CCNA Learning Journey

Day 5 : Ethernet LAN Switching Part 1

OSI Model - PDUs

Ethernet Frame

Preamble

• Length 7 bytes (56 bits)
• Alternating 1’s and 0’s
• 10101010 * 7
• Allows devices to synchronize their receiver clocks

SFD

• Start Frame Delimiter
• Length: 1 byte (8 bits)
• 10101011
• Marks the end of the preamble, and the beginning of the rest of the frame

Destination

• Indicate the devices sending and receiving the frame
• Consist of the destination and source “MAC address”
• MAC = Media Access Control
• 6 byte (48-bit) address of the physical device

Type / Length

• 2 byte (16-bit field)
• A value of 1500 or less in this field indicates the Length of the encapsulated packet (in bytes)
• A value of 1536 or greater in this field indicates the type of the encapsulated packet(usually IPv4 or IPv6), and the length is determined via other methods

IPv4 = 0x0800 (hexadecimal) (2048 in decimal)
IPv6 = 0x86DD (hexadecimal) (34525 in decimal)

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Frame Check Sequence

• 4 bytes (32 bits) in length
• Detects corrupted data by running a “CRC” algrithm over the received data
• CRC = “Cyclic Redundancy Check”

Length of frame