Ethernet LAN Switching (Part 1)| Day-5

• The Physical Layer of the OSI model defines physical characteristics of the medium used to transfer data between devices
• The Physical Layer includes voltage levels, maximum transmission distances, physical connectors, and cable specifications
• Digital bits are converted into electrical signals for wired connections or radio signals for wireless connections
• The Data Link Layer (Layer 2) provides node-to-node connectivity and data transfer
• Layer 2 formatting is for transmission over a physical medium and detects/corrects Physical Layer errors
• Layer 2 addressing is separate from Layer 3 addressing, with IP addresses being Layer 3 addresses
• Ethernet LAN switching involves Layer 1 and Layer 2 of the OSI model
• LANs (Local Area Networks) are networks contained within a relatively small area, like an office floor or home network
• Switches do not separate LANs, but adding more switches can expand an existing LAN
• Ethernet frames encapsulate packets with headers and trailers
• Ethernet headers include fields for preamble, destination, source, and type/length
• Ethernet trailers include the frame check sequence (FCS) field
• The preamble in an Ethernet frame is 7 bytes long and used for synchronization
• The start frame delimiter (SFD) in an Ethernet frame is 1 byte long and indicates the start of the frame
• Destination and source fields in Ethernet frames contain MAC addresses, which are 6-byte physical device addresses
• The Type or Length field in Ethernet frames is 2 bytes long and indicates the type or length of the encapsulated packet
• The FCS field in Ethernet trailers is 4 bytes long and is used to detect corrupted data using a CRC algorithm
• CRC stands for cyclic redundancy check
• CRC is used to detect corrupted data by running an algorithm over the received data
• Cyclic refers to cyclic codes, redundancy refers to the fact that the 4 bytes at the end of the message enlarge the message without adding new information, and check refers to verifying the data for errors
• Ethernet frame's Frame Check Sequence is a Cyclic Redundancy Check
• An Ethernet frame consists of various fields:

  • Preamble: 7 bytes
  • Start-frame delimiter: 1 byte
  • Destination: 6 bytes
  • Source: 6 bytes
  • Type or length field: 2 bytes
  • Frame Check Sequence in the trailer: 4 bytes
  • Total size, including header and trailer, is 26 bytes
• MAC address is a 6-byte or 48-bit physical address assigned to a device when it is made
• MAC address is different from an IP address, which is assigned in the CLI when configuring the device
• MAC address is globally unique, no two devices in the world should have the same MAC address
• There are locally-unique MAC addresses that don't have to be globally unique throughout the world
• First 3 bytes of the MAC address are the OUI (organizationally unique identifier) assigned to the company making the device
• Last 3 bytes of the MAC address are unique to the device itself
• MAC addresses are written as a series of 12 hexadecimal characters
• Hexadecimal system uses 16 possible digits: 0-9 and A-F
• Switches dynamically learn device locations on the network by looking at the source MAC address of frames they receive
• Unknown unicast frames are frames for which the switch doesn't have an entry in its MAC Address table
• When a switch doesn't know the destination of a frame, it floods the frame by forwarding it out of all interfaces except the one it received the packet on
• PCs drop frames that don't match their MAC address, while the intended recipient processes the frame normally up the OSI stack
• When the destination and source addresses of the frame are reversed, it is known as a KNOWN UNICAST frame
• UNKNOWN unicast frames are flooded, while known unicast frames are simply forwarded to the destination
• Dynamic MAC addresses on Cisco switches are removed from the MAC address table after 5 minutes of inactivity
• SW1 floods an UNKNOWN unicast frame out of all interfaces except the one it was received on
• SW2 also floods an UNKNOWN unicast frame out of all interfaces except the one it was received on
• SW2 uses the source MAC address field of the frame to dynamically learn MAC addresses and the corresponding interface