CCNA Practice Test 24 — 15 Questions

Question 1

A network administrator is troubleshooting slow file transfers between two servers in different access-layer switches. The administrator runs the 'show interface' command on the uplink connecting the two switches and notices a high number of CRC errors on both ends, but a high number of late collisions on only one interface; the other interface reports no late collisions.

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Duplex mismatch: the interface with late collisions is half-duplex while the other end is full-duplex. Configure both ends to auto-negotiate speed and duplex.. A is correct because a duplex mismatch causes one interface to operate in half-duplex (detecting late collisions due to CSMA/CD) while the other operates in full-duplex (no collisions). The half-duplex interface waits for the carrier sense before transmitting, but the full-duplex interface transmits immediately, causing the half-duplex side to detect collisions after the transmission window (late collisions). CRC errors occur on both ends because frames are corrupted when collisions happen. Configuring both ends to auto-negotiate ensures matching duplex and speed, resolving the issue.

Answer

Speed mismatch: one interface is set to 100 Mbps, the other to 1 Gbps. This causes frequent link flaps, resulting in CRC errors. Use the 'speed' command to match the rates.

Answer

Faulty Ethernet cable causing signal degradation, which leads to CRC errors. The late collisions are a result of the switch misdetecting collisions due to the degraded signal. Replace the cable.

Answer

A broadcast storm caused by a loop is flooding the uplink with frames, leading to CRC errors and late collisions as the switch discards excess traffic. Enable Spanning Tree Protocol to block the redundant path.

Question 2

A network engineer configures a static route: ip route 192.168.10.0 255.255.255.0 10.1.1.2. The next hop 10.1.1.2 is reachable via OSPF. Later, the engineer notices that the route to 192.168.10.0/24 has disappeared from the routing table. What is the most likely cause?

Accepted Answer

The OSPF route to 10.1.1.0/30 has been lost, making the next-hop address 10.1.1.2 unresolvable.. The static route to 192.168.10.0/24 uses 10.1.1.2 as the next-hop address. For a static route with a next-hop IP (rather than an exit interface) to be installed in the routing table, the next-hop must be reachable via a valid route in the routing table. If the OSPF route to the subnet containing 10.1.1.2 (e.g., 10.1.1.0/30) is lost, the next-hop becomes unreachable, and the static route is removed from the routing table. This is the most likely cause of the route disappearing.

Answer

The static route has a higher administrative distance than OSPF, so OSPF's route to 192.168.10.0/24 replaced it.

Answer

The static route uses a next-hop IP address that is not directly connected, which is unsupported on this platform.

Answer

A routing loop caused by recursive lookups has suppressed the static route to prevent loops.

Question 3

A network engineer notices that after removing a standard ACL that was applied inbound on the internet-facing interface, the router is now receiving IP packets from the internet with source IP addresses in the 10.0.0.0/8 range, which were previously blocked. What is the most likely cause?

Accepted Answer

Removing the ACL from the interface eliminates the implicit deny at the end and restores the default permit all behavior, allowing all incoming traffic.. When a standard ACL is removed from an interface using the 'no ip access-group' command, the implicit deny any at the end of the ACL is also removed. By default, Cisco routers permit all IP traffic on an interface unless an ACL is applied to filter it. Therefore, after removal, the router reverts to its default permit-all behavior, allowing previously blocked traffic like source IPs in the 10.0.0.0/8 range.

Answer

The original standard ACL only had a permit statement, so after removal the permit still takes effect because the ACL remains in the running configuration.

Answer

The ip access-group command on the interface remains but is missing the referenced ACL, causing the router to default to denying all ingress traffic except the previously permitted 10.0.0.0/8.

Answer

The ACL was reapplied in the outbound direction instead of inbound, so it now blocks traffic leaving the interface but not entering it.

Question 4

A network engineer notices that a newly connected switch-to-switch link is up, but traffic from multiple VLANs is not passing. When issuing the show interfaces trunk command, no trunk ports are listed. Both switch ports are configured with switchport mode dynamic auto. What is the most likely cause?

Accepted Answer

Both ports are set to dynamic auto, so neither switch initiates DTP negotiation.. When both switch ports are configured with switchport mode dynamic auto, neither actively initiates Dynamic Trunking Protocol (DTP) negotiation. DTP dynamic auto ports wait for the other side to send DTP frames to form a trunk; since both sides are passive, the link remains in access mode and does not become a trunk, preventing traffic from multiple VLANs from passing.

Answer

There is a native VLAN mismatch between the two switches.

Answer

The connecting cable is a straight-through Ethernet cable rather than a crossover cable.

Answer

One switch lacks a VLAN that exists on the other switch.

Question 5

A network engineer notices that Host A in VLAN 10 (10.10.10.50/24) can successfully ping its default gateway 10.10.10.1, but cannot ping the VLAN 20 SVI (10.20.20.1) or any hosts in VLAN 20. The SVIs for both VLAN 10 and VLAN 20 are in an up/up state, and the switch's trunk ports are correctly allowing both VLANs. What is the most likely cause?

