Network Design Your task is to draft a top-level design of an enterprise-wide te

ID: 3674296 • Letter: N

Question

Network Design

Your task is to draft a top-level design of an enterprise-wide telecommunications network for a fictitious company, including LAN topology and WAN infrastructure that will handle a constant stream of traffic between the company offices. The company specifications are as follows:

Locations: Five metropolitan geographic urban location; each location has 120 users over three floors; each Floor dimensions are as follows: Length = 210 feet ft), Height = 8 Ft, and Width = 80 ft

Platforms: Multiple (Windows, Mac, Linux, and Mobile Os)

Topology: Ethernet (Copper, Fiber, and Wi-Fi)

Bandwidth: To be determine by you, the network designer, for inbound and outbound

Servers: To be determined by you, the network designer

Intranet access: IP addresses are 172.17. y.x/24, where y is each location IP segment (use any octet that you wish for each location) and x = 1 to 254

Internet access: One access using T1 Link at each location; the public IP address is 96.68.111.x, where x = 1 to 5

Remote virtual private network access for users to telecommute

You should budget your time wisely and work on your project throughout the course. If you complete your course activities and use the feedback provided by the instructor, you will be on the right track to complete your project successfully.

Hint: Feel free to use any drawing that will depict the physical layout.

In this week, create a 4- to 5-page Microsoft Word document in response to the following:

Describe the LAN topology design that will be used at each office.

Provide a checklist of cabling specifications for a cabling contractor.

Include a minimum of two scholarly references to support your cabling and LAN topology choices.

Explanation / Answer

LAN Topology Design

The CCDA objectives covered in this section are as follows:

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

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

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.

Benefits of Hierarchical Models

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.

Hierarchical Network Design

As Figure 4-1 illustrates, a hierarchical network design has three layers:

A Hierarchical Network Design Has Three Layers: Core, Distribution, and Access

Each layer provides necessary functionality to the network. The layers do not need to be implemented as distinct physical entities. Each layer can be implemented in routers or switches, represented by a physical media, or combined in a single box. A particular layer can be omitted altogether, but for optimum performance, a hierarchy should be maintained.

Core Layer

The core layer is the high-speed switching backbone of the network, which is crucial to enable corporate communications. The core layer should have the following characteristics:

NOTE

When routers are used in a network, the number of router hops from edge to edge is called the diameter. As noted, it is considered good practice to design for a consistent diameter within a hierarchical network. This means that from any end station to another end station across the backbone, there should be the same number of hops. The distance from any end station to a server on the backbone should also be consistent.

Limiting the diameter of the internetwork provides predictable performance and ease of troubleshooting. Distribution layer routers and client LANs can be added to the hierarchical model without increasing the diameter because neither will affect how existing end stations communicate.

Distribution Layer

The distribution layer of the network is the demarcation point between the access and core layers of the network. The distribution layer can have many roles, including implementing the following functions:

Several Cisco IOS software features can be used to implement policy at the distribution layer, including the following:

Access Layer

The access layer provides user access to local segments on the network. The access layer is characterized by switched and shared bandwidth LANs in a campus environment. Microsegmentation, using LAN switches, provides high bandwidth to workgroups by dividing collision domains on Ethernet segments and reducing the number of stations capturing the token on Token Ring LANs.

For small office/home office (SOHO) environments, the access layer provides access for remote sites into the corporate network by using WAN technologies such as ISDN, Frame Relay, and leased lines. Features such as dial-on-demand routing (DDR) and static routing can be implemented to control costs.

Hierarchical Model Examples

For small- to medium-sized companies, the hierarchical model is often implemented as a hub-and-spoke topology, as shown in Figure 4-2. Corporate headquarters forms the hub and links to the remote offices form the spokes.

The Hierarchical Model Is Often Implemented as a Hub-and-Spoke Topology

You can implement the hierarchical model by using either routers or switches. Figure 4-3is an example of a switched hierarchical design, while Figure 4-4 shows examples of routed hierarchical designs.

