
</s>
<s>
Shortest	B-Protocol
Path	I-Protocol
bridging	I-Protocol
enables	O
shortest	O
path	O
trees	O
for	O
VLAN	B-General_Concept
Bridges	O
all	O
IEEE	O
802.1	O
data	O
planes	O
and	O
SPB	O
is	O
the	O
term	O
used	O
in	O
general	O
.	O
</s>
<s>
Recently	O
there	O
has	O
been	O
a	O
lot	O
of	O
focus	O
on	O
SPBM	O
as	O
explained	O
due	O
to	O
its	O
ability	O
to	O
control	O
the	O
new	O
PBB	O
data	O
plane	O
and	O
leverage	O
certain	O
capabilities	O
such	O
as	O
removing	O
the	O
need	O
to	O
do	O
B-MAC	O
learning	O
and	O
automatically	O
creating	O
individual	O
(	O
unicast	B-Protocol
)	O
and	O
group	O
(	O
multicast	B-Protocol
)	O
Trees	O
.	O
</s>
<s>
SPBV	O
was	O
actually	O
the	O
original	O
project	O
that	O
endeavored	O
to	O
enable	O
Ethernet	O
VLANs	B-General_Concept
to	O
better	O
utilize	O
mesh	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
A	O
primary	O
feature	O
of	O
Shortest	B-Protocol
Path	I-Protocol
bridging	I-Protocol
is	O
the	O
ability	O
to	O
use	O
Link	B-Protocol
State	I-Protocol
IS-IS	B-Protocol
to	O
learn	O
network	B-Architecture
topology	I-Architecture
.	O
</s>
<s>
In	O
SPBV	O
the	O
mechanism	O
used	O
to	O
identify	O
the	O
tree	O
is	O
to	O
use	O
a	O
different	O
Shortest	O
Path	O
VLAN	B-General_Concept
ID	O
(	O
VID	O
)	O
for	O
each	O
source	O
bridge	O
.	O
</s>
<s>
The	O
IS-IS	B-Protocol
topology	B-Architecture
is	O
leveraged	O
both	O
to	O
allocate	O
unique	O
SPVIDs	O
and	O
to	O
enable	O
shortest	O
path	O
forwarding	O
for	O
individual	O
and	O
group	O
addresses	O
.	O
</s>
<s>
Proponents	O
of	O
SPB	O
believe	O
that	O
Ethernet	O
can	O
leverage	O
link	B-Protocol
state	I-Protocol
and	O
maintain	O
the	O
attributes	O
that	O
have	O
made	O
Ethernet	O
one	O
of	O
the	O
most	O
encompassing	O
data	O
plane	O
transport	O
technologies	O
.	O
</s>
<s>
Ethernet	O
VLAN	B-General_Concept
bridging	O
IEEE	O
802.1Q	O
is	O
the	O
frame	O
forwarding	O
paradigm	O
that	O
fully	O
supports	O
higher	O
level	O
protocols	O
such	O
as	O
IP	O
.	O
</s>
<s>
SPB	O
defines	O
a	O
shortest	O
path	O
Region	O
which	O
is	O
the	O
boundary	O
of	O
the	O
shortest	O
path	O
topology	B-Architecture
and	O
the	O
rest	O
of	O
the	O
VLAN	B-General_Concept
topology	B-Architecture
(	O
which	O
may	O
be	O
any	O
number	O
of	O
legacy	O
bridges	O
.	O
)	O
</s>
<s>
SPBV	O
still	O
allows	O
learning	O
of	O
Ethernet	O
MAC	O
addresses	O
but	O
it	O
can	O
distribute	O
multicast	B-Protocol
address	O
that	O
can	O
be	O
used	O
to	O
prune	O
the	O
shortest	O
path	O
trees	O
according	O
to	O
the	O
multicast	B-Protocol
membership	O
either	O
through	O
Multiple	O
MAC	O
Registration	O
Protocol	O
(	O
MMRP	O
)	O
or	O
directly	O
using	O
IS-IS	B-Protocol
distribution	O
of	O
multicast	B-Protocol
membership	O
.	O
</s>
<s>
SPBV	O
builds	O
shortest	O
path	O
trees	O
but	O
also	O
interworks	O
with	O
legacy	O
bridges	O
running	O
Rapid	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
and	O
Multiple	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
SPBV	O
supports	O
shortest	O
path	O
trees	O
but	O
SPBV	O
also	O
builds	O
a	O
spanning	O
tree	O
which	O
is	O
computed	O
from	O
the	O
link	B-Protocol
state	I-Protocol
database	O
and	O
uses	O
the	O
Base	O
VID	O
.	O
</s>
<s>
SPBV	O
differs	O
from	O
SPBM	O
in	O
that	O
MAC	O
addresses	O
are	O
learned	O
on	O
all	O
bridges	O
that	O
lie	O
on	O
the	O
shortest	O
path	O
and	O
a	O
shared	O
VLAN	B-General_Concept
learning	O
is	O
used	O
since	O
destination	O
MACs	O
may	O
be	O
associated	O
with	O
multiple	O
SPVIDs	O
.	O
</s>
<s>
SPBM	O
is	O
very	O
similar	O
to	O
PLSB	O
(	O
Provider	O
Link	B-Protocol
State	I-Protocol
Bridging	O
)	O
using	O
the	O
same	O
data	O
and	O
control	O
planes	O
but	O
the	O
format	O
and	O
contents	O
of	O
the	O
control	O
messages	O
in	O
PLSB	O
are	O
not	O
compatible	O
.	O
</s>
<s>
Individual	O
MAC	O
frames	O
(	O
unicast	B-Protocol
traffic	O
)	O
from	O
an	O
