Ethernet Switching:

This module discusses Ethernet switching and how the concept of collision and broadcast domains effect network performance. Specific reference to different network devices extend, segment and bound different types of network traffic.

Vocabulary - Key Terms:
A switch is essentially a fast, multi-port bridge that can contain dozens of ports.
Latency is the delay between the time a frame begins to leave the source device and when the first part of the frame reaches its destination.
(CAM): content-addressable memory
(ASIC): application-specific integrated circuit
(FCS): Frame Check Sequence
STP: standards-based protocol
A collision domain is a logical network segment where data packets can "collide" with one another for being sent on a shared medium, in particular in the Ethernet networking protocol. This is an Ethernet term used to describe a network scenario wherein one particular device sends a packet on a network segment, forcing every other device on that same segment to pay attention to it.Collisions decrease network efficiency; if two devices transmit simultaneously, a collision occurs, and both devices must retransmit at a later time.
A broadcast domain is a logical network segment in which any computer or other device connected to the network can directly transmit to any other on the domain without having to go through a routing device, provided that they share the same subnet address and are in the same VLAN, default or installed.It is the area of the computer network composed of all the computers and networking devices that can be reached by sending a frame to the data link layer broadcast address.
Ethernet LANs are broadcast domains. Any devices attached to the LAN can transmit frames to any other device because the medium is a shared transmission system.
A network segment is a portion of a computer network where in every device communicates using the same physical layer. Devices that extend the physical layer, such as repeaters or hubs, are also considered to extend the segment. However, devices that operate at the data link layer level or higher create new physical layers and thus create rather than extend segments.

Related Articles:
http://www.microsoft.com/technet/community/columns/cableguy/cg0102.mspx
--Function of OSI layers

Key Concepts:

The Bridge uses the MAC address to determine where to send the data to.

Full Duplex is known was the ability to communite in both directions at the same time.

A collision domain can be broken up into Different segments by using a bridge. It uses two ports to create two different collision domains. A switch has many ports which in return it can create a different collision domain with each port.

Latency can be caused by:

• Media delays may be caused by the finite speed that signals can travel through the physical media.
• Circuit delays may be caused by the electronics that process the signal along the path.
• Software delays may be caused by the decisions that software must make to implement switching and protocols.
• Delays may be caused by the content of the frame and the location of the frame switching decisions. For example, a device cannot route a frame to a destination until the destination MAC address has been read.

Switched networks are often designed with redundant paths to provide for reliability and fault tolerance. Redundant paths are desirable but they can have undesirable side effects such as switching loops. Switching loops can lead to broadcast storms that will rapidly overwhelm a network. STP is a standards-based protocol that is used to avoid switching loops. Each switch in a LAN that uses STP sends messages called Bridge Protocol Data Units (BPDUs) out all its ports to let other switches know of its existence. This information is used to elect a root bridge for the network. The switches use the spanning-tree algorithm (STA) to resolve and shut down the redundant paths

Each port on a switch that uses STP exists in one of the following five states:

• Blocking
• Listening
• Learning
• Forwarding
• Disabled

A port moves through these five states as follows:

• From initialization to blocking
• From blocking to listening or to disabled
• From listening to learning or to disabled
• From learning to forwarding or to disabled
• From forwarding to disabled

Examples of shared media and directly connected networks:

• Shared media environment – This occurs when multiple hosts have access to the same medium. For example, if several PCs are attached to the same physical wire or optical fiber, they all share the same media environment.
• Extended shared media environment – This is a special type of shared media environment in which networking devices can extend the environment so that it can accommodate multiple access or longer cable distances.
• Point-to-point network environment – This is widely used in dialup network connections and is most common for home users. It is a shared network environment in which one device is connected to only one other device. An example is a PC that is connected to an Internet service provider through a modem and a phone line.

Course Materials:

CCNA1 - Module 8 - Study Guide (.pdf)