CCNA Network Infrastructure and Connectivity Practice Test 18 — 15 Questions

Question 1

A network engineer is troubleshooting a link between two Cisco Catalyst 9300 switches that are connected via a single-mode fiber optic cable. The link is up, but the interface counters show a high number of CRC errors and frame check sequence (FCS) errors. The interface is configured for 1000 Mbps and full duplex on both ends. What is the most likely cause of these errors?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Faulty or dirty fiber optic cable or connectors. CRC and FCS errors indicate corruption at the data-link layer, typically caused by physical-layer issues such as signal degradation. Since the link is up and both ends are configured for 1000 Mbps full duplex, a speed/duplex mismatch is ruled out. Dirty or faulty single-mode fiber connectors or the cable itself can introduce optical signal loss or reflections, leading to bit errors that manifest as CRC/FCS errors.

Answer

A speed or duplex mismatch between the two switches

Answer

Incorrect SFP+ module type inserted in the switch

Answer

Excessive cable length beyond the distance limit

Question 2

Which TWO statements about fiber optic cables and SFP/SFP+ transceivers are correct?

Accepted Answer

Single-mode fiber (SMF) uses a smaller core diameter than multimode fiber (MMF).. Single-mode fiber (SMF) has a core diameter of about 9 microns, which is smaller than multimode fiber (MMF) cores of 50 or 62.5 microns. Option C is correct because SFP+ transceivers are indeed designed for 10 Gigabit Ethernet, supporting data rates up to 10 Gbps. Option B is wrong because multimode fiber is used for shorter distances (up to a few hundred meters) due to higher modal dispersion, while single-mode fiber supports longer distances (tens of kilometers). Option D is wrong because standard SFP modules support up to 1 Gbps, not 10 Gbps; SFP+ modules handle 10 Gbps. Option E is wrong because a 9-micron core is characteristic of single-mode fiber, not multimode fiber.

Answer

Multimode fiber supports longer distances than single-mode fiber.

Answer

SFP modules support data rates up to 10 Gbps.

Answer

Multimode fiber typically has a core diameter of 9 microns.

Question 3

Which TWO statements about fiber optic cable types and SFP transceivers are correct?

Accepted Answer

Multimode fiber is commonly used in campus and data center environments for distances up to several hundred meters.. Multimode fiber (MMF) has a larger core (typically 50/125 µm or 62.5/125 µm) and is used for short-reach, cost-effective connections up to a few hundred meters, making option B correct. Single-mode fiber (SMF) uses a much smaller core (~9 µm) and laser-based optics to achieve distances of 10 km or more, so option D is correct. Option A is wrong because it reverses the core size characteristic: SMF has a smaller core, and the given dimensions (62.5/125 µm) refer to multimode. Option C is wrong because SFP+ is a 10 Gbps form factor and is not inherently backward compatible with 1 Gbps SFP modules unless the switch port explicitly supports dual-rate operation. Option E is incorrect because fiber optic cables are inherently immune to electromagnetic interference (EMI) and therefore never require EMI shielding in any environment; the false statement claims they need shielding.

Answer

Single-mode fiber uses a larger core (typically 62.5/125 µm) and supports longer distances than multimode fiber.

Answer

SFP+ transceivers support data rates up to 1 Gbps and are backward compatible with standard SFP modules.

Answer

Fiber optic cables must be run with EMI shielding when deployed in environments with high electromagnetic interference.

Question 4

Drag and drop the cable/transceiver types on the left to the correct descriptions on the right.

Accepted Answer

Cat5e: Ethernet cable supporting up to 1 Gbps at 100 MHz. Cat5e supports up to 1 Gbps at 100 MHz, standard for 1000BASE-T over 100 meters. Cat6 supports up to 10 Gbps at 250 MHz but is limited to 55 meters for 10GBASE-T. Cat6a supports 10 Gbps at 500 MHz over full 100 meters. Single-mode fiber uses laser light for long distances, typically beyond 1 km.

Question 5

Drag and drop the cable/transceiver types on the left to the correct distance and speed descriptions on the right.

Accepted Answer

Cat5e UTP: 100 meters, 1 Gbps. Option A is correct: Cat5e UTP supports 1000BASE-T at 100 m and 1 Gbps. Option B is incorrect: Cat6 supports 10GBASE-T only up to 55 m; it requires Cat6A for 100 m at 10 Gbps. Option C is correct: 1000BASE-LX over single-mode fiber has a standard reach of 5 km at 1 Gbps. Option D is correct: 1000BASE-SX over 50 µm multimode fiber has a standard reach of 550 m at 1 Gbps.