Ethernet	O
attached	O
device	O
that	O
are	O
received	O
at	O
the	O
SPBM	O
edge	O
are	O
encapsulated	O
in	O
a	O
PBB	O
(	O
mac-in-mac	O
)	O
IEEE	O
802.1ah	O
header	O
and	O
then	O
traverse	O
the	O
IEEE	B-Protocol
802.1aq	I-Protocol
network	O
unchanged	O
until	O
they	O
are	O
stripped	O
of	O
the	O
encapsulation	O
as	O
they	O
egress	O
back	O
to	O
the	O
non-participating	O
attached	O
network	O
at	O
the	O
far	O
side	O
of	O
the	O
participating	O
network	O
.	O
</s>
<s>
In	O
this	O
manner	O
Ethernet	O
MAC	O
addresses	O
are	O
never	O
looked	O
up	O
in	O
the	O
core	O
of	O
an	O
IEEE	B-Protocol
802.1aq	I-Protocol
network	O
.	O
</s>
<s>
PBB	O
does	O
not	O
specify	O
how	O
B-MAC	O
addresses	O
are	O
learned	O
and	O
PBB	O
may	O
use	O
a	O
spanning	O
tree	O
to	O
control	O
the	O
B-VLAN	O
.	O
</s>
<s>
The	O
forward	O
and	O
reverse	O
paths	O
used	O
for	O
unicast	B-Protocol
and	O
multicast	B-Protocol
traffic	O
in	O
an	O
IEEE	B-Protocol
802.1aq	I-Protocol
network	O
are	O
symmetric	B-Algorithm
.	O
</s>
<s>
This	O
symmetry	O
permits	O
the	O
normal	O
Ethernet	O
Continuity	O
Fault	O
Messages	O
(	O
CFM	O
)	O
IEEE	O
802.1ag	O
to	O
operate	O
unchanged	O
for	O
SPBV	O
and	O
SPBM	O
and	O
has	O
desirable	O
properties	O
with	O
respect	O
to	O
time	O
distribution	O
protocols	O
such	O
as	O
Precision	B-Protocol
Time	I-Protocol
Protocol	I-Protocol
(	O
PTP	B-Protocol
Version	I-Protocol
2	I-Protocol
)	O
.	O
</s>
<s>
IEEE	B-Protocol
802.1aq	I-Protocol
supports	O
the	O
creation	O
of	O
thousands	O
of	O
logical	O
Ethernet	O
services	O
in	O
the	O
form	O
of	O
E-LINE	O
,	O
E-LAN	O
or	O
E-TREE	O
constructs	O
which	O
are	O
formed	O
between	O
non-participating	O
logical	O
ports	O
of	O
the	O
IEEE	B-Protocol
802.1aq	I-Protocol
network	O
.	O
</s>
<s>
These	O
group	O
address	O
packets	B-Protocol
are	O
encapsulated	O
with	O
a	O
PBB	O
header	O
which	O
indicates	O
the	O
source	O
participating	O
address	O
in	O
the	O
SA	O
while	O
the	O
DA	O
indicates	O
the	O
locally	O
significant	O
group	O
address	O
this	O
frame	O
should	O
be	O
forwarded	O
on	O
and	O
which	O
source	O
bridge	O
originated	O
the	O
frame	O
.	O
</s>
<s>
The	O
IEEE	B-Protocol
802.1aq	I-Protocol
multicast	B-Protocol
forwarding	O
tables	O
are	O
created	O
based	O
on	O
computations	O
such	O
that	O
every	O
bridge	O
which	O
is	O
on	O
the	O
shortest	O
path	O
between	O
a	O
pair	O
of	O
bridges	O
which	O
are	O
members	O
of	O
the	O
same	O
service	O
group	O
will	O
create	O
proper	O
forwarding	O
database	O
(	O
FDB	O
)	O
state	O
to	O
forward	O
or	O
replicate	O
frames	O
it	O
receives	O
to	O
that	O
members	O
of	O
that	O
service	O
group	O
.	O
</s>
<s>
Since	O
the	O
group	O
address	O
computation	O
produce	O
shortest	O
path	O
trees	O
,	O
there	O
is	O
only	O
ever	O
one	O
copy	O
of	O
a	O
multicast	B-Protocol
packet	B-Protocol
on	O
any	O
given	O
link	O
.	O
</s>
<s>
Since	O
only	O
bridges	O
on	O
a	O
shortest	O
path	O
between	O
participating	O
logical	O
ports	O
create	O
forwarding	O
database	O
(	O
FDB	O
)	O
state	O
the	O
multicast	B-Protocol
makes	O
the	O
efficient	O
use	O
of	O
network	O
resources	O
.	O
</s>
<s>
The	O
actual	O
group	O
address	O
forwarding	O
operation	O
operates	O
more	O
or	O
less	O
identically	O
to	O
classical	O
Ethernet	O
,	O
the	O
backbone	O
destination	O
address	O
(	O
B-DA	O
)	O
+	O
backbone	O
VLAN	B-General_Concept
identifier	O
(	O
B-VID	O
)	O
combination	O
are	O
looked	O
up	O
to	O
find	O
the	O
egress	O
set	O
of	O
next	O
hops	O
.	O
</s>
<s>
Learning	O
is	O
however	O
implemented	O
at	O
the	O
edges	O
of	O
the	O
SPBM	O
multicast	B-Protocol
tree	O
to	O
learn	O
the	O
B-MAC	O
to	O
MAC	O
address	O
relationship	O
for	O
correct	O
individual	O
frame	O
encapsulation	O
in	O
the	O
reverse	O
direction	O
(	O
as	O
packets	B-Protocol
arrive	O
over	O
the	O
Interface	O
)	O
.	O
</s>
<s>
Properly	O
implemented	O
an	O
IEEE	B-Protocol
802.1aq	I-Protocol
