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AFAIK 802.1Q tag is optional in the FRame header. So it may or not be present in the header right? SO how does the Packet processing ASIC or processor identifies or parses to interpret the frame header as vlan tag present or not present ?

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With 802.1q payload in the packet unique ethertype value is used in the header of Ethernet frame. Normal IPv4 packets use 0x0800, normal IPv6 packets use 0x86DD, 802.1q packets use 0x8100 etc.

http://en.wikipedia.org/wiki/EtherType

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  • Still ethertype value is present after 802.1Q header right ? so by doing start of frame address plus 20(8bytes preamable+6 bytes SMac + 6 bytes DMAC) we won't be knowing whether we are interpreting "Ether type" or a "802.1Q" header right ?
    – Hemanth
    Apr 24, 2014 at 8:33
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    Sorry about the confusion . now understood. First 2 bytes after DEstination MAC is interpreted in common for normal ethernet frames and vlan tags. thanks
    – Hemanth
    Apr 24, 2014 at 11:46

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