Figure 4-3 An Example of a Switched Hierarchical Design

Redundant Models

When designing a network topology for a customer who has critical systems, services, or network paths, you should determine the likelihood that these components will fail and design redundancy where necessary.

Consider incorporating one of the following types of redundancy into your design:

Each of these types of redundancy is elaborated in the sections that follow.

Workstation-to-Router Redundancy

When a workstation has traffic to send to a station that is not local, the workstation has many possible ways to discover the address of a router on its network segment, including the following:

The sections that follow cover each of these methods.

ARP

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.

Explicit Configuration

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.

RDP

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.

RIP

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.

IPX

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.

AppleTalk

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.

HSRP

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.

HSRP routers on a LAN communicate among themselves to designate two routers as active and standby. The active router sends periodic hello messages. The other HSRP routers listen for the hello messages. If the active router fails and the other HSRP routers stop receiving hello messages, the standby router takes over and becomes the active router. Because the new active router assumes both the IP and MAC addresses of the phantom, end nodes see no change at all. They continue to send packets to the phantom router's MAC address, and the new active router delivers those packets.

HSRP also works for proxy ARP. When an active HSRP router receives an ARP request for a node that is not on the local LAN, the router replies with the phantom router's MAC address instead of its own. If the router that originally sent the ARP reply later loses its connection, the new active router can still deliver the traffic.

Figure 4-5 shows a sample implementation of HSRP.

An Example of HSRP: The Phantom Router Represents the Real Routers

In Figure 4-5, the following sequence occurs:

Server Redundancy

In some environments, fully redundant (mirrored) file servers should be recommended. For example, in a brokerage firm where traders must access data in order to buy and sell stocks, the data can be replicated on two or more redundant servers. The servers should be on different networks and power supplies.

If complete server redundancy is not feasible due to cost considerations, mirroring or duplexing of the file server hard drives is a good idea. Mirroring means synchronizing two disks, while duplexing is the same as mirroring with the additional feature that the two mirrored hard drives are controlled by different disk controllers.

Route Redundancy

Designing redundant routes has two purposes: load balancing and minimizing downtime.

Load Balancing

AppleTalk and IPX routers can remember only one route to a remote network by default, so they do not support load balancing. You can change this for IPX by using the ipx maximum-paths command and for AppleTalk by using the appletalk maximum-pathscommand on a Cisco router.

Most IP routing protocols can load balance across up to six parallel links that have equal cost. Use the maximum-paths command to change the number of links that the router will load balance over for IP; the default is four, the maximum is six. To support load balancing, keep the bandwidth consistent within a layer of the hierarchical model so that all paths have the same cost. (Cisco's IGRP and EIGRP are exceptions because they can load balance traffic across multiple routes that have different metrics by using a feature called variance.)

A hop-based routing protocol does load balancing over unequal bandwidth paths as long as the hop count is equal. After the slower link becomes saturated, the higher-capacity link cannot be filled; this is called pinhole congestion. Pinhole congestion can be avoided by designing equal bandwidth links within one layer of the hierarchy or by using a routing protocol that takes bandwidth into account.

IP load balancing depends on which switching mode is used on a router. Process switching load balances on a packet-by-packet basis. Fast, autonomous, silicon, optimum, distributed, and NetFlow switching load balance on a destination-by-destination basis because the processor caches the encapsulation to a specific destination for these types of switching modes.

Minimizing Downtime

In addition to facilitating load balancing, redundant routes minimize network downtime.

As already discussed, you should keep bandwidth consistent within a given layer of a hierarchy to facilitate load balancing. Another reason to keep bandwidth consistent within a layer of a hierarchy is that routing protocols converge much faster if multiple equal-cost paths to a destination network exist.

By using redundant, meshed network designs, you can minimize the effect of link failures. Depending on the convergence time of the routing protocols being used, a single link failure will not have a catastrophic effect.

