Ethernet Switching Module 7

Ethernet Frames

• Ethernet operates in the data link layer and the physical layer.
• It is a family of networking technologies defined in the IEEE 802.2 and 802.3 standards.
• The 802 LAN/MAN standards, including Ethernet, use two separate sublayers of the data link layer to operate:
  • LLC Sublayer (IEEE 802.2): Places information in the frame to identify which network layer protocol is used for the frame.
  • MAC Sublayer (IEEE 802.3, 802.11, or 802.15): Responsible for data encapsulation and media access control, and provides data link layer addressing.
• IEEE 802.3 data encapsulation includes:
  • Ethernet frame: The internal structure of the Ethernet frame.
  • Ethernet Addressing: The Ethernet frame includes both a source and destination MAC address to deliver the Ethernet frame from Ethernet NIC to Ethernet NIC on the same LAN.
  • Ethernet Error detection: The Ethernet frame includes a frame check sequence (FCS) trailer used for error detection.
• The minimum Ethernet frame size is 64 bytes and the maximum is 1518 bytes.
• Any frame less than 64 bytes in length is considered a “collision fragment” or “runt frame” and is automatically discarded.
• Frames with more than 1500 bytes of data are considered “jumbo” or “baby giant frames”.

Ethernet MAC Address

• An Ethernet MAC address consists of a 48-bit binary value, expressed using 12 hexadecimal values.
• MAC addressing provides a method for device identification at the data link layer of the OSI model.
• An Ethernet MAC address is a 48-bit address expressed using 12 hexadecimal digits.
• Because a byte equals 8 bits, a MAC address is 6 bytes in length.
• When the MAC address table of the switch contains the destination MAC address, it is able to filter the frame and forward out a single port.

The MAC Address Table

• A switch builds its MAC address table by learning the source MAC address of incoming frames and associating them with the port on which they were received.
• The switch forwards frames based on the content of their MAC address tables.
• When the destination MAC address is not listed in the switch’s MAC address table, it sends the frame to the default gateway.
• When the source MAC address is not listed in the switch’s MAC address table, it adds the source MAC address to the table.

Switch Speeds and Forwarding Methods

• Switches use one of the following forwarding methods for switching data between network ports:
  • Store-and-forward switching: Receives the entire frame and computes the CRC. If the CRC is valid, the switch looks up the destination address, which determines the outgoing interface. Then the frame is forwarded out of the correct port.
  • Cut-through switching: Forwards the frame before it is entirely received. At a minimum, the destination address of the frame must be read before the frame can be forwarded.
• Cut-through switching has two variants:
  • Fast-forward switching: Offers the lowest level of latency by immediately forwarding a packet after reading the destination address. Because fast-forward switching starts forwarding before the entire packet has been received, there may be times when packets are relayed with errors. The destination NIC discards the faulty packet upon receipt.

This quiz covers the basics of Ethernet switching in a network, including the Ethernet sublayers, frame fields, and MAC addresses. Learn how Ethernet works in a switched network and how a switch builds a MAC address table.