Ch08 Backbone Network Q&A

CHAPTER 8

BACKBONE NETWORKS

Chapter Summary This chapter examines backbone networks (BNs) that are used in the distribution layer (within-

building backbones) and the core layer (campus backbones). We discuss the three primary

backbone architectures and the recommended best practice design guidelines on when to use

them. The chapter ends with a discussion of how to improve BN performance and of the future

of BNs.

Learning Objectives After reading this chapter, students should:

Understand the Internetworking devices used in BNs

Understand the switched backbone architecture

Understand the routed backbone architecture

Understand Virtual LAN architecture

Understand the best practice recommendations for backbone design

Be aware of ways to improve BN performance

Key Terms chassis switch

forwarding equivalence

class (FEC)

IEEE 802.1q

label switched router

(LSR)

layer-2 switch

main distribution facility

(MDF)

module

multiprotocol label

switching (MPLS)

multiswitch VLAN

patch cables

rack

routed backbone

router

single-switch VLAN

switched backbone

virtual LAN (VLAN)9

VLAN ID

VLAN switch

VLAN tag

VLAN trunk

Instructors Manual to Accompany: Business Data Communications and Networking, 12e by Fitzgerald/Dennis

2015 John Wiley & Sons, Inc. Page 1

Chapter Outline

1. Introduction 2. Switched Backbones 3. Routed Backbones 4. Virtual LANs 5. The Best Practice Backbone Design 6. Improving Backbone Performance

a. Improving Device Performance b. Improving Circuit Capacity c. Reducing Network Demand

7. Implications for Management 8. Summary

Answers to Textbook Exercises

Answers to End-of-Chapter Questions

1. How does a layer-2 switch differ from a router?

Layer 2 switches operate by using the data link layer address or MAC address to forward

packets between network segments. They connect the same or different types of cable.

Layer-2 switches (or workgroup switches) operate at the Data Link layer, and typically provide

ports for a small set of 16 to 24 computers. Layer-2 switches enable all ports to be in use

simultaneously by managing paired combinations of ports as separate point-to-point circuits.

Layer-2 switches "learn" addresses; a layer-2 switch builds a forwarding table after it is first

turned on. To learn addresses, a layer-2 switch retransmits to all ports (except to the one from

which it was received) only for a packet with a destination address not already in the forwarding

table. The resulting ACK from the destination computer (that recognized its address) is then

used by the layer-2 switch to add the new port number and address to the forwarding table.

Routers operate at the network layer. They connect two different TCP/IP subnets. Routers

strip

off the data link layer packet, process the network layer packet, and forward only those

messages that need to go to other networks on the basis of their network layer address. In

general, they perform more processing on each message than switches and therefore operate

more slowly.

2. How does a layer-2 switch differ from a VLAN?

Layer 2 switches operate by using the data link layer address or MAC address to forward

packets between network segments. They connect the same or different types of cable.

Layer-2 switches (or workgroup switches) operate at the Data Link layer, and typically provide

ports for a small set of 16 to 24 computers. Layer-2 switches enable all ports to be in use

simultaneously by managing paired combinations of ports as separate point-to-point circuits.

Layer-2 switches "learn" addresses; a layer-2 switch builds a forwarding table after it is first



turned on. To learn addresses, a layer-2 switch retransmits to all ports (except to the one from

which it was received) only for a packet with a destination address not already in the forwarding

table. The resulting ACK from the destination computer (that recognized its address) is then

used by the layer-2 switch to add the new port number and address to the forwarding table.

VLAN switches work a little differently. When a VLAN switch receives a frame that is

destined for another computer in the same subnet on the same VLAN switch, the switch acts

as a traditional layer-2 switch: it forwards the frame unchanged to the correct computer.

VLAN switches use Ethernets 802.1qs tagging to move frames from one switch to another. When a VLAN switch receives an Ethernet frame that needs to go to a computer on another

VLAN switch, it changes the Ethernet frame by inserting the VLAN ID number and a

priority code into the VLAN tag field.

3. How does a router differ from a VLAN?

VLAN switches can create multiple subnets, so they act like 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.

4. Under what circumstances would you use a switched backbone?

Switched backbones can be used in situations where the network administrators wants to spread

the traffic around the network more efficiently. In addition, it also provides an architecture

where network capacity is no longer tied to the physical location of the computers, as computers

in

5. Under what circumstances would you want to use a routed backbone?

Routed backbones are good for connecting different buildings on the same enterprise campus

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

each part of the network connected to the backbone. Each segment has its own subnet

addresses that can be managed by a different network manager. Broadcast messages stay

within each subnet and do not move to other parts of the network.

