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Network Troubleshooting

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Question 421Question →

A network administrator is troubleshooting an issue where users are unable to access a secure website hosted on an internal Linux server. The administrator executes the command `netstat -an` on the server and receives the following output snippet:

text Active Internet connections (servers and established) Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 0.0.0.0:80 0.0.0.0:* LISTEN tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN

Based on this output, which of the following is the primary cause of the connectivity failure?

Show answer & explanation

Answer: The web server daemon is actively listening for unencrypted HTTP traffic on port 80, but the HTTPS service on port 443 is not running or listening.

Answer

The web server daemon is actively listening for unencrypted HTTP traffic on port 80, but the HTTPS service on port 443 is not running or listening.
The `netstat -an` output displays numerical network addresses and active socket states. The snippet shows TCP port 80 (HTTP) and TCP port 22 (SSH) in the `LISTEN` state. Because secure web traffic uses TCP port 443 (HTTPS), and port 443 does not appear in the active listening socket table, client requests over HTTPS will be refused at the transport layer.

Step-by-Step Solution

1
Analyze the command output
Identify active network socket listeners: TCP port 80 (HTTP) and TCP port 22 (SSH) bound to all IPv4 interfaces (0.0.0.0) in the LISTEN state.
The netstat -an command displays numeric network addresses and port numbers currently bound by local daemons.
2
Evaluate user access requirements against socket states
Users are attempting to connect via secure HTTPS (standard TCP port 443). However, netstat shows no service listening on TCP port 443.
For clients to establish a TCP handshake on port 443, the web server service must have a socket configured in the LISTEN state on that port.
3
Deduce the primary root cause
The web server service is either stopped, misconfigured, or missing a TLS listener binding for TCP port 443.
The absence of port 443 in netstat output directly accounts for the inability to establish secure connections.

Key Concept

Analyzing netstat listening sockets to verify network service availability and port bindings
Question 422Question →

A network technician is troubleshooting connectivity issues following a switch maintenance window. Workstations assigned to VLAN 45 on an edge switch can no longer communicate with the default gateway hosted on the core switch, whereas hosts on VLAN 10 attached to the same edge switch maintain uninterrupted access. The trunk link between the switches is configured using 802.1Q. Output from `show interfaces trunk` on the core switch shows the allowed VLAN list on interface GigabitEthernet0/1 is set to `10,20,30`. Which of the following configuration changes on the core switch will resolve the connectivity issue for VLAN 45?

Show answer & explanation

Answer: Add VLAN 45 to the allowed VLAN list on interface GigabitEthernet0/1.

Answer

Add VLAN 45 to the allowed VLAN list on interface GigabitEthernet0/1.
On 802.1Q trunk links, the allowed VLAN list specifies which VLAN tags the switch interface will accept and forward. Since `show interfaces trunk` shows the allowed list restricted to VLANs 10, 20, and 30, traffic tagged for VLAN 45 is dropped at the switch port. Adding VLAN 45 to the allowed VLAN list on interface GigabitEthernet0/1 permits frames tagged for VLAN 45 to traverse the trunk link to reach the default gateway.

Step-by-Step Solution

1
Analyze the symptom and output
VLAN 10 traffic succeeds while VLAN 45 traffic fails across the trunk link. Interface `GigabitEthernet0/1` trunk status explicitly lists allowed VLANs as `10,20,30`.
By default, 802.1Q trunks allow all VLANs (1-4094), but explicit `switchport trunk allowed vlan` lists prune unlisted VLANs from entering or exiting the interface.
2
Identify the missing configuration element
VLAN 45 is absent from the allowed list, causing the switch interface to drop all incoming and outgoing frames carrying the 802.1Q tag for VLAN 45.
Traffic for VLAN 45 cannot reach the core switch's default gateway because the trunk port prunes the traffic at Layer 2.
3
Select the corrective CLI configuration
Executing `switchport trunk allowed vlan add 45` on interface GigabitEthernet0/1 includes VLAN 45 in the trunk's active forwarding list.
Modifying the allowed list restores Layer 2 trunking for tagged VLAN 45 frames without disrupting existing allowed VLANs.

Key Concept

VLAN Trunking Allowed List Configuration and Pruning
Estimated Time:1m 30s
Question 423Question →

A network technician is troubleshooting an issue where a newly connected workstation on VLAN 30 fails to receive an IP address from a central DHCP server located on VLAN 10. Place the troubleshooting steps in the correct logical sequence according to standard network troubleshooting methodology to isolate and resolve the issue.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct logical order begins with Layer 1 physical link verification, followed by local client IP configuration inspection, then router DHCP relay (ip helper-address) verification, and lastly central DHCP server scope pool check.
The correct sequence follows standard structured troubleshooting: starting at the physical layer (verifying link LED), checking local operating system network configuration (confirming APIPA), inspecting intermediate network infrastructure (verifying IP helper/DHCP relay placement across VLAN boundaries), and finally validating end-service resource capacity (DHCP scope pool status).

