CCNA Network Infrastructure and Connectivity Practice Test 27 — 15 Questions

Question 1

Refer to the exhibit. A network engineer is investigating intermittent connectivity complaints on a gigabit uplink between two distribution switches. The engineer runs the show interfaces GigabitEthernet0/0 command on one of the switches. Based on the output, what is the most likely cause of the errors?

Accepted Answer

A damaged or faulty cable is causing excessive CRC errors.. The output shows a high number of CRC errors and runts, which typically indicate a Layer 1 physical-layer issue such as a damaged or faulty cable. CRC errors occur when frames fail the cyclic redundancy check due to signal degradation, noise, or physical damage to the cabling. On a gigabit uplink, this is the most likely cause of intermittent connectivity.

Answer

The interface is configured with an incorrect encapsulation type.

Answer

A duplex mismatch exists between the connected devices.

Answer

The interface is assigned to the wrong VLAN.

Question 2

A user reports that their computer cannot access the network. The technician checks the computer's IP configuration and finds an APIPA address (169.254.x.x). The computer is connected to a switch port on VLAN 20. The DHCP server is located on VLAN 1. The technician then examines the router's interfaces using 'show ip interface brief' and sees that all interfaces shown are up/up. What should the technician do next?

Accepted Answer

Verify that the ip helper-address command is configured on the router's VLAN 20 interface.. The APIPA address (169.254.x.x) indicates the host failed to obtain a DHCP lease. Since the DHCP server is on VLAN 1 and the host is on VLAN 20, a DHCP relay (ip helper-address) must be configured on the router's VLAN 20 interface to forward DHCP broadcast messages to the server. The 'show ip interface brief' shows all interfaces are up/up, so the next logical step is to verify the relay configuration.

Answer

Check the DHCP server logs to see if it is receiving Discover messages.

Answer

Attempt to ping the DHCP server's IP address from the host's APIPA address.

Answer

Restart the DHCP service on the server and recheck the host.

Question 3

Refer to the exhibit. A network engineer is troubleshooting a connectivity issue between two routers connected via a serial link. The engineer runs the show interfaces Serial0/0/0 command on R1. Based on the output, what is the most likely cause of the problem?

Accepted Answer

The DCE end of the serial link is missing a clock rate configuration.. The output shows that the interface is up (line protocol is down), and the serial cable is physically connected. The absence of a clock rate on the DCE end of a serial link causes the line protocol to remain down because the receiving router cannot synchronize bit timing. Option D is correct because the DCE device must provide a clock signal for the serial link to establish Layer 2 connectivity.

Answer

The interface is configured with the wrong encapsulation type.

Answer

The interface is administratively shut down.

Answer

The serial cable is disconnected or has a physical fault.

Question 4

A technician is troubleshooting a connectivity issue where a workstation connected to a Cisco switch port cannot ping other hosts that are in the same VLAN 10 segment. The technician runs the show mac address-table command and notices that the workstation's MAC address is listed on VLAN 1, not VLAN 10. What is the most likely cause?

Accepted Answer

The switch port is not configured with the switchport access vlan 10 command.. The workstation's MAC address appears in VLAN 1 instead of VLAN 10 because the switch port is operating in the default VLAN (VLAN 1). The most likely cause is that the port has not been explicitly assigned to VLAN 10 using the `switchport access vlan 10` command. Without this command, the port remains in its default access VLAN (VLAN 1), so all frames from the workstation are associated with VLAN 1, preventing communication with hosts in VLAN 10.

Answer

The switch port is configured as a trunk with native VLAN 1.

Answer

The MAC address table contains a stale entry that must be cleared.

Answer

Spanning Tree Protocol has placed the port in a blocking state.

Question 5

A network engineer notices that a switch port connected to a legacy server is experiencing late collisions and the server reports excessive retransmissions. The switch port is configured for auto-negotiation and shows a negotiated speed of 100 Mbps and duplex full. The server's NIC is manually set to 100 Mbps and half-duplex. What is the most likely cause?

Accepted Answer

A duplex mismatch exists between the switch port and the server NIC.. Option C is correct because the switch port is auto-negotiating to full-duplex while the server's NIC is manually set to half-duplex. This creates a duplex mismatch: the switch transmits expecting no collisions (full-duplex), but the server, operating in half-duplex, detects collisions when the switch sends frames while the server is transmitting. Late collisions occur because the collision is detected after the first 64 bytes of the frame, and the server's half-duplex CSMA/CD logic forces retransmissions, matching the symptoms described.

Answer

The switch port is incorrectly configured for auto-negotiation and should be manually set to match the server's NIC.

Answer

The server's NIC is failing, causing cyclic redundancy check (CRC) errors and forcing retransmissions.

Answer

The switch port is overloaded by a broadcast storm, causing an excessive number of collisions.

Question 6

A network technician replaced a faulty SFP transceiver on a switch port. After replacement, the port remains in a down/down state. The technician verifies the fiber cable is securely connected at both ends and observes that the remote switch port is also in a down/down state. What should the technician do next?

