802.1Q Frame
IEEE 802.1Q uses an internal tagging mechanism that inserts a four-byte tag ?¬?eld into the
original Ethernet frame between the Source Address and Type or Length ?¬?elds. Because
802.1Q alters the frame, the trunking device recomputes the frame check sequence (FCS)
on the modi?¬?ed frame.
It is the responsibility of the Ethernet switch to look at the four-byte tag ?¬?eld and determine
where to deliver the frame. An Ether Type of 0x8100 indicates to devices that the frame has
an 802.1Q tag. A tiny part of the four-byte tag ?¬?eld??”three bits to be exact??”is used to
specify the priority of the frame. The details of this are speci?¬?ed in the IEEE 802.1p
standard. The 802.1Q header contains the 802.1p ?¬?eld, so you must have 802.1Q to have
802.1p. Following the priority bit is a single ?¬‚ag to indicate whether the addressing is Token
Ring. This is because 802.1Q tagging could also be implemented in a Token Ring
environment; the ?¬‚ag will be 0 for an Ethernet frame. The remainder of the tag is used for
the VID. Figure 2-9 shows the 802.1Q frame format.
Cisco Catalyst
Series Switch
802.1Q
Trunk
802.1Q
Trunk
Catalyst
Series Switch
Catalyst
Series Switch
802.
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