Step-by-Step Solution

1
Verify physical connection and port status.
Physical link is active and connected.
Always verify basic layer 1 connectivity before diagnosing software or protocol issues.
2
Inspect local network adapter status on the client.
Client displays an APIPA address (169.254.x.x).
Confirms that the host attempted DHCP discovery but received no response.
3
Verify the DHCP relay configuration on the VLAN 30 gateway interface.
Ensures DHCP broadcast packets are converted to unicast and forwarded to the server on VLAN 10.
Routers drop broadcast packets by default; without a relay agent, DHCP traffic cannot cross subnets.
4
Check the central DHCP server scope status for VLAN 30.
Determines whether available leases exist in the scope.
If relaying is operational, scope exhaustion is the next common failure point.

Key Concept

Inter-VLAN DHCP Troubleshooting Methodology
Question 424Question →

A network technician is troubleshooting a client workstation that cannot access an internal web portal by its fully qualified domain name (FQDN). Following a standard bottom-up troubleshooting workflow to isolate network issues from local network configuration to name resolution, place the following command-line steps in the correct logical diagnostic sequence.

Drag items to arrange them in the correct order

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Answer

The correct sequence begins with checking local interface settings using ipconfig /all, testing local subnet routing using ping to the default gateway, testing remote host IP reachability using ping to the target IP, and finally diagnosing DNS name resolution using nslookup.
The logical sequence follows standard CompTIA network troubleshooting: first, check local host configuration using ipconfig /all; second, verify local subnet network reachability by pinging the default gateway IP; third, verify end-to-end network path reachability by pinging the destination server's IP address directly; fourth, test domain name lookup functionality using nslookup to confirm FQDN-to-IP mapping.

Step-by-Step Solution

1
Inspect local network configuration
The technician verifies the local IP parameters (IP address, subnet mask, default gateway, DNS server) using ipconfig /all.
This confirms basic NIC operational status and verifies assigned IP configuration settings.
2
Test local router reachability
The technician issues ping 192.168.1.1 to test connectivity to the local gateway router.
Pinging the default gateway confirms local link layer and Layer 3 subnet connectivity.
3
Test end-to-end IP routing reachability
The technician executes ping 10.0.5.25 using the target server's raw IP address.
Directly pinging the destination IP tests end-to-end routing without relying on domain name resolution.
4
Diagnose host name resolution
The technician uses nslookup portal.lab.internal to evaluate FQDN query responses from the DNS server.
Performing an explicit DNS lookup isolates application layer name resolution problems after IP reachability is confirmed.

Key Concept

Command-Line Troubleshooting Methodology & Tool Selection
Question 425Question →

A network administrator is troubleshooting an issue where workstation users are directed to an old server IP address for an internal web application hosted at `app.corp.internal`. The administrator suspects the local DNS server (`192.168.1.1`) is returning a cached, non-authoritative record. Which command should the administrator run to query the authoritative DNS server at `10.100.1.10` directly for the IPv4 address record?

Show answer & explanation

Answer: nslookup app.corp.internal 10.100.1.10

Answer

The command `nslookup app.corp.internal 10.100.1.10` directly queries the authoritative DNS server for the standard IPv4 address (A record) of the specified host.
The command `nslookup app.corp.internal 10.100.1.10` instructs `nslookup` to send a default IPv4 (A record) lookup query for `app.corp.internal` directly to the specified server address `10.100.1.10`, bypassing the host's default configured DNS server.

Step-by-Step Solution

1
Identify the required utility and target host name.
The target domain to resolve is `app.corp.internal` for standard IPv4 resolution (A record).
The administrator needs to verify the IPv4 mapping returned directly by the authoritative server.
2
Determine the proper syntax to bypass the default local resolver.
Appending `10.100.1.10` after the hostname in `nslookup` sends the query directly to `10.100.1.10`.
This bypasses cached or non-authoritative entries stored on the local DNS server (`192.168.1.1`).
3
Verify command flags and protocol ports.
`nslookup app.corp.internal 10.100.1.10` uses standard DNS port 53 and queries for an A record by default.
No additional flags altering port numbers or record types to IPv6 are needed.

Key Concept

Direct DNS Querying via Command-Line Utilities
Question 426Question →

A network administrator is setting up a new server on a subnet designed for database hosts: 172.16.10.128/26172.16.10.128/26. The default gateway router interface for this subnet is assigned IP address 172.16.10.129/26172.16.10.129/26. The administrator manually configures the server with IP address 172.16.10.66172.16.10.66 and subnet mask 255.255.255.192255.255.255.192 (/26/26). Upon testing, the server cannot ping its default gateway or reach any external networks. Which of the following best identifies the root cause of this connectivity failure?

Show answer & explanation

Answer: The host IP address assigned to the server resides on a different IP subnet than the default gateway.

Answer

The host IP address assigned to the server resides on a different IP subnet than the default gateway.
With a /26/26 subnet mask (255.255.255.192255.255.255.192), subnets increment in blocks of 64 addresses. The default gateway IP 172.16.10.129172.16.10.129 resides in the 172.16.10.128/26172.16.10.128/26 subnet (usable host range 172.16.10.129−172.16.10.190172.16.10.129 - 172.16.10.190). However, the static host IP 172.16.10.66172.16.10.66 resides in the adjacent 172.16.10.64/26172.16.10.64/26 subnet (usable host range 172.16.10.65−172.16.10.126172.16.10.65 - 172.16.10.126). Because the host and default gateway are on different logical IP subnets, the host cannot complete ARP resolution for the default gateway IP address, resulting in total unreachability.

