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Switch foundation

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: unicast, multicast and broadcast.

unicast, broadcast, multicast
unicast, broadcast, multicast

  1. Unicast: In a unicast transmission, a single frame is sent from a single source to a single destination on a network, such as sending message from a server to a LAN PC.
  2. multicast: a single data frame or a single source to multiple destinations packet is copied and sent to a specific subset of nodes on the network. Multicast is a one-to-many transmission method in which the network carries a message to multiple receivers at the same time. Multicast conservers network bandwidth by using shared transmission across a network. Multicasting sends the data to a predetermined endpoint, such as a switch, where the traffic is sent to each intended recipient, instead of each traffic stream being sent from start to finish across the network, independent of others.
  3. broadcast: Broadcast is a one-to-all transmission method in which the network carries a message to all devices at the same time. Broadcast message traffic is sent out to every node on the network where the broadcast is not filtered or blocked by a router. Broadcasts are issued by the ARP for address resolution when the location of a user or server is not known. 


There are 3 main switching methods:
3 switching methods
3 switching 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.