Module 2 - Design the Network Structure

Section 2 - Provision Hardware and Media for the LAN

Section Objectives

• 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 section to include hardware and media.

Time Required to Complete This Section

Completing This Section

1. Study the reading assignment.
2. Click on any links that you see in the reading assignment and review the information that appears.
3. Review any tables and job aids that appear in the reading assignment.
4. Review the case studies at the end of this section.
5. Complete the questions in each case study.
6. Review the answers provided by our internetworking experts.

Resources Required to Complete This Section

• Access to the World Wide Web and Cisco's Web site
• A downloaded, printed copy of this section
• Paper and pencil
• Topology drawings you created in Section 1 of Module 2
• Access to the Cisco Reseller Product Selection Tool
• Open the/tools directory on the "Designing Cisco Networks" CD
• Double-click Cisco Reseller Product Selection Tool (Crpst.exe) to start the tool
• "Small and Medium Business Solution Guide" included with this course

Reading Assignment

Switching versus Routing in Network Design

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

• High bandwidth
• Low cost
• Easy configuration

• Broadcast firewalling
• Hierarchical addressing
• Communication between dissimilar LANs
• Fast convergence
• Policy routing
• QoS routing
• Security
• Redundancy and load balancing
• Traffic flow management
• Multimedia group membership

Switching Services

Switching and bridging sometimes result in non optimal routing of packets because every packet must go through the root bridge of the spanning tree. 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.

Problems Solved with Internetworking Devices

The decision to use an internetworking device depends on which problems you are trying to solve for your client. As we discussed in the introduction to this course, customer problems can be categorized as follows:

Media Problems

Media problems refer to an excessive number of collisions on Ethernet and long waits for the token in Token Ring or FDDI. Media problems are caused by too many devices, all with a high offered 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 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

When using switches or bridges, everything associated with one port is a bandwidth domain. In the case of an Ethernet switch, a bandwidth domain is also known as a collision domain.

A broadcast domain includes associated with one port on a router. 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. Routers do not forward broadcasts.

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 can transmit at the same time, causing a collision.

Segmenting the Network Using Switches

When analyzing your customer's current network and future needs, as we discussed in Module 1, determine if bandwidth domains need to be segmented using switches. 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.

Broadcast Radiation

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.

Scalability for Flat Networks

• Broadcast/multicast load
• IP addressing constraints
• Inter-VLAN routing requirements
• Management and fault isolation constraints
• Traffic flow characteristics

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. We will talk more about router performance and capacity in the next section, "Provision Hardware and Media for the WAN."

Campus LAN Design

Cisco Switches

• Catalyst 5000 switching system
• Catalyst 1900/2820 ethernet switches

The following diagram shows a sample switched campus network using Catalyst 5000 series switches.
 

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

• Cisco Reseller Product Selection Tool, included on this CD or with the course on Cisco's Web site

• "Small and Medium Business Solution Guide," included with this course

• Price list (To obtain a copy of the current Cisco price list, log in to the Reseller's Web site using your reseller account.)

Provisioning Network Media

 

Scalability Constraints for IEEE 802.3 

	10Base5	10Base2	10BaseT	100BaseT
Topology	Bus	Bus	Star	Star
Maximum Segment Length (meters)	500	185	100 from hub to station	100 from hub to station
Maximum Number of Attachments per Segment	100	30	2 (hub and station or hub-hub)	2 (hub and station or hub-hub)
Maximum Collision Domain	2500 meters of 5 segments and 4 repeaters; only 3 segments can be populated	2500 meters of 5 segments and 4 repeaters; only 3 segments can be populated	2500 meters of 5 segments and 4 repeaters; only 3 segments can be populated	See the Ethernet Design Rules Section that follows

 

Ethernet Design Rules

The most significant design rule for Ethernet is that the round-trip propagation delay in one collision domain must not exceed 512 bit times, which is a requirement for collision detection to work correctly. This rule means that the maximum round-trip delay for a 10-Mbps Ethernet network is 51.2 microseconds. The maximum round-trip delay for a 100-Mbps Ethernet network is only 5.12 microseconds because the bit time on a 100-Mbps Ethernet network is 0.01 microseconds as opposed to 0.1 microseconds on 10-Mbps Ethernet.