Step-by-Step Solution

1
Analyze the subnet boundaries for a /26 subnet mask (255.255.255.192).
A /26 mask creates subnets with block sizes of 64 addresses: 172.16.10.0/26 (0-63), 172.16.10.64/26 (64-127), and 172.16.10.128/26 (128-191).
Determining exact subnet ranges identifies which addresses belong to the same local broadcast domain.
2
Map the configured server IP address to its corresponding subnet range.
The server IP 172.16.10.66 falls into the 172.16.10.64/26 subnet (usable range 172.16.10.65 to 172.16.10.126).
Confirms the host's actual local subnet.
3
Compare host IP subnet to default gateway IP subnet.
The gateway IP 172.16.10.129 resides in the 172.16.10.128/26 subnet (usable range 172.16.10.129 to 172.16.10.190).
Because the host and gateway are on separate subnets, the host cannot resolve the gateway's MAC address via local ARP requests, resulting in host unreachability.

Key Concept

Default Gateway Subnet Alignment and CIDR Boundary Troubleshooting
Estimated Time:1m 30s
Question 427Question →

A network administrator connects two enterprise switches via a fiber trunk link on interface GigabitEthernet 0/1. Shortly after enabling the link, console logs on both switches begin displaying periodic warning messages indicating a native VLAN mismatch. Interface status shows the trunk link is active, but untagged management traffic sent from VLAN 10 on Switch-A is being improperly received on VLAN 20 by Switch-B. Which of the following configuration changes will resolve this issue while restoring proper 802.1Q trunking functionality?

Show answer & explanation

Answer: Reconfigure the trunk link so that both switch interfaces are configured with matching native VLAN IDs.

Answer

Reconfigure the trunk link so that both switch interfaces are configured with matching native VLAN IDs.
On an 802.1Q trunk link, frames belonging to the native VLAN are sent without an 802.1Q VLAN tag. When the switches on opposite ends of the trunk have mismatched native VLAN configurations, untagged frames egressing one switch's native VLAN are ingested into the receiving switch's native VLAN, causing cross-VLAN traffic leakage and generating CDP/syslog mismatch warnings. Reconfiguring both switch interfaces to use the exact same native VLAN ID resolves the mismatch and restores proper frame processing.

Step-by-Step Solution

1
Analyze the log messages and symptoms reported on the switch interfaces.
Identified an 802.1Q native VLAN mismatch where Switch-A considers VLAN 10 native while Switch-B considers VLAN 20 native.
802.1Q trunking sends frames on the native VLAN without adding an explicit 802.1Q VLAN tag.
2
Determine the impact of untagged frame processing across a mismatched native VLAN link.
Untagged frames sent from Switch-A's native VLAN (VLAN 10) are received untagged by Switch-B and incorrectly placed into Switch-B's native VLAN (VLAN 20), causing cross-VLAN traffic leaking and log warnings.
Receiving switchports assume any untagged frame belongs to their locally configured native VLAN.
3
Identify the required remediation step.
Configure matching native VLAN IDs (e.g., switchport trunk native vlan 10 on both switches) to align Layer 2 VLAN tagging expectations.
Matching native VLAN configurations ensure untagged frames map to the same broadcast domain on both ends of the trunk link.

Key Concept

802.1Q Native VLAN Trunk Misconfiguration Troubleshooting
Question 428Question →

Match each switch status indicator or log message on the left with its corresponding underlying root cause on the right.

Click a left item, then click its matching right item

Items

%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on GigabitEthernet0/1 (10), with Switch2 GigabitEthernet0/1 (20).
Interface FastEthernet0/12 reports an increasing counter for CRC errors and late collisions during heavy traffic.
Interface GigabitEthernet0/2 transitions to err-disabled state after receiving BPDUs with a bridge priority of 0.
Trunk port operational state is up, but frames belonging to VLAN 30 are dropped while VLAN 10 frames pass normally.

Matches

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Answer

Each switch indicator matches its root cause: Native VLAN mismatch log matches untagged frame default VLAN mismatch; CRC errors and late collisions match duplex misconfiguration; Err-disabled port from priority 0 BPDU matches STP Root Guard enforcement; Trunk passing VLAN 10 but dropping VLAN 30 matches trunk allowed VLAN list exclusion.
Each indicator corresponds to a classic Layer 2 troubleshooting symptom: CDP log warnings indicate non-matching native VLAN IDs on 802.1Q trunks; late collisions with CRC errors indicate a full-duplex/half-duplex mismatch; an err-disabled state after receiving a priority 0 BPDU indicates STP Root Guard action; and selective VLAN dropping on an active trunk indicates an incomplete allowed VLAN list.

