<s>
Open	B-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
(	O
OSPF	B-Protocol
)	O
is	O
a	O
routing	B-Protocol
protocol	I-Protocol
for	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	B-Protocol
)	O
networks	O
.	O
</s>
<s>
It	O
uses	O
a	O
link	B-Protocol
state	I-Protocol
routing	I-Protocol
(	O
LSR	O
)	O
algorithm	O
and	O
falls	O
into	O
the	O
group	O
of	O
interior	B-Protocol
gateway	I-Protocol
protocols	I-Protocol
(	O
IGPs	O
)	O
,	O
operating	O
within	O
a	O
single	O
autonomous	B-Architecture
system	I-Architecture
(	O
AS	B-Architecture
)	O
.	O
</s>
<s>
OSPF	B-Protocol
gathers	O
link	B-Protocol
state	I-Protocol
information	O
from	O
available	O
routers	O
and	O
constructs	O
a	O
topology	O
map	O
of	O
the	O
network	O
.	O
</s>
<s>
The	O
topology	O
is	O
presented	O
as	B-Architecture
a	O
routing	B-Protocol
table	O
to	O
the	O
Internet	B-Protocol
Layer	I-Protocol
for	O
routing	B-Protocol
packets	O
by	O
their	O
destination	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
OSPF	B-Protocol
supports	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
and	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
networks	O
and	O
supports	O
the	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
addressing	O
model	O
.	O
</s>
<s>
OSPF	B-Protocol
is	O
widely	O
used	O
in	O
large	O
enterprise	B-General_Concept
networks	I-General_Concept
.	O
</s>
<s>
IS-IS	B-Protocol
,	O
another	O
LSR-based	O
protocol	O
,	O
is	O
more	O
common	O
in	O
large	O
service	O
provider	O
networks	O
.	O
</s>
<s>
Originally	O
designed	O
in	O
the	O
1980s	O
,	O
OSPF	B-Protocol
is	O
defined	O
for	O
IPv4	B-Protocol
in	O
protocol	O
version	O
2	O
by	O
RFC	O
2328	O
(	O
1998	O
)	O
.	O
</s>
<s>
The	O
updates	O
for	O
IPv6	B-Protocol
are	O
specified	O
as	B-Architecture
OSPF	B-Protocol
Version	O
3	O
in	O
RFC	O
5340	O
(	O
2008	O
)	O
.	O
</s>
<s>
OSPF	B-Protocol
supports	O
the	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
(	O
CIDR	B-Protocol
)	O
addressing	O
model	O
.	O
</s>
<s>
OSPF	B-Protocol
is	O
an	O
interior	B-Protocol
gateway	I-Protocol
protocol	I-Protocol
(	O
IGP	O
)	O
for	O
routing	B-Protocol
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	B-Protocol
)	O
packets	O
within	O
a	O
single	O
routing	B-Protocol
domain	O
,	O
such	O
as	B-Architecture
an	O
autonomous	B-Architecture
system	I-Architecture
.	O
</s>
<s>
It	O
gathers	O
link	B-Protocol
state	I-Protocol
information	O
from	O
available	O
routers	O
and	O
constructs	O
a	O
topology	O
map	O
of	O
the	O
network	O
.	O
</s>
<s>
The	O
topology	O
is	O
presented	O
as	B-Architecture
a	O
routing	B-Protocol
table	O
to	O
the	O
Internet	B-Protocol
Layer	I-Protocol
which	O
routes	O
packets	O
based	O
solely	O
on	O
their	O
destination	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
OSPF	B-Protocol
detects	O
changes	O
in	O
the	O
topology	O
,	O
such	O
as	B-Architecture
link	O
failures	O
,	O
and	O
converges	B-Protocol
on	O
a	O
new	O
loop-free	O
routing	B-Protocol
structure	O
within	O
seconds	O
.	O
</s>
<s>
It	O
computes	O
the	O
shortest-path	O
tree	O
for	O
each	O
route	O
using	O
a	O
method	O
based	O
on	O
Dijkstra	B-Algorithm
's	I-Algorithm
algorithm	I-Algorithm
.	O
</s>
<s>
The	O
OSPF	B-Protocol
routing	B-Protocol
policies	I-Protocol
for	O
constructing	O
a	O
route	O
table	O
are	O
governed	O
by	O
link	O
metrics	O
associated	O
with	O
each	O
routing	B-Protocol
interface	O
.	O
</s>
<s>
Cost	O
factors	O
may	O
be	O
the	O
distance	O
of	O
a	O
router	O
(	O
round-trip	O
time	O
)	O
,	O
data	O
throughput	O
of	O
a	O
link	O
,	O
or	O
link	O
availability	O
and	O
reliability	O
,	O
expressed	O
as	B-Architecture
simple	O
unitless	O
numbers	O
.	O
</s>
<s>
OSPF	B-Protocol
divides	O
the	O
network	O
into	O
routing	B-Protocol
areas	O
to	O
simplify	O
administration	O
and	O
optimize	O
traffic	O
and	O
resource	O
utilization	O
.	O
</s>
<s>
Areas	O
are	O
identified	O
by	O
32-bit	O
numbers	O
,	O
expressed	O
either	O
simply	O
in	O
decimal	O
,	O
or	O
often	O
in	O
the	O
same	O
octet-based	O
dot-decimal	B-Protocol
notation	I-Protocol
used	O
for	O
IPv4	B-Protocol
addresses	O
.	O
</s>
<s>
By	O
convention	O
,	O
area	O
0	O
(	O
zero	O
)	O
,	O
or	O
0.0.0.0	O
,	O
represents	O
the	O
core	O
or	O
backbone	O
area	O
of	O
an	O
OSPF	B-Protocol
network	O
.	O
</s>
<s>
While	O
the	O
identifications	O
of	O
other	O
areas	O
may	O
be	O
chosen	O
at	O
will	O
,	O
administrators	O
often	O
select	O
the	O
IP	B-Protocol
address	I-Protocol
of	O
a	O
main	O
router	O
in	O
an	O
area	O
as	B-Architecture
the	O
area	O
identifier	O
.	O
</s>
<s>
Each	O
additional	O
area	O
must	O
have	O
a	O
connection	O
to	O
the	O
OSPF	B-Protocol
backbone	O
area	O
.	O
</s>
<s>
Such	O
connections	O
are	O
maintained	O
by	O
an	O
interconnecting	O
router	O
,	O
known	O
as	B-Architecture
an	O
area	O
border	O
router	O
(	O
ABR	O
)	O
.	O
</s>
<s>
An	O
ABR	O
maintains	O
separate	O
link-state	B-Protocol
databases	O
for	O
each	O
area	O
it	O
serves	O
and	O
maintains	O
summarized	B-Protocol
routes	I-Protocol
for	O
all	O
areas	O
in	O
the	O
network	O
.	O
</s>
<s>
OSPF	B-Protocol
runs	O
over	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
and	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
,	O
but	O
does	O
not	O
use	O
a	O
transport	B-Architecture
