FitzGerald J., Dennis A., Durcikova A. Business Data Communications and Networking

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7.3 BACKBONE NETWORK ARCHITECTURES 245

Switched backbone networks use a star topology with one switch at its center.

Figure 7.6 shows a switched backbone connecting a series of LANs. There is a switch

serving each LAN (access layer) which is connected to the backbone switch at the bottom

of the ﬁgure (distribution layer). This ﬁgure implies that the LAN switches are close to

the computers in their LANs and farther from the backbone switch. Most organizations

now use switched backbones in which all network devices for one part of the building

are physically located in the same room, often in a rack of equipment. This form of

switched backbone is shown graphically in Figure 7.7. This has the advantage of placing

Switch

FIGURE 7.6 Switched backbone network design

246 CHAPTER 7 BACKBONE NETWORKS

Switch

FIGURE 7.7 Rack-mounted switched backbone network design

all network equipment in one place for easy maintenance and upgrade, but it does require

more cable. In most cases, the cost of the cable itself is only a small part of the overall cost

to install the network, so the cost is greatly outweighed by the simplicity of maintenance

and the ﬂexibility it provides for future upgrades.

The room containing the rack of equipment is sometimes called the main distribu-

tion facility (MDF) or central distribution facility (CDF). Figure 7.8 shows a photo of an

MDF room at Indiana University. Figure 7.9 shows the equipment diagram of this same

room. The cables from all computers and devices in the area served by the MDF (often

7.3 BACKBONE NETWORK ARCHITECTURES 247

FIGURE 7.8 An MDF with

rack-mounted equipment. A layer-2

chassis switch with six 100Base-T

modules (center of photo) connects

to four 24-port 100Base-T switches.

The chassis switch is connected to

the campus backbone using

1000Base-F over ﬁber-optic cable.

The cables from each room are

wired into the rear of the patch

panel (shown at the top of the

photo), with the ports on the front of

the patch panel labeled to show

which room is which. Patch cables

connect the patch panel ports to the

ports on the switches

hundreds of cables) are run into the MDF room. Once in the room, they are connected

into the various devices. The devices in the rack are connected among themselves using

very short cables called patch cables.

With rack-mounted equipment, it becomes simple to move computers from one

LAN to another. In the switched backbone design as shown in Figure 7.6, for example,

all the computers in the same general physical location are connected to the same switch

248 CHAPTER 7 BACKBONE NETWORKS

7.1

SWITCHED BACKBONES

INDIANA UNIVERSITY

MANAGEMENT

FOCUS

At Indiana University we commonly use switched

backbones in our buildings. Figure 7.10 shows a

typical design. Each ﬂoor in the building has a set

of switches and access points that serve the LANs

on that ﬂoor. Each of these LANs and WLANs are

connected into a switch for that ﬂoor, thus forming

a switched backbone on each ﬂoor. Typically, we

use switched 100Base-T within each ﬂoor.

The switch forming the switched backbone on

each ﬂoor is then connected into another switch

in the basement, which provides a switched

backbone for the entire building. The building

backbone is usually a higher speed network run-

ning over ﬁber-optic cable (e.g., 100Base-F or 1

GbE). This switch, in turn, is connected into a

high-speed router that leads to the campus back-

bone (a routed backbone design).

and thus share the capacity of the switch. Although this often works well, it can cause

problems if many of the computers on the switch are high-trafﬁc computers. For example,

in Figure 7.6, if all the busy computers on the network are located in the upper left area

of the ﬁgure, the switch in this area may become a bottleneck.

Layer-2 Chassis Switch

Serial

(1 Port)

100Base-T

(1 Port)

100Base-T

(1 Port)

100Base-T

(1 Port)

100Base-T

(1 Port)

100Base-T

(1 Port)

Empty Empty

1000Base-F

(1 Port)

To Building

Backbone

LAN

24 port 100Base-T Switch

LAN

24 port 100Base-T Switch

LAN

24 port 100Base-T Switch

LAN

24 port 100Base-T Switch

FIGURE 7.9 MDF network diagram

7.3 BACKBONE NETWORK ARCHITECTURES 249

LAN

WLAN

LAN

Third Floor

LAN

WLAN

LANLAN

Second Floor

LAN

WLAN

LANLAN

First FloorBasement

To Campus Backbone

Router

Switch

Switch Switch Switch Switch

Wireless AP

Switch Switch Switch Switch

FIGURE 7.10 Switched backbones at Indiana University

With an MDF, all cables run into the MDF. If one switch becomes overloaded, it

is straightforward to unplug the cables from several high-demand computers from the

overloaded switch and plug them into one or more less-busy switches. This effectively

spreads the trafﬁc around the network more efﬁciently and means that network capacity is

no longer tied to the physical location of the computers; computers in the same physical

area can be connected into different network segments.

