LAN Switching

The advances in switching technology combined with the decrease in switch prices have made computer networks a common and increasingly important aspect of business today.

Switches must learn about the network to make intelligent decisions. Due to the size and changing nature of networks, switches learned how to discover network address and keep track of network changes

Switches must make decisions about what to do with traffic. The decisions are based on the switch’s knowledge of the network.

Switches must also have mechanisms for segmenting users into logical groupings to allow efficient provisioning of services.

Broadcast and Collision Domains
From time to time, a device on the network wants to communicate with all other "local" devices at he same time. Typically, this communication occurs when a device wants to query the network for an address, when a device is newly added to a network, or when there is a change in the network.

A group of devices that receive all broadcast messages from members within that group is called a broadcast domain. Network broadcast domains are typically segmented with Layer 3 devices (routers).

A group of devices that share a common access medium, and can therefore interfere with each other when transmitting simultaneously, define a collision domain. Traditionally, each broadcast domain had multiple collision domains. Modern switches, however, have a low price/port ratio, making it feasible to dedicate a port to a single end device, effectively removing all collision domains.

Forwarding and Filtering
A switch always does something when it receives traffic. The preference is to send the traffic out a specific port (called filtering), but that only works when the location of the intended destination is known. When the destination address is not known, the switch forwards the traffic out every port, except the one on which the traffic was received. This process is called flooding.

From a network efficiency standpoint, it is much better for the network when the switch knows all the addresses on every port, but it is not always practical to enter this information manually. As the network grows and changes, all the port addresses are almost impossible to track.

Address Learning
A switch must therefore learn the addresses of the devices attached to it. It does so by inspecting the source address of all the traffic sent though it and then associates the port the traffic was received on with the Media Access Control (MAC) address listed. The following example illustrates this concept. (The MAC addresses, shown for clarity only, are not the correct format.)

Time 0 - The switch shown has an empty MAC address table.
Time 1 - The device attached to port 2 sends a message intended for the device on port 0. This message kicks off two actions within the switch: The switch now knows the address associated with the device on port 2, so it enters the information in its table; and because it does not have an association for the device the traffic is intended for (namely the computer on port 0), it floods the message Out all ports except the one on which it was received.
Time 2 -The device on port 0 replies to the message. The switch now associates the source address of the message with port 0.

This process happens all the time in every switch.

Frame Transmission Modes
Switches are typically Layer 2 devices. (Some switches now perform Layer 3 functions.) According to the OSI model, the data unit processed by a switch is called a frame. Switches must balance speed and accuracy (no errors) when processing frames because they are typically measured on both attributes.

The three primary frame switching modes follow:

Cut-through - The switch only checks the destination address and then immediately begins forwarding the frame. This process can decrease latency but can also transmit frames containing errors.
Store and forward - The switch reads the entire frame and performs a cyclic redundancy check (CRC) before forwarding. If the CRC is bad, the switch discards the frame. Although this method does increase latency (processing time), it tends to minimize errors.
Fragment-free (modified cut-through) - The switch reads the first 64 bytes before forwarding the frame. The switch needs a minimum of 64 bytes to detect and filter out collision frames.