protocol	I-Architecture
,	O
such	O
as	B-Architecture
UDP	B-Protocol
or	O
TCP	B-Protocol
.	O
</s>
<s>
It	O
encapsulates	O
its	O
data	O
directly	O
in	O
IP	B-Protocol
packets	O
with	O
protocol	B-Protocol
number	I-Protocol
89	I-Protocol
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
other	O
routing	B-Protocol
protocols	I-Protocol
,	O
such	O
as	B-Architecture
the	O
Routing	B-Protocol
Information	I-Protocol
Protocol	I-Protocol
(	O
RIP	O
)	O
and	O
the	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
(	O
BGP	B-Protocol
)	O
.	O
</s>
<s>
OSPF	B-Protocol
implements	O
its	O
own	O
transport	O
error	O
detection	O
and	O
correction	O
functions	O
.	O
</s>
<s>
OSPF	B-Protocol
uses	O
multicast	O
addressing	O
for	O
distributing	O
route	O
information	O
within	O
a	O
broadcast	B-Architecture
domain	I-Architecture
.	O
</s>
<s>
It	O
reserves	O
the	O
multicast	O
addresses	O
224.0.0.5	O
(	O
IPv4	B-Protocol
)	O
and	O
FF02::5	O
(	O
IPv6	B-Protocol
)	O
for	O
all	O
SPF/link	O
state	O
routers	O
(	O
AllSPFRouters	O
)	O
and	O
224.0.0.6	O
(	O
IPv4	B-Protocol
)	O
and	O
FF02::6	O
(	O
IPv6	B-Protocol
)	O
for	O
all	O
Designated	O
Routers	O
(	O
AllDRouters	O
)	O
.	O
</s>
<s>
For	O
non-broadcast	O
networks	O
,	O
special	O
provisions	O
for	O
configuration	O
facilitate	O
neighbor	O
discovery	O
.	O
</s>
<s>
OSPF	B-Protocol
multicast	B-Protocol
IP	I-Protocol
packets	O
never	O
traverse	O
IP	B-Protocol
routers	O
,	O
they	O
never	O
travel	O
more	O
than	O
one	O
hop	O
.	O
</s>
<s>
The	O
protocol	O
may	O
therefore	O
be	O
considered	O
a	O
link	O
layer	O
protocol	O
,	O
but	O
is	O
often	O
also	O
attributed	O
to	O
the	O
application	O
layer	O
in	O
the	O
TCP/IP	O
model	O
.	O
</s>
<s>
OSPF	B-Protocol
over	O
IPv4	B-Protocol
can	O
operate	O
securely	O
between	O
routers	O
,	O
optionally	O
using	O
a	O
variety	O
of	O
authentication	O
methods	O
to	O
allow	O
only	O
trusted	O
routers	O
to	O
participate	O
in	O
routing	B-Protocol
.	O
</s>
<s>
OSPFv3	B-Protocol
(	O
IPv6	B-Protocol
)	O
relies	O
on	O
standard	O
IPv6	B-Protocol
protocol	O
security	O
(	O
IPsec	B-Protocol
)	O
,	O
and	O
has	O
no	O
internal	O
authentication	O
methods	O
.	O
</s>
<s>
For	O
routing	B-Protocol
IP	B-Protocol
multicast	I-Protocol
traffic	O
,	O
OSPF	B-Protocol
supports	O
the	O
Multicast	B-Protocol
Open	I-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
(	O
MOSPF	B-Protocol
)	O
protocol	O
.	O
</s>
<s>
Cisco	O
does	O
not	O
include	O
MOSPF	B-Protocol
in	O
their	O
OSPF	B-Protocol
implementations	O
.	O
</s>
<s>
Protocol	B-Protocol
Independent	I-Protocol
Multicast	I-Protocol
(	O
PIM	O
)	O
in	O
conjunction	O
with	O
OSPF	B-Protocol
or	O
other	O
IGPs	O
,	O
is	O
widely	O
deployed	O
.	O
</s>
<s>
OSPF	B-Protocol
version	O
3	O
introduces	O
modifications	O
to	O
the	O
IPv4	B-Protocol
implementation	O
of	O
the	O
protocol	O
.	O
</s>
<s>
Except	O
for	O
virtual	O
links	O
,	O
all	O
neighbor	O
exchanges	O
use	O
IPv6	B-Protocol
link-local	O
addressing	O
exclusively	O
.	O
</s>
<s>
The	O
IPv6	B-Protocol
protocol	O
runs	O
per	O
link	O
,	O
rather	O
than	O
based	O
on	O
the	O
subnet	B-Protocol
.	O
</s>
<s>
All	O
IP	B-Protocol
prefix	I-Protocol
information	O
has	O
been	O
removed	O
from	O
the	O
link-state	B-Protocol
advertisements	I-Protocol
and	O
from	O
the	O
hello	O
discovery	O
packet	O
making	O
OSPFv3	B-Protocol
essentially	O
protocol-independent	O
.	O
</s>
<s>
Despite	O
the	O
expanded	O
IP	B-Protocol
addressing	I-Protocol
to	O
128	O
bits	O
in	O
IPv6	B-Protocol
,	O
area	O
and	O
router	O
Identifications	O
are	O
still	O
based	O
on	O
32-bit	O
numbers	O
.	O
</s>
<s>
OSPF	B-Protocol
supports	O
complex	O
networks	O
with	O
multiple	O
routers	O
,	O
including	O
backup	O
routers	O
,	O
to	O
balance	O
traffic	O
load	O
on	O
multiple	O
links	O
to	O
other	O
subnets	B-Protocol
.	O
</s>
<s>
Neighboring	O
routers	O
in	O
the	O
same	O
broadcast	B-Architecture
domain	I-Architecture
or	O
at	O
each	O
end	O
of	O
a	O
point-to-point	B-Architecture
link	I-Architecture
communicate	O
with	O
each	O
other	O
via	O
the	O
OSPF	B-Protocol
protocol	O
.	O
</s>
<s>
The	O
routers	O
in	O
an	O
Ethernet	O
or	O
Frame	O
Relay	O
network	O
select	O
a	O
designated	O
router	O
(	O
DR	O
)	O
and	O
a	O
backup	O
designated	O
router	O
(	O
BDR	O
)	O
which	O
act	O
as	B-Architecture
a	O
hub	O
to	O
reduce	O
traffic	O
between	O
routers	O
.	O
</s>
<s>
OSPF	B-Protocol
uses	O
both	O
unicast	B-Protocol
and	O
multicast	O
transmission	O
modes	O
to	O
send	O
"	O
hello	O
"	O
packets	O
and	O
link-state	B-Protocol
updates	O
.	O
</s>
<s>
As	B-Architecture
a	O
link-state	B-Protocol
routing	I-Protocol
protocol	I-Protocol
,	O
OSPF	B-Protocol
establishes	O
and	O
maintains	O
neighbor	O
relationships	O
for	O
exchanging	O
routing	B-Protocol
updates	O
with	O
other	O
routers	O
.	O
</s>
<s>
Two	O
OSPF	B-Protocol
routers	O
are	O
neighbors	O
if	O
they	O
are	O
members	O
of	O
the	O
same	O
subnet	B-Protocol
and	O
share	O
the	O
same	O
area	O
ID	O
,	O
subnet	B-Protocol
mask	I-Protocol
,	O
timers	O
and	O
authentication	O
.	O
</s>
<s>
In	O
essence	O
,	O
OSPF	B-Protocol
neighborship	O
is	O
a	O
relationship	O
between	O
two	O
routers	O
that	O
allow	O
them	O
to	O
see	O
and	O
understand	O
each	O
other	O
but	O
nothing	O
more	O
.	O
</s>
<s>
OSPF	B-Protocol
neighbors	O
