CCNA 640-802 Ver 22-09-2008 page12

QUESTION 111

Which two statements describe the operation of the CSMA/CD access method, which is in use on the Certkiller network? (Choose two)

A. After a collision, the station that detected the collision has first priority to resend the lost data.

B. In a CSMA/CD collision domain, stations must wait until the media is not in use before transmitting.

C. In a CSMA/CD collision domain, multiple stations can successfully transmit data simultaneously.

D. The use of hubs to enlarge the size of collision domains is one way to improve the operation of the CSMA/CD access method.

E. After a collision, all stations run a random backoff algorithm. When the backoff delay period has expired, all stations have equal priority to transmit data.

F. After a collision, all stations involved run an identical backoff algorithm and then synchronize with each other prior to transmitting data.

Answer: B, E

Explanation:

Ethernet networking uses Carrier Sense Multiple Access with Collision Detect (CSMA/CD), a protocol that helps devices share the bandwidth evenly without having two devices transmit at the same time on the network medium. CSMA/CD was created to overcome the problem of those collisions that occur when packets are transmitted simultaneously from different nodes. And trust me, good collision management is crucial, because when a node transmits in a CSMA/CD network, all the other nodes on the network receive and examine that transmission. Only bridges and routers can effectively prevent a transmission from propagating throughout the entire network! So, how does the CSMA/CD protocol work? Like this: when a host wants to transmit over the network, it first checks for the presence of a digital signal on the wire. If all is clear (no other host is transmitting), the host will then proceed with its transmission. But it doesn't stop there. The transmitting host constantly monitors the wire to make sure no other hosts begin transmitting. If the host detects another signal on the wire, it sends out an extended jam signal that causes all nodes on the segment to stop sending data (think, busy signal). The nodes respond to that jam signal by waiting a while before attempting to transmit again. Backoff algorithms determine when the colliding stations can retransmit. If collisions keep occurring after 15 tries, the nodes attempting to transmit will then time out.

QUESTION 112

Which three statements are true about the operation of a full-duplex Ethernet network, which is being used in the Certkiller LAN? (Choose three)

A. Ethernet hub ports are preconfigured for full-duplex mode.

B. The host network card and the switch port must be capable of operating in full-duplex mode.

C. There are no collisions in full-duplex mode.

D. In a full-duplex environment, the host network card must check for the availability of the network media before transmitting.

E. A dedicated switch port is required for each full-duplex node.

Answer: B, C, E

Explanation:

Half-duplex Etherne

t is defined in the original 802.3 Ethernet and Cisco says you only use one wire pair with a digital signal running in both directions on the wire. It also uses the CSMA/CD protocol to help prevent collisions and to permit retransmitting if a collision does occur. If a hub is attached to a switch, it must operate in half-duplex mode because the end stations must be able to detect collisions. Half-duplex Ethernet-typically 10BaseT-is only about 30 to 40 percent efficient as Cisco sees it, because a large 10BaseT network will usually only give you 3- to 4Mbps-at most. Full-duplex Ethernet uses two pairs of wires, instead of one wire pair like half duplex. Also, full duplex uses a point-to-point connection between the transmitter of the transmitting device and the receiver of the receiving device, which means that with full-duplex data transfer, you get a faster data transfer compared to half duplex. And because the transmitted data is sent on a different set of wires than the received data, no collisions occur. The reason you don't need to worry about collisions is because now Full-duplex Ethernet is like a freeway with multiple lanes instead of the single-lane road provided by half duplex. Full-duplex Ethernet is supposed to offer 100 percent efficiency in both directions; this means you can get 20Mbps with a 10Mbps Ethernet running full duplex, or 200Mbps for FastEthernet.

QUESTION 113

Exhibit:

You work as a network technician at Certkiller .com. Please study the exhibit carefully. Which statement describes the operational state of the Certkiller 2 FastEthernet 0/0 interface?

