<s>
Intermediate	B-Protocol
System	I-Protocol
to	I-Protocol
Intermediate	I-Protocol
System	I-Protocol
(	O
IS-IS	B-Protocol
,	O
also	O
written	O
ISIS	O
)	O
is	O
a	O
routing	B-Protocol
protocol	I-Protocol
designed	O
to	O
move	O
information	O
efficiently	O
within	O
a	O
computer	B-Architecture
network	I-Architecture
,	O
a	O
group	O
of	O
physically	O
connected	O
computers	O
or	O
similar	O
devices	O
.	O
</s>
<s>
It	O
accomplishes	O
this	O
by	O
determining	O
the	O
best	O
route	B-Protocol
for	O
data	O
through	O
a	O
packet	O
switching	O
network	O
.	O
</s>
<s>
The	O
IS-IS	B-Protocol
protocol	O
is	O
defined	O
in	O
ISO/IEC	O
10589:2002	O
as	O
an	O
international	O
standard	O
within	O
the	O
Open	O
Systems	O
Interconnection	O
(	O
OSI	O
)	O
reference	O
design	O
.	O
</s>
<s>
The	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
republished	O
IS-IS	B-Protocol
in	O
,	O
but	O
that	O
RFC	O
was	O
later	O
marked	O
as	O
historic	O
by	O
because	O
it	O
republished	O
a	O
draft	O
rather	O
than	O
a	O
final	O
version	O
of	O
the	O
(	O
International	O
Organization	O
for	O
Standardization	O
)	O
ISO	O
standard	O
,	O
causing	O
confusion	O
.	O
</s>
<s>
IS-IS	B-Protocol
has	O
been	O
called	O
"	O
the	O
de	O
facto	O
standard	O
for	O
large	O
service	O
provider	O
network	O
backbones.	O
"	O
</s>
<s>
IS-IS	B-Protocol
is	O
an	O
interior	O
gateway	O
protocol	O
,	O
designed	O
for	O
use	O
within	O
an	O
administrative	O
domain	O
or	O
network	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
exterior	O
gateway	O
protocols	O
,	O
primarily	O
Border	O
Gateway	O
Protocol	O
(	O
BGP	O
)	O
,	O
which	O
is	O
used	O
for	O
routing	B-Protocol
between	O
autonomous	O
systems	O
(	O
RFC	O
1930	O
)	O
.	O
</s>
<s>
IS-IS	B-Protocol
is	O
a	O
link-state	O
routing	B-Protocol
protocol	I-Protocol
,	O
operating	O
by	O
reliably	O
flooding	O
link	O
state	O
information	O
throughout	O
a	O
network	O
of	O
routers	O
.	O
</s>
<s>
Each	O
IS-IS	B-Protocol
router	O
independently	O
builds	O
a	O
database	O
of	O
the	O
network	O
's	O
topology	O
,	O
aggregating	O
the	O
flooded	O
network	O
information	O
.	O
</s>
<s>
Like	O
the	O
OSPF	B-Protocol
protocol	O
,	O
IS-IS	B-Protocol
uses	O
Dijkstra	B-Algorithm
's	I-Algorithm
algorithm	I-Algorithm
for	O
computing	O
the	O
best	O
path	O
through	O
the	O
network	O
.	O
</s>
<s>
Packets	O
(	O
datagrams	B-Protocol
)	O
are	O
then	O
forwarded	O
,	O
based	O
on	O
the	O
computed	O
ideal	O
path	O
,	O
through	O
the	O
network	O
to	O
the	O
destination	O
.	O
</s>
<s>
The	O
IS-IS	B-Protocol
protocol	O
was	O
developed	O
by	O
a	O
team	O
of	O
people	O
working	O
at	O
Digital	O
Equipment	O
Corporation	O
as	O
part	O
of	O
DECnet	B-Protocol
Phase	O
V	O
.	O
It	O
was	O
standardized	O
by	O
the	O
ISO	O
in	O
1992	O
as	O
ISO	B-Protocol
10589	I-Protocol
for	O
communication	O
between	O
network	O
devices	O
that	O
are	O
termed	O
Intermediate	O
Systems	O
(	O
as	O
opposed	O
to	O
end	O
systems	O
or	O
hosts	O
)	O
by	O
the	O
ISO	O
.	O
</s>
<s>
The	O
purpose	O
of	O
IS-IS	B-Protocol
was	O
to	O
make	O
possible	O
the	O
routing	B-Protocol
of	O
datagrams	B-Protocol
using	O
the	O
ISO-developed	O
OSI	O
protocol	B-Protocol
stack	I-Protocol
called	O
CLNS	B-Protocol
.	O
</s>
<s>
IS-IS	B-Protocol
was	O
developed	O
at	O
roughly	O
the	O
same	O
time	O
