CCNA Network Infrastructure and Connectivity Practice Test 26 — 15 Questions

Question 1

Two routers, R1 and R2, are connected via a serial link. The interface on R1 shows 'Serial0/0 is up, line protocol is down' and no pings succeed across the link. You check the configuration and notice R1 has 'encapsulation ppp' but R2's serial interface was mistakenly left at the default encapsulation hdlc.

Accepted Answer

The mismatched encapsulation types prevent the routers from forming a Layer 2 connection, so the line protocol remains down.. Option B is correct because the line protocol on a serial interface requires both ends to agree on the Layer 2 encapsulation type. R1 is configured with PPP (encapsulation ppp), while R2 defaults to HDLC. Since these encapsulations are incompatible, the routers cannot establish a valid Layer 2 connection, causing the line protocol to remain down despite the physical layer being up.

Answer

The IP addresses on the serial interfaces are in different subnets, so packets are dropped at Layer 3.

Answer

The serial cable is faulty, causing physical layer issues that trigger the line protocol down state.

Answer

The routers are missing a routing protocol configuration, so they cannot route traffic across the link.

Question 2

A network administrator notices that file transfers to a server are extremely slow, and on the switch interface connecting to the server, the output of 'show interfaces' indicates a high number of runts and CRC errors, but no collisions. Which of the following is the most likely cause?

Accepted Answer

The switch port is configured for full-duplex, but the server's NIC is set to half-duplex.. The combination of runts (frames smaller than 64 bytes) and CRC errors with zero collisions is a classic symptom of a duplex mismatch. When one side operates at full-duplex and the other at half-duplex, the half-duplex side will detect collisions and invoke its backoff algorithm, causing the full-duplex side to receive truncated frames (runts) and frames with invalid FCS (CRC errors). The switch interface statistics show no collisions because the switch port is full-duplex and does not detect collisions, while the server's half-duplex NIC is causing the corruption.

Answer

The cable connecting the server to the switch is faulty.

Answer

The switch port speed is set to 100 Mbps, but the server NIC is set to 10 Mbps.

Answer

The server's NIC driver is outdated, causing packet loss.

Question 3

A network technician configures a Windows 10 PC with a static IPv6 address of 2001:db8:acad:1::100/64 and a default gateway of 2001:db8:acad:2::1. The PC can communicate with other hosts in the 2001:db8:acad:1::/64 subnet, but it cannot access any resources on other subnets, even though IPv4 connectivity through the same network works normally. What is the most likely reason for this issue?

Accepted Answer

The PC's default gateway address is in a different subnet than the PC's IPv6 address.. The PC's IPv6 address is 2001:db8:acad:1::100/64, placing it in the 2001:db8:acad:1::/64 subnet. The configured default gateway is 2001:db8:acad:2::1, which belongs to the 2001:db8:acad:2::/64 subnet. For IPv6, a host will only consider a default gateway on the same link-local or on-link subnet; if the gateway address is not within the same /64 prefix as the host's address, the host cannot send packets to it directly, and all off-subnet traffic fails.

Answer

The PC's IPv6 stack has a corrupted binding that prevents routing.

Answer

The router's IPv6 routing table does not have a route back to the PC's subnet.

Answer

The DNS server for IPv6 resolution is misconfigured, causing all off-subnet traffic to fail.

Question 4

An administrator deploys a new WLAN on a Cisco 9800 WLC using WPA3-Personal (SAE) with AES encryption. A single 802.11ax laptop running Windows 10 fails to connect, displaying an authentication timeout despite entering the correct passphrase. Other clients, including legacy 802.11ac devices, connect without issue.

Accepted Answer

The laptop’s wireless adapter does not support Protected Management Frames, which are mandatory for WPA3-Personal.. WPA3-Personal (SAE) mandates the use of Protected Management Frames (PMF) as defined in IEEE 802.11w. If the laptop's wireless adapter or driver does not support PMF, it cannot complete the SAE handshake, resulting in an authentication timeout. Legacy 802.11ac clients can connect because they are using WPA2, which does not require PMF.

Answer

The WLC’s WLAN is misconfigured for WPA3-Enterprise, and the laptop lacks a supplicant for 802.1X authentication.

Answer

The WLC has disabled 802.11ax OFDMA on the 5 GHz band, preventing the 802.11ax laptop from associating.

Answer

The laptop’s driver is configured for 160 MHz channel width, which is incompatible with the WLC’s channel plan, causing authentication to fail.

Question 5

A user reports they cannot access any network resources. A network administrator runs ipconfig on the user's Windows PC and sees an IPv4 address of 169.254.45.3/16. The administrator then pings the default gateway 10.0.0.1, which fails, and uses traceroute to 10.0.0.1, which shows only '1 * * * Request timed out.' What is the most likely cause of the problem?

