Tech Info - Ethernet Frame Format

The generic format of an Ethernet frame is defined below, the 'options' fields may be present if the Type/Length field indicates a VLAN TPID. The VLAN extended Frame Format is defined here.

Standard Frame Format

Preamble	Destination MAC	Source MAC	Type/Length	Options	Data	FCS
8	6	6	2	?	46 - 1500	4
	64 - 1518 bytes

NOTES:

1. Preamble does not count toward the total block size.
2. Frames of less than 64 bytes are always padded to 64 bytes.
3. Maximum LAN block size on a non VLAN is 1514.
4. The Type/Length for non-VLAN values.
5. The options field is present depending on the value of the Type/Length field (and is used primarily in VLANs 802.1Q and 802.1D format LANs).

VLAN Extended Frame Format

The VLAN Extension is PRESENT if the the value of the first Type/Length field is x'8100 (TPID - Tag Protocol Identifier). The Frame Type/Length value is then replaced with the extended VLAN frame as defined below.

Type/Length	VLAN Descriptor	Options	Type/Length	Data....
2	2	?	2	46 - 1500
TPID (value x'8100)	TCI	Optional TCI indicates presence	Frame Type/Length

TCI (Tag Control Information)

The TCI is a 16 bit value defined as follows:

8	7	6	5	4	3	2	1	8	7	6	5	4	3	2	1
user_priority			CFI	12 bit VID (VLAN ID)

1. user_priority is in the range 0 to 7
2. CFI if 1 indicates there is one or more additional 'options' fields present
3. VID is the 12 bit VLAN ID

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