network	O
can	O
support	O
up	O
to	O
1000	O
participating	O
bridges	O
and	O
provide	O
tens	O
of	O
thousands	O
of	O
layer2	O
E-LAN	O
services	O
to	O
Ethernet	O
devices	O
.	O
</s>
<s>
As	O
new	O
members	O
come	O
and	O
go	O
,	O
the	O
IS-IS	B-Protocol
protocol	O
will	O
advertise	O
the	O
I-SID	O
membership	O
changes	O
and	O
the	O
computations	O
will	O
grow	O
or	O
shrink	O
the	O
trees	O
in	O
the	O
participating	O
node	O
network	O
as	O
necessary	O
to	O
maintain	O
the	O
efficient	O
multicast	B-Protocol
property	O
for	O
that	O
service	O
.	O
</s>
<s>
IEEE	B-Protocol
802.1aq	I-Protocol
has	O
the	O
property	O
that	O
only	O
the	O
point	O
of	O
attachment	O
of	O
a	O
service	O
needs	O
configuration	O
when	O
a	O
new	O
attachment	O
point	O
comes	O
or	O
goes	O
.	O
</s>
<s>
In	O
some	O
existing	O
implementations	O
this	O
property	O
is	O
used	O
to	O
automatically	O
(	O
as	O
opposed	O
to	O
through	O
configuration	O
)	O
add	O
or	O
remove	O
attachment	O
points	O
for	O
dual-homed	O
technologies	O
such	O
as	O
rings	O
to	O
maintain	O
optimum	O
packet	B-Protocol
flow	O
between	O
a	O
nonparticipating	O
ring	O
protocol	O
and	O
the	O
IEEE	B-Protocol
802.1aq	I-Protocol
network	O
by	O
activating	O
a	O
secondary	O
attachment	O
point	O
and	O
deactivating	O
a	O
primary	O
attachment	O
point	O
.	O
</s>
<s>
Failure	O
recovery	O
is	O
as	O
per	O
normal	O
IS-IS	B-Protocol
with	O
the	O
link	O
failure	O
being	O
advertised	O
and	O
new	O
computations	O
being	O
performed	O
,	O
resulting	O
in	O
new	O
FDB	O
tables	O
.	O
</s>
<s>
Fast	O
link	O
failure	O
detection	O
may	O
be	O
performed	O
using	O
IEEE	O
802.1ag	O
Continuity	O
Check	O
Messages	O
(	O
CCMs	O
)	O
which	O
test	O
link	O
status	O
and	O
report	O
a	O
failure	O
to	O
the	O
IS-IS	B-Protocol
protocol	O
.	O
</s>
<s>
This	O
allows	O
much	O
faster	O
failure	O
detection	O
than	O
is	O
possible	O
using	O
the	O
IS-IS	B-Protocol
hello	O
message	O
loss	O
mechanisms	O
.	O
</s>
<s>
Both	O
SPBV	O
and	O
SPBM	O
inherit	O
the	O
rapid	O
convergence	O
of	O
a	O
link	B-Protocol
state	I-Protocol
control	O
plane	O
.	O
</s>
<s>
A	O
special	O
attribute	O
of	O
SPBM	O
is	O
its	O
ability	O
to	O
rebuild	O
multicast	B-Protocol
trees	O
in	O
a	O
similar	O
time	O
to	O
unicast	B-Protocol
convergence	O
,	O
because	O
it	O
substitutes	O
computation	O
for	O
signaling	O
.	O
</s>
<s>
When	O
an	O
SPBM	O
bridge	O
has	O
performed	O
the	O
computations	O
on	O
a	O
topology	B-Architecture
database	O
,	O
it	O
knows	O
whether	O
it	O
is	O
on	O
the	O
shortest	O
path	O
between	O
a	O
root	O
and	O
one	O
or	O
more	O
leaves	O
of	O
the	O
SPT	O
and	O
can	O
install	O
state	O
accordingly	O
.	O
</s>
<s>
Convergence	O
is	O
not	O
gated	O
by	O
incremental	O
discovery	O
of	O
a	O
bridge	O
's	O
place	O
on	O
a	O
multicast	B-Protocol
tree	O
by	O
the	O
use	O
of	O
separate	O
signaling	O
transactions	O
.	O
</s>
<s>
However	O
,	O
SPBM	O
on	O
a	O
node	O
does	O
not	O
operate	O
completely	O
independently	O
of	O
its	O
peers	O
,	O
and	O
enforces	O
agreement	O
on	O
the	O
current	O
network	B-Architecture
topology	I-Architecture
with	O
its	O
peers	O
.	O
</s>
<s>
This	O
very	O
efficient	O
mechanism	O
uses	O
exchange	O
of	O
a	O
single	O
digest	O
of	O
link	B-Protocol
state	I-Protocol
covering	O
the	O
entire	O
network	O
view	O
,	O
and	O
does	O
not	O
need	O
agreement	O
on	O
each	O
path	O
to	O
each	O
root	O
individually	O
.	O
</s>
<s>
The	O
result	O
is	O
that	O
the	O
volume	O
of	O
messaging	O
exchanged	O
to	O
converge	O
the	O
network	O
is	O
in	O
proportion	O
to	O
the	O
incremental	O
change	O
in	O
topology	B-Architecture
and	O
not	O
the	O
number	O
of	O
multicast	B-Protocol
trees	O
in	O
the	O
network	O
.	O
</s>
<s>
Similarly	O
the	O
removal	O
of	O
a	O
bridge	O
,	O
which	O
might	O
involve	O
the	O
rebuilding	O
of	O
hundreds	O
to	O
thousands	O
of	O
trees	O
,	O
is	O
signaled	O
only	O
with	O
a	O
few	O