Accepted Answer

The ip routing global configuration command is missing.. Host A can reach its default gateway (10.10.10.1), which is the VLAN 10 SVI, but cannot reach the VLAN 20 SVI (10.20.20.1) or any hosts in VLAN 20. This indicates that Layer 2 connectivity is working (trunk allows both VLANs, SVIs are up/up), but inter-VLAN routing is failing. On a multilayer switch, inter-VLAN routing requires the global command 'ip routing' to enable the switch's IP routing engine; without it, the switch acts as a Layer 2 device only and cannot forward packets between different VLANs.

Answer

The VLAN 20 SVI is administratively shut down.

Answer

The trunk between the access switch and the Layer 3 switch is misconfigured as an access port.

Answer

The default gateway on Host A is configured incorrectly.

Question 6

After connecting a new switch to interface GigabitEthernet1/0/1 on a distribution switch, a network engineer notices that the interface is in err-disable state. The engineer checks the configuration and finds that spanning-tree portfast and spanning-tree bpduguard enable are applied to the interface. What is the most likely cause of the err-disable state?

Accepted Answer

Spanning-tree PortFast is enabled on a port that connects to another switch.. PortFast is designed for end-host ports that should not receive BPDUs. When PortFast is enabled on a port connecting to another switch, the switch will immediately transition the port to forwarding state, but if it then receives a BPDU from the connected switch, BPDU Guard will error-disable the port. This is the most common cause of err-disable state when both PortFast and BPDU Guard are configured on an inter-switch link.

Answer

BPDU Guard was incorrectly enabled on a port that should be a trunk link.

Answer

The connected switch is sending BPDUs with a lower bridge priority.

Answer

The port is configured as an access port, but should be a trunk.

Question 7

A network administrator is troubleshooting slow file transfers between two servers in different access-layer switches. The administrator runs the 'show interface' command on the uplink connecting the two switches and notices a high number of CRC errors on both ends, but a high number of late collisions on only one interface; the other interface reports no late collisions.

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

The switch port connecting the host is configured in the wrong VLAN (e.g., VLAN 20 instead of VLAN 10).. The captured traffic on the trunk shows that the router is not receiving any frames tagged with VLAN 10 from the host. If the switch port connecting the host is configured in VLAN 20 instead of VLAN 10, the host's frames will be tagged with VLAN 20 (or remain untagged in the access VLAN 20) and will not reach the router's SVI for VLAN 10, causing the ping to fail. This is the most direct cause given the symptom that the host cannot ping the default gateway.

Answer

The router's SVI for VLAN 10 is administratively down.

Answer

An inbound ACL on the router's SVI is blocking ICMP echo requests from the host.

Answer

The router has ICMP redirects enabled, causing it to ignore the pings.

Question 8

Match each operations or assurance technology to its most accurate purpose.

Accepted Answer

NetFlow - Flow analysis; SNMP - Device management; Syslog - Logging; IP SLA - Performance measurement; SPAN - Local traffic mirroring; RSPAN - Remote traffic mirroring. Each technology serves a specific assurance purpose: Syslog provides centralized event and message reporting, SNMP enables monitoring and management information exchange, NetFlow offers visibility into traffic flows and conversations, and NTP ensures clock synchronization for consistent timing across network devices.

Answer

NetFlow - Device management; SNMP - Flow analysis; Syslog - Performance measurement; IP SLA - Logging; SPAN - Remote traffic mirroring; RSPAN - Local traffic mirroring

Answer

NetFlow - Logging; SNMP - Performance measurement; Syslog - Flow analysis; IP SLA - Device management; SPAN - Remote traffic mirroring; RSPAN - Local traffic mirroring

Answer

NetFlow - Performance measurement; SNMP - Logging; Syslog - Device management; IP SLA - Flow analysis; SPAN - Local traffic mirroring; RSPAN - Remote traffic mirroring

Question 9

A network engineer configures an EtherChannel between two Cisco switches SW1 and SW2 using LACP. After configuration, hosts connected to SW1 report intermittent connectivity to hosts on SW2. The engineer checks the EtherChannel status and sees that the trunk is up but only allows VLAN 1, while the hosts communicate across VLANs 10 and 20. Which command should the engineer apply to both switches to resolve the issue?

Accepted Answer

switchport trunk allowed vlan 1,10,20. The output shows the EtherChannel is up but only VLAN 1 is allowed on the trunk, while the hosts on SW1 and SW2 communicate across VLANs 10 and 20. Applying 'switchport trunk allowed vlan 1,10,20' on both switches ensures all necessary VLANs are permitted over the EtherChannel, resolving the intermittent connectivity caused by dropped traffic for VLANs 10 and 20.

Answer

channel-group 1 mode active

Answer

lacp rate fast

Answer

switchport mode trunk

Question 10

Refer to the exhibit. An administrator is troubleshooting connectivity from a branch router R1 to the internet. A ping to 8.8.8.8 from R1 fails. The output of the show ip route command is shown. What is the most likely cause?

