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 can allocate bandwidth on a per-port basis. This will provide 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. The desired capacity of a vertical cable run is greater than that of a horizontal cable run. By installing a LAN switch at the MDF and IDF, the vertical cable run can manage the data traffic from the MDF to the IDF. The horizontal runs between the IDF and the workstations uses Category 5e UTP. No horizontal cable drop should be longer than 100 meters. In a normal environment, 10 Mbps is adequate for the horizontal drop. Use asymmetric LAN switches to allow for mixing 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 can be determined by reviewing 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 to each room. The IDF services a catchment area of 18 rooms. Therefore, four drops in each of the 18 rooms will equal 72 LAN switch ports. (4x18=72) The size of a collision domain is determined by how many hosts are physically connected to any single port on the switch. This also affects how much network bandwidth 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 are communicating with each other. Another way to implement LAN switching is to install shared LAN hubs on the switch ports, and connect multiple hosts to a single switch port. All hosts connected to the shared LAN hub share the same collision domain and bandwidth. Collisions would occur more frequently. Some older switches, such as the Catalyst 1700, do not properly support sharing the same collision domain and bandwidth. The older switches do not maintain multiple MAC addresses mapped to each port. As a result, there are many broadcasts and ARP requests. 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 requirements to the host must be provided according to the specifications gathered in the requirements phase of the network design process.
Content 5.1 LAN Design 5.1.6 Layer 3 design A router is a Layer 3 device and is considered one of the most powerful devices in the network topology.Layer 3 devices can be used to create unique LAN segments. Layer 3 devices allow communication between segments based on Layer 3 addressing, such as IP addressing. Implementation of Layer 3 devices allows for segmentation of the LAN into unique physical and logical networks. Routers also allow for connectivity to wide-area networks (WANs), such as the Internet. Layer 3 routing determines traffic flow between unique physical network segments based on Layer 3 addressing. A router forwards data packets based on destination addresses. A router does not forward LAN-based broadcasts such as ARP requests. Therefore, the router interface is considered the entry and exit point of a broadcast domain and stops broadcasts from reaching other LAN segments. Routers provide scalability because they serve as firewalls for broadcasts. They can also provide scalability by dividing networks into subnetworks, or subnets, based on Layer 3 addresses. When deciding whether to use routers or switches, remember to ask, "What is the problem that is to be solved?" If the problem is related to protocol rather than issues of contention, then routers are the appropriate solution. Routers solve problems with excessive broadcasts, protocols that do not scale well, security issues, and network layer addressing. Routers are more expensive and more difficult to configure than switches. Figure shows an example of an implementation that has multiple physical networks. All data traffic from Network 1 destined for Network 2 has to go through the router. In this implementation, there are two broadcast domains. The two networks have unique Layer 3 network addressing schemes. In a structured Layer 1 wiring scheme, multiple physical networks are easy to create by patching the horizontal cabling and vertical cabling into the appropriate Layer 2 switch. This can be done using patch cables. This implementation also provides robust security, because all traffic in and out of the LAN must pass through the router. Once an IP addressing scheme has been developed for a client, it should be clearly documented. A standard convention should be set for addressing important hosts on the network. This addressing scheme should be kept consistent throughout the entire network. Addressing maps provide a snapshot of the network. Creating physical maps of the network helps to troubleshoot the network. VLAN implementation combines Layer 2 switching and Layer 3 routing technologies to limit both collision domains and broadcast domains. VLANs can also be used to provide security by creating the VLAN groups according to function and by using routers to communicate between VLANs. A physical port association is used to implement VLAN assignment. Ports P1, P4, and P6 have been assigned to VLAN 1. VLAN 2 has ports P2, P3, and P5. Communication between VLAN 1 and VLAN 2 can occur only through the router. This limits the size of the broadcast domains and uses the router to determine whether VLAN 1 can talk to VLAN 2.
Content 5.2 LAN Switches 5.2.1 Switched LANs, access layer overview The construction of a LAN that satisfies the needs of both medium and large-sized organizations is more likely to be successful if a hierarchical design model is used. The use of a hierarchical design model will make it easier to make changes to the network as the organization grows. The hierarchical design model includes the following three layers: This hierarchical model applies to any network design. It is important to realize that these three layers may exist in clear and distinct physical entities. However, this is not a requirement. These layers are defined to aid in successful network design and to represent functionality that must exist in a network. The access layer is the entry point for user workstations and servers to the network. In a campus LAN the device used at the access layer can be a switch or a hub. If a hub is used, bandwidth is shared. If a switch is used, then bandwidth is dedicated. If a workstation or server is directly connected to a switch port, then the full bandwidth of the connection to the switch is available to the connected computer. If a hub is connected to a switch port, bandwidth is shared between all devices connected to the hub. Access layer functions also include MAC layer filtering and microsegmentation. MAC layer filtering allows switches to direct frames only to the switch port that is connected to the destination device. The switch creates small Layer 2 segments called microsegments. The collision domain can be as small as