04. Ethernet Switch

An Ethernet frame wraps an Internet Layer packet (like an IP packet) with a header and trailer.

• Ethernet II
  • Dominant format used today.
  • Identified by a 2-byte EtherType field that has a value of 0x0800 or greater. This field directly indicates the protocol of the encapsulated payload (e.g., 0x0800 for IPv4, 0x0806 for ARP).
• IEEE 802.3
  • An older standard.
  • In the same position as the EtherType field, it has a 2-byte Length field. If the value of this field is less than 0x0600, the receiving station knows it indicates the length of the data payload, not the protocol type.

An Ethernet switch (Layer 2) is the traffic controller of a local network. Its goal is to forward data frames only to their intended recipient.

A switch stores a MAC Address Table, implemented with a Content-Addressable Memory (CAM; offers high-speed lookups).

• Provide the data (MAC address) and it gives back the location (switch port number).
• Entry: (MAC address, port number, aging timer)

Algorithm

1. Learn: updates CAM table with source MAC address (resets aging timer).
2. Examines destination MAC address:
   1. Forward (unicast): if the dest. is known and on a different port, send the frame to that port.
   2. Filter: if the dest. is on the same port, drop the frame.
   3. Flood: if the dest. is unknown or is the broadcast address (FF:FF:FF:FF:FF:FF), send a copy out to every port (except source).