Sometimes a chassis switch is used instead of a rack. A chassis switch enables

users to plug modules directly into the switch. Each module is a certain type of network

device. One module might be a 16-port 100Base-T switch, another might be a router,

whereas another might be a 4-port 1000Base-F switch, and so on. The switch is designed

to hold a certain number of modules and has a certain internal capacity, so that all the

modules can be active at one time. For example, a switch with four 1000Base-T switches

(with 24 ports each), and one 1000Base-F port would have to have an internal switching

capacity of at least 97 Gbps ([4 × 24 × 1 Gbps] + [1 × 1 Gbps]).

The key advantage of chassis switches is their ﬂexibility. It becomes simple to add

new modules with additional ports as the LAN grows and to upgrade the switch to use

new technologies. For example, if you want to add gigabit Ethernet, you simply lay the

cable and insert the appropriate module into the chassis switch.

7.3.3 Routed Backbones

Routed backbones move packets along the backbone on the basis of their network

layer address (i.e., layer-3 address). Routed backbones are sometimes called subnetted

250 CHAPTER 7 BACKBONE NETWORKS

backbones or hierarchical backbones and are most commonly used to connect different

buildings within the same campus network (i.e., at the core layer).

Figure 7.11 illustrates a routed backbone used at the core layer. A routed backbone

is the basic backbone architecture we used to illustrate how TCP/IP worked in Chapter 5.

There are a series of LANs (access layer) connected to a switched backbone (distribution

layer). Each backbone switch is connected to a router. Each router is connected to a core

Internet

Router

Core

Router

Backbone

Switch

Computers

Switches

Backbone

Switch

Backbone

Switch

Router

FIGURE 7.11 Routed backbone design

7.3 BACKBONE NETWORK ARCHITECTURES 251

router (core layer). These routers break the network into separate subnets. The LANs

in one building are a separate subnet. Message trafﬁc stays within each subnet unless it

speciﬁcally needs to leave the subnet to travel elsewhere on the network, in which case

the network layer address (e.g., TCP/IP) is used to move the packet. For example, in a

switched backbone, a broadcast message (such as an ARP) would be sent to every single

computer in the network. A routed backbone ensures that broadcast messages stay in the

one-network segment (i.e., subnet) where they belong, and are not sent to all computers.

This leads to a more efﬁcient network.

Each set of LANs is usually a s eparate entity, relatively isolated from the rest of

the network. There is no requirement that all LANs share the same technologies. Each

set of LANs can contain its own server designed to support the users on that LAN, but

users can still easily access servers on other LANs over the backbone as needed.

The primary advantage of the routed backbone is that it clearly segments each

part of the network connected to the backbone. Each segment (usually a set of LANs

or switched backbone) has its own subnet addresses that can be managed by a different

A Day in the Life: Network Operations Manager

The job of the network operations manager is to ensure

that the network operates effectively. The operations

manager typically has several network administrators

and network managers that report to him or her and is

responsible for both day-to-day operations as well as

long-term planning for the network. The challenge is

to balance daily ﬁreﬁghting with longer-term planning;

they’re always looking for a better way to do things.

Network operations managers also meet with users to

ensure their needs are met. While network technicians

deal primarily with networking technology, a network

operations manager deals extensively with both tech-

nology and the users.

A typical day starts with administrative work that

includes checks on all servers and backup processes to

ensure that they are working properly and that there are

no security issues. Then it’s on to planning. One typi-

cal planning item includes planning for the acquisition

of new desktop or laptop computers, including meeting

with vendors to discuss pricing, testing new hardware

and software, and validating new standard conﬁgura-

tions for computers. Other planning is done around

network upgrades, such as tracking historical data to

monitor network usage, projecting future user needs,

surveying user requirements, testing new hardware and

software, and actually planning the implementation of

new network resources.

One recent example of long-term planning was the

migration from a Novell ﬁle server to Microsoft ADS

ﬁle services. The ﬁrst step was problem deﬁnition;

what were the goals and the alternatives? The key driv-

ing force behind the decision to migrate was to make it

simpler for the users (e.g., now the users do not need

to have different accounts with different passwords)

and to make it simpler for the network staff to pro-

vide technical support (e.g., now there is one less type

of network software to support). The next step was

to determine the migration strategy: a Big Bang (i.e.,

the entire network at once) or a phased implementa-

tion (several groups of users at a time). The migration

required a technician to access each individual user’s

computer, so it was impossible to do a Big Bang. The

next step was to design a migration procedure and

schedule whereby groups of users could be moved at

a time (e.g., department by department). A detailed

set of procedures and a checklist for network tech-

nicians were developed and extensively tested. Then

each department was migrated on a one-week sched-

ule. One key issue was revising the procedures and

checklist to account for unexpected occurrences during

the migration to ensure that no data were lost. Another

key issue was managing user relationships and dealing

with user resistance.

With thanks to Mark Ross

252 CHAPTER 7 BACKBONE NETWORKS

network manager. Broadcast messages stay within each subnet and do not move to other

parts of the network.

There are two primary disadvantages to routed backbones. First, the routers in

the network impose time delays. Routing takes more time than switching, so routed

networks can sometimes be slower. Second, routers are more expensive and require

more management than switches.

Figure 7.11 shows one core router. Many organizations actually use two core routers

to provide better security, as we discuss in Chapter 10.