do	O
not	O
exchange	O
any	O
routing	B-Protocol
information	I-Protocol
the	O
only	O
packets	O
they	O
exchange	O
are	O
hello	O
packets	O
.	O
</s>
<s>
OSPF	B-Protocol
adjacencies	O
are	O
formed	O
between	O
selected	O
neighbors	O
and	O
allow	O
them	O
to	O
exchange	O
routing	B-Protocol
information	I-Protocol
.	O
</s>
<s>
Two	O
routers	O
become	O
adjacent	O
if	O
at	O
least	O
one	O
of	O
them	O
is	O
designated	O
router	O
or	O
backup	O
designated	O
router	O
(	O
on	O
multiaccess-type	O
networks	O
)	O
,	O
or	O
they	O
are	O
interconnected	O
by	O
a	O
point-to-point	B-Architecture
or	O
point-to-multipoint	B-General_Concept
network	O
type	O
.	O
</s>
<s>
For	O
forming	O
a	O
neighbor	O
relationship	O
between	O
,	O
the	O
interfaces	O
used	O
to	O
form	O
the	O
relationship	O
must	O
be	O
in	O
the	O
same	O
OSPF	B-Protocol
area	O
.	O
</s>
<s>
When	O
configured	O
in	O
a	O
second	O
area	O
,	O
an	O
interface	O
must	O
be	O
configured	O
as	B-Architecture
a	O
secondary	O
interface	O
.	O
</s>
<s>
The	O
OSPF	B-Protocol
can	O
have	O
different	O
operation	O
modes	O
on	O
the	O
following	O
setups	O
on	O
an	O
interface	O
or	O
network	O
:	O
</s>
<s>
Point-to-point	B-Architecture
.	O
</s>
<s>
The	O
interface	O
can	O
be	O
IP	B-Protocol
unnumbered	O
(	O
without	O
a	O
unique	O
IP	B-Protocol
address	I-Protocol
assigned	O
to	O
it	O
)	O
.	O
</s>
<s>
Broadcast	B-Operating_System
(	O
default	O
)	O
,	O
each	O
router	O
advertises	O
itself	O
by	O
periodically	O
multicasting	O
hello	O
packets	O
.	O
</s>
<s>
Non-broadcast	B-Protocol
multi-access	I-Protocol
,	O
with	O
the	O
use	O
of	O
designated	O
routers	O
.	O
</s>
<s>
Packets	O
are	O
sent	O
as	B-Architecture
unicast	B-Protocol
.	O
</s>
<s>
Point-to-multipoint	B-General_Concept
,	O
where	O
OSPF	B-Protocol
treats	O
neighbours	O
as	B-Architecture
a	O
collection	O
of	O
point-to-point	B-Architecture
links	O
.	O
</s>
<s>
Point	B-General_Concept
to	I-General_Concept
Multipoint	I-General_Concept
Non	O
Broadcast	B-Operating_System
(	O
P2MP-NB	O
)	O
,	O
No	O
designated	O
router	O
is	O
elected	O
.	O
</s>
<s>
Separate	O
hello	O
packets	O
are	O
sent	O
to	O
each	O
neighbor	O
,	O
Packets	O
are	O
sent	O
as	B-Architecture
unicast	B-Protocol
.	O
</s>
<s>
Virtual	O
link	O
over	O
Virtual	O
links	O
,	O
tunneling	B-Protocol
and	O
shamelinks	O
,	O
are	O
a	O
form	O
of	O
connections	O
that	O
goes	O
over	O
the	O
routing	B-Protocol
engine	O
,	O
and	O
is	O
not	O
a	O
direct	O
connection	O
to	O
the	O
remote	O
host	O
.	O
</s>
<s>
Virtual	O
links	O
,	O
the	O
packets	O
are	O
sent	O
as	B-Architecture
unicast	B-Protocol
.	O
</s>
<s>
Endpoints	O
need	O
to	O
be	O
ABR	O
,	O
the	O
virtual	O
links	O
behave	O
as	B-Architecture
unnumbered	O
point-to-point	B-Architecture
connections	O
.	O
</s>
<s>
Virtual	O
link	O
over	O
tunneling	B-Protocol
(	O
like	O
GRE	B-Protocol
and	O
WireGuard	B-Application
)	O
.	O
</s>
<s>
Since	O
OSPF	B-Protocol
does	O
not	O
support	O
virtual	O
links	O
for	O
areas	O
other	O
than	O
the	O
backbone	O
,	O
a	O
workaround	O
is	O
to	O
use	O
of	O
tunneling	B-Protocol
.	O
</s>
<s>
If	O
the	O
same	O
IP	B-Protocol
or	O
router	O
ID	O
is	O
used	O
the	O
link	O
creates	O
two	O
equal-cost	O
routes	O
to	O
the	O
destination	O
.	O
</s>
<s>
Sham	O
link	O
A	O
link	O
that	O
connects	O
sites	O
that	O
belong	O
to	O
the	O
same	O
OSPF	B-Protocol
area	O
and	O
share	O
an	O
OSPF	B-Protocol
backdoor	O
link	O
via	O
MPLS	B-Protocol
VPN	O
backbone	O
.	O
</s>
<s>
Each	O
OSPF	B-Protocol
router	O
within	O
a	O
network	O
communicates	O
with	O
other	O
neighboring	O
routers	O
on	O
each	O
connecting	O
interface	O
to	O
establish	O
the	O
states	O
of	O
all	O
adjacencies	O
.	O
</s>
<s>
RFC	O
2328	O
specifies	O
the	O
protocol	O
for	O
initiating	O
these	O
conversations	O
(	O
Hello	O
Protocol	O
)	O
and	O
for	O
establishing	O
full	O
adjacencies	O
(	O
database	O
description	O
packets	O
,	O
link-state	B-Protocol
request	O
packets	O
)	O
.	O
</s>
<s>
Attempt	O
:	O
The	O
attempt	O
state	O
is	O
similar	O
to	O
the	O
down	O
state	O
,	O
except	O
that	O
a	O
router	O
is	O
in	O
the	O
process	O
of	O
efforts	O
to	O
establish	O
a	O
conversation	O
with	O
another	O
router	O
,	O
but	O
is	O
only	O
used	O
on	O
non-broadcast	B-Protocol
multiple-access	I-Protocol
networks	I-Protocol
(	O
NBMAs	B-Protocol
)	O
.	O
</s>
<s>
This	O
is	O
the	O
lowest	O
state	O
of	O
a	O
router	O
that	O
may	O
be	O
considered	O
as	B-Architecture
a	O
DR	O
.	O
</s>
<s>
Exchange	O
:	O
In	O
the	O
exchange	O
state	O
,	O
a	O
router	O
is	O
sending	O
its	O
link-state	B-Protocol
database	O
information	O
to	O
the	O
adjacent	O
neighbor	O
.	O
</s>
<s>
At	O
this	O
state	O
,	O
a	O
router	O
can	O
exchange	O
all	O
OSPF	B-Protocol
routing	B-Protocol
protocol	I-Protocol
packets	O
.	O
</s>
<s>
Loading	O
:	O
In	O
the	O
loading	O
state	O
,	O
a	O
router	O
requests	O
the	O
most	O
recent	O
link-state	B-Protocol
advertisements	I-Protocol
(	O
LSAs	O
)	O
from	O
its	O
neighbor	O
discovered	O
in	O
the	O
previous	O
state	O
.	O
</s>
<s>
The	O
link-state	B-Protocol
databases	O
of	O
the	O
neighbors	O
are	O
fully	O
synchronized	O
.	O
</s>
<s>
In	O
broadcast	B-Operating_System
multiple-access	O
networks	O
,	O
neighbor	O
adjacency	O
is	O
formed	O
dynamically	O
using	O
multicast	O
hello	O
packets	O
to	O
.	O