A. The interface is operational and currently handling traffic.

B. The interface has failed because of a media error.

C. The interface requires a no shutdown command to be issued.

D. The interface is

generating protocol errors.

E. None of the above.

Answer: A

Explanation:

The interface is up indicates whether the interface hardware is currently active and whether it has been taken down by an administrator. "Disabled" indicates that the router hasreceived more than 5,000 errors in a keepalive interval, which is 10 seconds, by default. The line protocol indicates whether the software processes that handle the line protocol believe that the interface is usable (that is, whether keepalives are successful) or if it has been taken down by an administrator. For a complete list of the field descriptions of this command as well as troubleshooting information for Ethernet interfaces refer to the reference link listed below. Reference: "Troubleshooting Ethernet"

QUESTION 114

The Certkiller network administrator wants to use a router named CK1 to segment the local network. What are some of the advantages of using CK1 to segment the network? (Choose two)

A. Routers generally cost les

s than switches

B. Filtering can occur based on Layer 3 information

C. Broadcasts are not forwarded across the router

D. Broadcasts are eliminated

E. Adding a router to the network decreases latency

F. Routers can generally support more LAN ports than switches

Answer: B, C

Explanation:

When the router's interface receives the broadcast, it discard the broadcast without forwarding it on to other networks. Even though routers are known for breaking up broadcast domains by default, it's important to remember that they break up collision domains as well. There are two advantages of using routers in your network:

* They don't forward broadcasts by default.

* They can filter the network based on layer 3 (Network layer) information (e.g., IP address) by using IOS based firewall ie.
ACL Four router functions in your network can be listed as follows:

* Packet switching

* Packet filtering

* Internetwork communication

* Path selection

QUESTION 115

Which of the following devices can the Certkiller network administrator use to segment the LAN? (Choose all that apply)

A. Hubs

B. Repeaters

C. Switches

D. Bridges

E. Routers

F. Media Converters

G. All of the above

Answer: C, D, E

Explanation:

Switches and bridges forward broadcast but routers do not forward broadcasts by default (they can via the "ip helper-address" command). Switches, bridges and routers can segment an Ethernet collision domain via the use of VLAN's

Incorrect Answers:

A. Hubs is incorrect because a hub doesn't segment a network, it only allows more hosts on one. Hubs operate at layer one, and is used primarily to physically add more stations to the LAN.

B. This also incorrect because the job of a repeater is to repeat a signal so it can exceed distance limitations. It also operates at layer one and provides no means for logical LAN segmentation.

F. This is incorrect because media converters work by converting data from a different media type to work with the media of a LAN. It also operates at layer one and provides no means for logical LAN segmentation.

QUESTION 116

Within the Certkiller office, The Sales and Production networks are separated by a router as shown in the diagram below:

Which of the following statements most accurately describes the characteristics of the above networks broadcast and collision domains? (Select the two best answer choices)

A. There are two broadcast domains in the network.

B. There are four broadcast domains in the network.

C. There are six broadcast domains in the network.

D. There are four collision domains in the network.

E. There are five collision domains in the network.

F. There are seven collision domains in the network.

Answer: A, F

Explanation:

In this network we have a hub being used in the Sales department, and a switch being used in the Production department. Based on this, we have two broadcast domains: one for each network being separated by a router. For the collision domains, we have 5 computers and one port for E1 so we have 6 collision domains total because we use a switch in the Production Department so 5 are created there, plus one collision domain for the entire Sales department because a hub is being used.

QUESTION 117

The Certkiller corporate LAN consists of one large flat network. You decide to segment this LAN into two separate networks with a router. What will be the affect of this change?

A. The number of broadcast domains will be decreased.

B. It will make the broadcasting of traffic between domains more efficient between segments.

C. It will increase the number of collisions.

D. It will prevent segment

1's broadcasts from getting to segment 2.