Accepted Answer

The next-hop IP address for the default route is not reachable.. The ping to 8.8.8.8 fails because the default route (0.0.0.0/0) points to a next-hop IP address that is not reachable. The routing table shows the default route is present, but if the next-hop address is not in the routing table via a connected or static route, the router cannot resolve the Layer 2 adjacency and will not forward packets. This is confirmed by the fact that the default route exists but the ping fails, indicating a reachability issue with the next hop.

Answer

The default route is missing from the routing table.

Answer

The static route has an incorrect administrative distance.

Answer

The ip routing process has not been enabled.

Question 11

A network engineer notices that traffic from the router to server 192.168.10.5 is being sent over a slow backup link, even though the primary high-speed link is up. The routing table has an OSPF route for 192.168.10.0/24 via the primary link and a static host route to 192.168.10.5/32 via the backup link. Why is the backup link used for traffic to the server?

Accepted Answer

The static /32 route is a more specific match than the OSPF /24 route, so it is selected regardless of administrative distance.. The router uses the most specific matching route in the routing table to forward traffic. The static host route to 192.168.10.5/32 has a longer prefix length (32 bits) than the OSPF route for 192.168.10.0/24 (24 bits), making it a more specific match. Even though OSPF has a higher administrative distance, the longest prefix match rule takes precedence over administrative distance when both routes are present in the routing table.

Answer

The static route has an administrative distance of 1, which is lower than OSPF's 110, so it is always preferred.

Answer

The router is load-balancing between the two routes, and traffic is being hashed to the backup link for this particular flow.

Answer

The OSPF route is not installed in the routing table because the static route has a better AD.

Question 12

A switchport is configured for 802.1X authentication. What is the usual role of the RADIUS server in that design?

Accepted Answer

It validates authentication credentials for the supplicant.. The switch acts as the authenticator and forwards authentication requests to the RADIUS server, which validates the user or device credentials.

Answer

It provides Layer 2 trunk negotiation for the supplicant.

Answer

It replaces the need for DHCP on the access layer.

Answer

It generates ARP replies on behalf of the default gateway.

Question 13

A network engineer notices that an NMS at 10.1.1.200 cannot poll a router that has SNMPv2c configured with community string 'public'. What is causing this issue?

Accepted Answer

The SNMP community 'public' has an access list that only permits host 10.1.1.100.. SNMPv2c community strings can be restricted by an access control list (ACL) that specifies which source IP addresses are allowed to poll the router. If the ACL only permits host 10.1.1.100, then the NMS at 10.1.1.200 will be denied access even though the community string 'public' is correct. This is a common configuration for security, but it prevents polling from unauthorized hosts.

Answer

SNMPv2c is not enabled on the router.

Answer

The NMS is using the wrong community string.

Answer

The router's SNMP agent is not listening on the interface facing 10.1.1.200.

Question 14

Two routers are directly connected over IPv6 and should form an OSPFv3 adjacency, but they do not. Link-local addressing is present on both interfaces. Which issue is most likely to prevent the adjacency?

Accepted Answer

The interfaces are assigned to different OSPFv3 areas.. An area mismatch is a strong and direct explanation. In plain language, even though the routers can have valid IPv6 addressing and proper link-local communication on the interface, OSPFv3 still requires the two ends of the shared segment to agree on the area context for the adjacency. If one side places the interface in one area and the other side places it in another, the routers will not treat each other as valid neighbors.

This is very similar in principle to OSPF for IPv4. Link-local addressing matters in OSPFv3, but the protocol still enforces key neighbor-formation checks. The correct answer is the one that focuses on a required protocol match rather than on a vague issue like hostname or cable color.

Answer

The routers need matching hostnames before OSPFv3 can start.

Answer

IPv6 requires a /64 only for routing protocols to function.

Answer

OSPFv3 cannot run on directly connected interfaces.

Question 15

Which TWO statements about fiber optic cable types and SFP transceivers are correct?

Accepted Answer

Multimode fiber is commonly used in campus and data center environments for distances up to several hundred meters.. Multimode fiber (MMF) has a larger core (typically 50/125 µm or 62.5/125 µm) and is used for short-reach, cost-effective connections up to a few hundred meters, making option B correct. Single-mode fiber (SMF) uses a much smaller core (~9 µm) and laser-based optics to achieve distances of 10 km or more, so option D is correct. Option A is wrong because it reverses the core size characteristic: SMF has a smaller core, and the given dimensions (62.5/125 µm) refer to multimode. Option C is wrong because SFP+ is a 10 Gbps form factor and is not inherently backward compatible with 1 Gbps SFP modules unless the switch port explicitly supports dual-rate operation. Option E is incorrect because fiber optic cables are inherently immune to electromagnetic interference (EMI) and therefore never require EMI shielding in any environment; the false statement claims they need shielding.

Answer

Single-mode fiber uses a larger core (typically 62.5/125 µm) and supports longer distances than multimode fiber.

Answer

SFP+ transceivers support data rates up to 1 Gbps and are backward compatible with standard SFP modules.

Answer

Fiber optic cables must be run with EMI shielding when deployed in environments with high electromagnetic interference.