link	B-Protocol
state	I-Protocol
updates	O
.	O
</s>
<s>
Following	O
are	O
three	O
animated	O
GIFs	O
which	O
help	O
to	O
show	O
the	O
behavior	O
of	O
802.1aq	B-Protocol
.	O
</s>
<s>
ECMT	O
in	O
an	O
IEEE	B-Protocol
802.1aq	I-Protocol
network	O
is	O
more	O
predictable	O
than	O
with	O
internet	B-Protocol
protocol	I-Protocol
(	O
IP	O
)	O
or	O
multiprotocol	B-Protocol
label	I-Protocol
switching	I-Protocol
(	O
MPLS	B-Protocol
)	O
because	O
of	O
symmetry	O
between	O
the	O
forward	O
and	O
reverse	O
paths	O
.	O
</s>
<s>
The	O
choice	O
as	O
to	O
which	O
ECMT	O
path	O
will	O
be	O
used	O
is	O
therefore	O
an	O
operator	O
assigned	O
head	O
end	O
decision	O
while	O
it	O
is	O
a	O
local	O
/	O
hashing	O
decision	O
with	O
IP/MPLS	B-Protocol
.	O
</s>
<s>
IEEE	B-Protocol
802.1aq	I-Protocol
,	O
when	O
faced	O
with	O
a	O
choice	O
between	O
two	O
equal	O
link	O
cost	O
paths	O
,	O
uses	O
the	O
following	O
logic	O
for	O
its	O
first	O
ECMT	O
tie	O
breaking	O
algorithm	O
:	O
first	O
,	O
if	O
one	O
path	O
is	O
shorter	O
than	O
the	O
other	O
in	O
terms	O
of	O
hops	O
,	O
the	O
shorter	O
path	O
is	O
chosen	O
,	O
otherwise	O
,	O
the	O
path	O
with	O
the	O
minimum	O
Bridge	O
Identifier	O
{	O
BridgePriority	O
concatenated	O
with	O
(	O
IS-IS	B-Protocol
SysID	O
)	O
}	O
is	O
chosen	O
.	O
</s>
<s>
The	O
upper	O
limit	O
of	O
multipath	O
permutations	O
is	O
gated	O
by	O
the	O
number	O
of	O
B-VIDs	O
delegated	O
to	O
802.1aq	B-Protocol
operation	O
,	O
a	O
maximum	O
of	O
4094	O
,	O
although	O
the	O
number	O
of	O
useful	O
path	O
permutations	O
would	O
only	O
require	O
a	O
fraction	O
of	O
the	O
available	O
B-VID	O
space	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
non-participating	O
packets	B-Protocol
associated	O
with	O
that	O
service	O
are	O
encapsulated	O
with	O
the	O
VID	O
associated	O
with	O
the	O
desired	O
ECMT	O
end	O
to	O
end	O
path	O
.	O
</s>
<s>
Essentially	O
the	O
operator	O
decides	O
which	O
services	O
go	O
in	O
which	O
ECMT	O
paths	O
,	O
unlike	O
a	O
hashing	O
solution	O
used	O
in	O
other	O
systems	O
such	O
as	O
IP/MPLS	B-Protocol
.	O
</s>
<s>
Trees	O
can	O
support	O
link	B-Architecture
aggregation	I-Architecture
(	O
LAG	O
)	O
groups	O
within	O
a	O
tree	O
"	O
branch	O
"	O
segment	O
where	O
some	O
form	O
of	O
hashing	O
occurs	O
.	O
</s>
<s>
This	O
symmetric	B-Algorithm
and	O
end	O
to	O
end	O
ECMT	O
behavior	O
gives	O
IEEE	B-Protocol
802.1aq	I-Protocol
a	O
highly	O
predictable	O
behavior	O
and	O
off	O
line	O
engineering	O
tools	O
can	O
accurately	O
model	O
exact	O
data	O
flows	O
.	O
</s>
<s>
Such	O
computations	O
are	O
used	O
by	O
time	O
distribution	O
protocols	O
such	O
as	O
IEEE	B-Protocol
1588	I-Protocol
for	O
frequency	O
and	O
time	O
of	O
day	O
synchronization	O
as	O
required	O
between	O
precision	O
clock	O
sources	O
and	O
wireless	O
base	O
stations	O
.	O
</s>
<s>
Shown	O
above	O
are	O
three	O
figures	O
 [ 5 , 6 , 7 ] 	O
which	O
show	O
8	O
and	O
16	O
equal	O
cost	O
tree	O
(	O
ECT	O
)	O
behavior	O
in	O
different	O
network	B-Architecture
topologies	I-Architecture
.	O
</s>
<s>
These	O
are	O
composites	O
of	O
screen	O
captures	O
of	O
an	O
802.1aq	B-Protocol
network	O
emulator	O
and	O
show	O
the	O
source	O
in	O
purple	O
,	O
the	O
destination	O
in	O
yellow	O
,	O
and	O
then	O
all	O
the	O
computed	O
and	O
available	O
shortest	O
paths	O
in	O
pink	O
.	O
</s>
<s>
It	O
is	O
expected	O
that	O
numerous	O
shared	O
tree	O
models	O
will	O
also	O
be	O
defined	O
,	O
as	O
will	O
hop	O
by	O
hop	O
hash	O
based	O
equal-cost	B-Protocol
multi-path	I-Protocol
(	O
ECMP	B-Protocol
)	O
style	O
behaviors	O
..	O
all	O
defined	O
by	O
a	O
VID	O
and	O
an	O
algorithm	O
that	O
every	O
node	O
agrees	O
to	O
run	O
.	O
</s>
<s>
802.1aq	B-Protocol
does	O
not	O
spread	O
traffic	O
on	O
a	O
hop	O
by	O
hop	O
basis	O
.	O
</s>
<s>
Instead	O
,	O
802.1aq	B-Protocol
allows	O
assignment	O
of	O
a	O
