Troubleshooting Scenario Questions

show interfaces status

Why it fails: show interfaces status displays speed, duplex, and VLAN assignment for each port, but it does not show Spanning Tree Protocol states. It can indicate connected or notconnected, but not whether STP has blocked a port. This command is useful for physical layer verification, not for detecting STP topology changes or loops.

show running-config

Why it fails: show running-config displays the current configuration, including STP settings, but it does not show operational STP states or port roles. It can confirm if STP is enabled, but not whether ports are currently blocking. For real-time STP status, operational commands like show spanning-tree are required.

show mac address-table

Why it fails: show mac address-table lists MAC addresses learned per port. A loop can cause MAC address flapping, where the same MAC appears on multiple ports, but this command alone does not show STP states. While useful for detecting flapping, it does not directly verify whether STP has blocked ports to prevent a loop.

Reconfigure the default gateway on all hosts in VLAN 20 to point to the VLAN 40 SVI address.

Why it fails: Hosts should point to the gateway address of their own VLAN, not to another VLAN's interface. Changing VLAN 20 clients to use the VLAN 40 SVI would break their existing connectivity to VLAN 30 as well. This misconfiguration would widen the outage instead of fixing the VLAN 40 reachability problem.

Replace the Layer 3 switch with a router that has physical interfaces for each VLAN.

Why it fails: Replacing hardware is a drastic action that ignores diagnosis. The existing Layer 3 switch already routes between two working VLANs, so its routing capability is proven. The failure is isolated to one destination VLAN, which points to a configuration or status issue rather than a hardware limitation. This step would disrupt the working network without addressing the root cause.

Disable inter-VLAN routing and rely on Layer 2 flooding to deliver traffic between VLANs.

Why it fails: Layer 2 flooding cannot cross VLAN boundaries because each VLAN is a separate broadcast domain. Disabling routing would eliminate the working connectivity between VLAN 20 and VLAN 30 and would not make VLAN 40 reachable. This action misunderstands how inter-VLAN communication actually occurs, which always requires a Layer 3 function.

show cdp neighbors gigabitEthernet 0/1

Why it fails: show cdp neighbors displays information about directly connected Cisco devices, including platform and port ID. It does not show duplex settings or error statistics for the local interface. While CDP can help map topology, it will not reveal a duplex mismatch or related errors on the port.

show controllers gigabitEthernet 0/1

Why it fails: show controllers is typically used on serial interfaces to display hardware-level information about the controller, such as cabling and clocking. It is not commonly used for Ethernet duplex or error statistics. On Ethernet ports, it may not provide duplex status or the specific error counters needed to diagnose a duplex mismatch.

show ip interface gigabitEthernet 0/1

Why it fails: show ip interface displays Layer 3 information such as IP address, subnet mask, and access lists applied to the interface. It does not show duplex settings or Layer 1/2 error statistics. Duplex mismatches are physical and data link layer issues, so this command will not provide the needed counters.

Dynamic Host Configuration Protocol (DHCP)

Why it fails: DHCP is responsible for assigning IP addresses to clients. Since the user can ping a public IP address, they clearly have a valid IP, subnet mask, and default gateway already assigned to their interface. If DHCP were failing, the user would likely have no connectivity at all or would be using an APIPA address.

Network Address Translation (NAT)

Why it fails: NAT allows private internal IP addresses to be translated into public addresses for internet access. If NAT were failing, the user would not be able to ping the public IP address '72.163.4.161' because their private packets would not be routable on the internet. Successful pinging proves that NAT is currently working correctly.

Address Resolution Protocol (ARP)

Why it fails: ARP resolves IP addresses to MAC addresses on the local network segment. If ARP were failing, the computer would not be able to send any packets to the local gateway, which would prevent both the ping to the IP and the attempt to reach the website. ARP is clearly functioning if any external traffic is successful.

Check the switch interface status.

Why it fails: The switch interface status tells you if the physical link is up or down. While this is important, it does not tell you if the workstation has been assigned a valid IP address by DHCP or static configuration. It only confirms that the physical layer is connected.

Perform a traceroute to the local gateway.

Why it fails: A traceroute to the local gateway is meant to test path reliability. If the PC does not have a valid IP address, it will not be able to generate the packets necessary for a traceroute, making this test useless until the basic IP addressing has been verified first.

Reset the switch configuration.

