Confused about the header size for a Ethernet frame

header frame lan ethernet
Bic B picture Bic B · Oct 7, 2012 · Viewed 17.7k times · Source

I was researching a few things about VLANs and came across the VLAN tag and also the headers.

If we have a MTU for a standard 802.3 Ethernet frame (1518 bytes) what is included in the header 802.3?

Also how do we calculate the header length for that?

What is the difference between 802.3 and 802.1q? I know that the VLAN tag requires extra bytes but how to calculate how many bytes needed to the 802.1q VLAN tag?

Thanks in advance

Answer

user1500049 picture user1500049 · Oct 9, 2012

A regular 802.3/Eth-II ethernet frame doesn't carry VLAN info.
802.1Q can carry VLAN (and QoS) info over to the receiving end.

If the ethertype is 0x8100 then you got yourself an 802.1Q tag which is another 4 bytes in addition to the 14 bytes (dmac+smac+type).

See wikipedia for reference. http://en.wikipedia.org/wiki/Ethernet_frame

EDIT:

Regular Eth-II/802.3 has a total length of:

dmac(6)+smac(6)+etype(2)+payload(1500)+crc(4) = 1518 bytes

For the case of Eth-II/802.3 with 802.1Q tagging:

dmac(6)+smac(6)+8100(2)+vlan/Qos(2)+etype(2)+payload(1500)+crc(4) = 1522 bytes

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