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Provisioning Hardware and Media for the LAN


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Provisioning Hardware and Media for the LAN



Switching Versus Routing in Network Design



Cisco's Catalyst Switches



Cisco's Routers



Selecting Switches, Routers, Access Servers, and Other Cisco Hardware



Provisioning Network Media






Case Studies

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Designing Cisco Networks

From: Designing Cisco Networks
Author: Diane Teare
Publisher: Cisco Press (53)
More Information

6. Provisioning Hardware and Media for the LAN

It will take you approximately two hours to complete this chapter and its exercises. Upon completion of this second chapter in Part IV you will be able to do the following:

  • Recognize scalability constraints and issues for standard LAN technologies.

  • Recommend Cisco products and LAN technologies that will meet a customer's requirements for performance, capacity, and scalability in small- to medium-sized networks.

  • Update the network topology drawing you created in the previous chapter to include hardware and media.

This chapter includes some tables and other job aids that you will find useful when completing the case studies at the end of the chapter. References to some Web sites are also included; relevant information has been extracted from these sites and is provided in the chapter. If you have access to the Internet, you might want to visit the sites mentioned to obtain detailed information related to specific topics. All the sites referenced in this chapter are also listed in Appendix C, “Interesting WWW Links and Other Suggested Readings.”

The Cisco Product Selection Tool, available on Cisco's web site, is also referenced in the case studies at the end of this chapter. An introduction to the tool is provided there, and you are encouraged to try the tool if you have access to the Internet.

Follow these steps to complete this chapter:

  1. Study the chapter content, including any tables and job aids that appear.

  2. Review the case studies at the end of this chapter.

  3. Complete the questions in each case study.

  4. Review the answers provided by our internetworking experts in Appendix B, “Answers to Chapter Questions, Case Studies, and Sample CCDA Exam.”

Provisioning LAN media and hardware involves making many decisions, including which devices to use and what media to use to interconnect these devices. This chapter discusses these issues in the following sections:

  • Switching versus Routing in Network Design

  • Cisco's Catalyst Switches

  • Cisco's Routers

  • Selecting Switches, Routers, Access Servers, and Other Cisco Hardware

  • Provisioning Network Media

Switching Versus Routing in Network Design

When provisioning internetworking devices for small- to medium-sized networks, you need to decide when LAN switches are appropriate and when routers are appropriate. The sections that follow provide some information on the different types of services offered by routers and switches, and how these devices can be used in your design.

Router Services

If you need internetworking services, routers are needed. Routers offer the following services:

  • Broadcast firewalling or filtering

  • Hierarchical addressing

  • Communication between dissimilar LANs

  • Fast convergence

  • Policy routing

  • Quality of Service (QoS) routing

  • Security

  • Redundancy and load balancing

  • Traffic flow management

  • Multimedia group membership

Switching Services

Some services are becoming available on switches also. For example, support for multimedia often requires a protocol such as the Internet Group Management Protocol (IGMP), which allows workstations to join a group that receives multimedia multicast packets. Cisco now allows Catalyst switches to participate in this process by using the Cisco Group Management Protocol (CGMP). One router will still be needed, but you will not need a router in every department of a company because with CGMP, switches can communicate with the router to determine whether any users attached to them are part of a multicast group.

Switching and bridging sometimes result in nonoptimal routing of packets because packets only travel on paths that are included in the Spanning Tree. (Recall that the spanning tree is running in order to prevent broadcast storms in a switched network). When routers are used, the routing of packets can be controlled and designed for optimal paths. Cisco supports improved routing and redundancy in switched environments by allowing one instance of the spanning tree per VLAN.

In general, incorporating switches in small-to medium-sized network designs will provide the following advantages:

  • High bandwidth

  • Low cost

  • Easy configuration

Problems Solved with Internetworking Devices

The decision to use an internetworking device depends on which problems you are trying to solve for your client. Recall from Chapter 2, “Analyzing Small- to Medium-Sized Business Networks” that customer problems can be categorized as follows:

  • Media problems

  • Protocol problems

  • Need to transport large payloads

The decision to use routing or switching depends on the problem to be solved. Figure 6-1 illustrates these problems and their solutions, as detailed in the sections that follow.

Figure 6-1. The Decision to Use Routing or Switching Depends on the Problem to be Solved

Media Problems

Media problems refer to an excessive number of collisions on Ethernet or long waits for the token in Token Ring or FDDI. Media problems are caused by too many devices on the media, all with a high load for the network segment. Media problems can be solved by dividing a network into separate segments, using one or more switches.