Accepted Answer

Check the running configuration for the no shutdown command on the interface.. The correct answer is D because the most common cause of a port remaining in a down/down state after replacing a faulty SFP is that the interface is administratively down. The 'no shutdown' command must be applied to bring the interface up. Since the technician already verified physical connectivity and both ends show down/down, the issue is likely at the configuration layer, not the physical layer.

Answer

Verify that the speed and duplex settings are set to auto-negotiation.

Answer

Check whether the SFP module type is incompatible with the switch.

Answer

Verify the VLAN assignment on the port.

Question 7

Refer to the exhibit. An engineer is troubleshooting a connectivity issue where packets from the local network destined to host 192.168.1.200 are being dropped. The host 192.168.1.200 is configured with IP address 192.168.1.200/25, while the router’s GigabitEthernet0/0 interface is on the same physical segment. Based on the output, what is the most likely cause of the problem?

Accepted Answer

The router interface Gi0/0 is using a /24 subnet mask while the host is in a /25 subnet, making 192.168.1.200 appear as a local address to the router but off-subnet to the host.. The router's GigabitEthernet0/0 interface is configured with a /24 subnet mask, which means its directly connected network is 192.168.1.0/24. The host 192.168.1.200/25 has a subnet mask of /25, so its local network is 192.168.1.128/25. When the router receives a packet destined for 192.168.1.200, it considers the address to be within its own directly connected /24 subnet and attempts to forward it locally via ARP. However, the host's /25 mask causes it to treat 192.168.1.200 as a local address, but the router's /24 mask means the router expects the host to be on the same subnet without a gateway. The mismatch prevents the host from responding to ARP requests from the router for addresses outside its /25 range, leading to dropped packets.

Answer

The host 192.168.1.200 has an incorrect default gateway set, preventing it from communicating with the router.

Answer

An inbound access list on GigabitEthernet0/0 is blocking ICMP traffic from host 192.168.1.200.

Answer

The GigabitEthernet0/0 interface is in a shutdown state, preventing traffic from being forwarded to the host.

Question 8

A wireless client associates to an AP and successfully authenticates to the correct SSID, but it does not obtain an IP address. The WLC is running in local mode. What should the technician do next?

Accepted Answer

Verify the VLAN mapping on the WLC for the client’s WLAN.. Option C is correct because when a wireless client authenticates to the SSID but fails to obtain an IP address, the most likely cause is a VLAN mapping mismatch on the WLC. In local mode, the WLC maps the WLAN to a specific VLAN (via the interface or VLAN tag), and if that VLAN does not have a DHCP relay or is not trunked to the correct switch, the client's DHCP requests will never reach the DHCP server. This is a common Layer 2 connectivity issue that prevents IP address assignment even though authentication succeeds.

Answer

Check the DHCP server to ensure it has available leases.

Answer

Verify the AP’s operating channel for interference.

Answer

Verify the WPA3 PSK on the client.

Question 9

Refer to the exhibit. A network engineer is troubleshooting a serial link between two routers that is not coming up. The engineer issues the show controllers command on one router and sees the output shown. What is the most likely cause of the issue?

Accepted Answer

The clock rate command is missing on the DCE serial interface.. The 'show controllers' output indicates the router is the DCE (Data Communications Equipment) on the serial link, but no clock rate has been configured. For a serial interface to come up, the DCE end must provide clocking via the 'clock rate' command. Without it, the interface will remain down (line protocol down) because no clock signal is present to synchronize data transmission.

Answer

The serial cable type is incorrectly identified.

Answer

The encapsulation mismatch is causing the line protocol to stay down.

Answer

The serial interface is administratively shut down.

Question 10

A technician is troubleshooting a dual-stack network where an IPv6-only host cannot reach an IPv4 resource. The technician issues the show ipv6 interface brief command on the local router and notices the interface facing the host has a link-local address but no global unicast address. The technician then checks the running configuration and finds that the command ipv6 unicast-routing is missing. What is the most likely cause?

Accepted Answer

IPv6 unicast routing has not been enabled on the router.. The missing `ipv6 unicast-routing` command means the router is not acting as an IPv6 router, so it does not send Router Advertisement (RA) messages. Without RAs, the host cannot autoconfigure a global unicast address or learn a default gateway, breaking IPv6 connectivity to any IPv4 resource even if NAT64 is present.

Answer

An IPv6 access list on the router is blocking Router Advertisement messages.

Answer

The IPv4 resource is not configured for NAT64 translation.

Answer

The host has an incorrect default gateway for IPv6.

Question 11

Refer to the exhibit. An administrator notices that all interfaces on R1 are in an administratively down state. The administrator issues the show version command and sees the following output. What is the most likely cause of the issue?

