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Link Layer & LANs

How a frame crosses one link, and how a LAN is built from many. The module starts with what the link layer is for, then reads the Ethernet frame and MAC address, and covers how a receiver spots a corrupted frame. From there it follows the frame through switches: how they learn, how spanning tree stops loops, how ARP finds the next-hop MAC, and how VLANs split one switch into several broadcast domains.

What you will be able to answer​

  • What does the link layer do, and why is the frame rewritten on every link while the IP packet is not?
  • What is in an Ethernet frame, how does a MAC address break down bit by bit, and where do 64, 1518 and 1522 bytes come from?
  • How do parity, the Internet checksum and CRC-32 each detect errors, and why is none of it security?
  • How does a switch learn, flood, forward and filter, and how do collision domains differ from broadcast domains?
  • Why does a loop of switches melt a LAN, and how do spanning tree, RSTP and LACP deal with it?
  • How does ARP turn a next-hop IP into a MAC address, and what do the gateway case, gratuitous ARP and spoofing change?
  • What do access and trunk ports, the 802.1Q tag and the native VLAN do, and how does traffic cross between VLANs?
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