A network can be designed as a full mesh or a partial mesh. In a full mesh network, every router has a link to every other router, as shown in Figure 4-6. A full mesh network provides complete redundancy and also provides good performance because there is just a single-hop delay between any two sites. The number of links in a full mesh is n(n–1)/2, where n is the number of routers. Each router is connected to every other router. (Divide the result by 2 to avoid counting Router X to Router Y and Router Y to Router X as two different links.)

Figure 4-6 Full Mesh Network: Every Router Has a Link to Every Other Router in the Network

A full mesh network can be expensive to implement in wide-area networks due to the required number of links. In addition, practical limits to scaling exist for groups of routers that broadcast routing updates or service advertisements. As the number of router peers increases, the amount of bandwidth and CPU resources devoted to processing broadcasts increases.

A suggested guideline is to keep broadcast traffic at less than 20 percent of the bandwidth of each link; this will limit the number of peer routers that can exchange routing tables or service advertisements. When planning redundancy, follow guidelines for simple, hierarchical design. Figure 4-7 illustrates a classic hierarchical and redundant enterprise design that uses a partial mesh rather than a full mesh architecture. For LAN designs, links between the access and distribution layer can be Fast Ethernet, with links to the core at Gigabit Ethernet speeds.

Figure 4-7 Partial Mesh Design with Redundancy

Media Redundancy

In mission-critical applications, it is often necessary to provide redundant media.

In switched networks, switches can have redundant links to each other. This redundancy is good because it minimizes downtime, but it may result in broadcasts continuously circling the network, which is called a broadcast storm. Because Cisco switches implement the IEEE 802.1d Spanning-Tree Algorithm, this looping can be avoided in the Spanning-Tree Protocol. The Spanning-Tree Algorithm guarantees that only one path is active between two network stations. The algorithm permits redundant paths that are automatically activated when the active path experiences problems.

Because WAN links are often critical pieces of the internetwork, redundant media is often deployed in WAN environments. As shown in Figure 4-8, backup links can be provisioned so they become active when a primary link goes down or becomes congested.

Backup Links Can Be Used to Provide Redundancy

Often, backup links use a different technology. For example, a leased line can be in parallel with a backup dialup line or ISDN circuit. By using floating static routes, you can specify that the backup route has a higher administrative distance (used by Cisco routers to select which routing information to use) so that it is not normally used unless the primary route goes down.

Backup links can be combined with load balancing and channel aggregation. Channel aggregation means that a router can bring up multiple channels (for example, Integrated Services Digital Network [ISDN] B channels) as bandwidth requirements increase.

Cisco supports the Multilink Point-to-Point Protocol (MPPP), which is an Internet Engineering Task Force (IETF) standard for ISDN B channel (or asynchronous serial interface) aggregation. MPPP does not specify how a router should accomplish the decision-making process to bring up extra channels. Instead, it seeks to ensure that packets arrive in sequence at the receiving router. Then, the data is encapsulated within PPP and the datagram is given a sequence number. At the receiving router, PPP uses this sequence number to re-create the original data stream. Multiple channels appear as one logical link to upper-layer protocols.

Secure Models

This section introduces secure topology models. The information in this book is not sufficient to learn all the nuances of internetwork security. To learn more about internetwork security, you might want to read the book Firewalls and Internet Security, by Bill Cheswick and Steve Bellovin, published by Addison Wesley. Also, by searching for the word "security" on Cisco's web site (http://www.cisco.com), you can keep up to date on security issues.

Secure topologies are often designed by using a firewall. A firewall protects one network from another untrusted network. This protection can be accomplished in many ways, but in principle, a firewall is a pair of mechanisms: One blocks traffic and the other permits traffic.

Some firewalls place a greater emphasis on blocking traffic, and others emphasize permitting traffic. Figure 4-9 shows a simple firewall topology using routers.

A Simple Firewall Network, Using Routers

You can design a firewall system using packet-filtering routers and bastion hosts. Abastion host is a secure host that supports a limited number of applications for use by outsiders. It holds data that outsiders access (for example, web pages) but is strongly protected from outsiders using it for anything other than its limited purposes.