Accepted Answer

The DHCP server is unreachable.. The 169.254.45.3/16 address is an Automatic Private IP Addressing (APIPA) address, assigned by Windows when a DHCP discovery fails. The failed ping and traceroute to the default gateway confirm that the PC has no IP connectivity to the network. Since the PC is on the same subnet as the DHCP server (10.0.0.0/24), the most likely cause is that the DHCP server is unreachable, preventing the PC from obtaining a valid IP address.

Answer

The PC's DNS server address is incorrect.

Answer

The switch port is in an administratively down state.

Answer

The Ethernet cable is unplugged from the PC.

Question 6

A network engineer replaces a failed 1000BASE-LX SFP on a core switch with a new transceiver of the same type. After connecting the single-mode fiber, the link remains down and a 'show interfaces gig1/0/49 transceiver' reveals an Rx power of –30 dBm, while the far-end SFP is transmitting at –3 dBm over a 2 km span. The fiber patch cord shows no visible damage.

Accepted Answer

Excessive attenuation due to a dirty or damaged fiber connector is preventing the link from coming up.. Option C is correct because the measured Rx power of –30 dBm is far below the receive sensitivity threshold for 1000BASE-LX (typically –19 to –22 dBm), even though the transmitter is outputting a healthy –3 dBm over only 2 km. This indicates excessive loss in the optical path, most commonly caused by a dirty or damaged fiber connector. Cleaning the connector ends with an appropriate fiber cleaning tool and inspecting with a microscope would likely resolve the issue.

Answer

The new SFP is a counterfeit Cisco transceiver that cannot establish a stable link.

Answer

The SFP is not fully seated in the switch port, causing an intermittent optical connection.

Answer

The single-mode fiber distance exceeds the 10 km maximum for 1000BASE-LX, leading to severe signal dispersion.

Question 7

A network administrator captures traffic on Server B and finds that ICMP echo requests from Host A arrive, and the server generates corresponding echo replies, but these replies never appear on the wire. The server's routing table has a valid default gateway, and no ACLs are blocking the traffic. What is the most likely cause?

Accepted Answer

The server's ARP cache contains an incorrect MAC address for the destination host.. The ICMP echo requests arrive at Server B, and the server generates replies, but they never appear on the wire. This indicates the server cannot deliver the frames to the next hop. Since the server has a valid default gateway and no ACLs are blocking, the most likely cause is an incorrect MAC address in the ARP cache for the destination (either the host or the default gateway). The server will encapsulate the IP packet into a frame using the wrong MAC, causing the switch to drop the frame or send it to the wrong device, so the reply never reaches the wire correctly.

Answer

The server's TCP stack is corrupt.

Answer

The server's routing table is missing a route to the source network.

Answer

The switch port connecting Server B has port security enabled and has learned a different MAC.

Question 8

A network engineer notices that users in VLAN 10 report intermittent connectivity and slow file transfers to a server on the same switch. The engineer issues the show interfaces fa0/1 command on the switch port connected to the server and observes a high number of runts, input errors, and CRC errors, while output errors are minimal. The interface configuration shows speed 100 and duplex full.

Accepted Answer

The server NIC is auto-negotiating to 100 Mbps half-duplex, resulting in a duplex mismatch.. Option B is correct because the symptoms—high runts, input errors, and CRC errors with minimal output errors—are classic indicators of a duplex mismatch. When the switch port is hardcoded to 100 Mbps full-duplex and the server NIC auto-negotiates to 100 Mbps half-duplex (a common fallback when one side is manually set), collisions occur on the full-duplex side, corrupting frames and causing input errors. The speed matches (both 100 Mbps), so the issue is purely duplex-related.

Answer

The server NIC is set to 10 Mbps half-duplex, causing a speed mismatch.

Answer

The cable connecting the server to the switch is faulty, introducing excessive noise.

Answer

The switch port is configured with an incorrect native VLAN, causing duplex negotiation issues.

Question 9

A network administrator has recently upgraded the corporate wireless LAN to support 802.11ax (Wi-Fi 6) and is using WPA3-Enterprise with a central WLC. Several users with new 802.11ax laptops report that they can connect to the SSID, but after a few minutes their connections drop and then re-establish, while legacy 802.11ac clients work without issues. Which action will resolve this problem?

Accepted Answer

Enable Protected Management Frames (PMF) as Required on the WLAN.. WPA3-Enterprise requires Protected Management Frames (PMF) to be set to 'Required' on the WLC. When PMF is not enabled or set to 'Optional', 802.11ax clients using WPA3 may experience intermittent disconnects because management frame protection is mandatory for WPA3 operation. Legacy 802.11ac clients using WPA2 do not require PMF, so they remain unaffected.

Answer

Downgrade the WLAN security to WPA2-Enterprise for backward compatibility.

Answer

Disable OFDMA and MU-MIMO on the WLC for the affected APs.

Answer

Adjust the 5 GHz channel width from 80 MHz to 40 MHz to avoid interference.

