<s>
TRILL	B-Protocol
(	O
Transparent	B-Protocol
Interconnection	I-Protocol
of	I-Protocol
Lots	I-Protocol
of	I-Protocol
Links	I-Protocol
)	O
is	O
an	O
Internet	O
Standard	O
implemented	O
by	O
devices	O
called	O
TRILL	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
TRILL	B-Protocol
combines	O
techniques	O
from	O
bridging	B-Protocol
and	O
routing	B-Protocol
,	O
and	O
is	O
the	O
application	O
of	O
link-state	B-Protocol
routing	I-Protocol
to	O
the	O
VLAN-aware	O
customer-bridging	O
problem	O
.	O
</s>
<s>
Routing	B-Protocol
bridges	I-Protocol
(	O
RBridges	B-Protocol
)	O
are	O
compatible	O
with	O
and	O
can	O
incrementally	O
replace	O
previous	O
IEEE	O
802.1	O
customer	O
bridges	O
.	O
</s>
<s>
TRILL	B-Protocol
Switches	I-Protocol
are	O
also	O
compatible	O
with	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
,	O
routers	O
and	O
end	B-Protocol
systems	I-Protocol
.	O
</s>
<s>
They	O
are	O
invisible	O
to	O
current	O
IP	O
routers	O
,	O
and	O
like	O
conventional	O
routers	O
,	O
RBridges	B-Protocol
terminate	O
the	O
broadcast	B-Protocol
,	I-Protocol
unknown-unicast	I-Protocol
and	I-Protocol
multicast	I-Protocol
traffic	I-Protocol
of	O
DIX	O
Ethernet	O
and	O
the	O
frames	O
of	O
IEEE	O
802.2	O
LLC	O
including	O
the	O
bridge	O
protocol	O
data	O
units	O
of	O
the	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
TRILL	B-Protocol
is	O
the	O
successor	O
to	O
Spanning	B-Protocol
Tree	I-Protocol
Protocol	I-Protocol
,	O
both	O
having	O
been	O
created	O
by	O
the	O
same	O
person	O
,	O
Radia	O
Perlman	O
.	O
</s>
<s>
The	O
catalyst	O
for	O
TRILL	B-Protocol
was	O
an	O
event	O
at	O
Beth	O
Israel	O
Deaconess	O
Medical	O
Center	O
which	O
began	O
on	O
13	O
November	O
2002	O
.	O
</s>
<s>
The	O
concept	O
of	O
Rbridges	B-Protocol
 [ sic ] 	O
was	O
first	O
proposed	O
to	O
the	O
Institute	O
of	O
Electrical	O
and	O
Electronics	O
Engineers	O
in	O
2004	O
,	O
who	O
in	O
2005	O
rejected	O
what	O
came	O
to	O
be	O
known	O
as	O
TRILL	B-Protocol
,	O
and	O
in	O
2006	O
through	O
2012	O
devised	O
an	O
incompatible	O
variation	O
known	O
as	O
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
.	O
</s>
<s>
TRILL	B-Protocol
switches	I-Protocol
run	O
a	O
link-state	B-Protocol
routing	I-Protocol
protocol	I-Protocol
amongst	O
themselves	O
.	O
</s>
<s>
A	O
link-state	B-Protocol
protocol	I-Protocol
is	O
one	O
in	O
which	O
connectivity	O
is	O
broadcast	B-Operating_System
to	O
all	O
the	O
RBridges	B-Protocol
,	O
so	O
that	O
each	O
RBridge	B-Protocol
knows	O
about	O
all	O
the	O
other	O
RBridges	B-Protocol
,	O
and	O
the	O
connectivity	O
between	O
them	O
.	O
</s>
<s>
This	O
gives	O
RBridges	B-Protocol
enough	O
information	O
to	O
compute	O
pair-wise	O
optimal	O
paths	O
for	O
unicast	B-Protocol
,	O
and	O
calculate	O
distribution	O
trees	O
for	O
delivery	O
of	O
frames	O
either	O
to	O
destinations	O
whose	O
location	O
is	O
unknown	O
or	O
to	O
multicast	O
or	O