Service	O
ID	O
(	O
ISID	O
)	O
to	O
a	O
VLAN	B-General_Concept
ID	O
(	O
VID	O
)	O
at	O
the	O
edge	O
of	O
the	O
network	O
.	O
</s>
<s>
The	O
deterministic	O
nature	O
of	O
the	O
routing	O
makes	O
offline	O
prediction/computation/experimentation	O
of	O
the	O
network	O
loading	O
much	O
simpler	O
since	O
actual	O
routes	O
are	O
not	O
dependent	O
on	O
the	O
contents	O
of	O
the	O
packet	B-Protocol
headers	O
with	O
the	O
exception	O
of	O
the	O
VLAN	B-General_Concept
identifier	O
.	O
</s>
<s>
We	O
will	O
work	O
through	O
SPBM	O
behavior	O
on	O
a	O
small	O
example	O
,	O
with	O
emphasis	O
on	O
the	O
shortest-path	O
trees	O
for	O
unicast	B-Protocol
and	O
multicast	B-Protocol
.	O
</s>
<s>
These	O
would	O
be	O
switches	O
or	O
routers	O
running	O
the	O
IEEE	B-Protocol
802.1aq	I-Protocol
protocol	O
.	O
</s>
<s>
Since	O
802.1aq	B-Protocol
does	O
not	O
support	O
parallel	O
interfaces	O
each	O
interface	O
corresponds	O
to	O
an	O
adjacency	O
.	O
</s>
<s>
The	O
port	O
/	O
interface	O
index	O
numbers	O
are	O
of	O
course	O
local	O
and	O
are	O
shown	O
because	O
the	O
output	O
of	O
the	O
computations	O
produce	O
an	O
interface	O
index	O
(	O
in	O
the	O
case	O
of	O
unicast	B-Protocol
)	O
or	O
a	O
set	O
of	O
interface	O
indexes	O
(	O
in	O
the	O
case	O
of	O
multicast	B-Protocol
)	O
which	O
are	O
part	O
of	O
the	O
forwarding	O
information	O
base	O
(	O
FIB	O
)	O
together	O
with	O
a	O
destination	O
MAC	O
address	O
and	O
backbone	O
VID	O
.	O
</s>
<s>
The	O
IS-IS	B-Protocol
protocol	O
runs	O
on	O
all	O
the	O
links	O
shown	O
since	O
they	O
are	O
between	O
participating	O
nodes	O
.	O
</s>
<s>
The	O
IS-IS	B-Protocol
hello	O
protocol	O
has	O
a	O
few	O
additions	O
for	O
802.1aq	B-Protocol
including	O
information	O
about	O
backbone	O
VIDs	O
to	O
be	O
used	O
by	O
the	O
protocol	O
.	O
</s>
<s>
We	O
will	O
assume	O
that	O
the	O
operator	O
has	O
chosen	O
to	O
use	O
backbone	O
VIDs	O
101	O
and	O
102	O
for	O
this	O
instance	O
of	O
802.1aq	B-Protocol
on	O
this	O
network	O
.	O
</s>
<s>
The	O
node	O
will	O
use	O
their	O
MAC	O
addresses	O
as	O
the	O
IS-IS	B-Protocol
SysId	O
and	O
join	O
a	O
single	O
IS-IS	B-Protocol
level	O
and	O
exchange	O
link-state	B-Protocol
packets	B-Protocol
(	O
LSPs	O
in	O
IS-IS	B-Protocol
terminology	O
)	O
.	O
</s>
<s>
The	O
LSPs	O
will	O
contain	O
node	O
information	O
and	O
link	O
information	O
such	O
that	O
every	O
node	O
will	O
learn	O
the	O
full	O
topology	B-Architecture
of	O
the	O
network	O
.	O
</s>
<s>
Since	O
we	O
have	O
not	O
specified	O
any	O
link	O
weights	O
in	O
this	O
example	O
,	O
the	O
IS-IS	B-Protocol
protocol	O
will	O
pick	O
a	O
default	O
link	O
metric	O
for	O
all	O
links	O
,	O
therefore	O
all	O
routing	O
will	O
be	O
minimum	O
hop	O
count	O
.	O
</s>
<s>
After	O
topology	B-Architecture
discovery	O
the	O
next	O
step	O
is	O
distributed	O
calculation	O
of	O
the	O
unicast	B-Protocol
routes	O
for	O
both	O
ECMP	B-Protocol
VIDs	O
and	O
population	O
of	O
the	O
unicast	B-Protocol
forwarding	O
tables	O
(	O
FIBs	O
)	O
.	O
</s>
<s>
802.1aq	B-Protocol
specifies	O
how	O
to	O
choose	O
two	O
of	O
them	O
:	O
the	O
first	O
is	O
referred	O
to	O
as	O
the	O
Low	O
PATH	O
ID	O
path	O
.	O
</s>
<s>
Therefore	O
,	O
each	O
node	O
on	O
that	O
path	O
will	O
create	O
a	O
forwarding	O
entry	O
toward	O
the	O
MAC	O
address	O
of	O
node	O
five	O
using	O
the	O
first	O
ECMP	B-Protocol
VID	O
101	O
.	O
</s>
<s>
Conversely	O
,	O
802.1aq	B-Protocol
specifies	O
a	O
second	O
ECMP	B-Protocol
tie-breaking	O
algorithm	O
called	O
High	O
PATH	O
ID	O
.	O
</s>
<s>
All	O
participating	O
devices	O
would	O
do	O
a	O
simple	O
DA+VID	O
lookup	O
to	O
determine	O
the	O
outgoing	O
interface	O
,	O
and	O
would	O
also	O
check	O
that	O
incoming	O
interface	O
is	O
the	O
proper	O
next	O
hop	O
for	O
the	O
packet	B-Protocol
's	O
SA+VID	O
.	O
</s>
<s>
The	O
addresses	O
of	O
the	O
participating	O