Accepted Answer

The configuration register is set to 0x2142, causing the router to ignore the startup configuration.. The configuration register value 0x2142 tells the router to ignore the startup configuration during boot, loading only the default factory configuration. This causes all interfaces to be in an administratively down state because no interface configurations from the startup-config are applied. The administrator sees the interfaces as administratively down because the default configuration does not include any 'no shutdown' commands.

Answer

The router IOS image is corrupted and needs to be reinstalled.

Answer

The interfaces have been manually shut down in the running configuration, and the startup-config file is empty.

Answer

The router experienced a memory error during boot and failed to load the startup configuration.

Question 12

A technician is troubleshooting a connection between two routers, R1 and R2, connected back-to-back using Ethernet cables. Both routers have their interfaces configured and are in an 'up/up' state. R1's interface uses 192.168.1.1/24, and R2's interface uses 192.168.2.1/24. When the technician attempts to ping R2 from R1, the ping fails. What is the most likely cause?

Accepted Answer

The IP addresses assigned to the interfaces belong to different subnets, preventing direct Layer 3 communication.. The ping fails because R1's interface is configured with IP address 192.168.1.1/24, which places it in the 192.168.1.0/24 subnet, while R2's interface uses 192.168.2.1/24, placing it in the 192.168.2.0/24 subnet. For two devices to communicate directly at Layer 3 over a single Ethernet link, their IP addresses must belong to the same subnet. Since these addresses are in different subnets, R1 will see the destination as unreachable and will not even attempt to send an ARP request for R2's MAC address, resulting in a failed ping.

Answer

The Ethernet cable is faulty, causing intermittent physical layer failures.

Answer

A duplex mismatch exists between R1 and R2, causing one-way communication.

Answer

A routing protocol is not configured to allow the routers to learn about each other's directly connected networks.

Question 13

A network engineer notices that after issuing the no shutdown command on interface GigabitEthernet0/0 of a router, the interface remains down. The output of show interfaces GigabitEthernet0/0 displays 'GigabitEthernet0/0 is up, line protocol is down'. The physically connected switch port is also administratively down. What is the most likely cause?

Accepted Answer

The connected switch port is administratively down.. The show interfaces output indicates that the router interface is physically up (Layer 1 is functional) but the line protocol is down (Layer 2 is not active). The most common cause for this specific combination is that the connected switch port is administratively down (shutdown), which prevents the switch from sending any keepalive frames or establishing a link. Since the switch port is not forwarding traffic, the router's line protocol cannot come up, even though the router interface itself is no longer in shutdown mode.

Answer

Mismatched encapsulation types on the router and switch.

Answer

Speed and duplex mismatch between the router and switch.

Answer

Incorrect native VLAN configuration on the trunk link.

Question 14

Refer to the exhibit. A network engineer is troubleshooting an OSPFv2 neighbor adjacency that remains in the DOWN state between R1 and R2. The engineer issues the show cdp neighbors detail command on R1 and confirms that R1's GigabitEthernet0/1 interface is configured with IP address 192.168.1.1/30. What is the most likely cause of the problem?

Accepted Answer

The IP addresses on the connected interfaces are in different subnets.. The correct answer is C because OSPFv2 requires that neighboring interfaces share a common subnet to form an adjacency. The show cdp neighbors detail output confirms R1's GigabitEthernet0/1 is configured with IP address 192.168.1.1/30, but if R2's interface is on a different subnet (e.g., 192.168.2.0/30), the routers will not be able to communicate OSPF Hello packets, leaving the neighbor state in DOWN. This is a fundamental OSPF neighbor formation requirement, and a subnet mismatch will prevent the adjacency from progressing past the DOWN state.

Answer

The OSPF area IDs configured on R1 and R2 do not match.

Answer

There is an MTU mismatch between the two routers.

Answer

OSPF authentication is configured incorrectly on one of the routers.

Question 15

Refer to the exhibit. A network engineer is troubleshooting a connectivity issue on R1. The serial link to R2 on interface Serial0/1 is using HDLC encapsulation, and the physical cable has been verified as good. The engineer has confirmed that the encapsulation type matches on both routers and that the clock rate is correctly configured on the DCE end. Based on the output, what is the most likely cause of the line protocol down state on Serial0/1?

Accepted Answer

The keepalive packets are not being received, causing the protocol to stay down.. The exhibit shows Serial0/1 with status 'up' and protocol 'down' (up/down). This indicates Layer 1 is operational but the Layer 2 Data Link layer is not. Since the engineer already verified that encapsulation (HDLC) matches on both ends and the clock rate is correctly set, the only remaining common cause for a serial interface in up/down state is a keepalive failure. HDLC sends periodic keepalive messages across the link; if they are not received, the line protocol remains down.

Answer

The encapsulation type is mismatched between R1 and R2.

Answer

The clock rate has not been configured on the DCE end of the serial link.

Answer

The IP address configured on Serial0/1 conflicts with another interface.

CCNA Network Infrastructure and Connectivity Practice Test 27 — 15 Questions

Exhibit

CCNA Network Infrastructure and Connectivity Practice Test 27 — 15 Questions

Exhibit