Step-by-Step Solution

1
Analyze the CDP error message indicating native VLAN mismatch.
Identified that native VLAN settings differ between trunk endpoints (VLAN 10 vs VLAN 20).
802.1Q trunking requires identical native VLAN IDs on both ends to ensure untagged traffic is processed consistently.
2
Analyze interface statistics showing late collisions and CRC errors.
Identified duplex mismatch as the root cause.
Late collisions occur when a full-duplex interface transmits while a half-duplex interface on the same segment attempts to transmit after its 512-bit collision window.
3
Evaluate the interface transitioning to err-disabled upon receiving a priority 0 BPDU.
Identified Root Guard protection mechanism as the root cause.
Root Guard protects the root bridge topology by blocking ports that receive superior BPDUs.
4
Evaluate selective VLAN traffic dropping on an active trunk link.
Identified trunk allowed VLAN list pruning or exclusion.
Trunk ports forward only VLANs explicitly allowed in their configuration; missing VLANs are silently dropped.

Key Concept

Switching and VLAN Troubleshooting Indicators
Question 429Question →

A network administrator installs a new 10Gbps LC duplex fiber patch cable between two core switches using matching SFP+ transceivers. After plugging in the cable, the link status LEDs on both switch interfaces remain completely dark. Testing with a visual fault locator (VFL) confirms light is passing through both fiber strands with no physical breaks. Which of the following is the most appropriate troubleshooting action to resolve this issue?

Show answer & explanation

Answer: Reverse the positions of the transmit (Tx) and receive (Rx) fiber strands on one end of the LC duplex connector.

Answer

Reverse the positions of the transmit (Tx) and receive (Rx) fiber strands on one end of the LC duplex connector.
Swapping the transmit (Tx) and receive (Rx) strands on one connector end resolves a fiber polarity inversion. Duplex fiber communications require a crossover layout (Tx to Rx). When connected straight-through (Tx-to-Tx / Rx-to-Rx), optical signals are directed into transmit lasers rather than receivers, preventing link establishment even when light continuity is verified.

Step-by-Step Solution

1
Analyze the diagnostic test results
The visual fault locator (VFL) verifies continuity and absence of physical glass breaks in both strands.
Eliminates physical cable damage or excessive bend loss as the root cause.
2
Identify duplex fiber transmission requirements
Duplex fiber links require transmit (Tx) at Node 1 to align with receive (Rx) at Node 2, and vice versa.
If Tx connects to Tx and Rx connects to Rx (straight-through polarity mismatch), neither optical transceiver receives incoming light.
3
Apply the corrective physical cable adjustment
Unclip and swap the Tx and Rx connectors at one end of the LC duplex fiber patch cable.
Establishes proper fiber polarity crossover, allowing optics to establish a link.

Key Concept

Fiber Optic Polarity and Duplex Connection Troubleshooting
Estimated Time:1m 30s
Question 430Question →

A network technician is conducting diagnostic tests on a newly deployed workstation. Match each command-line utility command to its correct network troubleshooting function or operational output.

Click a left item, then click its matching right item

Items

netstat -an
dig -x
pathping
arp -a

Matches

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Answer

netstat -an corresponds to displaying active TCP/UDP listening ports and numerical connections; dig -x corresponds to performing a reverse DNS lookup; pathping corresponds to measuring per-hop latency and packet loss over time; arp -a corresponds to displaying local IP-to-MAC address mappings.
Each command-line utility targets a specific diagnostic domain: netstat -an reports transport-layer listening ports and sockets numerically; dig -x performs reverse DNS PTR lookups; pathping assesses sustained hop-by-hop latency and packet loss; arp -a outputs local Layer 2 to Layer 3 address resolution entries.

Step-by-Step Solution

1
Analyze the primary function of protocol/session display tools.
Identify that netstat displaying socket statuses numerically matches listing listening ports and established TCP/UDP sockets.
The -n flag prevents hostname resolution, speeding up output and showing exact port numbers.
2
Evaluate the specialized DNS query flags.
Identify dig -x as the reverse DNS lookup invocation.
The -x flag automatically formats the target IP address into an in-addr.arpa or ip6.arpa PTR query.
3
Differentiate path diagnostic utilities.
Identify pathping as the tool combining traceroute with sustained ping stats.
Pathping monitors intermediate hops over several minutes to calculate exact packet loss percentages per hop.
4
Examine local address resolution tools.
Identify arp -a as the command for displaying the Address Resolution Protocol cache.
ARP links Layer 2 physical addresses to Layer 3 IP addresses for the local broadcast domain.

Key Concept

Command-Line Network Utilities & Operational Diagnostics
Question 431Question →

A network administrator is troubleshooting network connectivity issues for automated guided vehicles (AGVs) deployed on a newly provisioned warehouse floor subnet (VLAN 45: 10.45.0.0/2310.45.0.0/23). The AGVs cannot communicate with the central controller on another subnet. Inspecting the IP configuration on an AGV displays the following terminal output:

IPv4 Address. . . . . . . . . . . : 169.254.12.88
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . :

The central DHCP server is located on VLAN 10 (10.10.0.5010.10.0.50) with a valid, unexhausted scope for VLAN 45. Which TWO of the following statements correctly identify the underlying cause and the required resolution?

Select all that apply

Show answer & explanation

Answer: The host automatically generated an Automatic Private IP Addressing (APIPA) address because DHCP broadcast requests are not reaching the central DHCP server.; An IP helper address (DHCP relay) must be configured on the VLAN 45 default gateway router interface pointing to the DHCP server at 10.10.0.50.

