CCNA-1. Chapter 9. Ethernet

Ethernet Data Link sublayers:

• Logical Link Control (LLC)
  
  • connection to upper layers
  • frames the network layer packet
  • identifies network layer protocol
  • independen of physical enviroment
• Media Access Control (MAC)
  
  • Data Encapsulation
    
    • Frame delimiting
      
      • identify a group of bits that make up a frame
      • synchronization between the transmitting and receiving nodes
    • Addressing (header contains physical address (48bit MAC address))
    • Error detection (trailer with a cyclic redundancy check (CRC))
  • Media Access Control
    
    • controls the placement and removal of frames from the media
    • initiation of frame transmission
    • recovery from transmission failure due to collisions

Ethernet logical topology - multi-access bus (see CCNA-1. Chapter7. Data link OSI level) Ethernet frame size 64 bytes - 1518 (1522 - 802.3ac) bytes. Ethernet framing types:

• Ethernet (IEEE 802.3) Length field: equal to or greater than 0x0600 hexadecimal or 1536 decimal

• Ethernet II (DIX Ethernet standard) Type field: equal or less than 0x05DC hexadecimal or 1500 decimal

MAC address types:

• Unicast (destination MAC address)
• Broadcast (FF-FF-FF-FF-FF-FF)
• Multicast (01-00-5E-XX-XX-XX) XX-XX-XX converted to hex lower 23 bits of the IP multicast group address. The remaining bit in the MAC address is always a "0".

Purpose of inventing hubs (multi-port repeaters):

• enable more nodes to connect to the shared media
• migration from physical bus to physical star topology
• increased network reliability by allowing any single cable to fail without disrupting the entire network

Communication types:

• Synchronous (Ethernet >100Mbs) Timing not required. Preamble and Start Frame Delimiter (SFD) - only for compatibility.
• Asynchronous (Ethernet <10Mbs) Use the timing information to synchronize the receive circuit to the incoming data.

Ethernet LAN switches basic operations:

• Learning (source MAC address based)
• Aging (time stamped entries)
• Flooding (not in the MAC table - switch sends the frame to all ports except port frame arrived)
• Selective Forwarding (forward to the corresponding port based on MAC table)
• Filtering (not forward frame to port it arrived, drop a corrupt frame(fails a CRC check), port security)

ARP table is stored in the RAM



Proxy ARP (Router map MAC address of the gateway to any IP addresses not on the local network). Usage:

• Older implementation of IPv4 (ARP always sends requests for the destination)
• Improper mask configured (host believes that it is directly connected to the same network)
• Static routing with only exit interface configured on broadcast and NMBA networks

Use static ARP to prevent ARP spoofing (ARP poisoning)