Ethernet Frames Overview

Study Notes

Ethernet Frames

Ethernet operates in the data link layer (layer 2) and the physical layer (layer 1)
Ethernet is a family of networking technologies defined in the IEEE 802.2 and 802.3 standards
The MAC sublayer is responsible for data encapsulation and accessing the network media
Data encapsulation includes the Ethernet frame, Ethernet addressing, and Ethernet error detection
LLC (Logical Link Control) prepares the frame for the network layer, identifying what network protocol is used by the frame
MAC (Media Access Control) provides addressing and handles data encapsulation
The minimum Ethernet frame size is 64 bytes
The maximum Ethernet frame size is 1518 bytes
The preamble field is not included in the frame size
Frames less than 64 bytes are considered a “collision fragment” or “runt frame” and are discarded
Frames greater than 1500 bytes are considered “jumbo” or “baby giant frames”
Frames that are too small or too large are dropped, and considered invalid
Frames larger than 1500 bytes are supported by Fast Ethernet and Gigabit Ethernet switches and NICs ### Ethernet MAC Address
The Media Access Control (MAC) address is a unique identifier for a device’s network interface card (NIC).
The MAC address is a 48-bit address represented by 12 hexadecimal characters ranging from 0 to 9 and A to F.
The first 3 bytes of the address represent the Organizationally Unique Identifier (OUI). The OUI is assigned by the IEEE to the manufacturer of the NIC.
The OUI indicates the vendor who manufactured the NIC
The last 3 bytes of the address represent a unique identifier, assigned to the NIC by the manufacturer.
The MAC address for a device can be found under the device’s network settings in the operating system.

The MAC Address Table

A switch builds its MAC address table by learning the MAC addresses which are connected to its ports based on the traffic it sees.
The switch forwards traffic based on the destination MAC address and information contained in its MAC address table.
Whenever a device transmits a frame, the switch extracts the destination MAC address, and looks it up in its MAC address table.
If a match is found, the switch forwards the frame to the appropriate port.
If no match is found, the switch floods the frame out of all ports, except for the port from which the frame arrived.
By flooding, the switch allows the receiver to respond with its MAC address and update the switch’s MAC address table.

Switch Speeds and Forwarding Methods

Switches connect to various devices, such as computers, servers, and other switches.
Ethernet switches can operate at different speeds, including 10 Mbps, 100 Mbps, 1000 Mbps (Gigabit) and 10 Gbps.
Switches can use different forwarding methods to send Ethernet frames, including store-and-forward and cut-through forwarding.
Store-and-forward is a more reliable forwarding method, ensuring that the entire frame is received before it is forwarded.
Cut-through forwarding forwards the frame as soon as the destination MAC address is received, but does not check the entire frame for errors.
Cut-through can be faster than store-and-forward, but increases the reliability of the network.