This chapter is from the book
This chapter is from the book
This chapter is from the book
Exam Prep Questions
Question 1
Your company has upgraded all of its hubs to switches. One of the switches appears to be working overly hard, with its overall traffic indicator lit continuously, as are the port lights. Your boss is afraid there is a bottleneck and dispatches you with one of the company's $70,000 protocol analyzers to diagnose the problem. However, when you plug the analyzer into an open port, it reports extremely light and sporadic traffic. What, if anything, is the problem?
Indicator lights on networking equipment are notoriously inaccurate. If the protocol analyzer reports light traffic then the switch is well below capacity and there is no problem.
The switch probably has address filtering activated on the analyzer's port. Deactivating the filtering option will give the analyzer access to all traffic.
Switches build virtual circuits between two ports. If the analyzer's port is not part of the current circuit, it will not see activity.
There is no problem. Unlike a hub, activity lights on a switch should be on constantly.
Answer C is correct. Okay, this was a nasty way to start because we haven't even mentioned activity lights and protocol analyzers. Nevertheless, if you know how a switch works, it should not be hard to deduce the answer. Switches do create virtual circuits between two ports. If a port were not part of the circuit, it would not be aware of the activity. That makes answer C a likely candidate. Activity indicator lights are not accurate, but if every port's indicator is lit constantly, that switch has to be really working. Therefore, A is probably not correct. Filtering sounds good, but even without filtering, the analyzer would still only see activity broadcast or addressed to its port. That rules out B as a correct answer. Lastly, if every port activity indicator is constantly lit, it would be logical to assume there is activity. A switch only connects two ports at a time, so having all of the port activity lights continually on is probably not normal. Therefore D is questionable as a correct answer.
Question 2
Which type of switch can create multiple temporary collision domains when segments approach saturation?
Cut-through switches
Non-blocking switches
Switches running in fragment-free mode
Store-and-forward switches
Answer A is correct. Early cut-through switches read the source and destination addresses and dropped or forwarded frames based on that information alone. As segments approached saturation and the number of collisions escalated, cut-through switches could and did pass the remains of collisions provided the source and destination address fields survived intact. When this occurred, both segments would essentially be in the same collision domain connected by the switch. Answer B is incorrect because non-blocking means the switch has the capacity to switch frames between all segments at the same time. Answer C could be correct because a fragment-free switch examines only the first 64 bytes of the frame. However, it has been shown that if the first 64 bytes of a frame are correct then there is greater than a 90% chance the remainder of the frame is also correct, so few if any damaged frames would be passed. Answer D is incorrect because a store-and-forward switch is the only type of switch that will stop all damaged frames by performing a cyclical redundancy check (CRC) prior to passing the frame on.
Question 3
How did the 10BASE-T standard put an end to the cumbersome bus structure of early Ethernets?
10BASE-T eliminated the bus in favor of star wiring that was far less expensive and more reliable.
Networks using the 10BASE-T standard moved the signaling frequency below the radio spectrum so common twisted-pair wire could be used.
A network using the 10BASE-T standard was far more reliable than a bus network because severing a cable only brought down a segment of the network.
A network using the 10BASE-T standard is a bus network.
Answer D is correct. Networks using the 10BASE-T standard did not require coaxial cable. The bus structure however remained as a part of the Hub. The correct way of describing an Ethernet using the 10BASE-T standard is a "star wired, bus network." Answer A is incorrect because the 10BASE-T standard did not eliminate the use of a bus. Answer B is incorrect because signaling remained in the radio spectrum despite the use of cabling meeting the 10BASE-T standard. Answer C is incorrect because severing a cable would only bring down the node or station using the station.
Question 4
Your uncle has a problem with his 10BASE-2 Ethernet and he has asked for your help. He has several geographically dispersed offices in a shared tenant office building. There are only 10 stations connected to the network but 2 of the stations located in adjoining offices experience erratic network behavior when communicating with each other. With the exception of these 2 stations, the network performs well. You take a look at each of the problem stations and find that each station has the network cable attached to only one side of the "T" connector. The other side of the "T" connector has a 3-inch black plug attached. Knowing your uncle is short on cash, what would you recommend he buy to remedy the problem with the 2 adjoining stations?
A patch cable to connect the stations in the adjoining offices and complete the network path.
A 10BASE-2 repeater.
A non-filtering bridge.
A static or fixed configuration router.
Answer B is correct. Okay, this question really rambles but so will some of the questions on the exam. The key to the question is seeing the 10BASE-2 cable attached to only one side of each station's "T" connector. The plug attached to the other side of the "T" connector should be a giveaway. Although these stations are in adjoining offices, they are each at the end of the network cable. Even if you have never seen one, you could probably guess that the black plug is actually a terminating resistor, which confirms these stations as the ends of the network. To communicate, these stations must send data across the entire network. While we don't know the exact length of the cable, we do know it streams off to connect the other geographically diverse offices and then loops back almost to the starting point. That is probably a "pretty good distance" (technical term). As only the end stations are having problems, this is most likely an attenuation problem, which is a Layer 1 (Physical) issue. The most efficient way to deal with Layer 1 attenuation is with a Layer 1 repeater. Okay, there are a lot of assumptions here and not much in the way of cold hard facts, but that's the way it is in the real world and, of course, it did make you think. Answer A is incorrect because it would create a loop. Signals introduced to a loop would continue around the loop until dissipated by attenuation, which would effectively bring down the network. Answer C could be correct because bridges provide a Layer 1 repeater function, but they are really designed to provide Layer 2 capabilities, which would not be used. Besides, bridges are more expensive than repeaters and your uncle is strapped for cash. Answer D is incorrect because routers are even more expensive than bridges and they operate at Layer 3. Of course routers could be made to work, but you would need a pair of them and you would end up with two separate networks. Definitely a square peg in a round hole approach.