nodes	O
00:00:00:00:00:00	O
...	O
00:00:00:07:00	O
are	O
never	O
learned	O
but	O
are	O
advertised	O
by	O
IS-IS	B-Protocol
as	O
the	O
node	O
's	O
SysId	O
.	O
</s>
<s>
Unicast	B-Protocol
forwarding	O
to	O
a	O
non-participating	O
client	O
(	O
e.g.	O
</s>
<s>
Since	O
this	O
information	O
is	O
not	O
advertised	O
by	O
IEEE	B-Protocol
802.1aq	I-Protocol
it	O
has	O
to	O
be	O
learned	O
.	O
</s>
<s>
The	O
mechanism	O
for	O
learning	O
is	O
identical	O
to	O
IEEE	O
802.1ah	O
,	O
in	O
short	O
,	O
the	O
corresponding	O
outer	O
MAC	O
unicast	B-Protocol
DA	O
,	O
if	O
not	O
known	O
is	O
replaced	O
by	O
a	O
multicast	B-Protocol
DA	O
and	O
when	O
a	O
response	O
is	O
received	O
,	O
the	O
SA	O
of	O
that	O
response	O
now	O
tells	O
us	O
the	O
DA	O
to	O
use	O
to	O
reach	O
the	O
non-participating	O
node	O
that	O
sourced	O
the	O
response	O
.	O
</s>
<s>
Since	O
we	O
wish	O
to	O
group/scope	O
sets	O
of	O
non-participating	O
ports	O
into	O
services	O
and	O
prevent	O
them	O
from	O
multicasting	O
to	O
each	O
other	O
,	O
IEEE	B-Protocol
802.1aq	I-Protocol
provides	O
mechanism	O
for	O
per	O
source	O
,	O
per	O
service	O
multicast	B-Protocol
forwarding	O
and	O
defines	O
a	O
special	O
multicast	B-Protocol
destination	O
address	O
format	O
to	O
provide	O
this	O
.	O
</s>
<s>
Since	O
the	O
multicast	B-Protocol
address	O
must	O
uniquely	O
identify	O
the	O
tree	O
,	O
and	O
because	O
there	O
is	O
a	O
tree	O
per	O
source	O
per	O
unique	O
service	O
,	O
the	O
multicast	B-Protocol
address	O
contains	O
two	O
components	O
,	O
a	O
service	O
component	O
in	O
the	O
low	O
order	O
24	O
bits	O
and	O
a	O
network-wide	O
unique	O
identifier	O
in	O
the	O
upper	O
22	O
bits	O
.	O
</s>
<s>
Since	O
this	O
is	O
a	O
multicast	B-Protocol
address	O
the	O
multicast	B-Protocol
bit	O
is	O
set	O
,	O
and	O
since	O
we	O
are	O
not	O
using	O
the	O
standard	O
OUI	O
space	O
for	O
these	O
manufactured	O
addresses	O
,	O
the	O
Local	O
'	O
L	O
 '	O
bit	O
is	O
set	O
to	O
disambiguate	O
these	O
addresses	O
.	O
</s>
<s>
In	O
Figure	O
3	O
above	O
,	O
this	O
is	O
represented	O
with	O
the	O
DA=[7,O]	O
where	O
the	O
7	O
represents	O
packets	B-Protocol
originating	O
from	O
node	O
7	O
and	O
the	O
colored	O
O	O
represents	O
the	O
E-LAN	O
service	O
we	O
are	O
scoped	O
within	O
.	O
</s>
<s>
Prior	O
to	O
creating	O
multicast	B-Protocol
forwarding	O
for	O
a	O
service	O
,	O
nodes	O
with	O
ports	O
that	O
face	O
that	O
service	O
must	O
be	O
told	O
they	O
are	O
members	O
.	O
</s>
<s>
Since	O
node	O
0	O
is	O
a	O
tandem	O
between	O
nodes	O
7	O
and	O
5	O
it	O
creates	O
a	O
forwarding	O
entry	O
for	O
packets	B-Protocol
from	O
node	O
7	O
on	O
this	O
service	O
,	O
to	O
node	O
5	O
.	O
</s>
<s>
Likewise	O
,	O
since	O
it	O
is	O
a	O
tandem	O
between	O
nodes	O
7	O
and	O
4	O
it	O
creates	O
forwarding	O
state	O
from	O
node	O
7	O
for	O
packets	B-Protocol
in	O
this	O
service	O
to	O
node	O
4	O
this	O
results	O
in	O
a	O
true	O
multicast	B-Protocol
entry	O
where	O
the	O
DA/VID	O
have	O
outputs	O
on	O
two	O
interfaces	O
1	O
and	O
2	O
.	O
</s>
<s>
Node	O
2	O
on	O
the	O
other	O
hand	O
is	O
only	O
on	O
one	O
shortest	O
path	O
in	O
this	O
service	O
and	O
only	O
creates	O
a	O
single	O
forwarding	O
entry	O
from	O
node	O
7	O
to	O
node	O
6	O
for	O
packets	B-Protocol
in	O
this	O
service	O
.	O
</s>
<s>
Figure	O
3	O
only	O
shows	O
a	O
single	O
E-LAN	O
service	O
and	O
only	O
the	O
tree	O
from	O
one	O
of	O
the	O
members	O
,	O
however	O
very	O
large	O
numbers	O
of	O
E-LAN	O
services	O
with	O
membership	O
from	O
2	O
to	O
every	O
node	O
in	O
the	O
network	O
can	O
be	O
supported	O
by	O
advertising	O
the	O
membership	O
,	O
computing	O
the	O
tandem	O
behaviors	O
,	O
manufacturing	O
the	O
known	O
multicast	B-Protocol
addresses	O
and	O
populating	O
the	O
FIBs	O
.	O
</s>
<s>
802.1aq	B-Protocol
takes	O
IS-IS	B-Protocol
topology	B-Architecture
information	O
augmented	O