6. Under what circumstances would you use a VLAN backbone?

A VLAN backbone is useful when you want to put computers that are in different geographic

locations in the same subnet. In addition, VLANs make it much simpler to manage the

broadcast traffic and provide a better opportunity to prioritize traffic on the network.

7. Explain how routed backbones work.

Routed backbones move packets along the backbone based on their network layer address (i.e.,

layer 3 address). The most common form of routed backbone uses a bus topology (e.g., using

Ethernet 100Base-T). Routed backbones can be used at the core or distribution layers.



At the core layer routed backbones are sometimes called subnetted backbones or hierarchical

backbones and are most commonly used to connect different buildings within the same

campus network.

At the distribution layer a routed backbone uses routers or layer 3 switches to connect a series of

LANs (access layer) to a single shared media backbone network. Each of the LANs 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.

8. In Figure 8.5, would the network still work if we removed the routers in each building and just had one core router? What would be the advantages and disadvantages of doing this?

The network would still work, although the traffic on the network would be significantly

increased due to the creation of one large LAN instead of three subnets. The advantages of

this would be slightly lower costs due to only purchasing one router instead of four and less

maintenance and management. Each of these advantages are minor, and certainly not work

the additional traffic on the network.

9. Explain how switched backbones work.

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

The traditional backbone circuit and set of routers or bridges is replaced by one switch and a set

of circuits to each LAN. The collapsed backbone has more cable, but fewer devices. There is

no backbone cable. The backbone exists only in the switch, which is why this is called a collapsed backbone. The original collapsed backbone technology uses layer-2 switches and

suffers some disadvantage due to the load of data link layer overhead message traffic and

limitations on network segmentation. As this weakness has been recognized, collapsed

backbone technology is adapting by evolving to the use of layer-3 switches to overcome these

problems. The result is better performance and improved network management capabilities for

switched backbone networks.

Collapsed backbones are probably the most common type of backbone network used in the

distribution layer (i.e., within a building). Most new building backbone networks designed today

use collapsed backbones. They also are making their way into the core layer as the campus

backbone, but routed backbones still remain common.

10. What are the key advantages and disadvantages among routed and switched backbones?

Advantages Disadvantages

Routed

backbones Clear segmentation of parts of the

network connected to the backbone as

each network has a subnet address

and can be managed separately.

Slower performance as routing takes more time than bridging or switching.

Management and/or software overhead costs due to need to

establish subnet addressing and



provide reconfiguration when

computers are moved (or support

dynamic addressing).

Switched

backbones Performance is improved. With the

traditional backbone network, the

backbone circuit was shared among

many LANs; each had to take turns

sending messages. With the

collapsed backbone, each connection

into the switch is a separate point-to-

point circuit. The switch enables

simultaneous access, so that several

LANs can send messages to other

LANs at the same time. Throughput

is increased significantly, often by

200% to 600%, depending upon the

number of attached LANs and the

traffic pattern.

Since there are far fewer networking devices in the network, this reduces

costs and greatly simplifies network

management. All the key backbone

devices are in the same physical

location, and all traffic must flow

through the switch. If something goes

wrong or if new cabling is needed, it

can all be done in one place.

Software reconfiguration replaces hardware reconfiguration.

Because data link layer addresses are used to move packets, there is more

broadcast traffic flowing through the

network and it is harder to isolate and

separately manage the individually

attached LANs. Layer 3 switches can

use the network layer address, so

future collapsed backbones built with

layer 3 will not suffer from this

problem.

Collapsed backbones use more cable, and the cable must be run longer

distances, which often means that

fiber optic cables must be used.

If the switch fails, so does the entire backbone network. If the reliability of

the switch has the same reliability as

the reliability of the routers, then there

is less chance of an failure (because

there are fewer devices to fail).

For most organizations, the relatively

minor disadvantages of cable

requirements and impacts of potential

switch failure are outweighed by the

benefits offered by collapsed backbones.

11. Compare and contrast rack-based and chassis-switch based switched backbones.

The rack-based collapsed backbone has the advantage of placing all network equipment in

one place for easy maintenance and upgrade, but 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 distribution

facility (MDF) or central distribution facility (CDF). The cables from all computers and devices

in the area served by the MDF (often hundreds of cables) are run into the MDF room. Once in

the run 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-based equipment, it

becomes simple to move computers from one LAN to another. This convenience is used to

spread the traffic around the network more efficiently so that network capacity is no longer tied

to the physical location of the computers. Computers in the same physical area can be connected

into very different network segments conveniently in the MDF.



A chassis switch enables users to plug modules directly into the switch. Each module is a

certain type of network device. 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 or ATM

you simply lay the cable and insert the appropriate module into the switch.