To make 100-Mbps Ethernet work, there are much more severe distance limitations than those required for 10-Mbps Ethernet. The general rule is that a 100-Mbps Ethernet has a maximum diameter of 205 meters when UTP cabling is used, whereas 10-Mbps Ethernet has a maximum diameter of 2500 meters.

Scalability Constraints for 10-Mbps Fiber Ethernet Networks

10BaseF is based on the FOIRL specification, which includes 10BaseFP, 10BaseFB, 10BaseFL, and a revised FOIRL standard. The new FOIRL allows DTE (end-node) connections, rather than just repeaters, as allowed with the older FOIRL specification.

Details on 100-Mbps Ethernet Topologies

Understanding Collision Domains

For a 100-Mbps Ethernet to work, you must impose distance limitations, based on the type of repeaters used.

100BaseT Repeaters

• Class I repeaters have a latency of 0.7 microseconds or less. Only one repeater hop is allowed.

• Class II repeaters have a latency of 0.46 microseconds or less. One or two repeater hops are allowed.

Note: The Cisco FastHub 316 is a Class II repeater.

Example of 100BaseT Topology

Other topologies are possible as long as the round-trip propagation delay does not exceed 5.12 microseconds (512 bit times.) When the delay does exceed 5.12 microseconds, the network experiences illegal (late) collisions and CRC errors.

Checking the Propagation Delay

To check a path to make sure the path delay value (PDV) does not exceed 512 bit times, add up the following delays:

• All link segment delays
• All repeater delays
• DTE delay
• A safety margin (0 to 5 bit times)

     1. Determine the delay for each link segment (link segment delay value, or LSDV), including interrepeater
         links, using the following formula. (Multiply by two so that it is a round-trip delay.)

 
• LSDV = 2 x segment length x cable delay for this segment.

 
• For end-node segments, the segment length is the cable length between the PHY interface at the repeater and the PHY interface at the DTE. Use your two farthest DTEs for a worst-case calculation. For interrepeater links, the segment length is the cable length between the repeater PHY interfaces.

 
• Cable delay is the delay specified by the manufacturer if available. When actual cable lengths or propagation delays are not known, use the delay in bit times as specified in the following table.

 
• Cable delay must be specified in bit times per meter (BT/m).

     3. Determine the delay for each repeater in the path. If model-specific data is not available from the
         manufacturer, determine the class of repeater (I or II).
 
     4. MII cables for 100BaseT should not exceed 0.5 meters each in length. When evaluating system topology,
         MII cable lengths need not be accounted for separately. Delays attributable to the MII are incorporated
         into DTE and repeater delays.

     5. Use the DTE delay value shown in the following table unless your equipment manufacturer defines a
         different value.
 
     6. Decide on an appropriate safety margin from 0 to 5 bit times. Five bit times is a safe value.
 
     7. Insert the values obtained in the calculations described in the following formula for calculating the PDV:

•  PDV = link delays + repeater delays + DTE delay + safety margin

Round-Trip Delay

Example Network Cabling Implementation

To ensure that the PDV does not exceed 512 bit times, calculate a worst-case scenario using DTE 1 and DTE 2, which are 75 meters from their repeaters.

Assume that DTE 1 starts transmitting a minimum-sized frame of 64 bytes (512 bits). DTE 2 just barely misses hearing DTE 1's transmission and starts transmitting also. The collision happens on the far-right side of the network and must traverse back to DTE. These events must occur within 512 bit times. If they take any longer than 512 bit times, then DTE 1 will have stopped sending when it learns about the collision and will not know that its frame was damaged by the collision. To calculate the link delays for the Category 5 cable segments, the repeaters, and DTEs, we will use the values from the previous table titled "Network Component Delays." (The "Network Component Delays" table uses round-trip delay values, so you need not multiply by two.)