with	O
service	O
attachment	O
(	O
I-SID	O
)	O
information	O
,	O
does	O
a	O
series	O
of	O
computations	O
and	O
produces	O
a	O
forwarding	O
table	O
(	O
filtering	O
table	O
)	O
for	O
unicast	B-Protocol
and	O
multicast	B-Protocol
entries	O
.	O
</s>
<s>
The	O
IS-IS	B-Protocol
extensions	O
that	O
carry	O
the	O
information	O
required	O
by	O
802.1aq	B-Protocol
are	O
given	O
in	O
the	O
isis-layer2	O
IETF	O
document	O
listed	O
below	O
.	O
</s>
<s>
An	O
implementation	O
of	O
802.1aq	B-Protocol
will	O
first	O
modify	O
the	O
IS-IS	B-Protocol
hellos	O
to	O
include	O
an	O
NLPID	O
(	O
network	O
layer	O
protocol	O
identifier	O
)	O
of	O
0xC01	O
in	O
their	O
Protocols-Supported	O
type	B-Protocol
–	I-Protocol
length	I-Protocol
–	I-Protocol
value	I-Protocol
(	O
TLV	O
)	O
(	O
type	O
129	O
)	O
which	O
has	O
been	O
reserved	O
for	O
802.1aq	B-Protocol
.	O
</s>
<s>
Note	O
that	O
NLPID	O
of	O
IP	O
is	O
not	O
required	O
to	O
form	O
an	O
adjacency	O
for	O
802.1aq	B-Protocol
but	O
also	O
will	O
not	O
prevent	O
an	O
adjacency	O
when	O
present	O
.	O
</s>
<s>
The	O
links	O
are	O
assigned	O
802.1aq	B-Protocol
specific	O
metrics	O
which	O
travel	O
in	O
their	O
own	O
TLV	O
(	O
Type	O
Length	O
Value	O
)	O
which	O
is	O
more	O
or	O
less	O
identical	O
to	O
the	O
IP	O
link	O
metrics	O
.	O
</s>
<s>
The	O
calculations	O
will	O
always	O
use	O
the	O
maximum	O
of	O
the	O
two	O
unidirectional	O
link	O
metrics	O
to	O
enforce	O
symmetric	B-Algorithm
route	O
weights	O
.	O
</s>
<s>
The	O
node	O
is	O
assigned	O
a	O
mac	O
address	O
to	O
identify	O
it	O
globally	O
and	O
this	O
is	O
used	O
to	O
form	O
the	O
IS-IS	B-Protocol
SYSID	O
.	O
</s>
<s>
The	O
Area-Id	O
is	O
not	O
directly	O
used	O
by	O
802.1aq	B-Protocol
but	O
should	O
,	O
of	O
course	O
,	O
be	O
the	O
same	O
for	O
nodes	O
in	O
the	O
same	O
802.1aq	B-Protocol
network	O
.	O
</s>
<s>
The	O
SPSourceID	O
and	O
the	O
ECMT	O
assignments	O
to	O
B-VIDs	O
are	O
then	O
advertised	O
into	O
the	O
IS-IS	B-Protocol
network	O
in	O
their	O
own	O
802.1aq	B-Protocol
TLV	O
.	O
</s>
<s>
The	O
802.1aq	B-Protocol
computations	O
are	O
restricted	O
to	O
links	O
between	O
nodes	O
that	O
have	O
an	O
802.1aq	B-Protocol
link	O
weight	O
and	O
which	O
support	O
the	O
NLPID	O
0xC01	O
.	O
</s>
<s>
As	O
previously	O
discussed	O
the	O
link	O
weights	O
are	O
forced	O
to	O
be	O
symmetric	B-Algorithm
for	O
the	O
purpose	O
of	O
computation	O
by	O
taking	O
the	O
min	O
of	O
two	O
dissimilar	O
values	O
.	O
</s>
<s>
When	O
an	O
802.1aq	B-Protocol
node	O
receives	O
an	O
IS-IS	B-Protocol
update	O
it	O
will	O
compute	O
the	O
unique	O
shortest	O
path	O
to	O
all	O
other	O
IS-IS	B-Protocol
nodes	O
that	O
support	O
802.1aq	B-Protocol
.	O
</s>
<s>
There	O
will	O
be	O
one	O
unique	O
(	O
symmetric	B-Algorithm
)	O
shortest	O
path	O
per	O
ECMT	O
behavior	O
.	O
</s>
<s>
The	O
unicast	B-Protocol
FDB/FIB	O
will	O
be	O
populated	O
based	O
on	O
this	O
first	O
shortest	O
path	O
computation	O
.	O
</s>
<s>
The	O
transit	O
multicast	B-Protocol
computation	O
(	O
which	O
only	O
applies	O
when	O
transit	O
replication	O
is	O
desired	O
and	O
not	O
applicable	O
to	O
services	O
that	O
have	O
chosen	O
head	O
end	O
replication	O
)	O
can	O
be	O
implemented	O
in	O
many	O
ways	O
,	O
care	O
must	O
be	O
taken	O
to	O
keep	O
this	O
efficient	O
,	O
but	O
in	O
general	O
a	O
series	O
of	O
shortest	O
path	O
computations	O
must	O
be	O
done	O
.	O
</s>
<s>
For	O
ISIDs	O
that	O
have	O
chosen	O
head-end	O
replication	O
the	O
computation	O
is	O
trivial	O
and	O
involves	O
simply	O
finding	O
the	O
other	O
attachment	O
points	O
that	O
receive	O
that	O
ISID	O
and	O
creating	O
a	O
serial	O
unicast	B-Protocol
table	O
to	O
replicate	O
to	O
them	O
one	O
by	O
one	O
.	O
</s>
<s>
802.1aq	B-Protocol
must	O
produce	O
deterministic	O
symmetric	B-Algorithm
downstream	O
congruent	O
shortest	O
