<s>
Administrative	B-Protocol
distance	I-Protocol
(	O
AD	O
)	O
or	O
route	B-Protocol
preference	I-Protocol
is	O
a	O
number	O
of	O
arbitrary	O
unit	O
assigned	O
to	O
dynamic	B-Protocol
routes	I-Protocol
,	O
static	B-Protocol
routes	I-Protocol
and	O
directly-connected	O
routes	B-Protocol
.	O
</s>
<s>
The	O
value	O
is	O
used	O
in	O
routers	B-Protocol
to	O
rank	O
routes	B-Protocol
from	O
most	O
preferred	O
(	O
low	O
AD	O
value	O
)	O
to	O
least	O
preferred	O
(	O
high	O
AD	O
value	O
)	O
.	O
</s>
<s>
When	O
multiple	O
paths	O
to	O
the	O
same	O
destination	O
are	O
available	O
in	O
its	O
routing	B-Protocol
table	I-Protocol
,	O
the	O
router	B-Protocol
uses	O
the	O
route	O
with	O
the	O
lowest	O
administrative	B-Protocol
distance	I-Protocol
.	O
</s>
<s>
Router	B-Protocol
vendors	O
typically	O
design	O
their	O
routers	B-Protocol
to	O
assign	O
a	O
default	O
administrative	B-Protocol
distance	I-Protocol
to	O
each	O
kind	O
of	O
route	O
.	O
</s>
<s>
For	O
example	O
,	O
on	O
Cisco	O
routers	B-Protocol
,	O
routes	B-Protocol
issued	O
by	O
the	O
Open	B-Protocol
Shortest	I-Protocol
Path	I-Protocol
First	I-Protocol
routing	B-Protocol
protocol	I-Protocol
have	O
a	O
lower	O
default	O
administrative	B-Protocol
distance	I-Protocol
than	O
routes	B-Protocol
issued	O
by	O
the	O
Routing	B-Protocol
Information	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
This	O
is	O
because	O
,	O
by	O
default	O
on	O
Cisco	O
routers	B-Protocol
,	O
OSPF	B-Protocol
has	O
a	O
default	O
administrative	B-Protocol
distance	I-Protocol
of	O
110	O
and	O
RIP	B-Protocol
has	O
a	O
default	O
administrative	B-Protocol
distance	I-Protocol
of	O
120	O
.	O
</s>
<s>
Administrative	B-Protocol
distance	I-Protocol
values	O
can	O
,	O
however	O
,	O
usually	O
be	O
adjusted	O
manually	O
by	O
a	O
network	O
administrator	O
.	O
</s>
<s>
The	O
administrative	B-Protocol
distance	I-Protocol
(	O
AD	O
)	O
value	O
is	O
assigned	O
by	O
the	O
router	B-Protocol
on	O
a	O
per-protocol	O
basis	O
.	O
</s>
<s>
Routers	B-Protocol
,	O
by	O
design	O
,	O
should	O
not	O
install	O
multiple	O
routes	B-Protocol
into	O
the	O
routing	B-Protocol
table	I-Protocol
as	O
this	O
has	O
the	O
potential	O
to	O
cause	O
routing	B-Protocol
loops	O
.	O
</s>
<s>
While	O
a	O
router	B-Protocol
may	O
run	O
multiple	O
routing	B-Protocol
protocols	I-Protocol
on	O
the	O
same	O
device	O
,	O
it	O
is	O
necessary	O
for	O
the	O
router	B-Protocol
to	O
implement	O
a	O
process	O
to	O
ensure	O
that	O
multiple	O
routes	B-Protocol
,	O
pointing	O
to	O
the	O
same	O
destination	O
do	O
not	O
simultaneously	O
exist	O
in	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
Each	O
process	O
running	O
on	O
a	O
router	B-Protocol
advertises	O
its	O
administrative	B-Protocol
distance	I-Protocol
value	O
to	O
the	O
local	O
router	B-Protocol
.	O
</s>
<s>
The	O
router	B-Protocol
uses	O
this	O
value	O
to	O
determine	O
which	O
route	O
should	O
be	O
used	O