Why it fails: Resetting a switch is a destructive action that will cause significant network downtime. It is never the correct step for troubleshooting a single workstation's IP addressing issue. This would impact every other device connected to the switch and is completely disproportionate to the problem of a single host.

The MAC address of the DHCP server.

Why it fails: While the client learns the server's MAC address through the Ethernet header of the DHCP messages, it is not a 'parameter' provided within the DHCP ACK payload for the client's configuration. The client does not store the server's MAC address as part of its permanent networking configuration settings.

The SSH private key for the management console.

Why it fails: DHCP is used for basic network provisioning and does not handle the distribution of sensitive security credentials like SSH private keys. Security features like SSH require separate configuration or specialized deployment tools. Including such information in a DHCP broadcast would be a significant security risk to the entire network environment.

Incorrect DNS server

Why it fails: While an incorrect DNS server prevents users from resolving hostnames to IP addresses, it does not stop a technician from reaching remote resources via direct IP pings. Since the technician cannot reach any resources outside the subnet, the issue lies deeper in the routing logic than simple name resolution services.

Duplicate IP address

Why it fails: A duplicate IP address usually causes intermittent connectivity or total failure for the specific host involved, often triggering an ARP error message on the operating system. It would typically affect local connectivity as much as remote connectivity, which does not match the symptoms of local success but remote failure.

Faulty Cat6 cabling

Why it fails: Faulty cabling would generally impact all network traffic, regardless of whether the destination is local or remote. If the technician can successfully ping local servers, the physical layer connectivity between the host and the switch is confirmed to be working, ruling out a basic hardware failure in the cable.

The physical cable is disconnected.

Why it fails: If the physical cable were disconnected, the interface status would show as 'down/down' or 'administratively down'. The 'up' status for the first part of the output indicates that the hardware detects a carrier signal from the connected endpoint, confirming that the physical electrical connection is intact.

The switch port has been administratively disabled.

Why it fails: If the administrator had manually disabled the port using the shutdown command, the output would clearly state 'administratively down, line protocol is down'. Since the interface is 'up', it means the port is enabled and attempting to communicate with the device on the other end.

The MAC address on the server is incorrect.

Why it fails: MAC addresses are burned into the hardware and do not cause a line protocol to go down simply by being 'incorrect'. The switch learns MAC addresses from incoming frames; while a security policy like Port Security might disable a port, it would typically result in an err-disabled state.

The DNS server address is missing.

Why it fails: A missing DNS server would prevent the host from resolving hostnames to IP addresses, but it would not prevent communication with remote hosts if their IP addresses were used directly. The scenario describes a complete lack of reachability to other subnets, which is a fundamental routing issue at the network layer.

The subnet mask is too restrictive.

Why it fails: If the subnet mask were too restrictive, the host might misidentify local hosts as being on a remote network or vice versa. However, the host can already communicate with local hosts correctly. The inability to reach 'any' host on 'different' subnets points toward a failure to pass traffic to the router.

The NIC is configured for the wrong duplex mode.

Why it fails: A duplex mismatch would cause performance issues, collisions, and packet loss for all types of traffic, including local communication. Since the host can communicate perfectly with other hosts on the same subnet, the physical link and basic framing are operational, ruling out a simple duplex or physical layer mismatch.

The switch has learned the MAC address but the port is blocked.

Why it fails: If the switch had learned the MAC address, it would be listed in the table regardless of whether Spanning Tree was blocking the port. The absence of the MAC address indicates that no frames have reached the switch's processing engine from that specific physical port since the table was last cleared.

Port Fa0/2 is configured as a trunk port and is filtering traffic.

Why it fails: Trunk ports still learn MAC addresses for all the VLANs they carry. If Fa0/2 were a trunk, you would likely see multiple MAC addresses from various VLANs associated with it. The total absence of the port in the table indicates a lack of any incoming traffic, not a specific VLAN filtering issue.

The host on Fa0/2 is using a static IP address.

Why it fails: The use of a static IP address does not prevent a switch from learning a MAC address. As long as the host sends any Ethernet frame (such as an ARP request or a gratuitous ARP), the switch will record the source MAC address. The MAC table tracks hardware addresses, which are independent of IP configuration.

The routers are using different routing protocols.

Why it fails: Routing protocols like OSPF or EIGRP are used to share information about 'remote' networks. They are not required for two routers to ping each other on a directly connected link. Direct connectivity only requires correct Layer 1 and Layer 2 operation and matching Layer 3 subnet configurations on both interfaces.