that	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
IETF	O
was	O
developing	O
a	O
similar	O
protocol	O
called	O
OSPF	B-Protocol
.	O
</s>
<s>
IS-IS	B-Protocol
was	O
later	O
extended	O
to	O
support	O
routing	B-Protocol
of	O
datagrams	B-Protocol
in	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
,	O
the	O
Network	B-Application
Layer	I-Application
protocol	O
of	O
the	O
global	O
Internet	O
.	O
</s>
<s>
IS-IS	B-Protocol
adjacency	O
can	O
be	O
either	O
broadcast	O
or	O
point-to-point	O
.	O
</s>
<s>
The	O
IS-IS	B-Protocol
hello	O
packets	O
needs	O
to	O
be	O
exchanged	O
periodically	O
between	O
2	O
routers	O
to	O
establish	O
adjacency	O
.	O
</s>
<s>
This	O
contains	O
the	O
actual	O
route	B-Protocol
information	O
.	O
</s>
<s>
Both	O
IS-IS	B-Protocol
and	O
Open	B-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
(	O
OSPF	B-Protocol
)	O
are	O
link-state	O
protocols	O
,	O
and	O
both	O
use	O
the	O
same	O
Dijkstra	B-Algorithm
algorithm	I-Algorithm
for	O
computing	O
the	O
best	O
path	O
through	O
the	O
network	O
.	O
</s>
<s>
Both	O
support	O
Classless	O
Inter-Domain	O
Routing	B-Protocol
,	O
can	O
use	O
multicast	O
to	O
discover	O
neighboring	O
routers	O
using	O
hello	O
packets	O
,	O
and	O
can	O
support	O
authentication	O
of	O
routing	B-Protocol
updates	O
.	O
</s>
<s>
OSPF	B-Protocol
was	O
natively	O
built	O
to	O
route	B-Protocol
IP	O
and	O
is	O
itself	O
a	O
protocol	O
that	O
runs	O
on	O
top	O
of	O
IP	O
,	O
and	O
OSPFv2	B-Protocol
is	O
only	O
able	O
to	O
build	O
IPv4	B-Protocol
routing	B-Protocol
tables	O
.	O
</s>
<s>
IS-IS	B-Protocol
is	O
an	O
OSI	B-Application
Layer	I-Application
3	I-Application
protocol	O
initially	O
defined	O
for	O
routing	B-Protocol
CLNS	B-Protocol
.	O
</s>
<s>
However	O
,	O
IS-IS	B-Protocol
is	O
neutral	O
regarding	O
the	O
type	O
of	O
network	O
addresses	O
for	O
which	O
it	O
can	O
route	B-Protocol
,	O
and	O
was	O
easily	O
extended	O
to	O
support	O
IPv4	B-Protocol
routing	B-Protocol
,	O
using	O
mechanisms	O
described	O
in	O
,	O
and	O
later	O
IPv6	B-Protocol
as	O
specified	O
in	O
.	O
</s>
<s>
To	O
operate	O
with	O
IPv6	B-Protocol
networks	O
,	O
the	O
OSPF	B-Protocol
protocol	O
was	O
rewritten	O
in	O
OSPF	B-Protocol
v3	O
(	O
as	O
specified	O
in	O
)	O
.	O
</s>
<s>
Both	O
OSPF	B-Protocol
and	O
IS-IS	B-Protocol
routers	O
build	O
a	O
topological	O
representation	O
of	O
the	O
network	O
.	O
</s>
<s>
This	O
map	O
indicates	O
the	O
subnets	O
which	O
each	O
IS-IS	B-Protocol
router	O
can	O
reach	O
,	O
and	O
the	O
lowest-cost	O
(	O
shortest	O
)	O
path	O
to	O
a	O
subnet	O
is	O
used	O
to	O
forward	O
traffic	O
.	O
</s>
<s>
IS-IS	B-Protocol
differs	O
from	O
OSPF	B-Protocol
in	O
the	O
way	O
that	O
"	O
areas	O
"	O
are	O
defined	O
and	O
routed	B-Protocol
between	O
.	O
</s>
<s>
IS-IS	B-Protocol
routers	O
are	O
designated	O
as	O
being	O
:	O
Level	O
1	O
(	O
intra-area	O
)	O
;	O
Level	O
2	O
(	O
inter	O
area	O
)	O
;	O
or	O
Level	O
1	O
–	O
2	O
(	O
both	O
)	O
.	O
</s>
<s>
Routing	B-Protocol
information	I-Protocol
is	O
exchanged	O
between	O
Level	O
1	O
routers	O
and	O
other	O
Level	O
1	O
routers	O
of	O
the	O
same	O
area	O
,	O
and	O
Level	O
2	O
routers	O
can	O
only	O
form	O
relationships	O
and	O
exchange	O
information	O
with	O
other	O
Level	O
2	O
routers	O
.	O
</s>
<s>
In	O