Answer

The host automatically generated an APIPA address because DHCP broadcast requests were blocked across subnets, and an IP helper address (DHCP relay) must be configured on the VLAN 45 gateway router interface pointing to 10.10.0.50.
The presence of a 169.254.x.x address confirms that the client failed to receive a DHCP lease and assigned itself an Automatic Private IP Address (APIPA). Because the central DHCP server is situated on a different subnet (VLAN 10) than the client (VLAN 45), Layer 2 DHCP Discover broadcasts are blocked by default at the router interface. Configuring an IP helper address (DHCP relay) on the VLAN 45 router interface allows the router to relay client requests as unicast packets directly to the central DHCP server.

Step-by-Step Solution

1
Analyze the client IP configuration snippet.
The IP address 169.254.12.88 belongs to the link-local Automatic Private IP Addressing (APIPA) block.
Operating systems automatically assign an APIPA address when dynamic configuration via DHCP times out or fails.
2
Identify why DHCP communications are failing across subnets.
DHCP Discover packets are sent as Layer 2 broadcasts, which routers do not forward across VLAN boundaries.
The client resides on VLAN 45 while the central DHCP server is located on VLAN 10.
3
Determine the necessary corrective action.
Configure an IP helper address (DHCP relay) on the VLAN 45 router interface pointing to 10.10.0.50.
A DHCP relay agent captures local DHCP broadcast traffic, encapsulates it as unicast IP packets, and forwards it to the designated DHCP server.

Key Concept

Troubleshooting APIPA Assignment and Cross-Subnet DHCP Relay (IP Helper)
Estimated Time:1m 30s
Question 432Question →

An infrastructure team is auditing an 802.1Q trunk connection between Switch-1 and Switch-2 on interface FastEthernet 0/24. Syslog on Switch-1 displays `%CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on FastEthernet0/24 (1), with Switch-2 FastEthernet0/24 (30)`. Additionally, hosts assigned to VLAN 30 on Switch-1 cannot reach resources on VLAN 30 attached to Switch-2. A check of Switch-1 interface FastEthernet 0/24 shows that its trunk allowed VLAN list currently includes only VLANs 10 and 20. Which TWO of the following configuration steps must be performed on Switch-1 to resolve the native VLAN warning and allow VLAN 30 traffic to traverse the link? (Select TWO.)

Select all that apply

Show answer & explanation

Answer: Reconfigure the native VLAN on interface FastEthernet 0/24 of Switch-1 to VLAN 30.; Add VLAN 30 to the list of allowed VLANs on interface FastEthernet 0/24 of Switch-1.

Answer

The technician must reconfigure the native VLAN on Switch-1 FastEthernet 0/24 to VLAN 30 and append VLAN 30 to the trunk's allowed VLAN list.
Resolving this issue requires fixing two distinct misconfigurations on Switch-1's trunk port. First, changing the native VLAN to VLAN 30 eliminates the mismatch flagged by CDP. Second, adding VLAN 30 to the trunk's allowed VLAN list enables tagged frames for VLAN 30 to pass between the two switches.

Step-by-Step Solution

1
Analyze the CDP error log for the native VLAN configuration difference.
Switch-1 uses native VLAN 1, while Switch-2 uses native VLAN 30 on interface FastEthernet 0/24.
Matching native VLAN configurations across both ends of an 802.1Q trunk link is required to prevent CDP error logs and untagged traffic leaks.
2
Inspect the allowed VLAN trunk pruning list on Switch-1.
VLAN 30 is absent from Switch-1's allowed list (which only lists VLANs 10 and 20).
Trunk interfaces drop traffic for any VLAN not explicitly included in their allowed VLAN filter.
3
Formulate the required remediation commands for Switch-1 interface FastEthernet 0/24.
Reconfigure the native VLAN to 30 and append VLAN 30 to the allowed list.
Executing both changes resolves the native VLAN mismatch alert and permits VLAN 30 traffic across the trunk link.

Key Concept

802.1Q Trunking Native VLAN Matching and Allowed VLAN List Configuration
Question 433Question →

A network technician needs to pinpoint the precise location of a conductor break in a 90-meter Category 6 UTP cable run hidden behind drywall. A basic wire mapper confirms an open circuit on pin 3, but cannot determine how far along the cable the fault occurred. Which diagnostic tool should the technician use to measure the exact distance to the cable break?

Show answer & explanation

Answer: Time-Domain Reflectometer (TDR)

Answer

Time-Domain Reflectometer (TDR)
A Time-Domain Reflectometer (TDR) is specifically designed to send electrical pulses through copper media and analyze the reflected signals. By calculating the time delay of the reflection against the cable's velocity of propagation, a TDR precisely measures the distance to physical cable breaks, kinks, or impedance mismatches.

Step-by-Step Solution

1
Identify the physical layer fault and diagnostic goal
The goal is to determine the exact distance along a copper cable run where a conductor break (open circuit) is located.
Basic wire mappers only confirm electrical continuity across pins but lack signal reflection measurement capabilities.
2
Select the appropriate diagnostic tool designed for copper cable fault location
Select a Time-Domain Reflectometer (TDR).
A TDR transmits a signal pulse along copper conductors and measures the elapsed time for reflections caused by impedance changes (such as an open circuit or short) to return, calculating distance based on nominal velocity of propagation.

