802.1Q Encapsulation Explained

Lessons Discussion
83

Hi team,
Can someone explain to me the following regarding to the normal ethernet frame:

  • After the preamble, should it be SOF or SFD?
84

Hello Vanilson

The format of the Ethernet Header is specified in IEEE 802.3. In that specification, the official name for the single byte that comes after the 7 bytes of the Preamble is Start Frame Delimiter or SFD. In the lesson, Rene has used the acronym SOF for Start of Frame in the initial image of the Ethernet Header. I will inform Rene to make the change so that it more correctly reflects the official acronym. Thanks for pointing that out.

I hope this has been helpful!

Laz

85

Great explanation, appreciate it

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86

Hi, from this lesson, when we use a trunk link between switches, we need a trunking protocol. I am wondering if there is a protocol used for Ethernet frames themselves if it was just simple network connection between 2 hosts connected to a switch & without a vlan?

Thank you.

87

Hello Keder

Let’s first clarify some issues concerning terminology. Although it’s sometimes called a “protocol”, strictly speaking, 802.1Q is not a protocol. It is a standard that defines the structure of the VLAN tag. So in this sense, there is no trunking protocol used to employ the actual structure of the frame over a trunk.

Now there is a “trunking protocol” that exists, and that is Dynamic Trunking Protocol or DTP. This IS a protocol (Cisco proprietary) that is used to allow switchports to negotiate their role as either an access or trunk port. Best practice suggests that DTP should be disabled and the roles should be manually configured.

So, having said all of that, there is no protocol that is used for trunking and for access ports in the sense that you describe. However, what exists is the Ethernet frame standard, which defines the frames that are used on access ports and trunk ports. Specifically

  • The Ethernet frame format follows the IEEE 802.3/Ethernet II standard for frames sent to and from access ports
  • The 802.1Q standard defines the additional tag that is inserted into the IEEE 802.3/Ethernet II standard for frames sent to and from trunk ports.

I hope this has been helpful!

Laz

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88

Hi Laz, thanks for detailed explanation.

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