</s>
<s>
A	O
network	O
where	O
OSPF	B-Protocol
adverts	O
the	O
network	O
,	O
but	O
the	O
OSPF	B-Protocol
will	O
not	O
start	O
neighbour	O
adjacency	O
.	O
</s>
<s>
In	O
non-broadcast	O
multiple-access	O
network(NBMA )	O
,	O
neighbor	O
adjacency	O
is	O
formed	O
with	O
unicast	B-Protocol
packets	O
to	O
remote	O
host	O
.	O
</s>
<s>
A	O
network	O
may	O
have	O
more	O
than	O
two	O
routers	O
,	O
but	O
is	O
no	O
broadcast	B-Operating_System
support	O
.	O
</s>
<s>
Types	O
of	O
Non-broadcast	O
networks	O
:	O
</s>
<s>
A	O
network	O
is	O
divided	O
into	O
OSPF	B-Protocol
areas	O
that	O
are	O
logical	O
groupings	O
of	O
hosts	O
and	O
networks	O
.	O
</s>
<s>
Each	O
router	O
maintains	O
a	O
separate	O
link-state	B-Protocol
database	O
for	O
the	O
area	O
whose	O
information	O
may	O
be	O
summarized	O
towards	O
the	O
rest	O
of	O
the	O
network	O
by	O
the	O
connecting	O
router	O
.	O
</s>
<s>
This	O
reduces	O
the	O
routing	B-Protocol
traffic	O
between	O
parts	O
of	O
an	O
autonomous	B-Architecture
system	I-Architecture
.	O
</s>
<s>
OSPF	B-Protocol
can	O
handle	O
thousands	O
of	O
routers	O
with	O
more	O
a	O
concern	O
of	O
reaching	O
capacity	O
of	O
the	O
forwarding	B-Protocol
information	I-Protocol
base	I-Protocol
(	O
FIB	O
)	O
table	O
when	O
the	O
network	O
contains	O
lots	O
of	O
routes	O
and	O
lower-end	O
devices	O
.	O
</s>
<s>
Many	O
resources	O
refer	O
to	O
OSPF	B-Protocol
guides	O
from	O
over	O
20	O
years	O
ago	O
where	O
it	O
was	O
impressive	O
to	O
have	O
64MB	O
of	O
RAM	O
.	O
</s>
<s>
The	O
area	O
identifiers	O
are	O
commonly	O
written	O
in	O
the	O
dot-decimal	B-Protocol
notation	I-Protocol
,	O
familiar	O
from	O
IPv4	B-Protocol
addressing	O
.	O
</s>
<s>
However	O
,	O
they	O
are	O
not	O
IP	B-Protocol
addresses	I-Protocol
and	O
may	O
duplicate	O
,	O
without	O
conflict	O
,	O
any	O
IPv4	B-Protocol
address	O
.	O
</s>
<s>
The	O
area	O
identifiers	O
for	O
IPv6	B-Protocol
implementations	O
(	O
OSPFv3	B-Protocol
)	O
also	O
use	O
32-bit	O
identifiers	O
written	O
in	O
the	O
same	O
notation	O
.	O
</s>
<s>
When	O
dotted	O
formatting	O
is	O
omitted	O
,	O
most	O
implementations	O
expand	O
area	O
1	O
to	O
the	O
area	O
identifier	O
0.0.0.1	O
,	O
but	O
some	O
have	O
been	O
known	O
to	O
expand	O
it	O
as	B-Architecture
1.0.0.0	O
.	O
</s>
<s>
Several	O
vendors	O
(	O
Cisco	O
,	O
Allied	O
Telesis	O
,	O
Juniper	O
,	O
Alcatel-Lucent	O
,	O
Huawei	O
,	O
Quagga	B-Protocol
)	O
,	O
implement	O
totally	O
stubby	O
and	O
NSSA	O
totally	O
stubby	O
area	O
for	O
stub	O
and	O
not-so-stubby	O
areas	O
.	O
</s>
<s>
Although	O
not	O
covered	O
by	O
RFC	O
standards	O
,	O
they	O
are	O
considered	O
by	O
many	O
to	O
be	O
standard	O
features	O
in	O
OSPF	B-Protocol
implementations	O
.	O
</s>
<s>
OSPF	B-Protocol
defines	O
several	O
area	O
types	O
:	O
</s>
<s>
The	O
backbone	O
area	O
(	O
also	O
known	O
as	B-Architecture
area	O
0	O
or	O
area	O
0.0.0.0	O
)	O
forms	O
the	O
core	O
of	O
an	O
OSPF	B-Protocol
network	O
.	O
</s>
<s>
OSPF	B-Protocol
requires	O
this	O
to	O
prevent	O
routing	B-Protocol
loops	I-Protocol
.	O
</s>
<s>
Inter-area	O
routing	B-Protocol
happens	O
via	O
routers	O
connected	O
to	O
the	O
backbone	O
area	O
and	O
to	O
their	O
own	O
associated	O
areas	O
.	O
</s>
<s>
It	O
is	O
the	O
logical	O
and	O
physical	O
structure	O
for	O
the	O
'	O
OSPF	B-Protocol
domain	O
 '	O
and	O
is	O
attached	O
to	O
all	O
nonzero	O
areas	O
in	O
the	O
OSPF	B-Protocol
domain	O
.	O
</s>
<s>
In	O
OSPF	B-Protocol
the	O
term	O
autonomous	B-Architecture
system	I-Architecture
boundary	O
router	O
(	O
ASBR	O
)	O
is	O
historic	O
,	O
in	O
the	O
sense	O
that	O
many	O
OSPF	B-Protocol
domains	O
can	O
coexist	O
in	O
the	O
same	O
Internet-visible	O
autonomous	B-Architecture
system	I-Architecture
,	O
RFC	O
1996	O
.	O
</s>
<s>
All	O
OSPF	B-Protocol
areas	O
must	O
connect	O
to	O
the	O
backbone	O
area	O
.	O
</s>
<s>
A	O
stub	O
area	O
is	O
an	O
area	O
that	O
does	O
not	O
receive	O
route	O
advertisements	O
external	O
to	O
the	O
AS	B-Architecture
and	O
routing	B-Protocol
from	O
within	O
the	O
area	O
is	O
based	O
entirely	O
on	O
a	O
default	O
route	O
.	O
</s>
<s>
This	O
reduces	O
LSDB	O
and	O
routing	B-Protocol
table	O
size	O
for	O
internal	O
routers	O
.	O
</s>
<s>
Modifications	O
to	O
the	O
basic	O
concept	O
of	O
stub	O
area	O
have	O
been	O
implemented	O
by	O
systems	O
vendors	O
,	O
such	O
as	B-Architecture
the	O
totally	O
stubby	O
area	O
(	O
TSA	O
)	O
and	O
the	O
not-so-stubby	O
area	O
(	O
NSSA	O
)	O
,	O
both	O
an	O
extension	O
in	O
Cisco	O
Systems	O
routing	B-Protocol
equipment	O
.	O
</s>
<s>
The	O
only	O
way	O
for	O
traffic	O
to	O
get	O
routed	B-Protocol
outside	O
the	O
area	O
is	O
a	O
default	O
route	O
which	O
is	O
the	O
only	O
Type-3	O
LSA	O
advertised	O
into	O
the	O
area	O
.	O
</s>
<s>
When	O
there	O
is	O
only	O
one	O
route	O
out	O
of	O
the	O
area	O
,	O
fewer	O
routing	B-Protocol
decisions	O
have	O
to	O
be	O
made	O
by	O
the	O
route	O
processor	O
,	O
which	O
lowers	O
system	O
resource	O
utilization	O
.	O
</s>
<s>
A	O
not-so-stubby	O
area	O
(	O
NSSA	O
)	O
is	O
a	O
type	O
of	O
stub	O
area	O
that	O
can	O
import	O
autonomous	B-Architecture
system	I-Architecture
external	O
routes	O
and	O
send	O
them	O
to	O
other	O
areas	O
,	O
but	O
still	O
cannot	O
receive	O
AS-external	O
