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Chapter 5 Ethernet (8. Unicast, Multicast and Broadcast addresses…

- - - - The remaining portion of the multicast MAC address is created by converting the lower 23 bits of the IP multicast group address into 6 hexadecimal characters. 224.0.0.200 > 200 > 1100 1000 > 0xC8 > 01-00-5E-00-00-C8
- - - - >64: Collision fragment or runt frame<1500: jumbo or baby giant frames
      - 802.2 header and data uses Pad to increase this frame field to at least 64 bytes
- - - - Hexadecimal is usually represented in text by the value preceded by 0x (0x73=115). Less commonly, it may be followed by an H(for example 73H)
- - - - There are two variants of cut-through switching:
        
        Fast-forward switching
        
        Offers the lowest level of latency.
        
        Immediately forwards a packet after reading the destination address
        
        Fast-forward switching is the typical cut-through method of switching.
        
        Fragment-free switching
        
        The switch stores the first 64 bytes of the frame (where most network errors and collisions occur) before forwarding.
        
        Tries to enhance fast-forward switching by performing a small error check on the first 64 bytes of the frame to ensure that a collision has not occurred before forwarding the frame.
- - - - If the packet’s destination IPv4 address is on the same network as the source IPv4 address, the device will search the ARP table for the destination IPv4 address.
      - If the destination IPv4 address is on a different network than the source IPv4 address, the device will search the ARP table for the IPv4 address of the default gateway.
    - - An ARP table maps IP addresses to Ethernet hardware addresses
        
        arp table (or arp cache)
        Use the show ip arp command is used to display the ARP table
        On a Windows PC, the arp –a command is used to display the ARP table
      - NOTE: A MAC table maps Ethernet hardware addresses to switch ports
    - - ARP Broadcast
        
        An ARP request is received and processed by every device on the local network
        
        There could be some reduction in performance for a short period of time,
      - ARP Spoofing
        
        This is a technique used by an attacker to reply to an ARP request for an IPv4 address belonging to another device, such as the default gateway
        
        The attacker sends an ARP reply with its own MAC address. The receiver of the ARP reply will add the wrong MAC address to its ARP table and send these packets to the attacker.
- - - - • Frame Format
        • Frame Sizing
        • Timing
        • Encoding
    - - Ethernet is a contention-based method of networking
        
        The Carrier Sense Multiple Access/Collision Detection (CSMA/CD) process is used in half-duplex Ethernet LANs to detect and resolve collisions.
        
        Today’s Ethernet LANs use full-duplex switches, which allow multiple devices to send and receive simultaneously with no collisions.
- - - - LLC is implemented in software, and its implementation is independent of the hardware. In a computer, the LLC can be considered the driver software for the NIC.
- - - - Frame Delimiting: provides delimiters that are used to identify a group of bits that make up a frame.
      - Addressing: provides data link layer addressing
      - Error Detection: Each frame contains a trailer used to detect any errors in transmissions.
    - - Media recovery
      - Control of frame placement on and off the media
- - - - If there is no match, the device discards the frame. If there is a match, it passes the frame up the OSI layers, where the de-encapsulation process takes place.
      - Ethernet NICs will accept frames if the destination MAC address is a broadcast or a multicast group of which the host is a member.
- - - - Learning MAC Addresses
        
        The switch dynamically builds the MAC address table by examining the source MAC address of the frames received on a port.
        
        Forward – Examining the Destination MAC Address
        
        If the destination MAC address is in the table, it will forward the frame out the specified port.
        
        If the destination MAC address is not in the table, the switch will forward the frame out all ports except the incoming port.
        
        Learn – Examining the Source MAC Address
        
        If the source MAC address does not exist, it is added to the table along with the incoming port number.
        
        If the source MAC address does exist in the table but on a different port, the switch treats this as a new entry.
        
        If the source MAC address does exist, the switch updates the refresh timer for that entry. By default, most Ethernet switches keep an entry in the table for 5 minutes.
        
        Use S2#show mac address-table
        and S2#clear mac address-table dynamic to clear the table
        Use C:/ arp -a in a windows PC to see the IP-to-Physical table
- - - - Port-based Memory Buffering
        
        Frames are stored in queues that are linked to specific incoming and outgoing ports
      - Shared Memory Buffering
        
        It deposits all frames into a common memory buffer that all the ports on the switch share
        
        The frames in the buffer are linked dynamically to the destination port. This allows the packet to be received on one port and then transmitted on another port, without moving it to a different queue.
- - - - Full-duplex – Both ends of the connection can send and receive simultaneously.
      - Half-duplex – Only one end of the connection can send at a time.
    - - This occurs when one or both ports on a link are reset, and the autonegotiation process does not result in both link partners having the same configuration.
      - It also can occur when users reconfigure one side of a link and forget to reconfigure the other. Both sides of a link should have autonegotiation on, or both sides should have it off.