CCNA Network Troubleshooting Questions

The 'show vlan brief' command displays VLAN membership and ports assigned to each VLAN, but it does not show the allowed VLAN list on a trunk port. The trunk's allowed VLAN list is specifically shown by 'show interfaces trunk'.

The 'show running-config interface' command displays the current configuration of an interface, including VLAN assignments and trunk settings, but it does not show the dynamically learned allowed VLAN list or the operational trunk status. For verifying the trunk's allowed VLAN list, 'show interfaces trunk' is needed.

The 'show mac address-table' command displays MAC address entries learned by the switch, not the allowed VLAN list on a trunk port. It does not provide information about which VLANs are permitted on the trunk link.

The router can ping the DHCP server, so the server is reachable. The issue is that users get APIPA addresses, indicating DHCP requests are not reaching the server or replies are not returning, but reachability is confirmed.

The router can ping the DHCP server, indicating the ip helper-address is correctly configured and reachable. The issue is that the DHCP server's scope lacks a subnet for VLAN 10, not the helper-address configuration.

The question states users can ping the DHCP server from the router, indicating connectivity. Switch port misconfiguration for VLAN 10 would prevent all communication, but users can still obtain APIPA addresses, meaning they are on VLAN 10 but DHCP fails. The issue is at the DHCP server scope, not switch ports.

An unplugged network cable would result in no IP address or a disconnected status, not an Automatic Private IP Addressing (APIPA) address like 169.254.23.45. APIPA addresses are assigned when DHCP fails, but the link is still active.

The IP address 169.254.23.45 is an Automatic Private IP Addressing (APIPA) address, which indicates that the workstation failed to obtain an IP from a DHCP server. DNS issues do not cause APIPA addresses; they would result in name resolution failures but the workstation would still have a valid IP.

A static IP in the wrong subnet would typically result in a different IP address (not an APIPA 169.254.x.x address) and would not cause the workstation to self-assign an APIPA address. The 169.254.23.45 address indicates DHCP failure, not a static misconfiguration.

The default gateway on the server is misconfigured would prevent the server from reaching external networks, but here users can access external websites normally and can ping the server's IP, indicating the server is reachable. The issue is that users cannot reach the wiki server's web service, which is a higher-layer problem.

DNS cache poisoning would cause incorrect IP resolution, but the question states that DNS returns the correct IP and ping to the server succeeds, so the issue is not DNS-related.

The link lights are solid, indicating physical connectivity is present, so cable integrity is not the next logical step. The issue is likely at Layer 2 (switch port configuration) since the workstation has a valid IP but cannot reach the gateway.

DNS server settings affect name resolution, not IP-level connectivity to a gateway. Since the workstation has a valid IP but cannot ping the gateway, the issue is at Layer 2 or Layer 3, not DNS.

The workstation has a valid IP address and link lights are solid, indicating Layer 1 and Layer 3 configuration are likely correct. The inability to ping the gateway despite a valid IP suggests a Layer 2 issue, such as VLAN mismatch, not an ARP cache problem.

Incorrect DNS configuration would prevent name resolution but not direct IP pings; the user can ping the gateway (IP) but not a public IP, so the issue is routing, not DNS.

A duplicate IP address would cause intermittent connectivity or address conflict errors, but here pinging the default gateway succeeds, indicating the workstation has valid IP communication locally. The failure is specifically to external IPs, which points to a routing issue, not a duplicate address.

The issue is that pinging a public IP (8.8.8.8) fails while the default gateway is reachable, indicating a routing problem beyond the local network. Proxy server configuration affects application-layer traffic, not ICMP to a public IP, and the firewall permits outbound ICMP, so a proxy is not the cause.

The router's subinterfaces are configured, so they likely have IP addresses. The issue is the switch port is an access port in VLAN 1, preventing tagged traffic from VLANs 10 and 20 from reaching the router.

The question states that users within each VLAN can communicate, which means the VLANs are already created on the switch. The issue is inter-VLAN routing, which fails because the switch port to the router is an access port in VLAN 1, not a trunk carrying both VLANs.

The router's default route is used for reaching networks not directly connected, not for inter-VLAN routing. Since the router has subinterfaces for VLAN 10 and 20, it can route between them directly without a default route. The issue is that the switch port is an access port, preventing VLAN-tagged traffic from reaching the router.

The IP address 169.254.1.5 is an Automatic Private IP Addressing (APIPA) address, which indicates that the workstation failed to obtain an IP from a DHCP server. A down DNS server would not cause the workstation to self-assign an APIPA address; it would still receive a valid IP via DHCP.