routes	O
from	O
other	O
areas	O
.	O
</s>
<s>
It	O
visualizes	O
the	O
following	O
activities	O
:	O
the	O
ASBR	O
imports	O
external	O
addresses	O
with	O
a	O
type	O
7	O
LSA	O
,	O
the	O
ABR	O
converts	O
a	O
type	O
7	O
LSA	O
to	O
type	O
5	O
and	O
floods	O
it	O
to	O
other	O
areas	O
,	O
the	O
ABR	O
acts	O
as	B-Architecture
an	O
"	O
ASBR	O
"	O
for	O
other	O
areas	O
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
declare	O
an	O
area	O
both	O
totally	O
stubby	O
and	O
not-so-stubby	O
,	O
which	O
means	O
that	O
the	O
area	O
will	O
receive	O
only	O
the	O
default	O
route	O
from	O
area	O
0.0.0.0	O
,	O
but	O
can	O
also	O
contain	O
an	O
autonomous	B-Architecture
system	I-Architecture
boundary	O
router	O
(	O
ASBR	O
)	O
that	O
accepts	O
external	O
routing	B-Protocol
information	I-Protocol
and	O
injects	O
it	O
into	O
the	O
local	O
area	O
,	O
and	O
from	O
the	O
local	O
area	O
into	O
area	O
0.0.0.0	O
.	O
</s>
<s>
Redistribution	O
into	O
an	O
NSSA	O
area	O
creates	O
a	O
special	O
type	O
of	O
LSA	O
known	O
as	B-Architecture
type	O
7	O
,	O
which	O
can	O
exist	O
only	O
in	O
an	O
NSSA	O
area	O
.	O
</s>
<s>
An	O
NSSA	O
ASBR	O
generates	O
this	O
LSA	O
,	O
and	O
an	O
NSSA	O
ABR	O
router	O
translates	O
it	O
into	O
a	O
type	O
5	O
LSA	O
,	O
which	O
gets	O
propagated	O
into	O
the	O
OSPF	B-Protocol
domain	O
.	O
</s>
<s>
In	O
such	O
a	O
case	O
,	O
the	O
ASBR	O
does	O
send	O
externals	O
into	O
the	O
totally	O
stubby	O
area	O
,	O
and	O
they	O
are	O
available	O
to	O
OSPF	B-Protocol
speakers	O
within	O
that	O
area	O
.	O
</s>
<s>
By	O
declaring	O
the	O
totally	O
stubby	O
area	O
as	B-Architecture
NSSA	O
,	O
no	O
external	O
routes	O
from	O
the	O
backbone	O
,	O
except	O
the	O
default	O
route	O
,	O
enter	O
the	O
area	O
being	O
discussed	O
.	O
</s>
<s>
OSPF	B-Protocol
defines	O
the	O
following	O
overlapping	O
categories	O
of	O
routers	O
:	O
</s>
<s>
An	O
ABR	O
keeps	O
multiple	O
instances	O
of	O
the	O
link-state	B-Protocol
database	O
in	O
memory	O
,	O
one	O
for	O
each	O
area	O
to	O
which	O
that	O
router	O
is	O
connected	O
.	O
</s>
<s>
Autonomous	B-Architecture
system	I-Architecture
boundary	O
router	O
(	O
ASBR	O
)	O
An	O
autonomous	B-Architecture
system	I-Architecture
boundary	O
router	O
is	O
a	O
router	O
that	O
is	O
connected	O
by	O
using	O
more	O
than	O
one	O
routing	B-Protocol
protocol	I-Protocol
and	O
that	O
exchanges	O
routing	B-Protocol
information	I-Protocol
with	O
routers	O
autonomous	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
ASBRs	O
typically	O
also	O
run	O
an	O
exterior	O
routing	B-Protocol
protocol	I-Protocol
(	O
e.g.	O
,	O
BGP	B-Protocol
)	O
,	O
or	O
use	O
static	O
routes	O
,	O
or	O
both	O
.	O
</s>
<s>
An	O
ASBR	O
is	O
used	O
to	O
distribute	B-Protocol
routes	I-Protocol
received	O
from	O
other	O
,	O
external	O
ASs	O
throughout	O
its	O
own	O
autonomous	B-Architecture
system	I-Architecture
.	O
</s>
<s>
Routers	O
in	O
other	O
areas	O
use	O
ABRs	O
as	B-Architecture
next	O
hops	O
to	O
access	O
external	O
addresses	O
.	O
</s>
<s>
The	O
router	O
type	O
is	O
an	O
attribute	O
of	O
an	O
OSPF	B-Protocol
process	O
.	O
</s>
<s>
A	O
given	O
physical	O
router	O
may	O
have	O
one	O
or	O
more	O
OSPF	B-Protocol
processes	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
router	O
that	O
is	O
connected	O
to	O
more	O
than	O
one	O
area	O
,	O
and	O
which	O
receives	O
routes	O
from	O
a	O
BGP	B-Protocol
process	O
connected	O
to	O
another	O
AS	B-Architecture
,	O
is	O
both	O
an	O
area	O
border	O
router	O
and	O
an	O
autonomous	B-Architecture
system	I-Architecture
boundary	O
router	O
.	O
</s>
<s>
Each	O
router	O
has	O
an	O
identifier	O
,	O
customarily	O
written	O
in	O
the	O
dotted-decimal	B-Protocol
format	O
(	O
e.g.	O
,	O
1.2.3.4	O
)	O
of	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
This	O
identifier	O
must	O
be	O
established	O
in	O
every	O
OSPF	B-Protocol
instance	O
.	O
</s>
<s>
If	O
not	O
explicitly	O
configured	O
,	O
the	O
highest	O
logical	O
IP	B-Protocol
address	I-Protocol
will	O
be	O
duplicated	O
as	B-Architecture
the	O
router	O
identifier	O
.	O
</s>
<s>
However	O
,	O
since	O
the	O
router	O
identifier	O
is	O
not	O
an	O
IP	B-Protocol
address	I-Protocol
,	O
it	O
does	O
not	O
have	O
to	O
be	O
a	O
part	O
of	O
any	O
routable	B-Protocol
subnet	B-Protocol
in	O
the	O
network	O
,	O
and	O
often	O
is	O
n't	O
to	O
avoid	O
confusion	O
.	O
</s>
<s>
On	O
networks	O
(	O
same	O
subnet	B-Protocol
)	O
with	O
networks	O
type	O
of	O
:	O
</s>
<s>
A	O
system	O
of	O
designated	O
router	O
(	O
DR	O
)	O
and	O
backup	O
designated	O
router	O
(	O
BDR	O
)	O
,	O
is	O
used	O
to	O
reducing	O
network	O
traffic	O
by	O
providing	O
a	O
source	O
for	O
routing	B-Protocol
updates	O
.	O
</s>
<s>
,	O
all	O
routers	O
in	O
the	O
topology	O
will	O
listen	O
on	O
that	O
multicast	B-Protocol
address	I-Protocol
.	O
</s>
<s>
,	O
DR	O
and	O
BDR	O
will	O
listen	O
on	O
that	O
multicast	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Every	O
time	O
a	O
router	O
sends	O
an	O
update	O
,	O
it	O
sends	O
it	O
to	O
the	O
DR	O
and	O
BDR	O
on	O
the	O
multicast	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
DR	O
will	O
then	O
send	O
the	O
update	O
out	O
to	O
all	O
other	O
routers	O
in	O
the	O
area	O
,	O
to	O
the	O
multicast	B-Protocol
