Both ISL and IEEE 802.1Q tagging perform explicit tagging as the frame is tagged
with VLAN information explicitly. ISL uses an external tagging process that does not
modify the existing Ethernet frame whereas 802.1Q uses an internal tagging process
that does modify the Ethernet frame. This internal tagging process is what allows
IEEE 802.1Q tagging to work on both access and trunk links, because the frame
appears to be a standard Ethernet frame.
The IEEE 802.1Q header contains the following:
* A 4-byte tag header containing a tag protocol identifier (TPID) and tag control
information (TCI)
* A 2-byte TPID with a fixed value of 0x8100 that indicates that the frame carries
the 802.1Q/802.1p tag information.
A TCI containing the following elements:
* Three-bit user priority
* One-bit canonical format indicator (CFI)
* Twelve-bit VLAN identifier (VID)-Uniquely identifies the VLAN to which the frame
belongs
The 802.1Q standard can create an interesting scenario on the network. Recalling
that the maximum size for an Ethernet frame as specified by IEEE 802.3 is 1518
bytes, this means that if a maximum-sized Ethernet frame gets tagged, the frame
size will be 1522 bytes, a number that violates the IEEE 802.3 standard. To resolve
this issue, the 802.3 committee created a su/jointfilesconvert/449143/bgroup called 802.3ac to extend the
maximum Ethernet size to 1522 bytes. Network devices that do not support a larger
frame size will process the frame successfully but may report these anomalies as a
"baby giant."
Inter-Switch Link (ISL) is a Cisco proprietary protocol used to interconnect multiple
switches and maintain VLAN information as traffic travels between switches on trunk
links. This technology provides one method for multiplexing bridge groups (VLANs)
over a high-speed backbone. It is defined for Fast Ethernet and Gigabit Ethernet, as
is IEEE 802.1Q. ISL has been available on Cisco routers since Cisco IOS Software
Release 11.1.
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 CRC is appended to the
end of each frame. This CRC is in addition to any frame checking that the Ethernet
frame requires. The fields in an ISL header identify the frame as belonging to a
particular VLAN.
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.
Early network designers often configured VLANs with the aim of reducing the size of
the collision domain in a large single Ethernet segment and thus improving
performance. When Ethernet switches made this a non-issue (because each switch
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