Key Concept

Pinpointing Copper Cable Fault Locations with a TDR
Estimated Time:1m 0s
Question 434Question →

A network administrator configures a new DHCP scope on a centralized server to service clients on VLAN 20 (subnet 10.20.10.0/2410.20.10.0/24). Users on VLAN 20 report that they can successfully communicate with other local hosts on VLAN 20, but cannot access external networks or corporate servers located on other subnets. Running `ipconfig /all` on an affected client displays an IPv4 address of 10.20.10.4510.20.10.45 with a subnet mask of 255.255.255.0255.255.255.0, but the Default Gateway field is empty. Which of the following is the most likely cause of this issue?

Show answer & explanation

Answer: DHCP Option 003 (Router) was not configured within the DHCP scope settings.

Answer

DHCP Option 003 (Router) was not configured within the DHCP scope settings.
DHCP clients require specific options to configure network settings beyond basic IP address assignment. DHCP Option 003 delivers the default gateway IP address to clients. When Option 003 is omitted from a scope configuration, clients receive valid IP addresses and subnet masks allowing local Layer 2 traffic, but lack the default gateway setting required to route packets to external subnets.

Step-by-Step Solution

1
Analyze the IP configuration returned by the diagnostic command.
The client has acquired 10.20.10.45/2410.20.10.45/24, which is a valid lease from the configured 10.20.10.0/2410.20.10.0/24 pool.
Successful lease acquisition confirms physical layer 2 connectivity and functioning Layer 3 DHCP relay mechanisms.
2
Evaluate the symptom of intra-subnet versus inter-subnet communication.
Hosts can communicate within VLAN 20 but fail to reach external destinations.
Intra-subnet communication uses Layer 2 MAC addresses via ARP. Inter-subnet communication requires routing through a default gateway.
3
Correlate the missing Default Gateway field with DHCP server parameters.
DHCP Option 003 supplies the Default Gateway parameter to clients during the lease handshake.
Omitting Option 003 causes clients to successfully receive IP addresses and subnet masks while lacking the default route necessary to forward traffic outside the local network.

Key Concept

DHCP Option Configuration and Scope Parameters
Question 435Question →

A network engineer is troubleshooting a newly installed multi-strand MPO/MTP multimode fiber trunk cable connected between two high-density distribution switches. Although transceivers are installed and powered, the link status remains down, and switch logs indicate insufficient optical power reception. Which of the following diagnostic steps or corrective actions should the engineer take to resolve this physical layer issue? (Select TWO.)

Select all that apply

Show answer & explanation

Answer: Inspect the fiber ferrule endfaces with an optical inspection scope and clean contaminated connectors using specialized lint-free tools.; Measure the optical power loss across the fiber strands with an optical power meter and light source to verify correct polarity and signal strength.

Answer

The correct troubleshooting actions are inspecting and cleaning the fiber ferrule endfaces with an optical inspection scope, and measuring optical loss and polarity using an optical power meter paired with a light source.
Inspecting ferrule endfaces with a fiber inspection scope reveals microscopic debris causing signal insertion loss, and using an optical power meter with a light source quantifies decibel loss and confirms optical polarity across multi-fiber connectors.

Step-by-Step Solution

1
Identify the transmission media and symptom.
The media is multi-strand multimode fiber cabling (MPO/MTP) exhibiting optical attenuation and link down status.
Choosing diagnostic tools requires matching tool capabilities with optical fiber physical properties.
2
Select appropriate fiber optic diagnostic tools.
Optical inspection scopes identify physical contamination on ferrules, while an optical power meter (OPM) measures signal attenuation and confirms correct transmit/receive polarity.
Fiber optic troubleshooting relies on light transmission metrics and physical endface cleanliness rather than electrical conductivity testing.
3
Eliminate inappropriate copper testing tools.
Discard tone generators and copper TDRs.
Copper testing tools rely on electrical signals and metallic continuity, which cannot operate on optical glass fibers.

Key Concept

Fiber Optic Physical Layer Troubleshooting & Tool Selection
Question 436Question →

A network administrator is troubleshooting severe performance degradation on an enterprise network segment. Users report extreme file transfer delays when communicating with a local server connected to interface GigabitEthernet 0/5 on Switch-SW1. Interface metrics on Switch-SW1 reveal a rising count of late collisions, frame check sequence (FCS) errors, and alignment errors, despite low overall bandwidth utilization. Further inspection shows the server's network adapter is manually configured for 1000 Mbps Full-Duplex, whereas the connected switch port is set to auto-negotiation. Which of the following accounts for the root cause of this issue?

Show answer & explanation

Answer: The switch port auto-negotiation failed to detect full-duplex mode, defaulting to half-duplex and creating a duplex mismatch.