Three-Part Firewall System

The classic firewall system, called the three-part firewall system, has the following three specialized layers, as shown in Figure 4-10:

Structure and Components of a Three-Part Firewall System

Services available to the outside world are located on bastion hosts in the isolation LAN. Example services in these hosts include:

The isolation LAN has a unique network number that is different than the corporate network number. Only the isolation LAN network is visible to the outside world. On the outside filter, you should advertise only the route to the isolation LAN.

If internal users need to get access to Internet services, allow TCP outbound traffic from the internal corporate internetwork. Allow TCP packets back into the internal network only if they are in response to a previously sent request. All other TCP traffic should be blocked because new inbound TCP sessions could be from hackers trying to establish sessions with internal hosts.

NOTE

To determine whether TCP traffic is a response to a previously sent request or a request for a new session, the router examines some bits in the code field of the TCP header. If the acknowledgement field (ACK) is valid or reset the connection (RST) bits are set in a TCP segment header, the segment is a response to a previously sent request. The established keyword in Cisco IOS access lists (filters) is used to indicate packets with ACK or RST bits set.

The following list summarizes some rules for the three-part firewall system:

Always block traffic from coming in from between the firewall routers and hosts and the internal network. The firewall routers and hosts themselves are likely to be a jumping-off point for hackers, as shown in Figure 4-11.

Firewall Routers and Hosts May Make Your Network Vulnerable to Hacker Attacks

Keep bastion hosts and firewall routers simple. They should run as few programs as possible. The programs should be simple because simple programs have fewer bugs than complex programs. Bugs introduce possible security holes.

Do not enable any unnecessary services or connections on the outside filter router. A list of suggestions for implementing the outside filter router follows:

Cisco PIX Firewall

To provide stalwart security, hardware firewall devices can be used in addition to or instead of packet-filtering routers. For example, in the three-part firewall system illustrated earlier in Figure 4-10, a hardware firewall device could be installed on the isolation LAN. A hardware firewall device offers the following benefits:

Cisco's PIX Firewall is a hardware device that offers the features in the preceding list, as well as full outbound Internet access from unregistered internal hosts. IP addresses can be assigned from the private ranges, as defined in RFC 1918 (available atftp://ftp.isi.edu/in-notes/rfc1918.txt). The PIX Firewall uses a protection scheme calledNetwork Address Translation (NAT), which allows internal users access to the Internet while protecting internal networks from unauthorized access.

AN Types

The CCDA objective covered in this section is as follows:

Local-area networks can be classified as a large building LAN, campus LAN, or small/remote LAN. The large building LAN contains the major data center with high-speed access and floor communications closets; the large building LAN is usually the headquarters in larger companies. Campus LANs provide connectivity between buildings on a campus; redundancy is usually a requirement. Small/remote LANs provide connectivity to remote offices with a small number of nodes.

It is important to remember the Cisco hierarchical approach of network design. First, build a high-speed core backbone network. Second, build the distribution layer, where policy can be applied. Finally, build the access layer, where LANs provide access to the network end stations.

Large Building LANs

Large building LANs are segmented by floors or departments. Company mainframes and servers reside in a computing center. Media lines run from the computer center to the wiring closets at the various segments. From the wiring closets, media lines run to the offices and cubicles around the work areas. Figure 4-12 depicts a typical large building design.

Large Building LAN Design

Each floor may have more than 200 users. Following a hierarchical model of access, distribution, and core, Ethernet and Fast Ethernet nodes may connect to hubs and switches in the communications closet. Uplink ports from closet switches connect back to one or two (for redundancy) distribution switches. Distribution switches may provide connectivity to server farms that provide business applications, DHCP, DNS, intranet, and other services.

Campus LANs

A campus LAN connects two or more buildings located near each other using high-bandwidth LAN media. Usually the media (for example, copper or fiber) is owned. High-speed switching devices are recommended to minimize latency. In today's networks, Gigabit Ethernet campus backbones are the standard for new installations. In Figure 4-13, campus buildings are connected by using Layer 3 switches with Gigabit Ethernet media.

