N10-009 Network Troubleshooting Practice Test 3 — 15 Questions

Question 1

A user's workstation shows an IP address of 169.254.15.7 with a subnet mask of 255.255.0.0. The user cannot access any network resources. Which of the following is the most likely cause?

Accepted Answer

The DHCP server is unreachable. The IP address 169.254.15.7 with a /16 subnet mask is an Automatic Private IP Addressing (APIPA) address, assigned by Windows when a DHCP client fails to obtain a lease. This indicates the workstation cannot reach a DHCP server, so the most likely cause is that the DHCP server is unreachable.

Answer

The DNS server is not responding

Answer

The subnet mask is incorrect

Answer

The default gateway is down

Question 2

A user reports that they cannot connect to a file server on the same subnet. The technician checks the workstation's IP configuration: IP 192.168.1.10, subnet mask 255.255.255.0, default gateway 192.168.1.1. The workstation can ping the gateway but cannot ping the file server at 192.168.1.50. Which of the following is the most likely cause?

Accepted Answer

The file server is powered off or disconnected from the network. The workstation can ping the gateway (192.168.1.1) but not the file server (192.168.1.50) on the same subnet. Since both are on the same subnet, the default gateway is not needed for local traffic. The inability to ping the server suggests the server itself is not responding. The most likely causes are that the server is powered off, its network cable is disconnected, or it has a failed network interface. A firewall blocking ICMP would also prevent ping, but the question specifies that the correct answer is the server being powered off or disconnected (option C). Therefore, the most likely cause is that the file server is powered off or has a disconnected cable.

Answer

Incorrect subnet mask on the workstation

Answer

Incorrect default gateway on the workstation

Answer

A firewall on the workstation is blocking traffic to the server

Question 3

A user can access a website by its IP address (e.g., 203.0.113.5) but cannot access it by its domain name (example.com). Other users on the same subnet can access the website by domain name. Which of the following should the technician check FIRST?

Accepted Answer

Check the local hosts file. The issue is isolated to a single user who can reach the website by IP but not by domain name, while other users on the same subnet have no problem. This indicates a client-side DNS resolution problem. The first thing to check is the local hosts file (e.g., C:\Windows\System32\drivers\etc\hosts) for any incorrect or outdated entries that might be overriding DNS resolution. If the hosts file has an incorrect mapping for example.com, the browser will use that instead of querying the DNS server, causing the failure. Checking DNS server configuration is appropriate if the hosts file is clean.

Answer

Check the DNS server configuration on the network

Answer

Check the website's DNS records

Answer

Check the user's browser proxy settings

Question 4

A network technician is troubleshooting inter-VLAN routing. Hosts in VLAN 10 can communicate with hosts in VLAN 20, but cannot communicate with hosts in VLAN 30. All VLANs are configured on the same Layer 3 switch with SVIs. Which of the following should the technician verify FIRST?

Accepted Answer

The SVI for VLAN 30 is missing an IP address.. Since all VLANs are configured on the same Layer 3 switch with SVIs, inter-VLAN routing occurs internally. Hosts in VLAN 10 can reach VLAN 20, proving the Layer 3 switch is routing correctly for those VLANs. The failure to reach VLAN 30 most likely indicates that the SVI for VLAN 30 is missing an IP address, which prevents the switch from having a local interface to route traffic to that subnet.

Answer

VLAN 30 is not allowed on the trunk port to the switch.

Answer

The default gateway on hosts in VLAN 10 is incorrect.

Answer

The routing table does not have a route to VLAN 30.

Question 5

A user reports that they can access the company's intranet but not the internet. The technician checks the IP configuration and finds that the default gateway is set to 192.168.1.1, and the user can ping that IP. Which of the following is the most likely cause?

Accepted Answer

The router's NAT configuration is faulty.. Since the user can ping the default gateway (192.168.1.1) and access the intranet, Layer 3 connectivity to the local router is functional. The inability to reach the internet, despite having a valid gateway, points to a failure in the router's NAT (Network Address Translation) configuration. NAT is required to translate private RFC 1918 addresses (like 192.168.1.x) to a public IP for internet access; without it, packets are routed to the gateway but cannot be forwarded beyond the local network.

Answer

The DNS server address is incorrect.

Answer

The DHCP server is not providing a default gateway.

Answer

The user's workstation has a static IP address.

Question 6

Two routers are configured with OSPF in the same area, but they do not form an adjacency. Router A shows OSPF state EXSTART, and Router B shows state EXSTART. Which of the following is the most likely cause?

Accepted Answer

The MTU is mismatched between the two routers. When OSPF routers are stuck in the EXSTART state, it indicates that they have progressed past the 2-Way and ExStart phases but are unable to exchange Database Description (DBD) packets. The most common cause is an MTU mismatch, because OSPF will not proceed to the Exchange state if a DBD packet exceeds the interface MTU of the neighbor. This causes the routers to continuously renegotiate the master/slave relationship without completing the exchange.

Answer

The OSPF Hello and Dead intervals are mismatched

Answer

The OSPF area IDs are different

Answer

The network type is misconfigured (e.g., one side is broadcast, the other is point-to-point)

Question 7

A user reports that they can access the internet but cannot access the internal web server at 10.10.10.100. The technician can ping the server's IP from the router that serves as the user's default gateway. Which of the following is the MOST likely cause?

Accepted Answer

The server's default gateway is misconfigured. The user can reach the internet but not the internal web server at 10.10.10.100, while the technician can ping that server from the router (the default gateway). This indicates that the router has a valid route to the server's network and the server is reachable from the router's perspective. The most likely cause is that the server's default gateway is misconfigured: the server does not know how to send return traffic back to the user's subnet, so the TCP handshake fails (the SYN-ACK never reaches the user).

Answer

The user's workstation has a firewall blocking port 80

Answer

The router has a missing route to the server's network

Answer

The server's subnet mask is incorrect

Question 8

A technician is troubleshooting intermittent connectivity issues on a fiber link between two switches. The link light on both switches is green. Which of the following tools should the technician use to further investigate the issue?

Accepted Answer

Optical power meter. The correct tool is an optical power meter because the link lights are green, indicating Layer 1 signal presence, but intermittent connectivity suggests the signal strength may be marginal or fluctuating. An optical power meter measures the exact light level in dBm to verify it falls within the receiver's sensitivity range, which a simple link LED cannot detect.

Answer

Multimeter

Answer

Tone generator

Answer

Cable certifier

Question 9

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

Answer

Run ipconfig /flushdns on the workstation

Answer

Verify the default gateway configuration

Question 10

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

Answer

Replace the patch cables from the phones to the switches

Answer

Increase the DHCP lease time for the phones

Question 11

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

Hardware switching/forwarding is disabled or the forwarding table is corrupted. 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 data plane rather than the control plane. Hardware switching mechanisms (such as fast switching or generic forwarding engines) handle packet forwarding; if they are disabled or the forwarding information base (FIB) becomes corrupted, the router will not forward packets even though the routing table (RIB) is correct. Disabling hardware switching forces the router to use process switching, which can also cause forwarding failures.

Answer

The OSPF neighbor relationship is in the EXSTART state

Answer

The outbound interface has been administratively shut down

Answer

There is an ACL blocking the return traffic

Question 12

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

Answer

VLAN mismatch

Answer

Incorrect MTU setting

Question 13

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

Answer

The ARP cache on the client

Answer

The duplex settings on the client NIC

Question 14

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. 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

Answer

The DNS server configuration

Answer

The MAC address filtering on the switch

Question 15

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

Answer

Check the VLAN configuration

Answer

Check the speed/duplex settings