broadcast	B-Operating_System
groups	O
.	O
</s>
<s>
The	O
link-state	B-Protocol
routing	I-Protocol
protocol	I-Protocol
used	O
is	O
IS-IS	B-Protocol
because	O
:	O
</s>
<s>
it	O
runs	O
directly	O
over	O
layer	B-Protocol
2	I-Protocol
,	O
so	O
it	O
can	O
be	O
run	O
without	O
configuration	O
[	O
no	O
IP	O
addresses	O
need	O
be	O
assigned ]	O
,	O
as	O
it	O
is	O
inclusive	O
of	O
a	O
subset	O
of	O
the	O
Connectionless-mode	B-Protocol
Network	I-Protocol
Service	I-Protocol
(	O
CLNP	B-Protocol
)	O
.	O
</s>
<s>
it	O
is	O
easy	O
to	O
extend	O
by	O
defining	O
new	O
type	B-Protocol
–	I-Protocol
length	I-Protocol
–	I-Protocol
value	I-Protocol
(	O
TLV	O
)	O
data	O
elements	O
and	O
sub-elements	O
for	O
carrying	O
TRILL	B-Protocol
information	O
.	O
</s>
<s>
To	O
mitigate	O
temporary	O
loop	O
issues	O
,	O
RBridges	B-Protocol
forward	O
based	O
on	O
a	O
header	O
with	O
a	O
hop	B-Protocol
count	I-Protocol
.	O
</s>
<s>
RBridges	B-Protocol
also	O
specify	O
the	O
next-hop	O
RBridge	B-Protocol
as	O
the	O
frame	B-Protocol
destination	O
when	O
forwarding	O
unicast	B-Protocol
frames	O
across	O
a	O
shared-media	O
link	O
,	O
which	O
avoids	O
spawning	O
additional	O
copies	O
of	O
frames	O
during	O
a	O
temporary	O
loop	O
.	O
</s>
<s>
A	O
reverse-path	B-Protocol
forwarding	I-Protocol
check	O
and	O
other	O
checks	O
are	O
performed	O
on	O
multi-destination	O
frames	O
to	O
further	O
control	O
potentially	O
looping	O
traffic	O
.	O
</s>
<s>
The	O
first	O
RBridge	B-Protocol
that	O
a	O
unicast	B-Protocol
frame	B-Protocol
encounters	O
in	O
a	O
campus	O
,	O
RB1	O
,	O
encapsulates	O
the	O
received	O
frame	B-Protocol
with	O
a	O
TRILL	B-Protocol
header	O
that	O
specifies	O
the	O
last	O
RBridge	B-Protocol
,	O
RB2	O
,	O
where	O
the	O
frame	B-Protocol
is	O
decapsulated	O
.	O
</s>
<s>
RB1	O
is	O
known	O
as	O
the	O
"	O
ingress	O
RBridge	B-Protocol
"	O
and	O
RB2	O
is	O
known	O
as	O
the	O
"	O
egress	O
RBridge	B-Protocol
"	O
.	O
</s>
<s>
To	O
save	O
room	O
in	O
the	O
TRILL	B-Protocol
header	O
and	O
simplify	O
forwarding	O
lookups	O
,	O
a	O
dynamic	O
nickname	O
acquisition	O
protocol	O
is	O
run	O
among	O
the	O
RBridges	B-Protocol
to	O
select	O
two-octet	O
nicknames	O
for	O
RBridges	B-Protocol
,	O
unique	O
within	O
the	O
campus	O
,	O
which	O
are	O
an	O
abbreviation	O
for	O
the	O
six-octet	O
IS-IS	B-Protocol
system	O
ID	O
of	O
the	O
RBridge	B-Protocol
.	O
</s>
<s>
The	O
two-octet	O
nicknames	O
are	O
used	O
to	O
specify	O
the	O
ingress	O
and	O
egress	O
RBridges	B-Protocol
in	O
the	O
TRILL	B-Protocol
header	O
.	O
</s>
<s>
The	O
TRILL	B-Protocol
header	O
consists	O
of	O
six	O
octets	O
:	O
the	O
first	O
two	O
octets	O
include	O
a	O
six-bit	O
decrementing	O
hop	B-Protocol
count	I-Protocol
,	O
plus	O
flags	O
;	O
the	O
next	O
two	O
octets	O
contain	O
the	O
egress	O
RBridge	B-Protocol
nickname	O
;	O
the	O
final	O
two	O
octets	O
contain	O
the	O
ingress	O
RBridge	B-Protocol
nickname	O
.	O