Campus LANs

Ensure that a hierarchical design is implemented on the campus LAN and that network layer addressing is assigned to control broadcasts on the networks. Each building should have addressing assigned in such a way as to maximize address summarization. Apply contiguous subnets to buildings at the bit boundary to apply summarization and ease the design. Campus networks can support high-bandwidth applications such as video conferencing. Although most WAN implementations are configured to support only IP, legacy LANs may still be configured to support IPX and AppleTalk.

Small/Remote Site LANs

Small/remote sites usually connect back to the corporate network via a small router (Cisco 2500). The local-area network service is provided by a small hub or LAN switch (Catalyst 1900). The router filters broadcasts to the WAN circuit and forwards packets that require services from the corporate network. A server may be placed at the small/remote site to provide DHCP and other local applications such as NT backup domain controller and DNS; if not, the router will need to be configured to forward DHCP broadcasts and other types of services. Figure 4-14 shows a typical architecture of a small or remote LAN.Building Cisco Remote Access Networks from Cisco Press is an excellent resource for more information on remote access.

Small/Remote Office LAN

LAN Media

The CCDA objectives covered in this section are as follows:

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.

This section identifies some of the constraints that should be considered when provisioning various LAN media types. For additional reference material on this subject, refer to Appendix D, "LAN Media Reference."

Ethernet Design Rules

Table 4-1 provides scalability information that you can use when provisioning IEEE 802.3 networks.

Table 4-1 Scalability Constraints for IEEE 802.3

10Base5

10Base2

10BaseT

100BaseT

Topology

Bus

Star

Maximum Segment Length (meters)

500

185

100 from hub to station

Maximum Number of Attachments per Segment

100

2 (hub and station or hub-hub)

Maximum Collision Domain

2500 meters of 5 segments and 4 repeaters; only 3 segments can be populated

See the details in the section "100 Mbps Fast Ethernet Design Rules" later in this chapter.

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 a 10 Mbps Ethernet network.

To make 100 Mbps Ethernet work, distance limitations are much more severe than those required for 10 Mbps Ethernet. The general rule is that a 100 Mbps Ethernet has a maximum diameter of 205 meters when unshielded twisted-pair (UTP) cabling is used, whereas 10 Mbps Ethernet has a maximum diameter of 500 meters with 10BaseT and 2500 meters with 10Base5.

10 Mbps Fiber Ethernet Design Rules

Table 4-2 provides some guidelines to help you choose the right media for your network designs. 10BaseF is based on the fiber-optic interrepeater link (FOIRL) specification, which includes 10BaseFP, 10BaseFB, 10BaseFL, and a revised FOIRL standard. The new FOIRL allows data terminal equipment (DTE) end-node connections rather than just repeaters, which were allowed with the older FOIRL specification.

Table 4-2 Scalability Constraints for 10 Mbps Fiber Ethernet

10BaseFP

10BaseFB

10BaseFL

Old FOIRL

New FOIRL

Topology

Passive star

Backbone or repeater fiber system

Link

Link or star

Allows DTE (End Node) Connections?

Yes

Maximum Segment Length (Meters)

500

2000

1000 or 2000

1000

Allows Cascaded Repeaters?

Yes

Maximum Collision Domains in Meters

2500

100 Mbps Fast Ethernet Design Rules

100 Mbps Ethernet, or Fast Ethernet, topologies present some distinct constraints on the network design because of their speed. The combined latency due to cable lengths and repeaters must conform to the specifications in order for the network to work properly. This section discusses these issues and provides example calculations.

Understanding Collision Domains

The overriding design rule for 100 Mbps Ethernet networks is that the round-trip collision delay must not exceed 512 bit times. However, the bit time on a 100 Mbps Ethernet network is 0.01 microseconds, as opposed to 0.1 microseconds on a 10 Mbps Ethernet network. Therefore, the maximum round-trip delay for a 100 Mbps Ethernet network is 5.12 microseconds, as opposed to the more lenient 51.2 microseconds in a 10 Mbps Ethernet network.