Protocol Problems

Protocol problems are caused by protocols that do not scale well: for example, protocols that send an excessive number of broadcasts. Protocol problems can be solved by dividing a network into separate segments, using one or more routers.

Need to Transport Large Payloads

This category of problems includes the need to offer voice and video network services. These services may require much more bandwidth than is available on a customer's network or backbone. Transport problems can be solved by using high-bandwidth technologies, such as Fast Ethernet or ATM.

Bandwidth Domains and Broadcast Domains

A bandwidth domain, known as a collision domain for Ethernet LANs, includes all devices that share the same bandwidth. For example, when using switches or bridges, everything associated with one port is a bandwidth domain.

A broadcast domain includes all devices that see each other's broadcasts (and multicasts). For example, all the devices associated with one port on a router are in the same broadcast domain.

Devices in the same bandwidth domain are also in the same broadcast domain; however, devices in the same broadcast domain may be in different bandwidth domains.

All workstations within one bandwidth domain compete for the same LAN bandwidth resource. All traffic from any host in the bandwidth domain is visible to all the other hosts. In the case of an Ethernet collision domain, two stations may transmit at the same time, causing a collision. The stations then have to stop transmitting and try again at a later time, resulting in a delay in transmitting the traffic.

All broadcasts from any host in the same broadcast domain are visible to all other hosts in the same broadcast domain. Desktop protocols, such as AppleTalk, NetBIOS, IPX, and IP, require broadcasts or multicasts for resource discovery and advertisement. Hubs, switches, and bridges forward broadcasts and multicasts to all ports. Routers do not forward these broadcasts or multicasts to any ports. In other words, routers block broadcasts (destined for all networks) and multicasts; they only forward unicast packets (destined for a specific device) and directed broadcasts (destined for all devices on a specific network).

Segmenting the Network Using Switches

When analyzing your customer's current network and future needs, as discussed in Part III of this book, determine whether bandwidth domains need to be segmented using switches.

As noted in Chapter 3, “Characterizing the Existing Network,” Ethernet networks should be segmented if the network utilization is above approximately 40 percent for long periods of time. Token Ring and FDDI networks should be segmented if network utilization is above approximately 70 percent for long periods of time.

If there are too many hosts on a LAN, broadcast radiation can cause performance degradation. Broadcast radiation refers to the way that broadcasts and multicasts radiate from the source to all connected LANs in a flat network, causing all hosts on the LAN to do extra processing. When broadcasts and multicasts are more than approximately 20 percent of the traffic on a LAN, performance degrades.

As seen in Chapter 4, “Determining New Customer Requirements,” there are guidelines for an upper limit on the number of workstations on a LAN or VLAN, before broadcast radiation overwhelms the CPUs of the hosts. These limits are shown again in Table 6-1. Actual workstation limits depend on factors such as:

  • Broadcast/multicast load

  • IP addressing constraints

  • Inter-VLAN routing requirements

  • Management and fault isolation constraints

  • Traffic flow characteristics

Table 6-1. Scalability Constraints for Flat (Switched/Bridged) Networks


Maximum Number of Workstations











When connecting LANs or VLANs via one or more routers, you need to understand the characteristics of the network traffic. It is also important to understand the performance of the router(s) you plan to use in the network design. You can find out more about router performance and capacity in the next chapter, “Provisioning Hardware and Media for the WAN.”

The 80/20 Rule

Campus LANs are easiest to design when the traffic obeys the 80/20 rule, which states that 80 percent of traffic is local to a LAN or VLAN and only 20 percent of the traffic goes to a different LAN or VLAN. This is the case when users primarily access departmental servers and the LANs or VLANs are subdivided by department. However, with the emergence of server farms and corporate web servers, the 80/20 rule does not always apply. In these cases, it is important to provision bandwidth, switches, and routers carefully to avoid congestion and poor performance.

Summary of When to Use Switches Versus Routers

Table 6-2 summarizes the discussion of switching Versus routing.

Table 6-2. Summary of Considerations for Switches versus Routers



Problem Solved

Protocol problems

Media (LAN switches), and Transport of large payload (Fast Ethernet and ATM switches) problems

Key Features

Many, including filtering, addressing, connecting dissimilar LANs, security, load balancing, policy and QoS routing, multimedia

High bandwidth, low cost, ease of configuration

Broadcast and Bandwidth Domains

Reduces broadcast domain and bandwidth (collision) domain

Reduces bandwidth (collision) domain


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