IEEE 8021q

The IEEE standardizes many of the protocols relating to LANs today, and VLAN trunking is no exception. Years after Cisco created ISL, the IEEE completed work on the 802.1q standard, which defines a different way to do trunking.

802.1q uses a different style of header than does ISL for tagging frames with a VLAN number. In fact, 802.1q does not actually encapsulate the original frame—instead, it adds an extra 4-byte header to the middle of the original Ethernet header. That additional header includes a field with which to identify the VLAN number. Because the original header is now longer, 802.1q encapsulation forces a recalculation of the original FCS field in the Ethernet trailer because the FCS is based on the contents of the entire frame. Figure 10-5 shows the 802.1q header and framing of the revised Ethernet header.

Figure 10-5 802.1q Trunking Header

Dest

Src

Len/Etype

Data

X

Segment Frame

Dest

Src

Etype

Tag

Len/Etype

Data

VLAN-ID

Tagged Frame

VLAN-ID

0 0

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