Suppose you are to design a LAN for a company that has 150 employees, each given

Question

Suppose you are to design a LAN for a company that has 150 employees, each given a desktop computer attached to the LAN. What should be the data rate of the LAN if the typical use of the LAN is shown below?

A.     Each employee needs to retrieve a file of average size of 10 megabytes in a second. An employee may do this on average 10 times during the eight-hour working time.

B.      Each employee needs to access the Internet at 250 Kbps. This can happen for 10 employees simultaneously.

C.      Each employee may receive 10 e-mails per hour with an average size of 100 kilobytes. Half of the employees may receive e-mails simultaneously.

Explanation / Answer

The CCDA objectives covered in this section are as follows:

13

Describe the advantages, disadvantages, scalability issues, and applicability of standard internetwork topologies.

14

Draw a topology map that meets the customer's needs and includes a high-level view of internetworking devices and interconnecting media.

This section covers CCDA exam objectives about designing network topologies for the LAN. LANs provide data transfer rates that are typically much faster than wide-area networks (WANs). While most companies own their own LAN infrastructure, wide-area connections between LANs are usually leased on a monthly basis from an outside carrier. With the recent developments in Gigabit Ethernet technologies, LAN designs are now capable of 1000 Mbps speeds. High-speed Gigabit links can connect servers to LAN switches. At these speeds, the capacity is there to meet the performance requirements of current high-bandwidth applications.

Various speeds of Ethernet have evolved into the de facto standard for LANs. Ethernet uses a contention-based access method, meaning each device competes simultaneously for access to the network. All devices attached to the same Ethernet segment form a collision domain. Each device transmitting on that segment may attempt to transmit at the same time as another device on the same segment, resulting in a collision. As the number of devices in the same collision domain increases, so do the collisions, resulting in poorer performance.

Although not discussed in newer switched (bridged) networks, legacy Ethernet networks with repeaters and hubs should limit the size of the collision domain. To scale multiprotocol networks and networks with high-bandwidth applications, limit the size of collision domains using bridges, switches, and routers. This is covered in the section "LAN Hardware" later in the chapter.

Three different network topology models are discussed in the following sections:

Hierarchical models enable you to design internetworks in layers. To understand the importance of layering, consider the Open System Interconnection (OSI) reference model, which is a layered model for implementing computer communications. Using layers, the OSI model simplifies the tasks required for two computers to communicate. Hierarchical models for internetwork design also use layers to simplify the tasks required for internetworking. Each layer can be focused on specific functions, allowing you to choose the right systems and features for each layer. Hierarchical models apply to both LAN and WAN design.

The many benefits of using hierarchical models for your network design include the following:

After adopting hierarchical design models, many organizations report cost savings because they are no longer trying to do it all in one routing/switching platform. The modular nature of the model enables appropriate use of bandwidth within each layer of the hierarchy, reducing wasted capacity.

Keeping each design element simple and small facilitates ease of understanding, which helps control training and staff costs. Management responsibility and network management systems can be distributed to the different layers of modular network architectures, which also helps control management costs.

Hierarchical design facilitates changes. In a network design, modularity allows creating design elements that can be replicated as the network grows, facilitating easy network growth. As each element in the network design requires change, the cost and complexity of making the upgrade is contained to a small subset of the overall network. In large, flat, or meshed network architectures, changes tend to impact a large number of systems.

Improved fault isolation is facilitated by structuring the network into small, easy-to-understand elements. Network managers can easily understand the transition points in the network, which helps identify failure points.

Today's fast-converging protocols were designed for hierarchical topologies. To control the impact of routing overhead processing and bandwidth consumption, modular hierarchical topologies must be used with protocols designed with these controls in mind, such as EIGRP.

Route summarization is facilitated by hierarchical network design. Route summarization reduces the routing protocol overhead on links in the network and reduces routing protocol processing within the routers.

Some IP workstations send an ARP frame to find a remote station. A router running proxy ARP can respond with its data link layer address. Cisco routers run proxy ARP by default.

Most IP workstations must be configured with the IP address of a default router. This is sometimes called the default gateway.

In an IP environment, the most common method for a workstation to find a server is via explicit configuration (default router). If the workstation's default router becomes unavailable, the workstation must be reconfigured with the address of a different router. Some IP stacks enable you to configure multiple default routers, but many other IP stacks do not support redundant default routers.

RFC 1256 specifies an extension to the Internet Control Message Protocol (ICMP) that allows an IP workstation and router to run RDP to facilitate the workstation learning the address of a router.

An IP workstation can run RIP to learn about routers. RIP should be used in passive mode rather than active mode. (Active mode means that the station sends RIP frames every 30 seconds.) The Open Shortest Path First (OSPF) protocol also supports a workstation running RIP.

An IPX workstation broadcasts a find network number message to find a route to a server. A router then responds. If the client loses its connection to the server, it automatically sends the message again.

An AppleTalk workstation remembers the address of the router that sent the last Routing Table Maintenance Protocol (RTMP) packet. As long as there are one or more routers on an AppleTalk workstation's network, it has a route to remote devices.

Cisco's HSRP provides a way for IP workstations to keep communicating on the internetwork even if their default router becomes unavailable. HSRP works by creating a phantom router that has its own IP and MAC addresses. The workstations use this phantom router as their default router.

13

Describe the advantages, disadvantages, scalability issues, and applicability of standard internetwork topologies.

14

Draw a topology map that meets the customer's needs and includes a high-level view of internetworking devices and interconnecting media.

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