IEEE 802.1Q

IEEE 802.1Q is one of the VLAN tagging protocols compatible with Cisco Switches. This standard was created by the Institute of Electrical and Electronic Engineers (IEEE). Therefore, it is an open standard that can also be used in switches of other manufacturers.

To determine which VLAN a frame belongs to, a field is inserted in the frame header.

Original frame:

802.1Q frame:

An example will try to clarify the concept. Let's say we have a network of 2 switches and 4 hosts. Hosts A and Host D are in VLAN 2, while hosts B and C are in VLAN 3.

The segment between two switches uses a process called VLAN trunk. Suppose that host A sends a broadcast frame. SW1 "marks" the frame by inserting the VLAN ID in the frame header before sending the frame to SW2. SW2 receives the frame and knows that the frame belongs to VLAN 3. Therefore, it sends the frame to host D only because this host is in VLAN 3.
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