OSPF	B-Protocol
,	O
areas	O
are	O
delineated	O
on	O
the	O
interface	O
such	O
that	O
an	O
area	O
border	O
router	O
(	O
ABR	O
)	O
is	O
actually	O
in	O
two	O
or	O
more	O
areas	O
at	O
once	O
,	O
effectively	O
creating	O
the	O
borders	O
between	O
areas	O
inside	O
the	O
ABR	O
,	O
whereas	O
in	O
IS-IS	B-Protocol
area	O
borders	O
are	O
in	O
between	O
routers	O
,	O
designated	O
as	O
Level	O
2	O
or	O
Level	O
1	O
–	O
2	O
.	O
</s>
<s>
The	O
result	O
is	O
that	O
an	O
IS-IS	B-Protocol
router	O
is	O
only	O
ever	O
a	O
part	O
of	O
a	O
single	O
area	O
.	O
</s>
<s>
IS-IS	B-Protocol
also	O
does	O
not	O
require	O
Area	O
0	O
(	O
Area	O
Zero	O
)	O
to	O
be	O
the	O
backbone	O
area	O
through	O
which	O
all	O
inter-area	O
traffic	O
must	O
pass	O
.	O
</s>
<s>
The	O
logical	O
view	O
is	O
that	O
OSPF	B-Protocol
creates	O
something	O
of	O
a	O
spider	O
web	O
or	O
star	O
topology	O
of	O
many	O
areas	O
all	O
attached	O
directly	O
to	O
Area	O
Zero	O
and	O
IS-IS	B-Protocol
,	O
by	O
contrast	O
,	O
creates	O
a	O
logical	O
topology	O
of	O
a	O
backbone	O
of	O
Level	O
2	O
routers	O
with	O
branches	O
of	O
Level	O
1	O
–	O
2	O
and	O
Level	O
1	O
routers	O
forming	O
the	O
individual	O
areas	O
.	O
</s>
<s>
IS-IS	B-Protocol
also	O
differs	O
from	O
OSPF	B-Protocol
in	O
the	O
methods	O
by	O
which	O
it	O
reliably	O
floods	O
topology	O
and	O
topology	O
change	O
information	O
through	O
the	O
network	O
.	O
</s>
<s>
OSPF	B-Protocol
has	O
a	O
larger	O
set	O
of	O
extensions	O
and	O
optional	O
features	O
specified	O
in	O
the	O
protocol	O
standards	O
.	O
</s>
<s>
However	O
,	O
IS-IS	B-Protocol
is	O
easier	O
to	O
expand	O
:	O
its	O
use	O
of	O
TLV	O
data	O
allows	O
engineers	O
to	O
implement	O
support	O
for	O
new	O
techniques	O
without	O
redesigning	O
the	O
protocol	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
order	O
to	O
support	O
IPv6	B-Protocol
,	O
the	O
IS-IS	B-Protocol
protocol	O
was	O
extended	O
to	O
support	O
a	O
few	O
additional	O
TLVs	O
,	O
whereas	O
OSPF	B-Protocol
required	O
a	O
new	O
protocol	O
draft	O
(	O
OSPFv3	B-Protocol
)	O
.	O
</s>
<s>
In	O
addition	O
to	O
that	O
,	O
IS-IS	B-Protocol
is	O
less	O
"	O
chatty	O
"	O
and	O
can	O
scale	O
to	O
support	O
larger	O
networks	O
.	O
</s>
<s>
Given	O
the	O
same	O
set	O
of	O
resources	O
,	O
IS-IS	B-Protocol
can	O
support	O
more	O
routers	O
in	O
an	O
area	O
than	O
OSPF	B-Protocol
.	O
</s>
<s>
This	O
has	O
contributed	O
to	O
IS-IS	B-Protocol
as	O
an	O
ISP-scale	O
protocol	O
.	O
</s>
<s>
The	O
TCP/IP	O
implementation	O
,	O
known	O
as	O
"	O
Integrated	O
IS-IS	B-Protocol
"	O
or	O
"	O
Dual	O
IS-IS	B-Protocol
"	O
,	O
is	O
described	O
in	O
RFC	O
1195	O
.	O
</s>
<s>
IS-IS	B-Protocol
is	O
also	O
used	O
as	O
the	O
control	O
plane	O
for	O
IEEE	O
802.1aq	O
Shortest	O
Path	O
Bridging	O
(	O
SPB	O
)	O
.	O
</s>
<s>
IS-IS	B-Protocol
is	O
therefore	O
augmented	O
with	O
a	O
small	O
number	O
of	O
TLVs	O
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
and	O
802.1ah	O
Provider	O
Backbone	O
Bridges	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	B-Application
Layer	I-Application
Protocol	O
Identifier	O
(	O
NLPID	O
)	O
and	O
set	O
of	O
TLVs	O
.	O
</s>
<s>
This	O
extension	O
to	O
IS-IS	B-Protocol
is	O
defined	O
in	O
the	O
IETF	O
proposed	O
standard	O
RFC	O
6329	O
.	O
</s>
