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On Ethernet, the 8 octets occupied by the LLC and SNAP headers reduce the size of the available payload for protocols such as the Internet Protocol to 1492 bytes, compared to the use of the Ethernet II framing; therefore, for protocols that have EtherType values, packets are usually transmitted with Ethernet II headers rather than with LLC and ...
The SNAP header allows EtherType values to be used with all IEEE 802 protocols, as well as supporting private protocol ID spaces. In IEEE 802.3x-1997, the IEEE Ethernet standard was changed to explicitly allow the use of the 16-bit field after the MAC addresses to be used as a length field or a type field.
With the advent of the IEEE 802 suite of standards, a Subnetwork Access Protocol (SNAP) header combined with an IEEE 802.2 LLC header is used to transmit the EtherType of a payload for IEEE 802 networks other than Ethernet, as well as for non-IEEE networks that use the IEEE 802.2 LLC header, such as FDDI. However, for Ethernet, Ethernet II ...
In other words the VLAN tag follows the SNAP header. For 802.3 frames in LLC-SNAP format, the order is opposite; the VLAN tag is placed before the LLC-SNAP header. Because inserting the VLAN tag changes the frame, 802.1Q encapsulation forces a recalculation of the original frame check sequence field in the Ethernet trailer.
Insertion of 802.1ad tag in Ethernet-II frame. These examples are for an Ethernet II framing with EtherType field. The standard is also applicable to IEEE 802.3 frames with or without an LLC (i.e. Logical Link Control), LLC+SNAP header). The top frame is a simple Ethernet II frame. The middle frame has an 802.1Q tag added to it.
IEEE 802.2 is the original name of the ISO/IEC 8802-2 standard which defines logical link control (LLC) as the upper portion of the data link layer of the OSI Model. [1] The original standard developed by the Institute of Electrical and Electronics Engineers (IEEE) in collaboration with the American National Standards Institute (ANSI) was adopted by the International Organization for ...
The 0xAA fields of the LLC header indicate "SNAP", and the OUI 0x000000 in the SNAP header indicates an encapsulated EtherType. Ethernet II encapsulation comprises an Ethernet II frame header (destination MAC, source MAC, EtherType 0x8137) followed by IPX data. In non-Ethernet networks only 802.2 and SNAP frame types are available.
EtherType values are also used in Subnetwork Access Protocol (SNAP) headers. Ethernet is widely used in homes and industry, and interworks well with wireless Wi-Fi technologies. The Internet Protocol is commonly carried over Ethernet and so it is considered one of the key technologies that make up the Internet.