N10-009 Network Troubleshooting Practice Test 3 — 15 Questions

Question 1

A user reports that they can access the company's internal web server by IP address (10.10.10.100) but cannot access it by its hostname (intranet.company.com). The user's workstation is configured with the correct internal DNS server address. Which of the following should the technician do FIRST?

Accepted Answer

Check the DNS server's A record for intranet.company.com. The user can reach the server by IP but not by hostname, which indicates a DNS resolution problem. Since the workstation is configured with the correct internal DNS server, the most likely cause is a missing or incorrect A record for intranet.company.com on that DNS server. Checking the A record is the logical first step before other troubleshooting.

Answer

Review the firewall rules on the server

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Run ipconfig /flushdns on the workstation

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Verify the default gateway configuration

Question 2

A network engineer is troubleshooting intermittent call drops on a VoIP deployment. The network uses separate VLANs for voice (VLAN 20) and data (VLAN 10). Switch ports connecting the IP phones are configured with the correct voice VLAN. Which of the following is the MOST likely cause to check NEXT?

Accepted Answer

Verify that QoS markings and queuing are configured on the switches and routers. Intermittent call drops on a VoIP deployment with separate voice and data VLANs most likely stem from insufficient or misconfigured QoS markings and queuing. Voice traffic is sensitive to latency, jitter, and packet loss, which occur when data traffic competes for bandwidth without proper prioritization. Verifying that switches and routers have consistent QoS policies (e.g., marking with DSCP EF for RTP and queuing with strict priority) is the logical next step after ensuring the voice VLAN is correctly assigned.

Answer

Check the DNS server for the phone's name resolution

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Replace the patch cables from the phones to the switches

Answer

Increase the DHCP lease time for the phones

Question 3

A network technician is troubleshooting a router that is not forwarding packets to a remote destination network. The routing table shows a valid route learned via OSPF. The technician can successfully ping the next-hop IP address from the router. However, packets to the destination network are not being forwarded. Which of the following is the MOST likely cause?

Accepted Answer

CEF (Cisco Express Forwarding) is disabled or has a corruption. When a router has a valid OSPF-learned route and can ping the next-hop IP but still fails to forward packets, the issue is often with the forwarding plane rather than the control plane. CEF (Cisco Express Forwarding) is the default hardware-based switching mechanism that handles packet forwarding; if it is disabled or its Forwarding Information Base (FIB) becomes corrupted, the router will not forward packets even though the routing table (RIB) is correct. Disabling CEF forces the router to use process switching, which can also cause forwarding failures if the process is not properly handling the traffic.

Answer

The OSPF neighbor relationship is in the EXSTART state

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The outbound interface has been administratively shut down

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There is an ACL blocking the return traffic

Question 4

A user reports that their workstation cannot connect to the network. The technician checks the IP configuration and sees that the workstation has an IP address of 169.254.25.110. The DHCP server is operational and has available addresses. The switch port connected to the workstation shows a solid green link light. What should the technician check NEXT?

Accepted Answer

Check the switch port configuration for security features such as DHCP snooping or port security.. The workstation has an APIPA address (169.254.x.x), which indicates it failed to obtain a DHCP lease. Since the DHCP server is operational and has available addresses, the issue likely lies with the switch port configuration. Security features such as DHCP snooping or port security can block DHCP traffic or limit the number of MAC addresses, preventing the workstation from receiving a valid IP address. Checking the switch port configuration is the logical next step.

Answer

Check the DNS server configuration.

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Replace the network cable between the wall jack and the workstation.

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Verify the default gateway setting on the workstation.

Question 5

A user reports slow network performance on their workstation. The technician checks the switch port and sees a high number of CRC errors. Which of the following is the MOST likely cause of this issue?

Accepted Answer

Duplex mismatch. CRC errors indicate data corruption at the data link layer, typically caused by collisions or electrical interference. A duplex mismatch occurs when one device operates at full duplex and the other at half duplex, leading to collisions on a full-duplex link that corrupt frames and generate CRC errors. This is the most common cause of CRC errors on a switch port.

Answer

A faulty cable

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VLAN mismatch

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Incorrect MTU setting

Question 6

A technician is troubleshooting a connectivity issue. A client can successfully ping its default gateway but cannot ping a server located on a different subnet. The router's routing table shows a valid route to the server's network. What should the technician check NEXT?

Accepted Answer

The ACL applied to the router interface. Since the client can ping its default gateway but not a server on a different subnet, Layer 3 routing is working (the router has a valid route). The most likely cause is an ACL on the router interface that is blocking traffic to the server's network. ACLs filter traffic based on source/destination IP, protocol, or port, and can permit or deny packets even when a valid route exists.

Answer

The DNS resolution for the server

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The ARP cache on the client

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The duplex settings on the client NIC

Question 7

A network technician is troubleshooting a workstation that is experiencing intermittent connectivity and slow file transfers. The technician has confirmed that the patch cable passes a cable tester, the switch port is not error-disabled, and the workstation's NIC is configured for autonegotiation. The switch port is also set to autonegotiate. Which of the following should the technician check next?

Accepted Answer

The duplex and speed settings on both ends. Option B is correct because intermittent connectivity and slow file transfers, despite a passing cable test and autonegotiation on both ends, strongly indicate a duplex mismatch. When both sides are set to autonegotiate but one side fails to negotiate correctly (e.g., due to a faulty NIC or switch port), they may fall back to half-duplex while the other remains full-duplex, causing collisions, CRC errors, and retransmissions. The technician should verify the actual negotiated duplex and speed on both the workstation NIC and the switch port using commands like 'show interfaces' or NIC driver properties.

Answer

The route table on the default gateway

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The DNS server configuration

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The MAC address filtering on the switch

Question 8

A network technician sees that the link light on a switch port is not lit, even though the cable is connected to an active device. The port has been tested with a known good cable. Which of the following should the technician do next?

Accepted Answer

Check if the port is administratively disabled. When a link light is off despite a known good cable and an active device, the most likely cause is that the switch port is administratively disabled (shutdown). This is a common Layer 1/2 issue where the interface is configured with the 'shutdown' command, preventing the port from negotiating a link. Checking the interface status with 'show interfaces status' or 'show interfaces [interface]' will confirm if the port is in an 'administratively down' state.

Answer

Replace the switch

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Check the VLAN configuration

Answer

Check the speed/duplex settings

Question 9

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?

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.

Answer

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 10

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.

Answer

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 11

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.

Answer

DHCP server lease exhaustion

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Access point channel utilization

Answer

Switch port status on the distribution switch

Question 12

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.

Answer

Cable length exceeds 100 meters

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Faulty switch port

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Electromagnetic interference

Question 13

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.

Answer

Speed or duplex mismatch

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Spanning Tree Protocol blocking the port

Answer

VLAN 10 is not created on one of the switches

Question 14

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.

Answer

Check the duplex settings on both ends

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Verify the cable length is within specifications

Answer

Adjust the Spanning Tree Protocol priority

Question 15

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.

Answer

The DHCP server is not authorized in Active Directory

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The computer's NIC is faulty

Answer

The switch port is in the wrong VLAN