address	I-Protocol
.	O
</s>
<s>
DRs	O
and	O
BDRs	O
are	O
always	O
setup/elected	O
on	O
OSPF	B-Protocol
broadcast	B-Operating_System
networks	O
.	O
</s>
<s>
DR	O
's	O
can	O
also	O
be	O
elected	O
on	O
NBMA	B-Protocol
(	O
Non-Broadcast	B-Protocol
Multi-Access	I-Protocol
)	O
networks	O
such	O
as	B-Architecture
Frame	O
Relay	O
or	O
ATM	O
.	O
</s>
<s>
DRs	O
or	O
BDRs	O
are	O
not	O
elected	O
on	O
point-to-point	B-Architecture
links	O
(	O
such	O
as	B-Architecture
a	O
point-to-point	B-Architecture
WAN	O
connection	O
)	O
because	O
the	O
two	O
routers	O
on	O
either	O
side	O
of	O
the	O
link	O
must	O
become	O
fully	O
adjacent	O
and	O
the	O
bandwidth	O
between	O
them	O
cannot	O
be	O
further	O
optimized	O
.	O
</s>
<s>
A	O
designated	O
router	O
(	O
DR	O
)	O
is	O
the	O
router	O
interface	O
elected	O
among	O
all	O
routers	O
on	O
a	O
particular	O
multiaccess	O
network	O
segment	O
,	O
generally	O
assumed	O
to	O
be	O
broadcast	B-Operating_System
multiaccess	O
.	O
</s>
<s>
Special	O
techniques	O
,	O
often	O
vendor-dependent	O
,	O
may	O
be	O
needed	O
to	O
support	O
the	O
DR	O
function	O
on	O
non-broadcast	B-Protocol
multiaccess	I-Protocol
(	O
NBMA	B-Protocol
)	O
media	O
.	O
</s>
<s>
It	O
is	O
usually	O
wise	O
to	O
configure	O
the	O
individual	O
virtual	O
circuits	O
of	O
an	O
NBMA	B-Protocol
subnet	B-Protocol
as	B-Architecture
individual	O
point-to-point	B-Architecture
lines	O
;	O
the	O
techniques	O
used	O
are	O
implementation-dependent	O
.	O
</s>
<s>
The	O
BDR	O
is	O
the	O
OSPF	B-Protocol
router	O
with	O
the	O
second-highest	O
priority	O
at	O
the	O
time	O
of	O
the	O
last	O
election	O
.	O
</s>
<s>
If	O
no	O
router	O
is	O
a	O
DR	O
or	O
a	O
BDR	O
on	O
a	O
given	O
subnet	B-Protocol
,	O
the	O
BDR	O
is	O
first	O
elected	O
,	O
and	O
then	O
a	O
second	O
election	O
is	O
held	O
for	O
the	O
DR	O
.	O
</s>
<s>
If	O
the	O
priority	O
setting	O
on	O
an	O
OSPF	B-Protocol
router	O
is	O
set	O
to	O
0	O
,	O
that	O
means	O
it	O
can	O
NEVER	O
become	O
a	O
DR	O
or	O
BDR	O
.	O
</s>
<s>
NOTE	O
:	O
a	O
RID	O
is	O
the	O
highest	O
logical	O
(	O
loopback	O
)	O
IP	B-Protocol
address	I-Protocol
configured	O
on	O
a	O
router	O
,	O
if	O
no	O
logical/loopback	O
IP	B-Protocol
address	I-Protocol
is	O
set	O
then	O
the	O
router	O
uses	O
the	O
highest	O
IP	B-Protocol
address	I-Protocol
configured	O
on	O
its	O
active	O
interfaces	O
(	O
e.g.	O
</s>
<s>
If	O
a	O
higher	O
priority	O
OSPF	B-Protocol
router	O
comes	O
online	O
after	O
the	O
election	O
has	O
taken	O
place	O
,	O
it	O
will	O
not	O
become	O
DR	O
or	O
BDR	O
until	O
(	O
at	O
least	O
)	O
the	O
DR	O
and	O
BDR	O
fail	O
.	O
</s>
<s>
Unlike	O
other	O
routing	B-Protocol
protocols	I-Protocol
,	O
OSPF	B-Protocol
does	O
not	O
carry	O
data	O
via	O
a	O
transport	B-Architecture
protocol	I-Architecture
,	O
such	O
as	B-Architecture
the	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	B-Protocol
)	O
or	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	B-Protocol
)	O
.	O
</s>
<s>
Instead	O
,	O
OSPF	B-Protocol
forms	O
IP	B-Protocol
datagrams	O
directly	O
,	O
packaging	O
them	O
using	O
protocol	B-Protocol
number	I-Protocol
89	I-Protocol
for	O
the	O
IP	B-Protocol
Protocol	I-Protocol
field	I-Protocol
.	O
</s>
<s>
OSPF	B-Protocol
defines	O
five	O
different	O
message	O
types	O
,	O
for	O
various	O
types	O
of	O
communication	O
.	O
</s>
<s>
OSPF	B-Protocol
uses	O
the	O
following	O
packets	O
5	O
type	O
:	O
</s>
<s>
OSPF	B-Protocol
's	O
Hello	O
messages	O
are	O
used	O
as	B-Architecture
a	O
form	O
of	O
greeting	O
,	O
to	O
allow	O
a	O
router	O
to	O
discover	O
other	O
adjacent	O
routers	O
on	O
its	O
local	O
links	O
and	O
networks	O
.	O
</s>
<s>
The	O
messages	O
establish	O
relationships	O
between	O
neighboring	O
devices	O
(	O
called	O
adjacencies	O
)	O
and	O
communicate	O
key	O
parameters	O
about	O
how	O
OSPF	B-Protocol
is	O
to	O
be	O
used	O
in	O
the	O
autonomous	B-Architecture
system	I-Architecture
or	O
area	O
.	O
</s>
<s>
Database	O
description	O
messages	O
contain	O
descriptions	O
of	O
the	O
topology	O
of	O
the	O
autonomous	B-Architecture
system	I-Architecture
or	O
area	O
.	O
</s>
<s>
They	O
convey	O
the	O
contents	O
of	O
the	O
link-state	B-Protocol
database	O
(	O
LSDB	O
)	O
for	O
the	O
area	O
from	O
one	O
router	O
to	O
another	O
.	O
</s>
<s>
Communicating	O
a	O
large	O
LSDB	O
may	O
require	O
several	O
messages	O
to	O
be	O
sent	O
by	O
having	O
the	O
sending	O
device	O
designated	O
as	B-Architecture
a	O
master	O
device	O
and	O
sending	O
messages	O
in	O
sequence	O
,	O
with	O
the	O
slave	O
(	O
recipient	O
of	O
the	O
LSDB	O
information	O
)	O
responding	O
with	O
acknowledgments	O
.	O
</s>
<s>
Link	B-Protocol
state	I-Protocol
request	O
(	O
LSR	O
)	O
Link	B-Protocol
state	I-Protocol
request	O
messages	O
are	O
used	O
by	O
one	O
router	O
to	O
request	O
updated	O
information	O
about	O
a	O
portion	O
of	O
the	O
LSDB	O
from	O
another	O
router	O
.	O
</s>
<s>
Link	B-Protocol
state	I-Protocol
update	O
(	O
LSU	O
)	O
Link-state	B-Protocol
update	O
messages	O
contain	O
updated	O
information	O
about	O
the	O
state	O
of	O
certain	O
links	O
on	O
the	O
LSDB	O
.	O
</s>
<s>
They	O
are	O
sent	O
in	O
response	O
to	O
a	O
link	B-Protocol
state	I-Protocol
request	O
message	O
,	O
and	O
also	O
broadcast	B-Operating_System