Answer

The switch port auto-negotiation failed to detect full-duplex mode, defaulting to half-duplex and creating a duplex mismatch.
When an Ethernet port configured for auto-negotiation is connected to a port with manually hardcoded speed and duplex, IEEE 802.3 standards specify that the auto-negotiating port can detect speed through parallel detection but cannot negotiate duplex. Consequently, the auto-negotiating switch port defaults to half-duplex mode. Because the connected server operates in full-duplex mode, it transmits data without checking for carrier sense. The switch port, operating in half-duplex, perceives these simultaneous transmissions as collisions. When collisions occur after the collision window (64 bytes), they are logged as late collisions and cause FCS/CRC errors, resulting in extreme performance degradation.

Step-by-Step Solution

1
Analyze the reported interface statistics and symptoms.
Identified late collisions, FCS errors, and alignment errors alongside slow transfer rates.
Late collisions occur when a device operating in half-duplex detects a collision after transmitting the first 64 bytes of a frame.
2
Evaluate the interface speed and duplex configuration settings on both connected ends.
Determined that the server adapter is hardcoded to full-duplex while the switch port is set to auto-negotiation.
Ethernet auto-negotiation standards dictate that if link partner parameters cannot be negotiated, speed can be detected via parallel detection, but duplex defaults to half-duplex.
3
Synthesize the configuration mismatch with the observed symptoms.
The server transmits continuously in full-duplex mode, while the switch port expects half-duplex CSMA/CD operation, causing the switch to detect collisions during server transmission.
This operational mismatch directly generates late collisions, FCS errors, and severe packet retransmissions.

Key Concept

Duplex auto-negotiation behavior and symptom identification in Ethernet switching
Question 437Question →

A network administrator installs a multimode fiber optic patch cable between two rack-mounted switches in a data center. Although physical link connectivity is established, the switch management interface logs a high rate of bit errors and optical power loss. An OTDR scan reveals a sudden, localized non-reflective loss at a point where the cable is tightly zip-tied around a sharp corner of the cable tray, with no glass break detected. Which of the following physical layer issues is the most likely cause of this signal degradation?

Show answer & explanation

Answer: Exceeding the minimum bend radius of the fiber optic cable

Answer

Exceeding the minimum bend radius of the fiber optic cable is the most likely cause of the optical signal attenuation.
Exceeding the cable's minimum bend radius causes macrobending, where light escapes from the core into the cladding due to micro-distortions in total internal reflection. This manifests as localized optical power loss and high bit error rates on an OTDR scan while still maintaining a physical link.

Step-by-Step Solution

1
Analyze the symptom and diagnostic findings reported in the scenario.
The link establishes physically, but experiences elevated optical signal loss and bit errors, with OTDR pointing to a sharp corner where the cable is tightly secured.
Identifying the localized physical anomaly helps isolate cause between physical breakage, connector fault, or geometry distortion.
2
Evaluate the optical properties of fiber under mechanical stress.
Bending fiber cable beyond its minimum bend radius alters the internal angle of incidence, causing light rays to leak out of the core into the cladding (macrobending loss).
Macrobending creates non-reflective attenuation at the bend site without causing a total optical discontinuity.
3
Select the corrective action and physical cause.
Loosening the zip-ties and re-routing the fiber cable to respect its minimum bend radius will restore full optical signal strength.
Restoring proper bend radius eliminates macrobending optical losses.

Key Concept

Fiber Optic Bend Radius and Macrobending Loss
Estimated Time:1m 15s
Question 438Question →

A network administrator is diagnosing several switching, trunking, and physical layer issues across access and core switches in an enterprise environment. Match each switch console symptom or log output on the left with its corresponding underlying root cause on the right.

Click a left item, then click its matching right item

Items

Console log: %CDP-4-NATIVE_VLAN_MISMATCH: Native VLAN mismatch discovered on interface GigabitEthernet0/1 (1), with Switch-B GigabitEthernet0/1 (99).
Inter-VLAN traffic for VLAN 20 fails across a trunk link, while trunk status shows operational and configuration displays 'switchport trunk allowed vlan 10,30,40'.
Interface FastEthernet0/12 reports high counts of late collisions and FCS errors after connecting a server with a manually forced 100 Mbps Full-Duplex network adapter.
An unconfigured switch connected to the network unexpectedly becomes the STP Root Bridge for VLAN 1 without any explicit priority modification.

Matches

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Answer

1. CDP Native VLAN mismatch log matches mismatched untagged 802.1Q native VLAN numbers on opposing ends of the trunk.
2. VLAN 20 traffic failure with allowed list '10,30,40' matches omission of the target VLAN ID from the allowed list.
3. Late collisions with forced full-duplex server match duplex mismatch caused by auto-negotiation falling back to half-duplex.
4. Unconfigured switch becoming STP root bridge matches default STP priority tie (3276832768) resolved by lower MAC address.
Each switch symptom directly corresponds to a distinct Layer 2 operational failure mode: CDP log warnings indicate mismatched untagged native VLAN IDs on 802.1Q trunks; missing VLAN IDs in the allowed trunk string cause explicit frame dropping for that VLAN; forced full-duplex on a connected device breaks auto-negotiation, defaulting the switch port to half-duplex and producing late collisions; and unconfigured switches claiming root bridge status stem from default STP priority ties resolved by MAC address comparisons.

