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Basic Questions

June 2nd, 2017 Go to comments

Question 1

Explanation

Star topology is the most popular topology for the network which allows all traffic to flow through a central device.

star_topology.jpg

Question 2

Explanation

Carrier sense multiple access with collision detection (CSMA/CD) is a media access control method used most notably in early Ethernet (IEEE 802.3) technology for local area networking. When collision detection (CD) observes a collision (excess current above what it is generating, i.e. > 24 mA for coaxial Ethernet), it stops transmission immediately and instead transmits a 32-bit jam sequence.

Note: CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is a protocol for carrier transmission used in wireless networks. Unlike CSMA/CD (Carrier Sense Multiple Access/Collision Detect) which deals with transmissions after a collision has occurred, CSMA/CA acts to prevent collisions before they happen.

Question 3

Explanation

Each port of a router is a broadcast domain. This router has 2 ports so it has 2 broadcast domains.

Question 4

Question 5

Explanation

Ethernet frame size refers to the whole Ethernet frame, including the header and the trailer while MTU size refers only to Ethernet payload. Baby giant frames refer to Ethernet frame size up to 1600 bytes, and jumbo frame refers to Ethernet frame size up to 9216 bytes (according to this link: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-4000-series-switches/29805-175.html)

For example, standard Ethernet frame MTU is 1500 bytes. This does not include the Ethernet header and Cyclic Redundancy Check (CRC) trailer, which is 18 bytes in length, to make the total Ethernet frame size of 1518.

So according to strict definition, MTU size of 1600 cannot be classified as baby giant frames as the whole Ethernet frames will surely larger than 1600 -> Answer C is not correct.

Answer D is a better choice as the MTU is 1518, so the whole Ethernet frame would be 1536 (1518 + 18 Ethernet header and CRC trailer). This satisfies the requirement of baby giant frames “Baby giant frames refer to Ethernet frame size up to 1600 bytes”.

Question 6

Explanation

To understand more about TCP please read our TCP and UDP tutorial.

Note: Answer F is not correct because TCP does not require applications to determine the retranmission. TCP itself will determine if the data packets should be retransmitted or not.

Question 7

Explanation

Only router can break up broadcast domains so in the exhibit there are 2 broadcast domains: from e0 interface to the left is a broadcast domain and from e1 interface to the right is another broadcast domain -> A is correct.

Both router and switch can break up collision domains so there is only 1 collision domain on the left of the router (because hub doesn’t break up collision domain) and there are 6 collision domains on the right of the router (1 collision domain from e1 interface to the switch + 5 collision domains for 5 PCs in Production) -> F is correct.

Question 8

Explanation

Modern Ethernet networks built with switches and full-duplex connections no longer utilize CSMA/CD. CSMA/CD is only used in obsolete shared media Ethernet (which uses repeater or hub).

Question 9

Explanation

Each port of a switch is a separate collision domain. This switch uses two ports (for 5 hosts) so there are two collision domains in total.

Question 10

Explanation

At the end of each frame there is a Frame Check Sequence (FCS) field. FCS can be analyzed to determine if errors have occurred. FCS uses cyclic redundancy check (CRC) algorithm to detect errors in the transmitted frames. Before sending data, the sending host generates a CRC based on the header and data of that frame. When this frame arrives, the receiving host uses the same algorithm to generate its own CRC and compare them. If they do not match then a CRC error will occur.

Ethernet802.3_Frame_Format.jpg

Preamble is used to indicate the start of the frame by arranging the first 62 bits as alternating “1/0s” and the last two bits as “1”s. Like so, 010101010101010………………………10101011. Therefore when the receiving end sees the “11” it knows where the actual Ethernet header starts. The alternating 1s and 0s will also allow the two endpoints to sync their internal clocks. In summary, preamble is used for synchronization.

The “Type/Length” field is used to indicate the “Type”of the payload (Layer 3 protocol) which is indicated as a Hexadecimal value.

Note: Ethernet II uses “Type” while the old Ethernet version use “Length”

Question 11

Explanation

MDI stands for “Medium Dependent Interface”. MDI is a type of Ethernet port found on network devices. When connecting two devices with MDI ports (two hosts, for example) an Ethernet crossover cable is required. The crossover cable switches the send and receive ports on the two connectors, allowing data to flow correctly between two MDI ports.

Question 12

Explanation

Physical addresses or MAC addresses are used to identify devices at layer 2 -> A is correct.

MAC addresses are only used to communicate on the same network. To communicate on different network we have to use Layer 3 addresses (IP addresses) -> B is not correct; E is correct.

Layer 2 frame and Layer 3 packet can be recognized via headers. Layer 3 packet also contains physical address -> C is not correct.

On Ethernet, each frame has the same priority to transmit by default -> D is not correct.

All devices need a physical address to identify itself. If not, they can not communicate -> F is not correct.

Question 13

Question 14

Explanation

Only router can break up broadcast domains but in this exhibit no router is used so there is only 1 broadcast domain.

For your information, there are 7 collision domains in this exhibit (6 collision domains between hubs & switches + 1 collision between the two switches).

Comments (30) Comments
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  1. Student
    June 22nd, 2017

    I took the CCNA exam this week. When I took the simulation questions and pressed continue, it asked me am I sure I want to continue. I was told that once enter is continued y ou can’t return to previous questions. Can someone tell me if there was another button to push before hitting continue because I don’t think I got credit for my simulation answers. The simulation questions were the only questions I could go to the previous answer and go an message asking me If I would like to continue.

    If someone can answer this, please help. Send an answer to {email not allowed}

    Please reply.

  2. Ceros
    June 22nd, 2017

    why I only see then explanation to to the questions? the question it selve doesn’t shows up

  3. @Ceros
    June 23rd, 2017

    The website was asked to remove the questions probably because of Cisco’s request. But explanations dont count, so they are still there.

  4. 9tut
    June 23rd, 2017
  5. 9tut
    June 23rd, 2017

    @Student: In the sim, you have to look carefully to make sure you answered all the questions in that sim (there are small number boxes represented for each question in the sim) before pressing “Next” button. If you press “Next” then you will ignore all unanswered questions so be careful.

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