The DNS server is not configured on the routers.

Why it fails: DNS is used to resolve hostnames to IP addresses. Since the technician is attempting to ping the interface IP address directly, DNS is not involved in the process. The failure of an IP-to-IP ping indicates a more fundamental problem with the network layer configuration or the path between the two devices.

The 'no shutdown' command was not issued.

Why it fails: The scenario states that both interfaces are in an 'up/up' state. If the 'no shutdown' command had not been issued, the interfaces would show as 'administratively down'. Since they are 'up', the physical and data link layers are functional, and the issue must lie within the logical IP configuration settings.

The cable is disconnected.

Why it fails: If the cable were disconnected, the interface status would show as 'down/down'. The 'up' portion of the status indicates that the physical layer (Layer 1) detects a signal, meaning the link is physically connected, but the data link layer is failing.

The IP address is incorrectly configured.

Why it fails: IP configuration errors are Layer 3 issues. If the IP address were incorrect or duplicated, the interface status would still show as 'up/up', as Layer 3 errors do not prevent the Layer 1 and Layer 2 link from initializing and establishing connectivity.

The router is administratively shut down.

Why it fails: If an interface is administratively shut down, it is explicitly disabled by the administrator. This causes both the physical and data link layers to be down, resulting in an 'administratively down' status, which differs from the 'up/down' state observed in the exhibit.

The specific MAC address of the destination device.

Why it fails: The ping command operates at Layer 3 of the OSI model and does not display the MAC address of the destination in its standard output. While the ARP table may be updated as a side effect of the ping, the ICMP reply itself only confirms IP-level reachability and packet timing.

The list of every router hop along the path.

Why it fails: Identifying every router hop is the function of the 'traceroute' command, not 'ping'. Ping only confirms that the destination is reachable and provides statistics on that specific end-to-end path. It does not provide visibility into the intermediate devices or the specific path taken by the packets through the internetwork.

The TCP port numbers that are currently open on the host.

Why it fails: ICMP is a separate protocol from TCP and does not use port numbers. A ping cannot determine which applications or services are listening on the destination host. To check for open ports, a technician would need to use a port scanner or attempt a specific TCP connection like Telnet or SSH.

The switch needs the ip routing global configuration command enabled so that it can forward frames between VLAN 10 and VLAN 20.

Why it fails: The ip routing command enables Layer 3 routing functionality on a multilayer switch that supports it, but this branch switch is described as a Layer 2 access switch with no routing capability. Enabling that command on a Layer 2-only platform has no effect and would not create a path between VLANs. The real issue is the absence of any Layer 3 device, not a disabled feature on the access switch.

The access ports must be configured as trunk ports so that frames from both VLANs can be carried between them.

Why it fails: Trunk ports carry traffic for multiple VLANs between network devices such as switches, but they do not route traffic between VLANs. Configuring an access port as a trunk would not allow a host in VLAN 10 to reach a host in VLAN 20, because the frames still need a Layer 3 decision. The problem is the missing router or multilayer switch, not the port mode of the access ports.

Spanning Tree Protocol has blocked the redundant path between the two VLANs, preventing traffic from flowing between them.

Why it fails: Spanning Tree Protocol operates within a Layer 2 topology to prevent loops by blocking redundant links; it does not create or block paths between separate VLANs. VLAN 10 and VLAN 20 do not share a Layer 2 forwarding path at all, so STP state is irrelevant to their mutual reachability. The connectivity failure stems from the lack of Layer 3 routing, not from a blocked spanning-tree port.

The local Layer 2 switch

Why it fails: Layer 2 switches operate by mapping MAC addresses to ports and do not possess the routing tables or logic required to forward packets between different IP subnets. They rely on destination MAC addresses, which are only significant within the local broadcast domain, leaving the inter-subnet traffic unrouted.

The network interface card (NIC)

Why it fails: The NIC is a physical layer component responsible for converting data into electrical, optical, or radio signals for transmission onto the medium. It lacks the network layer intelligence to perform routing calculations or participate in path determination across different IP subnets in a routed network.

The ARP cache on the workstation

Why it fails: The ARP cache maps IP addresses to MAC addresses for local communication within the same subnet. When a destination IP is outside the local range, the device uses the default gateway address, but the cache itself cannot perform the routing function across different subnets.