Module Practice and Quiz

2.3.1

What did I learn in this module?

Frame Forwarding

The decision on how a switch forwards traffic is based on the flow of that traffic. The term ingress describes the port where a frame enters a device. The term egress describes the port that frames will use when leaving the device. An Ethernet frame will never be forwarded out the port where it entered. For a switch to know which port to use to transmit a frame, it must first learn which devices exist on each port. As the switch learns the relationship of ports to devices, it builds a table called a MAC address table. Every frame that enters a switch is checked for new information to learn by examining the source MAC address of the frame and port number where the frame entered the switch. If the destination MAC address is a unicast address, the switch will look for a match between the destination MAC address of the frame and an entry in its MAC address table. Switch forwarding methods include store-and-forward and cut-through. Store-and-forward uses error-checking and automatic buffering. Cut-through does not error check. Instead it performs rapid frame switching. This means the switch can make a forwarding decision as soon as it has looked up the destination MAC address of the frame in its MAC address table.

Switching Domains

If an Ethernet switch port is operating in half-duplex, each segment is in its own collision domain. There are no collision domains when switch ports are operating in full-duplex. By default, Ethernet switch ports will autonegotiate full-duplex when the adjacent device can also operate in full-duplex. A collection of interconnected switches forms a single broadcast domain. Only a network layer device, such as a router, can divide a Layer 2 broadcast domain. The Layer 2 broadcast domain is referred to as the MAC broadcast domain. The MAC broadcast domain consists of all devices on the LAN that receive broadcast frames from a host. When a switch receives a broadcast frame, it forwards the frame out each of its ports, except the ingress port where the broadcast frame was received. Each device connected to the switch receives a copy of the broadcast frame and processes it. Switches can: interconnect LAN segments, use a MAC address table to determine egress ports, and can lessen or eliminate collisions entirely. Characteristics of switches that alleviate network congestion are fast port speeds, fast internal switching, large frame buffers, and high port density.