Answer

Cat6 UTP: 100 meters, 10 Gbps

Question 6

A network technician is troubleshooting a connectivity issue where a host cannot communicate with a remote server. The technician notices that frames are being dropped at an intermediate switch. At which OSI model layer does the switch primarily operate, and what is the Protocol Data Unit (PDU) used at that layer?

Accepted Answer

Layer 2; frames. Switches primarily operate at Layer 2 (Data Link layer) of the OSI model, where they make forwarding decisions based on MAC addresses. The Protocol Data Unit (PDU) at this layer is the frame, which includes the MAC header, payload, and trailer. When frames are dropped at an intermediate switch, it indicates a Layer 2 issue such as a MAC address table problem, VLAN mismatch, or duplex mismatch.

Answer

Layer 1; bits

Answer

Layer 3; packets

Answer

Layer 4; segments

Question 7

A network engineer is troubleshooting a link between two Cisco Catalyst 9300 switches that are connected via a single-mode fiber optic cable. The link is up, but the interface counters show a high number of CRC errors and frame check sequence (FCS) errors. The interface is configured for 1000 Mbps and full duplex on both ends. What is the most likely cause of these errors?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

PDU is a segment; IP address is added at the Network layer.. At the Transport layer, TCP divides data into segments, so the PDU is called a segment, making B correct. The destination IP address (192.168.2.10) is added at the Network layer, where the IP header encapsulates the segment into a packet. Option A is wrong because a frame is a Data Link layer PDU, and IP addresses are not added at that layer. Option C is wrong because the PDU at the Transport layer is a segment, not a packet, and IP addresses are added at the Network layer, not the Transport layer. Option D is wrong because 'datagram' typically refers to UDP’s Transport layer PDU (not TCP), and IP addresses are not added at the Transport layer.

Answer

PDU is a frame; IP address is added at the Data Link layer.

Answer

PDU is a packet; IP address is added at the Transport layer.

Answer

PDU is a datagram; IP address is added at the Transport layer.

Question 8

A network technician is troubleshooting a connectivity issue where a user's email client cannot send messages, but the client can receive emails. The technician uses a protocol analyzer and sees that the client is successfully resolving the mail server's domain name to an IP address and establishing a TCP connection, but the server responds with an application-layer error. At which layers of the OSI model are the problem and the successful operations occurring, respectively?

Accepted Answer

The problem is at the Application layer (Layer 7); successful operations are at the Application layer (Layer 7) and Transport layer (Layer 4).. The problem is at the Application layer (Layer 7) because the email client can resolve the domain name (DNS, Layer 7), establish a TCP connection (Transport layer, Layer 4), but the mail server returns an application-layer error (e.g., SMTP 550 or 554), indicating the issue lies in the email protocol itself (e.g., authentication failure, mailbox full, or rejected sender). Successful operations include DNS resolution (Application layer) and TCP three-way handshake (Transport layer), confirming layers 7 and 4 are functioning correctly.

Answer

The problem is at the Transport layer (Layer 4); successful operations are at the Application layer (Layer 7) only.

Answer

The problem is at the Network layer (Layer 3); successful operations are at the Data Link layer (Layer 2) and Physical layer (Layer 1).

Answer

The problem is at the Presentation layer (Layer 6); successful operations are at the Session layer (Layer 5) and Transport layer (Layer 4).

Question 9

A network administrator is troubleshooting a connectivity issue between two hosts on different subnets. The administrator captures packets on the source host and notices that the frames contain the correct source and destination MAC addresses but the encapsulated packets have incorrect source and destination IP addresses. According to the OSI model, which layer is most likely responsible for this issue?

Accepted Answer

Network Layer (Layer 3). The Network Layer (Layer 3) is responsible for logical addressing (IP addresses) and routing packets between different subnets. Since the captured frames have correct MAC addresses (Layer 2) but incorrect source and destination IP addresses, the issue lies in how the IP headers are being constructed or assigned, which is a Layer 3 function. This could be caused by misconfigured IP addresses, subnet masks, or default gateways on the source host.

Answer

Physical Layer (Layer 1)

Answer

Data Link Layer (Layer 2)

Answer

Transport Layer (Layer 4)

Question 10

Which TWO statements correctly describe the OSI model layers and their corresponding PDU names during encapsulation?

Accepted Answer

The Transport layer encapsulates data into segments.. Both A and B are correct. During encapsulation, the Transport layer (Layer 4) adds a header to upper-layer data to form segments, and the Network layer (Layer 3) encapsulates those segments into packets by adding a network-layer header (e.g., IP). Option C is incorrect because the Data Link layer encapsulates packets into frames, not bits; bits are the raw transmission unit at the Physical layer. Option D is incorrect because the Session layer manages sessions but does not produce a PDU called 'sessions'; encapsulation of segments only occurs at Layers 4 and below. Option E is incorrect because the Physical layer transmits frames as bits but does not perform encapsulation; encapsulation ends at the Data Link layer.

