Building Ethernet LANs with Switches

HUB
1. When receive an electrical signal in one port,repeats the signal out all other ports
2. When two or more devices send at the same time, an electrical collision occurs, making both signals corrupt
3. devices must take turns by using carrier sense multiple access with collision detection (CSMA/CD) logic
Bridge
1. Separated devices into groups called collision domains.
2. Reduced the number of collisions that occurred in the network, because frames inside one collision domain did not collide with frames in another collision domain.
3. Increased bandwidth by giving each collision domain its own separate bandwidth, with one sender at a time per collision domain.
4. Buffer or queue the frame until the outgoing interface can send the frame
Switches
1. Like bridges, switches segment a LAN into separate collision domains, each with its own capacity
  1. Switching Logic
    1. Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address
    2. Learning MAC addresses by examining the source MAC address of each frame received by the switch.
    3. Creating a (Layer 2) loop-free environment with other bridges by using Spanning Tree Protocol (STP)
2. How Switches Learn MAC Addresses
  1. Listening to incoming frames and examining the source MAC addres in the frame.
    1. Flooding Frames
      1. When there is no matching entry in the table, switches forward the frame out all interfaces (except the incoming interface)
  2. Inactivity timer
3. Avoiding Loops Using Spanning Tree Protocol
  1. Without STP, any flooded frames would loop for an indefinite period of time in Ethernet networks with physically redundant links
    1. Blocks some ports from forwarding frames so that only one active path exists between any pair of LAN segments.
4. Switch Internal Processing
  1. Switching Method
    1. Fragment-free
      1. The switch forwards the frame after receiving the first 64 bytes of the frame, thereby avoiding forwarding frames that were errored because of a collision.
    2. Cut-through
      1. The switch forwards the frame as soon as it can. This reduces latency but does not allow the switch to discard frames that fail the FCS check.
    3. Store-and-forward
      1. The switch fully receives all bits in the frame (store) before forwarding the frame (forward). This allows the switch to check the FCS before forwarding the frame.

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