or	O
multicast	O
by	O
routers	O
on	O
a	O
regular	O
basis	O
.	O
</s>
<s>
Link	B-Protocol
state	I-Protocol
acknowledgment	O
(	O
LSAck	O
)	O
Link-state	B-Protocol
acknowledgment	O
messages	O
provide	O
reliability	O
to	O
the	O
link-state	B-Protocol
exchange	O
process	O
,	O
by	O
explicitly	O
acknowledging	O
receipt	O
of	O
a	O
Link	B-Protocol
State	I-Protocol
Update	O
message	O
.	O
</s>
<s>
The	O
link-state	B-Protocol
ID	O
of	O
the	O
type	O
1	O
LSA	O
is	O
the	O
originating	O
router	O
ID	O
.	O
</s>
<s>
Virtual	O
link	O
2	O
Network-LSAs	O
The	O
DR	O
200px|frameless|rightOriginated	O
for	O
broadcasts	B-Operating_System
and	O
NBMA	B-Protocol
networks	O
by	O
the	O
designated	O
router	O
.	O
</s>
<s>
The	O
link-state	B-Protocol
ID	O
of	O
the	O
type	O
2	O
LSA	O
is	O
the	O
IP	B-Protocol
interface	O
address	O
of	O
the	O
DR	O
.	O
3	O
Summary-LSAs	O
The	O
ABR200px|frameless|rightType	O
3	O
summary-LSAs	O
describe	O
routes	O
to	O
networks	O
.	O
</s>
<s>
4	O
ASBR-summary	O
The	O
ABRType	O
4	O
describe	O
routes	O
to	O
AS	B-Architecture
boundary	O
routers	O
beyond	O
its	O
area	O
.	O
</s>
<s>
The	O
area	O
border	O
router	O
(	O
ABR	O
)	O
generates	O
this	O
LSA	O
to	O
inform	O
other	O
routers	O
in	O
the	O
OSPF	B-Protocol
domain	O
,	O
that	O
the	O
matching	O
router	O
is	O
an	O
autonomous	B-Architecture
system	I-Architecture
boundary	O
router	O
(	O
ASBR	O
)	O
,	O
so	O
that	O
the	O
external	O
LSAs	O
(	O
Type	O
5	O
/	O
Type	O
7	O
)	O
it	O
sent	O
may	O
be	O
properly	O
resolved	O
outside	O
its	O
own	O
area	O
.	O
</s>
<s>
5	O
AS-external-LSAs	O
The	O
ASBR200px|frameless|rightType	O
5	O
These	O
describe	O
routes	O
advertised	O
by	O
the	O
ASBR	O
.	O
</s>
<s>
LSAs	O
contain	O
information	O
imported	O
into	O
OSPF	B-Protocol
from	O
other	O
routing	B-Protocol
processes	O
.	O
</s>
<s>
7	O
NSSA	O
external	O
link-state	B-Protocol
advertisements	I-Protocol
The	O
ASBR	O
,	O
within	O
a	O
not-so-stubby	O
area	O
Type	O
7-LSAs	O
are	O
identical	O
to	O
type-5	O
LSAs	O
.	O
</s>
<s>
OSPF	B-Protocol
uses	O
path	O
cost	O
as	B-Architecture
its	O
basic	O
routing	B-Protocol
metric	O
,	O
which	O
was	O
defined	O
by	O
the	O
standard	O
not	O
to	O
equate	O
to	O
any	O
standard	O
value	O
such	O
as	B-Architecture
speed	O
,	O
so	O
the	O
network	O
designer	O
could	O
pick	O
a	O
metric	O
important	O
to	O
the	O
design	O
.	O
</s>
<s>
In	O
practice	O
,	O
it	O
is	O
determined	O
by	O
comparing	O
the	O
speed	O
of	O
the	O
interface	O
to	O
a	O
reference-bandwidth	O
for	O
the	O
OSPF	B-Protocol
process	O
.	O
</s>
<s>
Here	O
is	O
an	O
example	O
table	O
that	O
shows	O
the	O
routing	B-Protocol
metric	O
or	O
'	O
cost	O
calculation	O
 '	O
on	O
an	O
interface	O
.	O
</s>
<s>
OSPF	B-Protocol
is	O
a	O
layer	O
3	O
protocol	O
:	O
if	O
a	O
layer	O
2	O
switch	O
is	O
between	O
the	O
two	O
devices	O
running	O
OSPF	B-Protocol
,	O
one	O
side	O
may	O
negotiate	O
a	O
speed	O
different	O
from	O
the	O
other	O
side	O
.	O
</s>
<s>
This	O
can	O
create	O
an	O
asymmetric	O
routing	B-Protocol
on	O
the	O
link	O
(	O
Router	O
1	O
to	O
Router	O
2	O
could	O
cost	O
'	O
1	O
 '	O
and	O
the	O
return	O
path	O
could	O
cost	O
'	O
10	O
 '	O
)	O
,	O
which	O
may	O
lead	O
to	O
unintended	O
consequences	O
.	O
</s>
<s>
OSPF	B-Protocol
version	O
3	O
introduces	O
modifications	O
to	O
the	O
IPv4	B-Protocol
implementation	O
of	O
the	O
protocol	O
.	O
</s>
<s>
Despite	O
the	O
expansion	O
of	O
addresses	O
to	O
128	O
bits	O
in	O
IPv6	B-Protocol
,	O
area	O
and	O
router	O
identifications	O
are	O
still	O
32-bit	O
numbers	O
.	O
</s>
<s>
Except	O
for	O
virtual	O
links	O
,	O
all	O
neighbor	O
exchanges	O
use	O
IPv6	B-Protocol
link-local	O
addressing	O
exclusively	O
.	O
</s>
<s>
The	O
IPv6	B-Protocol
protocol	O
runs	O
per	O
link	O
,	O
rather	O
than	O
based	O
on	O
the	O
subnet	B-Protocol
.	O
</s>
<s>
All	O
IP	B-Protocol
prefix	I-Protocol
information	O
has	O
been	O
removed	O
from	O
the	O
link-state	B-Protocol
advertisements	I-Protocol
and	O
from	O
the	O
hello	O
discovery	O
packet	O
,	O
making	O
OSPFv3	B-Protocol
essentially	O
protocol-independent	O
.	O
</s>
<s>
Area	O
scope	O
:	O
LSA	O
is	O
flooded	O
throughout	O
a	O
single	O
OSPF	B-Protocol
area	O
.	O
</s>
<s>
AS	B-Architecture
scope	O
:	O
LSA	O
is	O
flooded	O
throughout	O
the	O
routing	B-Protocol
domain	O
.	O
</s>
<s>
Use	O
of	O
IPv6	B-Protocol
link-local	O
addresses	O
,	O
for	O
neighbor	O
discovery	O
,	O
auto-configuration	O
.	O
</s>
<s>
"	O
R-bit	O
"	O
:	O
allows	O
for	O
multi-homed	O
hosts	O
to	O
participate	O
in	O
the	O
routing	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Add	O
"	O
instance	O
ID	O
"	O
,	O
which	O
allows	O
multiple	O
OSPF	B-Protocol
protocol	O
instances	O
on	O
the	O
same	O
logical	O
interface	O
.	O
</s>
<s>
With	O
IPv6	B-Protocol
,	O
addresses	O
in	O
LSAs	O
are	O
expressed	O
as	B-Architecture
prefix	O
and	O
prefix	O
length	O
.	O
</s>
<s>
A	O
new	O
LSA	O
type	O
is	O
added	O
,	O
link-LSA	O
,	O
which	O
provides	O
the	O
router	O
's	O
link-local	O
address	O
to	O
all	O
other	O
routers	O
attached	O
to	O
the	O
logical	O
interface	O
,	O
provides	O
a	O
list	O
of	O
IPv6	B-Protocol
prefixes	O
to	O
associate	O
with	O
the	O
link	O
,	O
and	O
can	O
send	O
information	O
that	O
reflect	O
the	O
router	O
's	O
capabilities	O
.	O
</s>
