CCNA Network Infrastructure and Connectivity Practice Test 19 — 15 Questions

Question 1

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?

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

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 2

A network engineer is troubleshooting a connectivity issue between two hosts on different subnets. The sending host has constructed a packet with a destination IP address of 192.168.2.10. As the packet travels down the OSI model layers on the sending host, which Protocol Data Unit (PDU) name is assigned to the data at the Transport layer after TCP segments are created, and at which layer does the IP address get encapsulated?

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 3

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?

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

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 4

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 5

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 6

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 7

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 8

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 9

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 10

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.

Question 11

Which TWO interface errors are most likely caused by a mismatch in duplex settings between two connected switches?

Accepted Answer

Runts. A duplex mismatch occurs when one switch operates at full duplex while the other operates at half duplex. On the half-duplex side, frames arriving while the interface is transmitting are considered collisions, causing the frame to be truncated into fragments (runts). On the full-duplex side, the switch does not detect collisions but may receive incomplete frames, which are counted as runts if they are less than 64 bytes. CRC errors also spike because the truncated or corrupted frames fail the Frame Check Sequence (FCS) validation.

Answer

Giants

Answer

Input errors

Answer

Output errors

Answer

Flaps

Question 12

Which TWO statements correctly describe the causes or implications of CRC errors, runts, giants, or output errors as seen in the output of 'show interface' or 'show interface status'?

Accepted Answer

A high number of runts on an interface typically indicates excessive collisions or a faulty NIC.. Runts—frames smaller than 64 bytes—often result from collisions truncating frames on half-duplex links or a faulty NIC. Option D is correct because duplex mismatch can cause late collisions, which appear as output errors in 'show interface'; a device on one side full-duplex and the other half-duplex leads to collisions and framing errors. Option A is wrong because CRC errors can stem from faulty cabling, interference, or a mismatched NIC, not exclusively a bad switch port. Option C is wrong because giants (frames over maximum MTU) may be forwarded if the interface is configured with jumbo frames or the switch is set to accept oversize frames. Option E is wrong because 'show controllers' displays frame-size errors like runts and giants, including details beyond CRC errors.

Answer

CRC errors are always caused by a faulty switch port and require port replacement.

Answer

Giants are frames that exceed the maximum transmission unit (MTU) and are always discarded by the switch.

Answer

The 'show controllers' command provides a detailed view of CRC errors but does not show runts or giants.

Question 13

A network engineer is troubleshooting an issue where a Windows 10 workstation (Host-A) cannot reach the internet, but can ping the local default gateway. The engineer runs 'ipconfig /all' on Host-A and reviews the output. What is the most likely cause of the problem?

Accepted Answer

The DNS server is configured as a public DNS server that may be unreachable due to network policy or firewall.. Host-A can ping the default gateway, which confirms that Layer 3 connectivity to the local network is working and that the subnet mask and default gateway are correctly configured. The inability to reach the internet despite this connectivity points to a name resolution failure, likely caused by an incorrect or unreachable DNS server. A public DNS server (e.g., 8.8.8.8) may be blocked by corporate firewall policy, preventing Host-A from resolving internet domain names.

Answer

The subnet mask is incorrect.

Answer

The default gateway is missing or incorrect.

Answer

The host has obtained an APIPA address (169.254.x.x).

Question 14

A network administrator is troubleshooting an IPv6 connectivity issue on a newly deployed router. The router's G0/0/0 interface is configured with an IPv6 address using EUI-64, but hosts on that subnet cannot reach the router's link-local address. The administrator runs 'show ipv6 interface g0/0/0' and sees that the interface is up/up but the IPv6 address is not in the expected format. What is the most likely cause of the problem?

Accepted Answer

The IPv6 address was not configured correctly; the 'ipv6 address' command was likely omitted or misconfigured.. The router's G0/0/0 interface is up/up, but the IPv6 address is not in the expected EUI-64 format. This indicates that the 'ipv6 address' command was likely omitted entirely, because if it were simply misconfigured (e.g., without the `eui-64` keyword), the router would still automatically generate a link-local address, and hosts would be able to reach it. Since hosts cannot reach the link-local address, IPv6 is not enabled on the interface at all. The correct configuration requires the `ipv6 address` command with the appropriate prefix and the `eui-64` keyword.

Answer

The interface is administratively down.

Answer

The MAC address of the interface is invalid, preventing EUI-64 from generating a proper address.

Answer

The router is not sending Router Advertisements, so hosts cannot autoconfigure.

Question 15

Which TWO statements correctly describe IPv4 and IPv6 host configuration?

Accepted Answer

APIPA assigns an IPv4 address in the 169.254.0.0/16 range when a DHCP server is unavailable.. APIPA (Automatic Private IP Addressing) automatically assigns an IPv4 address from the 169.254.0.0/16 range when a DHCP server is unavailable, enabling local subnet communication without manual configuration. Option E is correct because the ipconfig command on Windows displays both IPv4 and IPv6 addresses, subnet masks, default gateways, and DNS servers. Option B is incorrect because while IPv6 link-local addresses start with FE80::/10, they are not always assigned using EUI-64; they can also be randomly generated (privacy extensions) or manually configured. Option C is incorrect because EUI-64 generates the interface identifier (host portion) of an IPv6 address from the MAC address, not the network prefix — the prefix is provided via SLAAC, DHCPv6, or manual configuration. Option D is incorrect because a host can have multiple default gateways configured for redundancy, though only one is active at a time per routing table; additionally, IPv4 and IPv6 default gateways are independent and can coexist.

Answer

IPv6 link-local addresses are always assigned using EUI-64 and start with FE80::/10.

Answer

EUI-64 is used to generate the network prefix of an IPv6 address from the interface's MAC address.

Answer

A host can have only one default gateway configured at a time for both IPv4 and IPv6.