</s>
<s>
For	O
multi-destination	O
frames	O
,	O
the	O
"	O
egress	O
RBridge	B-Protocol
nickname	O
"	O
specifies	O
a	O
distribution	O
tree	O
for	O
the	O
frame	B-Protocol
,	O
where	O
the	O
(	O
nick	O
)	O
named	O
RBridge	B-Protocol
is	O
the	O
root	O
of	O
the	O
distribution	O
tree	O
.	O
</s>
<s>
The	O
ingress	O
RBridge	B-Protocol
selects	O
which	O
distribution	O
tree	O
the	O
frame	B-Protocol
should	O
travel	O
along	O
.	O
</s>
<s>
Even	O
though	O
RBridges	B-Protocol
are	O
transparent	O
to	O
Layer	B-Application
3	I-Application
devices	O
,	O
and	O
all	O
the	O
links	O
interconnected	O
by	O
RBridges	B-Protocol
appear	O
to	O
Layer	B-Application
3	I-Application
devices	O
to	O
be	O
a	O
single	O
link	O
,	O
RBridges	B-Protocol
act	O
as	O
link	O
routers	O
in	O
the	O
sense	O
that	O
,	O
in	O
the	O
forwarding	O
of	O
a	O
frame	B-Protocol
by	O
a	O
transit	O
RBridge	B-Protocol
,	O
the	O
outer	O
Layer	B-Protocol
2	I-Protocol
header	O
is	O
replaced	O
at	O
each	O
hop	B-Protocol
with	O
an	O
appropriate	O
Layer	B-Protocol
2	I-Protocol
header	O
for	O
the	O
next	O
hop	B-Protocol
,	O
and	O
the	O
hop	B-Protocol
count	I-Protocol
is	O
decreased	O
.	O
</s>
<s>
Despite	O
these	O
modifications	O
of	O
the	O
outer	O
Layer	B-Protocol
2	I-Protocol
header	O
and	O
the	O
hop	B-Protocol
count	I-Protocol
in	O
the	O
TRILL	B-Protocol
Header	O
,	O
the	O
original	O
encapsulated	O
frame	B-Protocol
is	O
preserved	O
,	O
including	O
the	O
original	O
frame	B-Protocol
's	O
VLAN	B-General_Concept
tag	O
.	O
</s>
<s>
Multipathing	O
of	O
multi-destination	O
frames	O
through	O
alternative	O
distribution	O
tree	O
roots	O
and	O
equal-cost	B-Protocol
multi-path	I-Protocol
routing	I-Protocol
(	O
ECMP	O
)	O
of	O
unicast	B-Protocol
frames	O
are	O
supported	O
.	O
</s>
<s>
Networks	O
with	O
a	O
more	O
mesh-like	O
structure	O
benefit	O
to	O
a	O
greater	O
extent	O
from	O
the	O
multipathing	O
and	O
optimal	O
paths	O
provided	O
by	O
TRILL	B-Protocol
than	O
networks	O
with	O
a	O
more	O
tree-like	O
structure	O
.	O
</s>
<s>
A	O
host	O
with	O
multiple	O
interfaces	O
running	O
the	O
Internet	O
Protocol	O
requires	O
that	O
each	O
of	O
the	O
interfaces	O
[	O
or	O
the	O
group	O
of	O
interfaces	O
in	O
a	O
team ]	O
have	O
a	O
unique	O
address	O
,	O
whereas	O
when	O
using	O
TRILL	B-Protocol
the	O
multiple-interface	O
host	O
can	O
have	O
a	O
single	O
IP	O
address	O
for	O
all	O
of	O
its	O
interfaces	O
attached	O
to	O
a	O
common	O
broadcast	B-Operating_System
domain	O
—	O
akin	O
to	O
the	O
case	O
of	O
a	O
network	B-Protocol
service	I-Protocol
access	I-Protocol
point	I-Protocol
address	I-Protocol
(	O
NSAP	B-Protocol
)	O
on	O
an	O
end	B-Protocol
system	I-Protocol
in	O
CLNP	B-Protocol
.	O
</s>
<s>
From	O
the	O
point	O
of	O
view	O
of	O
TRILL	B-Protocol
,	O
a	O
link	O
can	O
be	O
any	O
of	O
a	O
wide	O
variety	O
of	O
link	O
technologies	O
,	O
including	O
IEEE	O
802.3	O
(	O
Ethernet	O
)	O
,	O
PPP	B-Protocol