The workstation has an APIPA address (169.254.x.x), which indicates DHCP failure, not an incorrect default gateway. An incorrect default gateway would still allow local network access but not external routing, and the IP would be valid from DHCP or static.

A mismatched subnet mask would not cause the workstation to obtain an APIPA address (169.254.x.x); it would instead cause connectivity issues within the subnet while the IP address would still be assigned via DHCP or static configuration.

The question asks to identify the specific router causing packet loss after the third hop. Ping only tests end-to-end connectivity and does not show the path or which hop is failing.

nslookup is used for DNS resolution, not for tracing the network path or identifying packet loss at specific hops. It cannot show where packet loss occurs along the route.

The ipconfig /all command displays local network configuration details (IP address, MAC, DNS, etc.) but does not perform any path tracing or connectivity testing to remote hosts, so it cannot identify which router is causing packet loss.

CRC errors on a fiber link with link lights on typically indicate signal degradation or physical layer issues, not duplex mismatches, which usually cause frame check sequence errors or late collisions on copper links, not on single-mode fiber.

The question states the link is 10 km, which is within the typical range for single-mode fiber (up to 40 km or more). CRC errors are more likely due to signal degradation or dirty connectors, not cable length.

Duplex mismatch typically causes constant errors or complete link failure, not intermittent flapping. The symptoms here (up/down flapping with acceptable optical power) point to a physical layer issue like dirty connectors, not a duplex configuration problem.

Speed mismatch typically causes a link to not come up at all or to flap continuously, but the question states optical power levels are within range, and the issue is intermittent. Speed mismatch would usually be constant, not intermittent, and is less common on fiber links with auto-negotiation.

A VLAN mismatch would cause connectivity issues but typically results in a stable link state (up/up) with no communication, not intermittent flapping of the link status. The link flapping indicates a physical layer problem, not a layer 2 configuration issue.

Duplex mismatch typically causes CRC errors and frame check sequence failures, which would appear in interface error counters. Since the technician checked and found no errors, duplex mismatch is unlikely.

Incorrect MTU typically causes packet fragmentation or drops, not varying latency with no interface errors. The symptoms of high and varying latency without errors point to CPU overload, not MTU issues.

Route flapping typically causes intermittent connectivity and routing table instability, not consistently high latency with varying ping responses and no interface errors.

High output power can cause co-channel interference and cell overlap, but the question states channel utilization is above 80%, which points to congestion from too many APs on the same or overlapping channels, not power levels.

Intermittent disconnections with high channel utilization are more likely caused by co-channel interference from too many access points on the same or overlapping channels, not by microwave ovens, which typically cause non-Wi-Fi interference that is constant rather than intermittent.

Weak signal strength causes low throughput or disconnections at the client, but the question states channel utilization is consistently above 80% on the 2.4 GHz band, indicating congestion from many devices or access points, not weak signal.

The issue is that only one user cannot resolve the domain name, while others can. This points to a client-side DNS issue, not a server-side DNS configuration problem. Option B (DNS server configuration) would affect all users on the subnet, not just one.

Option C is not specified, but assuming it refers to checking the DNS server settings on the client, the issue is that other users on the same subnet can access the website by domain name, so the DNS server is working correctly for the subnet. The problem is isolated to the single user's machine, so checking DNS server settings is not the first step.

The issue is that only one user cannot resolve the domain name while others can, pointing to a client-side DNS issue. Option D (checking the DNS server configuration) is unnecessary because the DNS server is working for other users on the same subnet.

The question states that the default route points to an unreachable next-hop IP, and the administrator can ping the ISP's gateway. This indicates a static route misconfiguration, not a redistribution issue. Redistribution would only matter if a dynamic routing protocol were involved, but the problem is with a static default route.

The WAN interface is up (as stated), so it is not administratively down. An administratively down interface would show as 'down' or 'disabled', not 'up'.

The firewall blocking traffic to the cloud would not cause the router to have an unreachable next-hop IP for the default route; it would instead prevent traffic from passing through the firewall, but the router would still have a valid route.

The user can access the internet but not the internal web app, and nslookup returns 'Non-existent domain', indicating a DNS resolution failure. The web server being down on port 443 would not cause a DNS lookup to fail; it would cause a connection timeout or refusal after successful resolution.

The technician can ping the server's IP address successfully, indicating that network connectivity and firewall rules are not blocking traffic to the server. The issue is DNS resolution, not firewall filtering.

The user can access the internet and ping the server IP, but nslookup returns 'Non-existent domain', indicating a DNS resolution failure, not a browser misspelling. A misspelling would still result in a DNS query, not a 'Non-existent domain' error.