What does a Layer 2 switch primarily use to make forwarding decisions?

A Layer 2 switch primarily uses MAC addresses to make forwarding decisions. At Layer 2 of the OSI model, the primary function of a switch is to relay frames based on the hardware addresses (MAC addresses) associated with the devices connected to its ports.

When a switch receives a frame, it examines the source MAC address to update its MAC address table, which maps MAC addresses to the corresponding switch ports. This process allows the switch to learn which devices are connected to which ports, thus enabling efficient frame forwarding. When the switch receives a frame destined for a specific MAC address, it refers to its MAC address table to determine the correct port to forward the frame to.

Utilizing MAC addresses allows Layer 2 switches to maintain efficient communication within a local area network (LAN) without the need for more complex routing decisions based on IP addresses. In contrast, IP addresses are relevant for Layer 3 devices (routers), while UDP ports and TCP segments pertain to higher layers (transport layer) that deal with the flow of data between applications rather than the frame forwarding decisions made by Layer 2 switches. This distinction highlights the importance of MAC addresses in the operation and design of Layer 2 networking devices.