paths	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
keep	O
this	O
in	O
mind	O
when	O
trying	O
to	O
design	O
other	O
ECMT	O
algorithms	O
for	O
802.1aq	B-Protocol
as	O
this	O
is	O
an	O
easy	O
trap	O
to	O
fall	O
into	O
.	O
</s>
<s>
It	O
begins	O
by	O
taking	O
the	O
unidirectional	O
link	O
metrics	O
that	O
are	O
advertised	O
by	O
ISIS	O
for	O
802.1aq	B-Protocol
and	O
ensuring	O
that	O
they	O
are	O
symmetric	B-Algorithm
.	O
</s>
<s>
The	O
802.1aq	B-Protocol
standard	O
describes	O
a	O
mechanism	O
called	O
a	O
PATHID	O
which	O
is	O
a	O
network-wide	O
unique	O
identifier	O
for	O
a	O
path	O
.	O
</s>
<s>
802.1aq	B-Protocol
simply	O
always	O
picks	O
the	O
lowest	O
PATHID	O
path	O
when	O
a	O
choice	O
presents	O
itself	O
in	O
the	O
shortest	O
path	O
computations	O
.	O
</s>
<s>
The	O
low	O
PATHID	O
algorithm	O
is	O
the	O
basis	O
of	O
all	O
802.1aq	B-Protocol
tie	O
breaking	O
.	O
</s>
<s>
ECT-MASK[0]	O
is	O
reserved	O
for	O
a	O
common	O
spanning	O
tree	O
algorithm	O
,	O
while	O
ECT-MASK[1]	O
creates	O
the	O
Low	O
PATHID	O
set	O
of	O
shortest	B-Algorithm
path	I-Algorithm
first	I-Algorithm
trees	O
,	O
ECT-MASK[2]	O
creates	O
the	O
High	O
PATHID	O
set	O
of	O
shortest	O
path	O
trees	O
and	O
the	O
other	O
indexes	O
create	O
other	O
relatively	O
diverse	O
permutations	O
of	O
shortest	B-Algorithm
path	I-Algorithm
first	I-Algorithm
trees	O
.	O
</s>
<s>
The	O
above	O
description	O
gives	O
an	O
easy	O
to	O
understand	O
way	O
to	O
view	O
the	O
tie	O
breaking	O
;	O
an	O
actual	O
implementation	O
simply	O
backtracks	O
from	O
the	O
fork	O
point	O
to	O
the	O
join	O
point	O
in	O
two	O
competing	O
equal-cost	O
paths	O
(	O
usually	O
during	O
the	O
Dijkstra	B-Algorithm
shortest	I-Algorithm
path	I-Algorithm
computation	O
)	O
and	O
picks	O
the	O
path	O
traversing	O
the	O
lowest	O
(	O
after	O
masking	O
)	O
BridgePriority|SysId	O
.	O
</s>
<s>
The	O
first	O
public	O
interoperability	O
tests	O
of	O
IEEE	B-Protocol
802.1aq	I-Protocol
were	O
held	O
in	O
Ottawa	O
in	O
October	O
2010	O
.	O
</s>
<s>
Two	O
vendors	O
provided	O
SPBM	O
implementations	O
and	O
a	O
total	O
of	O
5	O
physical	O
switches	O
and	O
32	O
emulated	O
switches	O
were	O
tested	O
for	O
control/data	O
and	O
OA&M	B-Architecture
.	O
</s>
<s>
MC-LAG	B-Protocol
,	O
VXLAN	B-Protocol
,	O
and	O
QFabric	B-Architecture
have	O
all	O
been	O
proposed	O
,	O
but	O
the	O
IETF	O
TRILL	B-Protocol
standard	O
(	O
Transparent	O
Interconnect	O
of	O
Lots	O
of	O
Links	O
)	O
is	O
considered	O
the	O
major	O
competitor	O
of	O
IEEE	B-Protocol
802.1aq	I-Protocol
,	O
and	O
:	O
"	O
the	O
evaluation	O
of	O
relative	O
merits	O
and	O
difference	O
of	O
the	O
two	O
standards	O
proposals	O
is	O
currently	O
a	O
hotly	O
debated	O
topic	O
in	O
the	O
networking	O
industry.	O
"	O
</s>
<s>
Extreme	O
Networks	O
,	O
by	O
virtue	O
of	O
their	O
acquisition	O
of	O
the	O
Avaya	O
Networking	O
business	O
and	O
assets	O
,	O
is	O
currently	O
the	O
leading	O
exponent	O
of	O
SPB-based	O
deployments	O
;	O
their	O
enhanced	O
and	O
extended	O
implementation	O
of	O
SPB	O
-	O
including	O
integrated	O
Layer	O
3	O
IP	O
Routing	O
and	O
IP	B-Protocol
Multicast	I-Protocol
functionality	O
-	O
is	O
marketed	O
under	O
the	O
banner	O
of	O
the	O
"	O
Fabric	B-General_Concept
Connect	I-General_Concept
"	O
technology	O
.	O
</s>
<s>
Additionally	O
,	O
Extreme	O
Networks	O
is	O
supporting	O
an	O
IETF	O
Internet	O
Draft	O
Draft	O
that	O
defines	O
a	O
means	O
of	O
automatically	O
extended	O
SPBM-based	O
services	O
to	O
end-devices	O
via	O
conventional	O
Ethernet	O
Switches	O
,	O
leveraging	O
an	O
802.1AB	O
LLDP-based	O
communications	O
protocol	O
;	O
this	O
capability	O
-	O
marketing	O
"	O
Fabric	O
Attach	O
"	O
technology	O
-	O
allows	O
for	O
the	O
automatic	O
attachment	O
of	O
end-devices	O
,	O
and	O
includes	O
dynamic	O
configuration	O
of	O
VLAN/I	O
-SID	O
(	O
VSN	O
)	O
mappings	O
.	O
</s>