Answer

The Data Link layer encapsulates packets into bits.

Answer

The Session layer encapsulates segments into sessions.

Answer

The Physical layer encapsulates frames into bits.

Question 11

Which TWO statements correctly describe the encapsulation process at the OSI model Transport layer?

Accepted Answer

It breaks data into smaller units called segments (for TCP) or datagrams (for UDP).. At the Transport layer (Layer 4), TCP segments data into segments and UDP segments data into datagrams. This segmentation allows efficient transmission and reassembly at the destination. The Transport layer header includes source and destination port numbers to identify the communicating applications, not IP or MAC addresses.

Answer

It adds source and destination IP addresses to the data.

Answer

It adds a header that includes source and destination MAC addresses.

Answer

It converts data into bits for transmission over the physical medium.

Question 12

Which TWO statements accurately describe the responsibilities of the OSI model's Transport layer?

Accepted Answer

It segments data from the upper layers and manages end-to-end flow control.. The Transport layer (Layer 4) segments data from upper layers and manages end-to-end flow control using mechanisms like TCP's sliding window to prevent overwhelming a slow receiver. Option D is correct because the Transport layer provides either reliable delivery (TCP) or unreliable delivery (UDP) between applications on different hosts, ensuring data reaches the appropriate application via port numbers. Option A is incorrect because logical addressing and routing are functions of the Network layer (Layer 3). Option C is incorrect because converting data into electrical signals is the responsibility of the Physical layer (Layer 1). Option E is incorrect because encapsulating data into frames with MAC addresses is a function of the Data Link layer (Layer 2).

Answer

It provides logical addressing and routing to determine the best path for data.

Answer

It converts data into electrical signals for transmission over the physical medium.

Answer

It encapsulates data into frames and adds source and destination MAC addresses.

Question 13

A network engineer is troubleshooting intermittent connectivity on an access switch port connected to a server. The output of 'show interfaces gigabitEthernet 1/0/24' shows an increasing number of runts and giants, but no CRC errors. The 'show interfaces status' command indicates the port is in 'err-disabled' state every few hours and must be manually re-enabled. What is the most likely cause of this issue?

Accepted Answer

Faulty cable or connector causing physical layer errors. Runts and giants without CRC errors indicate a physical-layer issue that corrupts the frame preamble or interframe gap but not the actual data payload. A faulty cable or connector can cause signal degradation leading to these framing errors. The intermittent err-disabled state is typically triggered by link-flap (repeated link up/down events) caused by the unstable physical connection, not directly by alignment or frame-check error counters.

Answer

Duplex mismatch between the switch port and the server NIC

Answer

Incorrect VLAN configuration on the switch port

Answer

Speed mismatch between the switch port and the server NIC

Question 14

A network engineer notices that users on VLAN 100 are experiencing intermittent connectivity to the server farm. The switch connecting these users shows no errors on the uplink interface, but the server farm switch reports a high number of input errors on its connected interface. The engineer runs 'show controllers' on the server farm switch. What is the most likely cause of the issue?

Accepted Answer

The SFP module is faulty or incompatible with the cable type.. The 'show controllers' command on the server farm switch reveals physical-layer issues such as framing errors, CRC errors, or alignment errors, which are often caused by faulty or incompatible SFP modules. Since the uplink interface on the user switch shows no errors, the problem is isolated to the server farm switch's interface, and a faulty SFP can introduce signal degradation or electrical issues without necessarily causing complete link failure. Option B is correct because SFP incompatibility or defects commonly produce input errors at the physical layer, even when the link appears up.

Answer

The interface is configured with the wrong duplex setting.

Answer

The cable is too long, causing attenuation.

Answer

Auto-negotiation is disabled, causing a speed mismatch.

Question 15

Which TWO of the following are valid interpretations of errors seen in the output of the 'show interface' command?

Accepted Answer

CRC errors indicate that frames were received with an invalid checksum, often due to cabling issues.. CRC errors (option A) indicate frames with an invalid checksum, often due to cabling issues, which is correct. Option D is also correct: 'Input errors' is a cumulative counter that includes runts, giants, CRC errors, and frame errors. Option B is wrong because runts are frames smaller than 64 bytes, not larger. Option C is wrong because giants are frames larger than the maximum allowed size (typically 1518 bytes), not smaller than 64 bytes. Option E is wrong because 'flaps' refer to an interface going up and down repeatedly, not necessarily physically disconnected; it could be due to duplex mismatch or other reasons.

Answer

Runts are frames that are larger than the maximum allowed size.

Answer

Giants are frames that are smaller than 64 bytes.

Answer

Flaps indicate that the interface is physically disconnected.