100BaseT Repeaters

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

The IEEE 100BaseT specification defines two types of repeaters: Class I and Class II. Class I repeaters have a latency (delay) of 0.7 microseconds or less. Only one repeater hop is allowed. Class II repeaters have a latency (delay) of 0.46 microseconds or less. One or two repeater hops are allowed.

Table 4-3 shows the maximum size of collision domains, depending on the type of repeater.

Table 4-3 Maximum Size of Collision Domains for 100BaseT

Copper

Mixed Copper and Multimode Fiber

Multimode Fiber

DTE-DTE (or Switch-Switch)

100 meters

412 meters (2000 if full duplex)

One Class I Repeater

200 meters

260 meters

272 meters

One Class II Repeater

200 meters

308 meters

320 meters

Two Class II Repeaters

205 meters

216 meters

228 meters

The Cisco FastHub 316 is a Class II repeater, as are all the Cisco FastHub 300 series hubs. These hubs actually exceed the Class II specifications, which means that they have even lower latencies and therefore allow longer cable lengths. For example, with two FastHub 300 repeaters and copper cable, the maximum collision domain is 223 meters.

Example of 100BaseT Topology

Figure 4-15 shows examples of 100BaseT topologies with different media.

Figure 4-15 Examples of 100BaseT Topologies with Various Media and Repeaters

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 determine whether configurations other than the standard ones shown in Figure 4-15will work, use the following information from the IEEE 802.3u specification.

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

Use the following steps to calculate the PDV:

For end-node segments, the segment length is the cable length between the physical interface at the repeater and the physical 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 physical 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 Table 4-4.

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

Round-Trip Delay

Table 4-4 shows round-trip delay in bit times for standard cables and maximum round-trip delay in bit times for DTEs, repeaters, and maximum-length cables.

NOTE

Note that the values shown in Table 4-4 have been multiplied by two to provide a round-trip delay. If you use these numbers, you need not multiply by two again in the LSDV formula (LSDV = 2 _ segment length _ cable delay for this segment).

Table 4-4 Network Component Delays1

Component

Round-Trip Delay in Bit Times per Meter

Maximum Round-Trip Delay in Bit Times

Two TX/FX DTEs

N/A

100

Two T4 DTEs

N/A

138

One T4 DTE and one TX/FX DTE

N/A

127

Category 3 cable segment

1.14

114 (100 meters)

Category 4 cable segment

1.14

114 (100 meters)

Category 5 cable segment

1.112

111.2 (100 meters)

STP cable segment

1.112

111.2 (100 meters)

Fiber-optic cable segment

1.0

412 (412 meters)

Class I repeater

N/A

140

Class II repeater with all ports TX or FX

N/A

Class II repeater with any port T4

N/A

Example Network Cabling Implementation

See Figure 4-16 for this example. Company ABC has all UTP Category 5 cabling. Two Class II repeaters are separated by 20 meters instead of the standard 5 meters. The network administrators are trying to determine whether this configuration will work.

Figure 4-16 An Example Network Cabling Implementation for Company ABC (Showing the Two Most Distant DTEs)

To ensure that the PDV does not exceed 512 bit times, the network administrators must 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 1. 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, the administrators use the values from Table 4-4. (Remember that Table 4-4 uses round-trip delay values, so you need not multiply by two.)

To test whether this network will work, the network administrators filled in Table 4-5.

Table 4-5 Delays of Components in Company ABC's Network

Delay Cause

Calculation of Network Component Delay

Total (Bit Times)

Link 1

75m _ 1.112 bit times/m

83.4

Link 2

75m _ 1.112 bit times/m

83.4

Interrepeater link

20m _ 1.112 bit times/m

22.24

Repeater A

92 bit times

Repeater B

92 bit times

DTE 1 and 2

100 bit times

100

Safety margin

5 bit times

Grand Total

Add Individual Totals

478.04

The grand total in Table 4-5 is fewer than 512 bit times, so this network will work.

