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Layer 2 design

Layer 2 design 
5.1.5 This page will discuss some important Layer 2 design considerations.
The purpose of Layer 2 devices in the network is to switch frames based on destination MAC address information, provide error detection, and to reduce congestion in the network. The two most common Layer 2 network devices are bridges and LAN switches. Devices at Layer 2 determine the size of the collision domains. 
Collisions and collision domain size are two factors that negatively affect the performance of a network. Microsegmentation of the network reduces the size of collision domains and reduces collisions.  Micro segmentation is implemented through the use of bridges and switches. The goal is to boost performance for a workgroup or a backbone. Switches can be used with hubs to provide the appropriate level of performance for different users and servers.
Another important characteristic of a LAN switch is how it allocates bandwidth on a per-port basis. This provides more bandwidth to vertical cabling, uplinks, and servers. This type of switching is referred to as asymmetric switching. Asymmetric switching provides switched connections between ports of unlike bandwidth, such as a combination of 10-Mbps and 100-Mbps ports. Symmetric switching provides switched connections between ports of similar bandwidth.
The desired capacity of a vertical cable run is greater than that of a horizontal cable run. The installation of a LAN switch at the MDF and IDF allows the vertical cable run to manage the data traffic from the MDF to the IDF. The horizontal runs between the IDF and the workstations use Category 5e UTP. A horizontal cable drop should not be longer than 100 meters (328 ft.). In a normal environment, 10 Mbps is adequate for the horizontal drop. Asymmetric LAN switches allow 10-Mbps and 100-Mbps ports on a single switch.
The next task is to determine the number of 10 Mbps and 100 Mbps ports needed in the MDF and every IDF. This is accomplished by a review of the user requirements for the number of horizontal cable drops per room and the number of total drops in any catchment area. This includes the number of vertical cable runs. For example, suppose that user requirements dictate four horizontal cable runs to be installed in each room. The IDF services a catchment area of 18 rooms. Therefore, four drops in each of the 18 rooms equals 4x18, or 72 LAN switch ports.
The size of a collision domain is determined by the number of hosts that are physically connected to any single port on the switch. This also affects the bandwidth that is available to any host. In an ideal situation, there is only one host connected on a LAN switch port. The collision domain would consist only of the source host and destination host. The size of the collision domain would be two. Because of the small size of this collision domain, there should be virtually no collisions when any two hosts communicate with each other. Another way to implement LAN switching is to install shared LAN hubs on the switch ports. This allows multiple hosts to connect to a single switch port. All hosts connected to the shared LAN hub share the same collision domain and bandwidth. That means that collisions would occur more frequently. 
Shared media hubs are generally used in a LAN switch environment to create more connection points at the end of the horizontal cable runs. This is an acceptable solution, but care must be taken. Collision domains should be kept small and bandwidth to the host must be provided in accordance to the specifications gathered in the requirements phase of the network design process.
The next page will discuss Layer 3 design issues.

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