E. It will connect segment 1's broadcasts to segment 2.

Answer: D

Explanation

A router does not forward broadcast traffic. It therefore breaks up a broadcast domain, reducing unnecessary network traffic. Broadcasts from one segment will not be seen on the other segment.

Incorrect Answers:

A. This will actually increase the number of broadcast domains from one to two.

B. All link level traffic from segment one to segment two will now need to be routed between the two interfaces of the router. Although this will reduce the traffic on the LAN links, it does also provide a less efficient transport between the segments.

C. Since the network size is effectively cut into half, the number of collisions should decrease dramatically.

E. Broadcasts from one segment will be completely hidden from the other segment.

QUESTION 118

You want to influence the root bridge selection within the Certkiller LAN. Which two values are used by spanning tree protocol to elect a root bridge? (Choose two.)

A. Amount of RAM

B. Speed of the links

C. IOS Version

D. MAC Address

E. Bridge Priority

F. IP Address

G. Router ID

Answer: D, E

Explanation:

Switch having lowest bridge ID will become the root bridge. The bridge ID is how STP keeps track of all the switches in the network. It is determined by a combination of the bridge priority (32,768 by default on all Cisco switches) and the base MAC address. The bridge with the lowest bridge ID becomes the root bridge in the network.

QUESTION 119

A Certkiller Ethernet switch receives a unicast frame with a destination MAC that IS listed in the switch table. What will this switch do with the frame?

A. The switch will forward the frame to a specific port

B. The switch will forward the frame to all ports except the port on which it was received

C. The switch will return a copy of the frame out the source port

D. The switch will not forward unicast frames

E. The switch will remove the destination MAC from the switch table

F. None of the above.

Answer: A

Explanation:

How Does the Switch Find Host MACs? Let's use the diagram below to help us understand how address learning process takes place.

Let's assume that we have just powered on the switch. It has nothing in its MAC table. We connect the cables from the hosts on the switch interfaces as shown in the diagram. Host A initiates a connection towards Host D, and the following takes place:

1. Host A (interface fe0/0) sends a frame to Host D (MAC address:0000.43c5.334c).

2. The switch inspects the Source Address in the frame and notes in its table the MAC address of Host A along with the Interface number from which the frame originated.

3. The switch inspects the Destination Address in the frame. Since it does not have Hosts D MAC address in its table, it constructs a broadcast frame and forwards out all interfaces except the interface from where the original frame arrived.

4. Host D identifies itself as the expected recipient and responds back to Host A. The switch receives the respond frame on interface fe0/11 and places the SA in its table along with the interface number where the frame came from.

5. From now on, further communication between the two hosts will be switched to the appropriate interfaces based on the MAC tables entries. This process takes place every time a new host is attached on the switch and initiates traffic. The switch tries to keep its MAC table up-to-date, therefore if some hosts do not initiate traffic for a certain amount of time, the switch removes them from its table and reinserts them when they begin sending traffic.

QUESTION 120

In which circumstance are multiple copies of the same unicast frame likely to be transmitted in the Certkiller Switched LAN?

A. During high traffic periods

B. In an improperly implemented redundant topology

C. After broken links are re-established

D. When upper-layer protocols require high reliability

E. When a dual ring topology is in use

F. None of the above

Answer: B

Explanation:

A redundant topology eliminates single points of failure, but it also causes broadcast storms, multiple frame copies, and MAC address table instability problems. Multiple Frame Copies--when a new switch is added, the other switches may not have learned its correct MAC address. The host may send a unicast frame to the new switch. The frame is sent through several paths at the same time. The new switch will receive several copies of the frame. This causes MAC database instability. MAC database instability results when multiple copies of a frame arrive on different ports of a switch. Layer 2 has no mechanism to stop the loop. This is the main reason for the Spanning Tree Protocol(STP) IEEE 802.1d which was developed to prevent routing loops. If multiple connections between switches are created for redundancy purposes, network loops can occur in an improperly designed topology. Spanning Tree Protocol (STP) is used to stop network loops while still permitting redundancy.