.	O
</s>
<s>
Once	O
a	O
route	O
has	O
been	O
selected	O
,	O
the	O
routing	B-Protocol
information	I-Protocol
database	O
is	O
updated	O
.	O
</s>
<s>
If	O
two	O
routes	B-Protocol
have	O
the	O
same	O
administrative	B-Protocol
distance	I-Protocol
,	O
the	O
router	B-Protocol
uses	O
its	O
vendor-specific	O
algorithm	O
to	O
determine	O
which	O
route	O
should	O
be	O
installed	O
.	O
</s>
<s>
Cisco	O
routers	B-Protocol
simply	O
ignore	O
the	O
values	O
and	O
fall	O
back	O
to	O
the	O
default	O
values	O
,	O
which	O
are	O
never	O
the	O
same	O
.	O
</s>
<s>
The	O
router	B-Protocol
will	O
usually	O
compare	O
administrative	B-Protocol
distances	I-Protocol
to	O
determine	O
which	O
protocol	O
has	O
the	O
lowest	O
value	O
.	O
</s>
<s>
The	O
router	B-Protocol
prefers	O
protocols	O
that	O
have	O
a	O
lower	O
assigned	O
administrative	B-Protocol
distance	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
OSPF	B-Protocol
has	O
a	O
default	O
distance	O
of	O
110	O
,	O
so	O
it	O
is	O
preferred	O
by	O
the	O
router	B-Protocol
process	O
,	O
over	O
RIP	B-Protocol
,	O
which	O
has	O
a	O
default	O
distance	O
of	O
120	O
.	O
</s>
<s>
The	O
administrator	O
can	O
arbitrarily	O
reconfigure	O
the	O
administrative	B-Protocol
distances	I-Protocol
,	O
which	O
affects	O
the	O
ranking	O
of	O
the	O
preferred	O
routes	B-Protocol
by	O
the	O
routing	B-Protocol
process	O
.	O
</s>
<s>
On	O
Cisco	O
routers	B-Protocol
,	O
static	B-Protocol
routes	I-Protocol
have	O
an	O
administrative	B-Protocol
distance	I-Protocol
of	O
1	O
,	O
making	O
them	O
preferred	O
over	O
routes	B-Protocol
issued	O
by	O
a	O
dynamic	B-Protocol
routing	I-Protocol
protocol	I-Protocol
.	O
</s>
<s>
The	O
administrative	B-Protocol
distance	I-Protocol
is	O
a	O
value	O
that	O
is	O
always	O
only	O
referenced	O
by	O
the	O
local	O
router	B-Protocol
itself	O
.	O
</s>
<s>
The	O
administrative	B-Protocol
distance	I-Protocol
is	O
not	O
advertised	O
on	O
the	O
network	O
.	O
</s>
<s>
The	O
following	O
table	O
lists	O
the	O
default	O
administrative	B-Protocol
distances	I-Protocol
for	O
various	O
routing	B-Protocol
protocols	I-Protocol
used	O
on	O
Cisco	O
routers	B-Protocol
.	O
</s>
<s>
The	O
following	O
table	O
lists	O
the	O
default	O
administrative	B-Protocol
distances	I-Protocol
for	O
various	O
routing	B-Protocol
protocols	I-Protocol
used	O
on	O
Juniper	O
routers	B-Protocol
.	O
</s>
<s>
The	O
following	O
table	O
lists	O
the	O
default	O
administrative	B-Protocol
distances	I-Protocol
used	O
on	O
ExtremeXOS	O
/	O
Switch-Engine	O
.	O
</s>
<s>
The	O
following	O
table	O
lists	O
the	O
default	O
administrative	B-Protocol
distances	I-Protocol
used	O
on	O
Extreme	O
VOSS	O
/	O
Fabric-Engine	O
.	O
</s>
<s>
The	O
network	O
administrator	O
may	O
modify	O
the	O
administrative	B-Protocol
distance	I-Protocol
to	O
change	O
the	O
desired	O
ranking	O
of	O
router	B-Protocol
protocols	I-Protocol
.	O
</s>
<s>
This	O
may	O
be	O
