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ISL is a vendor-specific, proprietary
protocol used to interconnect multiple switches and maintain VLAN
information as traffic travels between switches on trunk links.
With ISL, an Ethernet frame is
encapsulated with a header that transports VLAN IDs between switches
and routers. ISL does add overhead to the packet as a 26-byte header
containing a 10-bit VLAN ID. In addition, a
4-byte cyclic redundancy check (CRC) is appended to the end of each
frame. This CRC is in addition to any frame checking that the
Ethernet frame requires.
A VLAN ID is added only if the frame
is forwarded out a port configured as a trunk link. If the frame is
to be forwarded out a port configured as an access link, the ISL
encapsulation is removed. Figure
illustrates the ISL frame format. Figure
lists the sizes of the various ISL fields and Figure
describe the ISL fields contained within the frame.
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