Calculating Cable Delays

Some cable manufacturers specify propagation delays relative to the speed of light or in nanoseconds per meter (ns/m). To convert these values to bit times per meter (BT/m), use Table 4-6.

Table 4-6 Conversion to Bit Times per Meter for Cable Delays1

Speed Relative to Speed of Light

Nanoseconds per Meter (ns/m)

Bit Times per Meter (BT/m)

0.4

8.34

0.834

0.5

6.67

0.667

0.51

6.54

0.654

0.52

6.41

0.641

0.53

6.29

0.629

0.54

6.18

0.618

0.55

6.06

0.606

0.56

5.96

0.596

0.57

5.85

0.585

0.58

5.75

0.575

0.5852

5.70

0.570

0.59

5.65

0.565

0.6

5.56

0.556

0.61

5.47

0.547

0.62

5.38

0.538

0.63

5.29

0.529

0.64

5.21

0.521

0.65

5.13

0.513

0.654

5.10

0.510

0.66

5.05

0.505

0.666

5.01

0.501

0.67

4.98

0.498

0.68

4.91

0.491

0.69

4.83

0.483

0.7

4.77

0.477

0.8

4.17

0.417

0.9

3.71

0.371

Token Ring Design Rules

Table 4-7 lists some scalability concerns when designing Token Ring segments. Refer to IBM's Token Ring planning guides for more information on the maximum segment sizes and maximum diameter of a network.

Table 4-7 Scalability Constraints for Token Ring

IBM Token Ring

IEEE 802.5

Topology

Star

Not specified

Maximum Segment Length (Meters)

Depends on type of cable, number of MAUs, and so on

Maximum Number of Attachments per Segment

260 for STP, 72 for UTP

250

Maximum Network Diameter

Depends on type of cable, number of MAUs, and so on

Gigabit Ethernet Design Rules

The most recent development in the Ethernet arena is Gigabit Ethernet. Gigabit Ethernet is specified by two standards: IEEE 802.3z and 802.3ab. The 802.3z standard specifies the operation of Gigabit Ethernet over fiber and coaxial cable and introduces the Gigabit Media Independent Interface (GMII). The 802.3z standard was approved in June 1998.

The 802.3ab standard specifies the operation of Gigabit Ethernet over Category 5 UTP. Gigabit Ethernet still retains the frame formats and frame sizes and it still uses CSMA/CD. As with Ethernet and Fast Ethernet, full duplex operation is possible. Differences can be found in the encoding; Gigabit Ethernet uses 8B/10B coding with simple nonreturn to zero (NRZ). Because of the 20 percent overhead, pulses run at 1250 MHz to achieve a 1000 Mbps. Table 4-8 covers Gigabit Ethernet scalability constraints.

Table 4-8 Gigabit Ethernet Scalability Constraints

Type

Speed

Maximum segment length

Encoding

Media

1000BaseT

1000 Mbps

100m

5-level

Cat 5 UTP

1000BaseLX (long wave)

1000 Mbps

550m

8B/10B

Single/multiple mode fiber

1000BaseSX (short wave)

1000 Mbps

62.5 micrometers: 220m

50 micrometers: 500m

8B/10B

Multimode fiber

1000BaseCX

1000 Mbps

25m

8B/10B

Shielded balanced copper

FDDI Design Rules

The FDDI specification does not actually specify the maximum segment length or network diameter. It specifies the amount of allowed power loss, which works out to the approximate distances shown in Table 4-9.

Table 4-9 Scalability Constraints for FDDI

Multimode Fiber

Single-Mode Fiber

UTP

Topology

Dual ring, tree of concentrators, and others

Star

Maximum Segment Length

2km between stations

60km between stations

100m from hub to station

Maximum Number of Attachments per Segment

1000 (500 dual-attached stations)

2 (hub and station or hub-hub)

Maximum Network Diameter

200km

LAN Hardware

The CCDA objectives covered in this section are as follows:

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

Recognize scalability constraints and issues for standard LAN technologies.

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

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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