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Tech Stuff - 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 and the MAC format 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. Numeric values are in bytes (octets).

  2. Preamble does not count toward the total block size.

  3. Frames of less than 64 bytes are always padded to 64 bytes.

  4. Maximum LAN block size on a non VLAN is 1514.

  5. The Type/Length for non-VLAN values.

  6. 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).

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Media Access Control (MAC) Address Format

The MAC address (aka EU-48) is an IEEE 802 administered format consisting of 48 bits:

Note: Bits are numbered 1 to 48 starting on left using the standard IETF convention. Note: IEEE uses a 8 - 1 bit per byte notation with 1 on the RIGHT.

Organizationally Unique Id (OUI) 24 bits (3 octets) Unique Interface Value 24 bits (3 octets)
Unique value assigned to an organization by IEEE (aka Vendor ID). The top (leftmost) octet has the following format:
Bit Usage/Meaning
1 - 6 Part of OUI assigned by IEEE
7 U/L bit (Universal/Local) If 0 address is assigned Universally (all IEEE assigned addresses ranges use 0). If 1 address has been assigned by local administrator. Numbered bit 2 in IEEE notation.
8 Unicast/Multicast. If 0 address is unicast, if 1 address is multicast. Set at run time by software. Numbered bit 1 in IEEE notation.
Unique Address assigned by OUI (vendor) if bit 7 (U/L) of first MAC octet is 0 or local administrator if bit 7 (U/L) of first MAC octet is 1.

Note: When the MAC is used to construct an IPv6 EUI-64 address using Stateless configuration the meaning and values of the U/L (bit 7) are reversed.

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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
 

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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 12bit 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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