(	O
Point	B-Protocol
to	I-Protocol
Point	I-Protocol
Protocol	I-Protocol
)	O
.	O
,	O
or	O
a	O
Pseudo-wire	B-Protocol
.	O
</s>
<s>
Ethernet	O
links	O
between	O
RBridges	B-Protocol
can	O
incorporate	O
IEEE	O
customer	O
or	O
provider	O
802.1	O
bridges	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
an	O
arbitrary	O
bridged	O
LAN	B-General_Concept
appears	O
to	O
an	O
RBridge	B-Protocol
as	O
a	O
multi-access	O
link	O
.	O
</s>
<s>
It	O
is	O
essential	O
that	O
only	O
one	O
RBridge	B-Protocol
act	O
as	O
the	O
ingress	O
RBridge	B-Protocol
for	O
any	O
given	O
native	O
frame	B-Protocol
and	O
TRILL	B-Protocol
has	O
an	O
Appointed	O
Forwarder	O
mechanism	O
to	O
assure	O
this	O
.	O
</s>
<s>
TRILL	B-Protocol
does	O
allow	O
load	O
splitting	O
of	O
this	O
duty	O
on	O
a	O
link	O
based	O
on	O
VLAN	B-General_Concept
,	O
so	O
that	O
only	O
one	O
RBridge	B-Protocol
on	O
each	O
link	O
encapsulates	O
and	O
decapsulates	O
native	O
frames	O
for	O
each	O
VLAN	B-General_Concept
.	O
</s>
<s>
RBridge	B-Protocol
ports	O
can	O
compatibly	O
implement	O
a	O
wide	O
variety	O
of	O
existing	O
and	O
proposed	O
link	O
level	O
and	O
IEEE	O
802.1	O
port	O
level	O
protocols	O
including	O
PAUSE	O
(	O
IEEE	O
802.3	O
Annex	O
31B	O
)	O
,	O
the	O
Link	B-Protocol
Layer	I-Protocol
Discovery	I-Protocol
Protocol	I-Protocol
(	O
IEEE	B-Protocol
802.1AB	I-Protocol
)	O
,	O
link	B-Architecture
aggregation	I-Architecture
(	O
IEEE	O
802.1AX	O
)	O
,	O
MAC	O
security	O
(	O
IEEE	O
802.1AE	O
)	O
,	O
or	O
port	O
based	O
access	O
control	O
(	O
IEEE	O
802.1X	O
)	O
.	O
</s>
<s>
This	O
is	O
because	O
RBridges	B-Protocol
are	O
layered	O
above	O
the	O
IEEE	O
802.1	O
EISS	O
(	O
Extended	O
Internal	O
Sublayer	O
Service	O
)	O
with	O
the	O
exception	O
that	O
an	O
RBridge	B-Protocol
port	O
handles	O
spanning	O
tree	O
and	O
VLAN	B-General_Concept
registration	O
PDUs	O
differently	O
.	O
</s>
<s>
Cisco	O
FabricPath	O
is	O
a	O
proprietary	O
implementation	O
of	O
TRILL	B-Protocol
that	O
utilizes	O
the	O
TRILL	B-Protocol
control	O
plane	O
(	O
including	O
IS-IS	B-Protocol
for	O
Layer	B-Protocol
2	I-Protocol
)	O
,	O
but	O
a	O
non-interoperable	O
data	O
plane	O
.	O
</s>
<s>
Brocade	O
Virtual	B-Architecture
Cluster	I-Architecture
Switching	I-Architecture
,	O
uses	O
the	O
TRILL	B-Protocol
data	O
plane	O
but	O
a	O
proprietary	O
control	O
plane	O
and	O
so	O
is	O
not	O
interoperable	O
with	O
standards	O
conformant	O
TRILL	B-Protocol
.	O
</s>
<s>
The	O
TRILL	B-Protocol
protocol	O
provides	O
mandatory	O
support	O
for	O
the	O
usual	O
4K	O
VLANs	B-General_Concept
and	O
can	O
optionally	O
support	O
24-bit	O
Fine	O
Grain	O
Labels	O
(	O
FGL	O
)	O
in	O
addition	O
to	O
VLANs	B-General_Concept
.	O
</s>
<s>
The	O
IEEE	B-Protocol
802.1aq	I-Protocol
standard	O
(	O
Shortest	B-Protocol
Path	I-Protocol
Bridging	I-Protocol
–	O
SPB	O
)	O
is	O
considered	O
the	O
major	O
competitor	O
of	O
TRILL	B-Protocol
.	O
</s>
