N10-009 Network Troubleshooting Practice Test 4 — 15 Questions

Question 1

A user reports that they cannot access the internal web server by its fully qualified domain name (intranet.company.com). The workstation's IP configuration shows a DNS server of 8.8.8.8, but the internal DNS server is 10.0.0.10. The user can successfully ping the server's IP address (10.0.0.50). What is the MOST likely cause of the issue?

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

Accepted Answer

A: The workstation is using the wrong DNS server address. The workstation's DNS server is set to 8.8.8.8 (a public Google DNS server), which cannot resolve the internal domain name 'intranet.company.com' because that zone is only hosted on the internal DNS server at 10.0.0.10. Since the user can ping the server's IP address (10.0.0.50), network connectivity is fine, confirming the issue is name resolution. The most likely cause is that the workstation is using the wrong DNS server address.

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B: The subnet mask on the workstation is incorrect

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C: The network cable is faulty

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D: The default gateway is misconfigured

Question 2

A network technician is troubleshooting a switch port that shows a link light but has a high number of CRC errors in the interface statistics. The port is connected to a workstation's network interface card (NIC). Both devices are set to autonegotiate. What is the MOST likely cause of the CRC errors?

Accepted Answer

A: Duplex mismatch between the switch port and the workstation NIC. CRC errors indicate that frames received by the switch have failed the Ethernet frame check sequence (FCS) validation, meaning the data was corrupted during transmission. When both devices are set to autonegotiate but one fails to correctly negotiate the duplex setting, a duplex mismatch occurs: one side operates at full duplex while the other operates at half duplex. This causes collisions on the half-duplex side, which corrupts frames and generates CRC errors, while the full-duplex side does not detect collisions and retransmits, leading to a high error count.

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B: VLAN mismatch between the switch and the workstation

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C: Incorrect MTU setting on the switch port

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D: Broadcast storm from a loop in the network

Question 3

A user reports that their laptop is connected to the corporate Wi-Fi network but cannot access the internet. Other users in the same area are able to access the internet without issues. The laptop shows a valid IP address of 192.168.1.25/24 and the default gateway is 192.168.1.1. Which of the following should the technician check NEXT?

Accepted Answer

DNS server configuration on the laptop. Since the laptop has a valid IP address (192.168.1.25/24) and default gateway (192.168.1.1), and other users can access the internet, the issue is isolated to this laptop. A common cause is incorrect or missing DNS server configuration, which prevents domain name resolution even though IP-level connectivity exists. The technician should verify the DNS server addresses in the laptop's IPv4 settings or run 'nslookup' to confirm resolution.

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DHCP server lease exhaustion

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Access point channel utilization

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Switch port status on the distribution switch

Question 4

A network technician notices that a switch port connected to a user's computer is showing a high number of CRC errors and late collisions. The link is operating at 100 Mbps, full duplex according to the switch. Which of the following is the most likely cause of these errors?

Accepted Answer

Duplex mismatch. CRC errors and late collisions in a full-duplex link are classic symptoms of a duplex mismatch. When one side is set to full duplex and the other to half duplex, the half-duplex side does not sense the carrier before transmitting, leading to collisions that are detected late in the frame. The switch reports full duplex, so the user's NIC is likely stuck at half duplex, causing these errors.

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Cable length exceeds 100 meters

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Faulty switch port

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Electromagnetic interference

Question 5

A network technician is troubleshooting communication between two switches. The trunk link between them is up, and both switches have the same list of allowed VLANs. However, devices in VLAN 10 on one switch cannot communicate with devices in VLAN 10 on the other switch. What is the MOST likely cause of this issue?

Accepted Answer

Native VLAN mismatch. A native VLAN mismatch is the most likely cause because when two switches have different native VLANs configured on a trunk, they will incorrectly tag or fail to tag frames for that VLAN. In this scenario, devices in VLAN 10 cannot communicate because the native VLAN frames (which are sent untagged) are being dropped or misinterpreted by the receiving switch, even though both switches list VLAN 10 as allowed. The trunk is up and the allowed VLAN list matches, so the issue points directly to a mismatch in the native VLAN configuration.