12. What is a module and why are modules important?

A module is any of certain types of network devices that can be plugged directly into a chassis

switch. Since a chassis switch must be able to support simultaneous activities of all connected

module, each switch has an internal capacity (in Mbps) which limits the maximum number of

modules that can be accepted by the switch. Modules can be switches, hubs, or routers.

13. Explain how single-switch VLANs work.

In a single switch VLAN 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. 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; the computers in the same VLAN act as though they are

connected to the same physical switch or hub.

14. Explain how multiswitch VLANs work.

A multi-switch VLAN works the same way as a single switch VLAN, except that several

switches are used to build the VLANs. The switches must be able to send packets among

themselves in a way that identifies the VLAN to which the packet belongs. There are two

approaches to this: packet encapsulation and modifying the Ethernet packet.

In the encapaulation approach a proprietary protocol encapsulates the packet. When a packet needs to go from one VLAN switch to another VLAN switch, the first switch puts a new VLAN

packet around the outside of the Ethernet packet. The VLAN packet contains the VLAN

information and is used to move the packet from switch to switch within the VLAN network..

When the packet arrives at the final destination switch, the VLAN packet is stripped off and the

unchanged Ethernet packet inside is sent to the destination computer.

In the modification approach the Ethernet packet itself is to modified to carry the VLAN information. 16-bytes of VLAN information (according to emerging standard IEEE 802.1q) are

added to the standard Ethernet (IEEE 802.3) packet. The additional VLAN information is used

to move the packet from switch to switch within the VLAN network. The original Ethernet

packet is restored from the modified packet at the final destination switch and then sent to the

destination computer.

15. What is IEEE 802.1q?

IEEE 802.1q is an emerging standard that inserts 16-bytes of VLAN information into the

normal IEEE 802.3 Ethernet packet. When a packet needs to go from one VLAN switch to

another VLAN switch, the first switch replaces the incoming Ethernet packet with an 802.1q



packet that contains all the information in the original 802.3 Ethernet packet, plus 16-bytes of

VLAN information. The additional VLAN information is used to move the packet from switch

to switch within the VLAN network. When the packet arrives at the final destination switch, the

IEEE 802.1q packet is stripped off and replaced with a new Ethernet packet that is identical to

the one with which it entered the VLAN.

16. What are the advantages and disadvantages of VLANs? Advantages:

VLANs are often faster and provide greater opportunities to manage the flow of traffic on the LAN and BN than do the traditional LAN and routed BN architecture.

Allow the ability to prioritize traffic

They allow computers in separate geographic locations to be placed on the same LAN.

Disadvantages:

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

Cost

17. How can you improve the performance of a BN?

Improving the performance of backbone networks is similar to improving LAN performance.

First, find the bottleneck, and then solve it (or more accurately, move the bottleneck somewhere

else). You can improve the performance of the network by improving the computers and other

devices in the network, by upgrading the circuits between computers, and by changing the

demand placed on the network.

Network performance can be improved by upgrading the computers and other devices in the

network, by using static rather than dynamic routing if there are few routes through the network,

by reducing switch-to-switch traffic in networks without standard protocols, by using the same

protocols in the backbone network as in the attached LANs, by encapsulating rather than

translating between different protocols, and by increasing the memory in backbone devices.

Performance can also be improved by adding additional circuits to increase capacity, by going

to a switched network, and by increasing the circuits on high traffic circuits. In addition,

performance can be enhanced by reducing demand or by restricting applications that use lots of

network capacity, and by using switches that filter certain broadcast messages.

18. Why are broadcast messages important?

Some application software packages and network operating system modules written for use

on LANs broadcast status messages to all computers on the LAN (but not necessarily all

computers served by a BN). For example, broadcast messages inform users when printers are

out of paper, or when the network manager is about to shut down the server. These types of

messages require filtering in a backbone network if their broadcast scope should be restricted to

a particular LAN or segment.



19. What are the preferred architectures used in each part of the backbone?

Answers can vary depending on preferences, but suggestions include:

Access switched backbones

Distribution layer Virtual LAN

Core layer routed backbones

20. Some experts are predicting that Ethernet will move into the WAN. What do you think?

The new Ethernet/IP packet networks have become dominant for high-traffic networks (2 Mbps

to 1Gbps), even though SONET and ATM remain preferred for some requirements. Since WAN

required a network with high network capacity, I believe that Ethernet will move into the WAN

into the near future.

Mini-Cases

I. Pats Engineering Works

Pats Engineering Works is a small company that specializes in complex engineering consulting projects. The company is moving into new offices and want you to design their network. They

have a staff of eight engineers (which is expected to grow to 12 over the next five years), plus

another eight management and clerical employees who also need network connections, but

whose needs are less intense. Design the network. Be sure to include a diagram.