7.3.4 Virtual LANs

For many years, the design of LANs remained relatively constant. However, in recent

years, the introduction of high-speed switches has begun to change the way we think

about LANs. Switches offer the opportunity to design radically new types of LANs.

Most large organizations today have traditional LANs, but many are considering the

virtual LAN (VLAN), a new type of LAN-BN architecture made possible by intelligent,

high-speed switches.

Virtual LANs are networks in which computers are assigned to LAN segments

by software rather than by hardware. In the preceding section, we described how in

rack-mounted collapsed BNs a computer could be moved from one hub to another by

unplugging its cable and plugging it into a different hub. VLANs provide the same

capability via software so that the network manager does not have to unplug and replug

physical cables to move computers from one segment to another.

Often, VLANs are faster and provide greater opportunities to manage the ﬂow of tra-

fﬁc on the LAN and BN than do the traditional LAN and routed BN architecture. However,

VLANs are signiﬁcantly more complex, so they usually are used only for large networks.

The simplest example is a single-switch VLAN, which means that the VLAN

operates only inside one switch. The computers on the VLAN are connected into the

one switch and assigned by software into different VLANs (Figure 7.12). The network

manager uses special software to assign the dozens or even hundreds of computers

attached to the switch to different VLAN segments. The VLAN segments function in the

same way as physical LAN segments or subnets; the computers in the same VLAN act as

though they are connected to the same physical switch or hub in a certain subnet. Because

VLAN switches can create multiple subnets, they act like layer-3 switches or routers,

except the subnets are inside the switch, not between switches. Therefore, broadcast

messages sent by computers in one VLAN segment are sent only to the computers on

the same VLAN.

Virtual LANs can be designed so that they act as though computers are connected

via hubs (i.e., several computers share a given capacity and must take turns using it) or

via switches (i.e., all computers in the VLAN can transmit simultaneously). Although

switched circuits are preferred to the shared circuits of hubs, VLAN switches with the

capacity to provide a complete set of switched circuits for hundreds of computers are

more expensive than those that permit shared circuits.

We should also note that it is possible to have just one computer in a given VLAN.

In this case, that computer has a dedicated connection and does not need to share the

network capacity with any other computer. This is commonly done for servers.

7.3 BACKBONE NETWORK ARCHITECTURES 253

Switch

FIGURE 7.12 VLAN-based backbone network design

Beneﬁts of VLANs Historically, we have assigned computers to subnets based on

geographic location; all computers in one part of a building have placed in the same sub-

net. With VLANs, we can put computers in different geographic locations in the same

subnet. For example, in Figure 7.12 a computer in the lower left could be put on the

same subnet as one in the upper right—a separate subnet from all the other computers.

254 CHAPTER 7 BACKBONE NETWORKS

A more common implementation is a multiswitch VLAN, in which several

switches are used to build the VLANs (Figure 7.13). VLANs are most commonly found

in building backbone networks (i.e., access and distribution layers) but are starting to

move into core backbones between buildings. In this case, we can now create subnets

that span buildings. For example, we could put one of the computers in the upper left of

Figure 7.13 in the same subnet as the computers in the lower right, which could be in a

completely different building. This enables us to create subnets based on who you are,

rather than on where you are; we have an accounting subnet and a marketing subnet, not

a Building A and a Building B subnet. We now manage security and network capacity

by who you are, not by where your computer is. Because we have several subnets, we

need to have a router—but more on that shortly.

Virtual LANs offer two other major advantages compared to the other network

architectures. The ﬁrst lies in their ability to manage the ﬂow of trafﬁc on the LAN and

backbone very precisely. VLANs make it much simpler to manage the broadcast trafﬁc

that has the potential to reduce performance and to allocate resources to different types of

trafﬁc more precisely. The bottom line is that VLANs often provide faster performance

than the other backbone architectures.

The second advantage is the ability to prioritize trafﬁc. The VLAN tag information

included in the Ethernet packet deﬁnes the VLAN to which the packet belongs and also

speciﬁes a priority code based on the IEEE 802.1q standard (see Chapter 4). As you will

recall from Chapter 5, the network and transport layers can use RSVP quality of service

(QoS), which enables them to prioritize trafﬁc using different classes of service. RSVP

is most effective when combined with QoS capabilities at the data link layer. (Without

QoS at the hardware layers, the devices that operate at the hardware layers [e.g., layer-2

VLAN

Switch 1

Router

VLAN ID: 20; IP: 179.58.7.1

VLAN ID: 10; IP: 179.58.10.1

VLAN ID: 30; IP: 179.58.11.1

VLAN ID: 10

IP: 179.58.10.101

VLAN ID: 10

IP: 179.58.10.102

VLAN ID: 10

IP: 179.58.10.103

VLAN ID: 30

IP: 179.58.7.30

VLAN ID: 20

IP: 179.58.11.20

VLAN ID: 10

IP: 179.58.10.50

VLAN Switch 3

VLAN

Switch 2

FIGURE 7.13 Multiswitch VLAN-based backbone network design