Question 5
How does a transmitting station on an Ethernet recognize a collision when it takes place?
A CRC is performed and if results do not match then a collision has occurred.
The transmitting station cannot determine that a collision has taken place while it is transmitting. It must wait until the receiving interface recognizes the collision and sends an error report in the form of a jam signal.
The interface does not monitor network errors. That is done at Layer 2 when the frame is checked for accuracy.
A voltage comparator on the transmitting interface senses an over-voltage condition on the line.
Answer D is correct. This is an area where there are a lot of old wives' tales and any one of them will get you in trouble. Collision detection is a Layer 1 issue for Ethernet dealing with voltage levels on the line. A voltage comparator on the interface (it is really a $.22 chip) monitors the voltage of the line even when the interface is transmitting. So long as the voltage stays within a predetermined range, everything is okay. However, when two or more signals are on the line at the same time, the combined voltage exceeds this level. The comparator senses the over voltage and it initiates the recovery process. Answer A is incorrect because the Cyclical Redundancy Check is performed at Layer 2 and indicates a fault with the data, which may not have been caused by a collision. Answer B is incorrect because the voltage is monitored during transmission. Answer C is incorrect because the question is asking about a Layer 1 collision, not Layer 2 error detection.
Question 6
What is the Protocol Data Unit (PDU) used at Layer 4?
Frame
Segment
Packet
Data
Answer B is correct. Layer 4 is the Transport layer and Segment is the name of its PDU. Answer A, Frame, is the PDU for Layer 2, Data Link. Answer C, Packet, is the PDU for Layer 3, Network. Answer D, Data, generally describes the PDUs for Layers 5 through 7.
Question 7
A network administrator is charged with connecting LANs from the Memphis production site to LANs at the Detroit headquarters. The administrator leases a 56Kb line between the locations and terminates the line with bridges to make the link as maintenance free and as transparent as possible. The link meets all expectations but it is carrying far more traffic than anticipated. In fact, the administrator is growing increasingly concerned about degradation of response times. What would you recommend?
Replace the bridges with cut through switches to increase throughput.
Lease a second line to cover peak loads.
All of the above.
None of the above.
Answer D is correct. The administrator is linking networks at Layer 3 with a bridge, which is a Layer 2 device. Bridges would pass all of the administrative traffic from both networks across the link, which could easily fill whatever bandwidth was available. Although there are special purpose bridges that could work in this situation, the real solution would be to use routers. Routers would eliminate Layer 2 administrative traffic from the link and free up bandwidth for data traffic. Answer A is incorrect because a switch would also pass administrative traffic only faster. Answer B is incorrect because it is addressing the symptom not the problem. And now, will the administrator and all those choosing A, B, or C please report to your new job in Marketing tomorrow morning.
Question 8
Which protocols are Layer 3 routable protocols? (Choose all that apply.)
Internet Protocol (IP)
Routing Information Protocol (RIP)
Internet Packet Exchange (IPX)
AppleTalk.
Answers A, C, and D are routable protocols that carry data between networks. Answer B is a distance-vector routing protocol that is used by routers to exchange routing table information and determine optimum paths for packets.
Question 9
People working at the end stations of a 10BASE-5 network are constantly complaining that information they send each other is slow and often has errors. Which devices may alleviate the problem? (Choose all that apply.)
A bridge
A hub
A repeater
A static or fixed configuration router
Answers A and C are correct. The most likely cause of the problem is attenuation of the signal as it crosses the cable. The end stations would experience a majority of the problems because they have the most cable between them. A repeater which rebuilds, retimes, and then retransmits the signal would be the ideal short-term solution. Although a bridge is designed to separate collision domains, its operation includes rebuilding, retiming, and retransmitting frames just like a repeater. The bridge provides more than is needed, but its repeater function would fix the problem. Hubs are not used in a 10BASE-5 network so answer B is wrong and answer D could go either way. If you could find a router with both ports configured for 10BASE-5 networks, it could conceivably work in this situation by rebuilding the signal as it was sent between networks. The user, however, would have to be willing to accept the higher costs of the router and the need to readdress many of the stations and configure them all to use the router as a gateway. Certainly not an elegant solution, and if this were the test I would not select it as a viable solution.
Question 10
A wiring technician was directed to extend a 10BASE-2 network cable to provide service to a new suite of offices. The existing cable was too short so the technician spliced a piece of cable that looked exactly like the network cable but had different numbers printed on the jacket. When the network was initialized the following morning, the entire network experienced erratic operation. What would be the most likely problem?
10BASE-2 cable cannot be cut without introducing a high level of attenuation. This attenuation can be great enough to stop all but the strongest signals.
Joining even slightly dissimilar cables (the numbers didn't match) would almost certainly create an impedance mismatch. The resulting reflected signals would create erratic performance and could bring the whole network down.
10BASE-2 cable is extremely tolerant of mismatches so it is doubtful the extension cable, even with a less than perfect splice, would cause erratic operation. A more likely culprit would be a malfunctioning vampire tap on the new cable.
None of the above.
Answer B is correct. We know that even a slight impedance mismatch will reflect signals on any cable. Dissimilar cables, joined with a splice, would be an invitation for disaster. So answer B would be the most likely candidate. A splice would have to be unbelievably awful to create an attenuation problem. Impedance mismatch yes, attenuation, not very likely, which would make answer A incorrect. No cable is tolerant of mismatched characteristics and besides that, vampire taps are not used on 10BASE-2 cable so C is also incorrect. Answer B is related to the material presented in the chapter and consistent with other examples. So answer D can be ruled out.