The recommended design should be based on the Rack-based collapsed backbone network

design or on the VLAN-based collapsed backbone network design.

II. Hospitality Hotel

Hospitality Hotel is a luxury hotel that whose guests are mostly business travelers. To improve

its quality of service, it has decided to install network connections in each of its 600 guest rooms

and 12 conference meeting rooms. Your task is to design the network for the public network and

decide how to connect the two networks together. Be sure to include a diagram.

The recommended design should be based on the Rack-based collapsed backbone network

design shown in the text. Each floor should have a layer-2 switch. Each conference room should

"home run" to the main closet switch. All floors should use an uplink port to interconnect to the

main switch.



The two networks should be connected together with a router in between the two networks. This

will allow the networks to be connected so that traffic can pass from one to the other, but the

router can also keep local traffic local, thus reducing traffic on the network.

III. Indiana University

Reread Management Focus 8-1. What other alternatives do you think Indiana University

considered? Why do you think they did what they did?

They probably also considered a routed backbone and a virtual LAN. Either of these could offer

some advantages, but the switched design is a good design that allows for flexibility in design.

IV. Shangri-La

Reread Management Focus 8-2. What other alternatives do you think the Shangri-La Resort

considered? Why do you think they did what they did?

The other alternatives could be numerous. They probably considered both routed and switched

backbone architectures as well. Although the VLAN is more costly, it does provide much more

efficiency, especially under heavy loads.

V. Chicago Consulting

You are the network manager for a consulting firm that needs to install a backbone to connect

four 100Base-T Ethernet LANs (each using a 24-port switch). Develop a simple backbone and

determine the total cost (i.e., select the device and price it). Prices are available at

www.cdw.com, but you can use any source that is convenient.

The answers can vary depending on the design by the student. A routed backbone architecture

(similar to Figure 8-5) would be a good option to consider.

VI. Western Trucking

Western Trucking operates a large fleet of trucks that deliver shipments for commercial shippers

such as food stores, retailers, and wholesalers. Their main headquarters building and secondary

building are shown in Figure 8-10. They want to upgrade to a faster network. Design a new

network for them, including the specific backbone and LAN technologies to be used. Assume

that the main office building is 170 feet by 100 feet in size and that the secondary building is 100

feet by 50 feet. The two buildings are 100 feet apart.

I would install a collapsed backbone network using a star topology with a Gigabit Ethernet

switch at the center. This backbone would support a series of LANs (as needed). In this new architecture, all circuits, routers, and bridges are replaced by one switch.



Next Day Air Service Case Study

1. For this case, one may assume that there are LANs in four department offices (Data

Processing, Accounts Payable, Information Services, and Agent Operations) and at Fleet

Maintenance and Dispatch in the Secondary Building. What type of backbone network do

you recommend for NDAS headquarters? Be prepared to justify your recommendation.

Remember to consider the expected growth of the company.

The determining factors in recommending a choice for a backbone network for NDAS are:

(1) throughput, (2) type of application, (3) network management requirements, and (4)

flexibility and potential for growth.

Examining these factors, Gigabit Ethernet is the recommended backbone technology for

NDAS headquarters. It will mesh well with their existing networks and allow them to

continue to grow going forward. It is a good choice for NDAS.

Note that student answers may differ. There is no single correct answer, so any reasonable

answer is acceptable.

2. Price the network you have designed. Prices are available at www.datacommwarehouse.com,

but use any source that is convenient. For simplicity, assume that Cat 5, Cat 5e, Cat 6 and

fiber optic cable have a fixed cost per circuit to buy and install, regardless of distance, of $80,

$100, $250 and $400.



3. It is very important for you to explain to President Coone that there will be significant business benefits derived from continuing to grow the network. Explain in detail what some

of these will be.



Additional Content

Teaching Notes

I cover most material evenly, but I omit the selecting backbone networks section. I no longer

cover FDDI and token ring as they are faded technologies, but I may mention these briefly in

class to let the students know about these technologies just in case they encounter these older

approaches in a job somewhere.



Fast Ethernet, gigabit Ethernet and switched networks are the new hot technologies so I spend a significant amount of time on them.

I also spend some amount of time on ATM. It is quite different from the other technologies we

have examined and students have a harder time understanding it. It is useful for them to

understand the concepts of an edge switch.

Gigabit Ethernet and switch technologies (especially layer-3 and layer-4 switches) are the least

mature of the technologies in this chapter. They will probably need careful watching to stay

current.

Ch08 Backbone Network Q&A

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