<s>
Cisco	B-Protocol
Inter-Switch	I-Protocol
Link	I-Protocol
(	O
ISL	O
)	O
is	O
a	O
Cisco	O
Systems	O
proprietary	O
protocol	O
that	O
maintains	O
VLAN	B-General_Concept
information	O
in	O
Ethernet	O
frames	O
as	O
traffic	O
flows	O
between	O
switches	B-Protocol
and	O
routers	B-Protocol
,	O
or	O
switches	B-Protocol
and	O
switches	B-Protocol
.	O
</s>
<s>
ISL	O
is	O
Cisco	O
's	O
VLAN	B-General_Concept
encapsulation	O
protocol	O
and	O
is	O
supported	O
only	O
on	O
some	O
Cisco	O
equipment	O
over	O
the	O
Fast	O
and	O
Gigabit	O
Ethernet	O
links	O
.	O
</s>
<s>
It	O
is	O
offered	O
as	O
an	O
alternative	O
to	O
the	O
IEEE	O
802.1Q	O
standard	O
,	O
a	O
widely	O
used	O
VLAN	B-General_Concept
tagging	O
protocol	O
,	O
although	O
the	O
use	O
of	O
ISL	O
for	O
new	O
sites	O
is	O
deprecated	O
by	O
Cisco	O
.	O
</s>
<s>
With	O
ISL	O
,	O
an	O
Ethernet	O
frame	B-Protocol
is	O
encapsulated	O
with	O
a	O
header	O
that	O
transports	O
VLAN	B-General_Concept
IDs	O
between	O
switches	B-Protocol
and	O
routers	B-Protocol
.	O
</s>
<s>
ISL	O
does	O
add	O
overhead	O
to	O
the	O
frame	B-Protocol
as	O
a	O
26-byte	O
header	O
containing	O
a	O
10-bit	O
VLAN	B-General_Concept
ID	O
.	O
</s>
<s>
In	O
addition	O
,	O
a	O
4-byte	O
CRC	O
is	O
appended	O
to	O
the	O
end	O
of	O
each	O
frame	B-Protocol
.	O
</s>
<s>
This	O
CRC	O
is	O
in	O
addition	O
to	O
any	O
frame	B-Protocol
checking	O
that	O
the	O
Ethernet	O
frame	B-Protocol
requires	O
.	O
</s>
<s>
The	O
fields	O
in	O
an	O
ISL	O
header	O
identify	O
the	O
frame	B-Protocol
as	O
belonging	O
to	O
a	O
particular	O
VLAN	B-General_Concept
.	O
</s>
<s>
A	O
VLAN	B-General_Concept
ID	O
is	O
added	O
only	O
if	O
the	O
frame	B-Protocol
is	O
forwarded	O
out	O
a	O
port	O
configured	O
as	O
a	O
trunk	O
link	O
.	O
</s>
<s>
If	O
the	O
frame	B-Protocol
is	O
to	O
be	O
forwarded	O
out	O
a	O
port	O
configured	O
as	O
an	O
access	O
link	O
,	O
the	O
ISL	O
encapsulation	O
is	O
removed	O
.	O
</s>
<s>
The	O
size	O
of	O
an	O
Ethernet	O
encapsulated	O
ISL	O
frame	B-Protocol
can	O
be	O
expected	O
to	O
start	O
from	O
94	O
bytes	O
and	O
increase	O
up	O
to	O
1548	O
bytes	O
because	O
of	O
the	O
overhead	O
(	O
additional	O
fields	O
)	O
the	O
protocol	O
creates	O
via	O
encapsulation	O
.	O
</s>
<s>
ISL	O
adds	O
a	O
26-byte	O
header	O
(	O
containing	O
a	O
15-bit	O
VLAN	B-General_Concept
identifier	O
)	O
and	O
a	O
4-byte	O
CRC	O
trailer	B-Device
to	O
the	O
frame	B-Protocol
.	O
</s>
<s>
ISL	O
functions	O
at	O
the	O
data-link	B-Protocol
layer	I-Protocol
of	O
the	O
OSI	O
model	O
.	O
</s>
<s>
Fast	O
EtherChannel	B-Protocol
technology	O
enables	O
aggregation	O
of	O
two	O
full-duplex	O
Fast	O
Ethernet	O
links	O
for	O
high-capacity	O
backbone	O
connections	O
.	O
</s>
<s>
DISL	O
minimizes	O
VLAN	B-General_Concept
trunk	O
configuration	O
procedures	O
because	O
only	O
one	O
end	O
of	O
a	O
link	O
needs	O
to	O
be	O
configured	O
as	O
a	O
trunk	O
.	O
</s>
