Ethernet Switching

# Ethernet Switching (Switching Concepts) > Exam relevance: Switching concepts appear in the Network Fundamentals domain (20%) and the Network Access domain (20%) — together they make up 40% of the CCNA 200-301 v1.1 exam (120 minutes). Switching concepts underpin nearly every simulation question in the Network Access domain. Understanding how a switch learns, forwards, and handles MAC addresses is foundational knowledge you cannot skip. --- ## Why Switches Exist Before switches, networks used hubs — dumb devices that flooded every incoming frame out every port, creating collisions and wasted bandwidth. A switch is a Layer 2 device that makes intelligent forwarding decisions based on MAC addresses, sending traffic only to the port where the destination device lives. This dramatically reduces unnecessary traffic and collision domains. --- ## The CAM Table (Content Addressable Memory) The heart of a switch is its CAM table, also called the MAC address table. This table maps MAC addresses to the physical switch port on which that device was last seen. ### How a switch builds the CAM table (MAC learning): 1. A frame arrives on a port. 2. The switch reads the source MAC address and records it alongside the incoming port number in the CAM table. 3. The switch then looks up the destination MAC address in the CAM table. ### Three forwarding behaviors: | Behavior | When it happens | What the switch does | |---|---|---| | Unicast known (forward) | Destination MAC is in the CAM table | Sends frame out only the matching port | | Flood | Destination MAC is NOT…