<s>
A	O
routing	B-Protocol
bridge	I-Protocol
or	O
RBridge	B-Protocol
,	O
also	O
known	O
as	O
a	O
TRILL	B-Protocol
switch	I-Protocol
,	O
is	O
a	O
network	O
device	O
that	O
implements	O
the	O
TRILL	B-Protocol
protocol	O
,	O
as	O
specified	O
by	O
the	O
IETF	O
and	O
should	O
not	O
be	O
confused	O
with	O
BRouters	B-Device
(	O
Bridging	O
Routers	O
)	O
.	O
</s>
<s>
RBridges	B-Protocol
are	O
compatible	O
with	O
previous	O
IEEE	O
802.1	O
customer	O
bridges	O
as	O
well	O
as	O
IPv4	O
and	O
IPv6	O
routers	O
and	O
end	O
nodes	O
.	O
</s>
<s>
They	O
are	O
invisible	O
to	O
current	O
IP	O
routers	O
and	O
,	O
like	O
routers	O
,	O
RBridges	B-Protocol
terminate	O
the	O
bridge	O
spanning	O
tree	O
protocol	O
.	O
</s>
<s>
The	O
RBridges	B-Protocol
in	O
a	O
campus	O
share	O
connectivity	O
information	O
amongst	O
themselves	O
using	O
the	O
IS-IS	B-Protocol
link-state	O
protocol	O
.	O
</s>
<s>
A	O
link-state	O
protocol	O
is	O
one	O
in	O
which	O
connectivity	O
is	O
broadcast	O
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	O
,	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	O
groups	O
.	O
</s>
<s>
IS-IS	B-Protocol
was	O
chosen	O
as	O
for	O
this	O
purpose	O
because	O
:	O
</s>
<s>
it	O
is	O
easy	O
to	O
extend	O
by	O
defining	O
new	O
TLV	O
(	O
type-length-value	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	O
count	O
.	O
</s>
<s>
RBridges	B-Protocol
also	O
specify	O
the	O
next	O
hop	O
RBridge	B-Protocol
as	O
the	O
frame	O
destination	O
when	O
forwarding	O
unicast	O
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>