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Speed or duplex mismatch

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Spanning Tree Protocol blocking the port

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VLAN 10 is not created on one of the switches

Question 6

A network engineer is troubleshooting intermittent packet loss on a 10 km single-mode fiber link between two buildings. The link lights are on, but the interface shows a high number of CRC errors. The engineer has cleaned the fiber connectors and replaced the patch cables. What should the engineer check NEXT?

Accepted Answer

Check the transmit/receive optical power levels. CRC errors on a single-mode fiber link typically indicate physical-layer issues such as excessive attenuation or dispersion. Since cleaning connectors and replacing patch cables did not resolve the problem, the next logical step is to measure the optical power levels at both the transmitter and receiver using an optical power meter. This will confirm whether the received signal is within the acceptable range (e.g., -3 dBm to -20 dBm for 10GBASE-LR) and identify if a damaged transceiver or a splice loss is causing the errors.

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Check the duplex settings on both ends

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Verify the cable length is within specifications

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Adjust the Spanning Tree Protocol priority

Question 7

A technician is troubleshooting a user's computer that cannot access any network resources. The technician runs ipconfig and sees an IP address of 169.254.18.33 with a subnet mask of 255.255.0.0. The computer is connected to a switch port configured for VLAN 10. The DHCP server is located in a different subnet (VLAN 200) and is reachable via a router. The technician confirms that the switch port is in the correct VLAN and that the cabling is good. Which of the following is the MOST likely cause of the issue?

Accepted Answer

The router does not have a DHCP relay (ip helper-address) configured. The 169.254.x.x address is an Automatic Private IP Addressing (APIPA) address, assigned when a DHCP client fails to receive a lease. Since the DHCP server is on VLAN 200 and the client is on VLAN 10, a DHCP relay (ip helper-address) must be configured on the router interface facing VLAN 10 to forward DHCP broadcast requests as unicast to the DHCP server. Without this relay, the DHCP server never receives the client's discover message, causing the client to self-assign an APIPA address.

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The DHCP server is not authorized in Active Directory

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The computer's NIC is faulty

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The switch port is in the wrong VLAN

Question 8

A network technician is troubleshooting an issue where Server A can ping Server B by IP address, but Server B cannot ping Server A. Both servers are in the same VLAN and subnet, connected to the same switch. The switch ports are configured identically, and there are no ACLs or firewalls between them. Which of the following is the MOST likely cause?

Accepted Answer

Server A's firewall is blocking incoming ICMP. Server A can ping Server B by IP address, meaning ICMP echo requests from Server A reach Server B and echo replies return successfully. However, Server B cannot ping Server A, which indicates that ICMP echo requests from Server B are not reaching Server A or their replies are blocked. Since both servers are in the same VLAN/subnet with no ACLs or firewalls between them, the most likely cause is that Server A's host-based firewall is blocking incoming ICMP (echo requests), preventing Server B's pings from being processed. This is a classic symptom of a one-way firewall rule that permits outbound ICMP but denies inbound ICMP.

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Server B's firewall is blocking outgoing ICMP

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The cable connecting Server A is faulty

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There is a duplex mismatch on Server B's switch port

Question 9

A user reports that they cannot access the internet. The network technician checks the workstation's IP configuration and finds the IP address 169.254.15.77. What does this address indicate?

Accepted Answer

The DHCP server is unreachable.. The IP address 169.254.15.77 falls within the Automatic Private IP Addressing (APIPA) range (169.254.0.0/16, per RFC 3927). This address is automatically assigned by the operating system when a DHCP client fails to receive a lease from a DHCP server, indicating that the DHCP server is unreachable or not responding.

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The workstation has a static IP address configured.

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The DNS server is down.

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The default gateway is misconfigured.

Question 10

A network administrator is troubleshooting an intermittent link between two switches connected by single-mode fiber. The interface log shows "Link up / Link down" events multiple times per hour. Which of the following is the most likely cause?