Question 10

A user reports that they cannot access a remote server at IP address 10.10.20.50. The user's PC has IP address 192.168.1.25/24, and the default gateway is 192.168.1.1. The user can successfully ping the default gateway and other hosts on the local subnet. However, pings to 10.10.20.50 fail, and a traceroute shows only the first hop (192.168.1.1) followed by timeouts. Which of the following is the most likely cause?

Accepted Answer

The default gateway lacks a route to the 10.10.20.0/24 network.. The user can reach local hosts and the default gateway, confirming that the PC's IP configuration and local switching are functional. The traceroute stopping at 192.168.1.1 with subsequent timeouts indicates that the default gateway receives the packets but does not know how to forward them to the 10.10.20.0/24 network. Therefore, the most likely cause is that the default gateway lacks a route to that remote subnet.

Answer

The user's PC has an incorrect subnet mask.

Answer

The remote server at 10.10.20.50 is powered off.

Answer

The user's DNS server is unreachable.

Question 11

Which TWO statements about SFP transceivers and their associated cable types are correct?

Accepted Answer

SFP-10G-LR transceivers use 1310 nm wavelength and can reach up to 10 km over single-mode fiber.. Option B is correct because the SFP-10G-LR transceiver operates at a 1310 nm wavelength and is designed for single-mode fiber, supporting distances up to 10 km. This is a standard specification defined by IEEE 802.3ae for 10GBASE-LR, making it the accurate description of this transceiver's capabilities.

Answer

SFP-10G-SR transceivers require single-mode fiber.

Answer

SFP-10G-LR transceivers can only be used with multimode fiber.

Answer

SFP-10G-LR transceivers support distances up to 40 km over single-mode fiber.

Question 12

Which TWO statements are correct regarding Protocol Data Units (PDUs) and data encapsulation in the OSI model?

Accepted Answer

Data is encapsulated with a TCP header at the Transport layer to form a segment.. Option B is correct because at the Transport layer, the TCP protocol adds a TCP header to the payload data, forming a segment. This encapsulation process is fundamental to reliable data delivery, as the TCP header includes source and destination ports, sequence numbers, and acknowledgment numbers for connection-oriented communication.

Answer

The PDU at the Transport layer is called a frame.

Answer

The Data Link layer adds a header and trailer to create a bit.

Answer

Encapsulation occurs as data moves up the OSI layers from Physical to Application.

Question 13

A network engineer notices that a workstation in VLAN 10 cannot communicate with hosts in VLAN 20. The workstation is connected to an access layer switch port that shows 'up/up' in show ip interface brief. The switch's trunk port to the router is up, and the router's sub-interface for VLAN 10 is also in an up/up state. The router-on-a-stick configuration appears operational, but inter-VLAN traffic still fails. What is the most likely cause?

Accepted Answer

The router sub-interface for VLAN 10 is missing the encapsulation dot1q command.. The router-on-a-stick configuration requires each sub-interface to use the `encapsulation dot1q <vlan-id>` command to tag traffic with the correct VLAN ID. Without this command, the sub-interface will not accept or forward frames tagged for VLAN 10, even if the interface is administratively up. Since the workstation in VLAN 10 can reach its local gateway but not VLAN 20, the missing encapsulation is the most likely cause.

Answer

The switch port to the workstation is incorrectly configured as a trunk instead of an access port.

Answer

The native VLAN on the trunk between the switch and router is mismatched.

Answer

VLAN 10 is not allowed on the trunk link.

Question 14

A network engineer notices that the router interface GigabitEthernet0/1 is in an 'administratively down' state in the output of the show ip interface brief command, preventing connectivity to the subnet connected to that interface. What is the most likely cause?

Accepted Answer

The shutdown command is configured on the interface.. The 'administratively down' state in the output of 'show ip interface brief' indicates that the interface has been manually disabled using the 'shutdown' command. This is a Layer 1/2 administrative state, not a protocol or connectivity issue. To bring the interface up, the 'no shutdown' command must be applied in interface configuration mode.

Answer

The interface does not have an IP address assigned.

Answer

The interface is configured with an incorrect subnet mask.

Answer

An access list is blocking traffic on the interface.

Question 15

A host cannot communicate with its default gateway. The technician uses the show arp command on the host and sees that the ARP entry for the gateway IP is incomplete. The technician has already verified that the Ethernet cable is securely connected and the switch port is active. What should the technician do next?

Accepted Answer

Check the switch port’s VLAN configuration and ensure the host and router interface are in the same VLAN.. An incomplete ARP entry for the default gateway indicates that the host sent an ARP request but never received a reply. Since the physical layer (cable and switch port) is verified as operational, the most likely cause is a Layer 2 mismatch: the host and the router interface are in different VLANs, preventing the ARP reply from reaching the host. Checking the switch port's VLAN configuration ensures both devices are in the same broadcast domain, which is required for ARP to function.

Answer

Replace the Ethernet cable between the host and the switch.

Answer

Clear the ARP cache on the host and attempt to ping the gateway again.

Answer

Check the router’s routing table for a route to the host’s subnet.