Step-by-Step Solution

1
Analyze the CDP syslog error message for native VLAN parameters.
Identify that GigabitEthernet0/1 on the local switch uses VLAN 1 as native while Switch-B uses VLAN 99.
CDP detects mismatched native VLAN tags between link endpoints on 802.1Q trunks.
2
Examine the trunk interface allowed VLAN configuration string.
Observe that VLAN 20 is omitted from 'switchport trunk allowed vlan 10,30,40'.
Trunk ports drop traffic for any VLAN not explicitly included in the allowed VLAN list.
3
Evaluate the interface statistics for late collisions and FCS errors.
Determine that forced full-duplex on the server side causes Ethernet auto-negotiation on the switch port to fail and default to half-duplex.
Duplex mismatch occurs because the auto-negotiating side defaults to half-duplex when link pulses are detected without FLP (Fast Link Pulse) negotiation packets.
4
Investigate why a new switch with default configuration became the STP Root Bridge.
Conclude that all switches share the default priority of 3276832768, so the lowest MAC address wins the election.
STP root bridge selection evaluates Bridge Priority first, using MAC address as the secondary tie-breaker.

Key Concept

Diagnosing VLAN tagging, trunking allowed lists, Ethernet duplex auto-negotiation, and STP root election behaviors from switch symptoms.
Question 439Question →

A network technician is responding to a user ticket reporting a duplicate IP address error notification on VLAN 12 (10.12.0.0/2410.12.0.0/24). Place the following diagnostic and remediation steps in the correct chronological order from initial investigation to final resolution.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct sequence begins with gathering client interface configuration details (`ipconfig /all`), releasing the conflicting IP (`ipconfig /release`), conducting an ARP scan to locate the rogue static host, adding a DHCP scope exclusion for that static IP address, and renewing the workstation lease (`ipconfig /renew`).
Following standard operating procedure, the technician first gathers diagnostic info (`ipconfig /all`) to identify the conflicting IP. Next, releasing the address (`ipconfig /release`) stops active IP collisions on the client. With the client quieted, an ARP query isolates the MAC address of the statically assigned rogue device. Defining a DHCP scope exclusion for that IP prevents the server from issuing it again. Finally, `ipconfig /renew` requests a clean IP address for the workstation.

Step-by-Step Solution

1
Inspect local network settings on the affected host.
The exact conflicting IP address (10.12.0.4510.12.0.45) and current lease status are confirmed.
CompTIA troubleshooting methodology mandates identifying problem symptoms and gathering information prior to making changes.
2
Release the IP address lease on the client workstation using `ipconfig /release`.
The workstation stops broadcasting and listening on the conflicting IP address.
Releasing the lease clears local address binding so traffic on the subnet can be cleanly probed without interference from the client workstation.
3
Perform an ARP scan or ping test for 10.12.0.4510.12.0.45 from a remote administrative terminal.
The ARP table maps 10.12.0.4510.12.0.45 to the hardware MAC address of the statically misconfigured device.
Identifying the MAC address isolates the unmanaged static device causing the collision within the DHCP scope.
4
Configure an exclusion for 10.12.0.4510.12.0.45 in the DHCP scope settings.
The DHCP pool marks 10.12.0.4510.12.0.45 as unavailable for dynamic assignment.
Scope exclusions guarantee the DHCP server will not attempt to lease out statically assigned IP addresses.
5
Execute `ipconfig /renew` on the workstation.
The workstation successfully receives an unallocated, unique IP address from the DHCP pool.
Renewing after fixing the scope configuration restores full network connectivity without IP address collisions.

Key Concept

DHCP Address Conflict Remediation Workflow
Question 440Question →

A network administrator is troubleshooting connectivity to an internal updates repository on a Windows workstation. The workstation is failing to resolve the host name update.service.internal. To resolve the issue, the administrator wants to purge any cached domain name resolution records locally on the workstation and directly test hostname resolution using a specific internal DNS server located at 192.168.10.50. Which of the following commands should the administrator execute to accomplish these tasks? (Select TWO)

Select all that apply

Show answer & explanation

Answer: ipconfig /flushdns; nslookup update.service.internal 192.168.10.50

Answer

The administrator must execute 'ipconfig /flushdns' to clear the local DNS resolver cache and 'nslookup update.service.internal 192.168.10.50' to send a targeted query to the designated DNS server.
Executing 'ipconfig /flushdns' purges the Windows local DNS resolver cache, removing potentially stale or incorrect host records. Appending the IP address '192.168.10.50' after the domain name in 'nslookup' instructs the utility to query that designated DNS server directly rather than using the system's default DNS server configuration.

Step-by-Step Solution

1
Purge stale local DNS cache
Clears outdated or corrupted hostname records stored locally by Windows.
Executing 'ipconfig /flushdns' forces the client resolver to discard existing cached records so subsequent queries check the DNS server directly.
2
Test name resolution against a specific server
Sends a direct DNS lookup request for update.service.internal to 192.168.10.50.
Passing the target IP address as the second parameter in 'nslookup' overrides the default configured DNS server on the network interface.

Key Concept

Utilizing Windows CLI utilities for DNS resolver cache clearing and targeted DNS lookup queries.
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