necessary	O
in	O
cases	O
where	O
routing	B-Protocol
redistribution	O
has	O
to	O
be	O
used	O
,	O
otherwise	O
,	O
routing	B-Protocol
loops	O
could	O
occur	O
.	O
</s>
<s>
The	O
Cisco	O
Internetwork	O
Operating	O
System	O
enables	O
network	O
administrators	O
to	O
modify	O
the	O
distance	O
by	O
changing	O
the	O
distance	O
value	O
in	O
sub-router	O
configuration	O
mode	O
.	O
</s>
<s>
In	O
the	O
example	O
below	O
,	O
RIP	B-Protocol
's	O
administrative	B-Protocol
distance	I-Protocol
is	O
changed	O
to	O
89	O
so	O
that	O
it	O
used	O
in	O
preference	O
to	O
OSPF	B-Protocol
.	O
</s>
<s>
Manually	O
configuring	O
the	O
administrative	B-Protocol
distance	I-Protocol
is	O
also	O
required	O
when	O
configuring	O
a	O
floating	O
static	B-Protocol
route	I-Protocol
.	O
</s>
<s>
Floating	O
static	B-Protocol
routes	I-Protocol
are	O
used	O
to	O
provide	O
an	O
alternate	O
path	O
when	O
a	O
primary	O
link	O
fails	O
.	O
</s>
<s>
In	O
order	O
for	O
static	B-Protocol
routes	I-Protocol
to	O
be	O
configured	O
as	O
a	O
backup	O
,	O
the	O
static	B-Protocol
route	I-Protocol
's	O
administrative	B-Protocol
distance	I-Protocol
would	O
need	O
to	O
be	O
adjusted	O
.	O
</s>
<s>
Otherwise	O
,	O
it	O
will	O
take	O
precedence	O
over	O
all	O
routing	B-Protocol
protocols	I-Protocol
and	O
routes	B-Protocol
issued	O
from	O
a	O
routing	B-Protocol
protocol	I-Protocol
will	O
not	O
be	O
inserted	O
into	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
The	O
example	O
below	O
shows	O
how	O
to	O
configure	O
the	O
administrative	B-Protocol
distance	I-Protocol
to	O
254	O
to	O
specify	O
that	O
it	O
should	O
only	O
be	O
used	O
as	O
a	O
last	O
resort	O
.	O
</s>
<s>
In	O
the	O
event	O
that	O
two	O
routing	B-Protocol
protocols	I-Protocol
are	O
configured	O
with	O
the	O
same	O
administrative	B-Protocol
distance	I-Protocol
,	O
the	O
Cisco	O
router	B-Protocol
will	O
ignore	O
the	O
configured	O
values	O
and	O
instead	O
use	O
the	O
default	O
values	O
.	O
</s>
<s>
Verifying	O
the	O
configuration	O
of	O
the	O
administrative	B-Protocol
distance	I-Protocol
is	O
done	O
on	O
Cisco	O
equipment	O
using	O
the	O
show	O
ip	O
route	O
command	O
in	O
privileged	O
exec	O
mode	O
on	O
the	O
console	O
of	O
the	O
Cisco	O
router	B-Protocol
.	O
</s>
<s>
In	O
the	O
example	O
shown	O
below	O
,	O
the	O
administrative	B-Protocol
distance	I-Protocol
is	O
1	O
.	O
</s>
<s>
The	O
letter	O
"	O
S	O
"	O
indicates	O
that	O
the	O
route	O
is	O
a	O
static	B-Protocol
route	I-Protocol
that	O
has	O
,	O
for	O
all	O
intents	O
and	O
purposes	O
,	O
been	O
added	O
manually	O
to	O
the	O
router	B-Protocol
process	O
by	O
the	O
administrator	O
and	O
installed	O
into	O
the	O
routing	B-Protocol
table	I-Protocol
.	O
</s>
<s>
The	O
do	O
show	O
ip	O
route	O
command	O
will	O
display	O
the	O
following	O
,	O
confirming	O
that	O
a	O
static	B-Protocol
route	I-Protocol
has	O
an	O
administrative	B-Protocol
distance	I-Protocol
of	O
1	O
.	O
</s>
