Switch foundation

Posted on July 22, 2011. Filed under: ICND2 break down |


In this class, we will review the basic concepts learned in ICND1 – Lession 3 Switching.

  • Switch is a Layer 2 network device, it forward frames based on the destination MAC address.
  • MAC Address table: When switch receives frame from new source MAC address, it creates an entry in its MAC address table or content addressable memory (CAM) table. This entry is the receiving port and the source MAC address of the frame.
  • Flooding: when switch forward a frame, it will check its CAM table for the destination MAC address. If there is an entry for the destination MAC, it forwards to that port, otherwise it forwards to all ports, except the port the frame was originally received from, this is called flooding.
  • Switching Loops and Broadcast storm: in muti-linked networks, redundent links can form a loop, called switching loop. A frame with new source MAC address can tranverse a switching loop and gets repeatly broadcasted, which is called broadcast storm. To prevent broadcast storm, STP protocal is used.

There are 3 main switch transmission methods:

  • Store-and-Forward: stores a receiving frame in memory and runs CRC check, and only forward frame if CRC passes.
  • Cut-Trough: only looks at enough of the frame for destination and forward.
  • Fragment-Free: checks the first 64 tytes of a frame before forwarding, because most of the collision happened in the first 64 bypes.
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