Accepted Answer

Dirty fiber connectors. Dirty fiber connectors cause intermittent signal loss by scattering or absorbing light, which leads to CRC errors and repeated link flaps as the optical transceiver struggles to maintain synchronization. This matches the 'Link up / Link down' pattern seen in the logs, especially on single-mode fiber where precise alignment is critical.

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Incorrect VLAN configuration on the switch ports

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Crossed fiber pairs

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Duplex mismatch between the switches

Question 11

A user reports that they cannot access the internet. The technician is able to ping the default gateway successfully but unable to ping a public IP address (e.g., 8.8.8.8). Which of the following is the most likely cause?

Accepted Answer

The router's default route is missing or misconfigured. Successful ping to the default gateway confirms Layer 2 and Layer 3 connectivity within the local subnet, but failure to ping a public IP like 8.8.8.8 indicates that traffic cannot leave the local network. This is most commonly caused by a missing or misconfigured default route on the router, which prevents the router from forwarding packets destined for external networks to the next-hop ISP gateway.

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Incorrect DNS server configuration

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Duplicate IP address on the network

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Faulty network interface card

Question 12

A network technician runs the command "traceroute 8.8.8.8" from a workstation. The output shows the first hop as the default gateway, the second hop as an internal router, and then a series of asterisks (* * *) before reaching the destination. What does the series of asterisks indicate?

Accepted Answer

The intermediate routers are not responding to ICMP time-exceeded messages. The series of asterisks indicates that intermediate routers beyond the second hop are not responding with ICMP Time-Exceeded messages when the TTL expires. Traceroute relies on these ICMP responses to identify each hop; if a router is configured to drop ICMP or not send the message, the output shows asterisks for that hop. The destination is still reachable, as the final hop succeeds, so the asterisks do not indicate unreachability.

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The destination is unreachable

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The TTL expired at the last hop

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The connection is encrypted

Question 13

A user reports that they can connect to the internet by IP address but cannot access any websites by domain name. Which command-line tool should a technician use first to isolate the issue?

Accepted Answer

nslookup. The user can reach the internet by IP address but not by domain name, which indicates a DNS resolution failure. The `nslookup` command queries DNS servers directly to test name resolution, making it the correct first step to isolate whether the issue is with the DNS server, the client's DNS configuration, or a network path to the DNS server.

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ping

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tracert

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netstat

Question 14

A network technician is troubleshooting an intermittent link between two switches connected via single-mode fiber. The interface logs show frequent 'link up / link down' events, sometimes several times per hour. The technician has verified that the SFPs are compatible and the fiber cable is within distance specifications. Which of the following is the most likely cause of the issue?

Accepted Answer

Dirty or contaminated fiber connectors. Intermittent link flaps on single-mode fiber, despite compatible SFPs and correct distance, are most often caused by dirty or contaminated fiber connectors. Even microscopic dust or oil on the end face can scatter light, causing signal loss that triggers the switch's optical receiver to lose sync and flap the link. This matches the symptom of frequent 'link up / link down' events without any configuration mismatch.

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Electromagnetic interference (EMI) from nearby power cables

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Mismatched VLAN configurations on the switches

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Duplex mismatch between the two switch ports

Question 15

A user reports that they cannot access the internal web server by its fully qualified domain name (intranet.company.com). The workstation's IP configuration shows a DNS server of 8.8.8.8, but the internal DNS server is 10.0.0.10. The user can successfully ping the server's IP address (10.0.0.50). What is the MOST likely cause of the issue?

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

Accepted Answer

Missing default route on the router. The user can ping the default gateway (local connectivity) but cannot ping a public IP like 8.8.8.8. This indicates that the workstation has a valid route to its local subnet, but the router lacks a default route (0.0.0.0/0) to forward traffic to the internet. Firewall logs confirm outbound ICMP is permitted, so the issue is at Layer 3 routing, not filtering.

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Incorrect DNS configuration on the workstation

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Duplicate IP address on the local network

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Proxy server configuration is required