Like ISL, 802.1Q is a protocol used to allow a single physical link
to carry traffic for multiple VLANs. It is the IEEE standard VLAN trunking
protocol. Rather than encapsulating the original Layer 2 frame in its entirety,
802.1Q inserts a tag into the original Ethernet header, then recalculates and
updates the FCS in the original frame and transmits the frame over the trunk
link.
The 802.1Q protocol, often referred to as "dot1q", offers the
clear benefit of being the first IEEE standards-based trunking protocol for
Ethernet. It allows multiple VLANs to traverse infrastructure equipment where
cross vendor links exist.

Some
additional benefits of the 802.1Q protocol are:
- Support for Ethernet and Token Ring
- Support for 4096 VLANs
- Support for Common Spanning Tree (CST), Multiple Spanning Tree (MST) and
Rapid Spanning Tree (RST)
- Point-to-multipoint topology support
- Support for untagged traffic over the trunk link via Native VLAN
- Extended quality of service (QoS) support
- Growing standard for IP Telephony links
The 802.1Q Tagging Process
To identify a frame with a given
VLAN, the 802.1Q protocol adds a tag, or a field, to the standard Layer 2
Ethernet data frame. The components of this tag are shown in Figure
. Because
inserting the tag alters the original frame, the switch must recalculate and
alter the CRC value for the original frame before sending it out the 802.1Q
trunk port. In comparison, ISL does not modify the original frame at all.
The new 802.1Q Tag field has the following components:
-
PRI – 3 bits carries priority information for the frame.
-
Token Ring Field – Indicates the canonical interpretation of the
frame if it is passed from Ethernet to Token Ring.
-
VLAN ID – VLAN association of the frame. By default, all normal and
extended range VLANs are supported.
If a non-802.1Q-enabled device or an access port receives an 802.1Q
frame, the tag data is ignored, and the packet is switched at Layer 2 as a
standard Ethernet frame. This allows for the placement of Layer 2 intermediate
devices, such as other switches or bridges, along the 802.1Q trunk path. To
process an 802.1Q tagged frame, a device must allow an MTU of 1522 or
higher.
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NOTE:
An Ethernet frame that has a larger MTU than expected (1518 by default
for Ethernet) but no larger than 1600 bytes, will register as a Layer 2 error
frame called a "baby giant." For ISL, the original frame plus ISL
encapsulation can generate a frame as large as 1548 bytes and 1522 bytes for an
802.1Q tagged frame.
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