To test whether this network will work, we filled in the following table:
 

Delay Cause	Calculation	Total
Link 1	75 x 1.112	83.4
Link 2	75 x 1.112	83.4
Interrepeater link	20 x 1.112	22.24
Repeater A	92	92
Repeater B	92	92
DTE 1 and 2	100	100
Safety margin	5	5
GRAND TOTAL	Add individual totals	478.04

 

The grand total is fewer than 512 bit times, so this network will work.

Calculating Cable Delays

Scalability for Token Ring Networks

Scalability Constraints for FDDI Networks

Case Studies

In this section, you will provision media and products for the LANs in the case studies.

Read each case study and complete the questions that follow. Keep in mind that there are potentially several correct answers to each question.

When you complete each question, you can refer to the solutions provided by our internetworking experts. The case studies and solutions will help prepare you for the Sylvan exam following the course.

In this section, you will review the following case studies:

1. CareTaker Publications, a publishing company
2. PH Network Services Corporation, a health care company
3. Pretty Paper Ltd., a European wall covering company
4. Jones, Jones, & Jones, an international law firm

Case Study: CareTaker Publications

Refer to the topology drawing you created for CareTaker Publications in the previous section. In this section, you will provision LAN hardware and media for CareTaker Publications.

Mr. Smith has indicated that the IS budget has been reduced to provide funding for another project. Mr. Smith wants to complete the project correctly, but needs to lower the costs.
 
     1. What media would you select between the switch and the servers?

• Start the Cisco Reseller Product Selection Tool (refer to the instructions at the beginning of Module 2 - Section 2.) You will see the initial Cisco Reseller Product Selection Tool window.

 
• Click on "Switches and Hubs." The High Performance LAN Product Selection Tool window appears.

 
• Click on the product features you want to select in each category, based on the case study requirements. A short list of potential products appears.

 
• Once you have the short list of products, use the "Small and Medium Business Solutions Guide" that came with this course to select the right product that meets this customer's needs.

 





 

Case Study: PH Network Services Corporation

Refer to the topology drawing you created for Mr. Pero in the previous section. In this section, you will provision LAN hardware and media for PH Network.
 
     1. What media would you select between the switch and the servers?

• Start the Cisco Reseller Product Selection Tool (refer to the instructions at the beginning of Module 2 - Section 2.) You will see the initial Cisco Reseller Product Selection Tool window.

 
• Click on "Switches and Hubs." The High Performance LAN Product Selection Tool window appears.

 
• Click on the product features you want to select in each category, based on the case study requirements. A short list of potential products appears.

 
• Once you have the short list of products, use the "Small and Medium Business Solutions Guide" that came with this course to select the right product that meets this customer's needs.

Case Study: Pretty Paper Ltd.

Refer to the topology drawing you created for Pretty Paper in the previous section. In this section we will provision LAN hardware and media for Pretty Paper.
 
     1. What media would you select between the switch and the servers?

• Start the Cisco Reseller Product Selection Tool (refer to the instructions at the beginning of Module 2 - Section 2.) You will see the initial Cisco Reseller Product Selection Tool window.

 
• Click on "Switches and Hubs." The High Performance LAN Product Selection Tool window appears.

 
• Click on the product features you want to select in each category, based on the case study requirements. A short list of potential products appears.

 
• Once you have the short list of products, use the "Small and Medium Business Solutions Guide" that came with this course to select the right product that meets this customer's needs.





 

Case Study: Jones, Jones, & Jones

Refer to the topology drawing you created for Mr. Jones in the previous section. In this section we will provision LAN hardware and media for Jones, Jones, & Jones.
 
     1. What media would you select between the switch and the servers?

• Start the Cisco Reseller Product Selection Tool (refer to the instructions at the beginning of Module 2 - Section 2.) You will see the initial Cisco Reseller Product Selection Tool window.

 
• Click on "Switches and Hubs." The High Performance LAN Product Selection Tool window appears.

 
• Click on the product features you want to select in each category, based on the case study requirements. A short list of potential products appears.

 
• Once you have the short list of products, use the "Small and Medium Business Solutions Guide" that came with this course to select the right product that meets this customer's needs.

If you are finished with this section, click here to go on to Section 3 and provision the WAN media.