<s>
Intra-area-prefix-LSA	O
is	O
added	O
,	O
an	O
LSA	O
that	O
carries	O
all	O
IPv6	B-Protocol
prefix	O
information	O
.	O
</s>
<s>
A	O
customer	O
can	O
use	O
OSPF	B-Protocol
over	O
a	O
MPLS-VPN	O
,	O
where	O
the	O
service	O
provider	O
uses	O
BGP	B-Protocol
or	O
RIP	O
as	B-Architecture
their	O
interior	B-Protocol
gateway	I-Protocol
protocol	I-Protocol
.	O
</s>
<s>
When	O
using	O
OSPF	B-Protocol
over	O
MPLS-VPN	O
,	O
the	O
VPN	O
backbone	O
becomes	O
part	O
of	O
the	O
OSPF	B-Protocol
backbone	O
area	O
0	O
.	O
</s>
<s>
The	O
MPLS-VPN	O
is	O
transparent	O
to	O
the	O
customer	O
's	O
OSPF	B-Protocol
standard	O
routing	B-Protocol
.	O
</s>
<s>
Customer	O
's	O
equipment	O
only	O
needs	O
to	O
support	O
OSPF	B-Protocol
.	O
</s>
<s>
Reduce	O
the	O
need	O
for	O
tunnels	O
(	O
Generic	B-Protocol
Routing	I-Protocol
Encapsulation	I-Protocol
,	O
IPsec	B-Protocol
,	O
wireguard	B-Application
)	O
to	O
use	O
OSPF	B-Protocol
.	O
</s>
<s>
To	O
achieve	O
this	O
,	O
a	O
non-standard	O
OSPF-BGP	O
redistribution	O
is	O
used	O
.	O
</s>
<s>
All	O
OSPF	B-Protocol
routes	O
retain	O
the	O
source	O
LSA	O
type	O
and	O
metric	O
.	O
</s>
<s>
OSPF-TE	B-Protocol
is	O
an	O
extension	O
to	O
OSPF	B-Protocol
extending	O
the	O
expressivity	O
to	O
allow	O
for	O
traffic	O
engineering	O
and	O
use	O
on	O
non-IP	O
networks	O
.	O
</s>
<s>
Using	O
OSPF-TE	B-Protocol
,	O
more	O
information	O
about	O
the	O
topology	O
can	O
be	O
exchanged	O
using	O
opaque	O
LSA	O
carrying	O
type	B-Protocol
–	I-Protocol
length	I-Protocol
–	I-Protocol
value	I-Protocol
elements	O
.	O
</s>
<s>
These	O
extensions	O
allow	O
OSPF-TE	B-Protocol
to	O
run	O
completely	O
out	O
of	O
band	O
of	O
the	O
data	O
plane	O
network	O
.	O
</s>
<s>
This	O
means	O
that	O
it	O
can	O
also	O
be	O
used	O
on	O
non-IP	O
networks	O
,	O
such	O
as	B-Architecture
optical	O
networks	O
.	O
</s>
<s>
OSPF-TE	B-Protocol
is	O
used	O
in	O
GMPLS	O
networks	O
as	B-Architecture
a	O
means	O
to	O
describe	O
the	O
topology	O
over	O
which	O
GMPLS	O
paths	O
can	O
be	O
established	O
.	O
</s>
<s>
In	O
the	O
Resource	B-Protocol
Reservation	I-Protocol
Protocol	I-Protocol
(	O
RSVP	B-Protocol
)	O
,	O
OSPF-TE	B-Protocol
is	O
used	O
for	O
recording	O
and	O
flooding	O
RSVP	B-Protocol
signaled	O
bandwidth	O
reservations	O
for	O
label	O
switched	O
paths	O
within	O
the	O
link-state	B-Protocol
database	O
.	O
</s>
<s>
documents	O
work	O
in	O
optical	O
routing	B-Protocol
for	O
IP	B-Protocol
based	O
on	O
extensions	O
to	O
OSPF	B-Protocol
and	O
IS-IS	B-Protocol
.	O
</s>
<s>
The	O
Multicast	B-Protocol
Open	I-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
(	O
MOSPF	B-Protocol
)	O
protocol	O
is	O
an	O
extension	O
to	O
OSPF	B-Protocol
to	O
support	O
multicast	O
routing	B-Protocol
.	O
</s>
<s>
MOSPF	B-Protocol
allows	O
routers	O
to	O
share	O
information	O
about	O
group	O
memberships	O
.	O
</s>
<s>
D-Link	B-General_Concept
implements	O
OSPFv2	B-Protocol
on	O
Unified	O
Services	O
Router	O
.	O
</s>
<s>
NetWare	B-Operating_System
implements	O
OSPF	B-Protocol
in	O
its	O
Multi	O
Protocol	O
Routing	B-Protocol
module	O
.	O
</s>
<s>
OpenBSD	B-Operating_System
includes	O
OpenOSPFD	B-Application
,	O
an	O
OSPFv2	B-Protocol
implementation	O
.	O
</s>
<s>
Windows	B-Device
NT	I-Device
4.0	I-Device
Server	O
,	O
Windows	B-Application
2000	I-Application
Server	O
and	O
Windows	B-Application
Server	I-Application
2003	I-Application
implemented	O
OSPFv2	B-Protocol
in	O
the	O
Routing	B-Library
and	I-Library
Remote	I-Library
Access	I-Library
Service	I-Library
,	O
although	O
the	O
functionality	O
was	O
removed	O
in	O
Windows	B-Device
Server	I-Device
2008	I-Device
.	O
</s>
<s>
OSPF	B-Protocol
is	O
a	O
widely	O
deployed	O
routing	B-Protocol
protocol	I-Protocol
that	O
can	O
converge	O
a	O
network	O
in	O
a	O
few	O
seconds	O
and	O
guarantee	O
loop-free	O
paths	O
.	O
</s>
<s>
IS-IS	B-Protocol
,	O
in	O
contrast	O
,	O
can	O
be	O
tuned	O
for	O
lower	O
overhead	O
in	O
a	O
stable	O
network	O
,	O
the	O
sort	O
more	O
common	O
in	O
ISP	O
than	O
enterprise	B-General_Concept
networks	I-General_Concept
.	O
</s>
<s>
There	O
are	O
some	O
historical	O
accidents	O
that	O
made	O
IS-IS	B-Protocol
the	O
preferred	O
IGP	O
for	O
ISPs	O
,	O
but	O
ISPs	O
today	O
may	O
well	O
choose	O
to	O
use	O
the	O
features	O
of	O
the	O
now-efficient	O
implementations	O
of	O
OSPF	B-Protocol
,	O
after	O
first	O
considering	O
the	O
pros	O
and	O
cons	O
of	O
IS-IS	B-Protocol
in	O
service	O
provider	O
environments	O
.	O
</s>
<s>
OSPF	B-Protocol
can	O
provide	O
better	O
load-sharing	O
on	O
external	O
links	O
than	O
other	O
IGPs	O
.	O
</s>
<s>
When	O
the	O
default	O
route	O
to	O
an	O
ISP	O
is	O
injected	O
into	O
OSPF	B-Protocol
from	O
multiple	O
ASBRs	O
as	B-Architecture
a	O
Type	O
I	O
external	O
route	O
and	O
the	O
same	O
external	O
cost	O
specified	O
,	O
other	O
routers	O
will	O
go	O
to	O
the	O
ASBR	O
with	O
the	O
least	O
path	O
cost	O
from	O
its	O
location	O
.	O
</s>
<s>
If	O
the	O
default	O
route	O
from	O
different	O
ISPs	O
is	O
injected	O
with	O
different	O
external	O
costs	O
,	O
as	B-Architecture
a	O
Type	O
II	O
external	O
route	O
,	O
the	O
lower-cost	O
default	O
becomes	O
the	O
primary	O
exit	O
and	O
the	O
higher-cost	O
becomes	O
the	O
backup	O
only	O
.	O
</s>
