CCNA Mock Exam 77 — 25 Questions

Question 1

Exhibit: A static route to 172.16.40.0/24 is configured, but traffic still follows the default route. Which two explanations are plausible?

Accepted Answer

The next hop for the static route may not be reachable. If a static route points to an unreachable next hop or the outgoing interface is down, the route may not be installed as usable. In that case the router can still use a less specific route such as the default route.

Answer

The default route always overrides more specific routes

Answer

Static routes require a metric lower than 1

Answer

The destination prefix must be configured under router ospf first

Question 2

You are connected to the console of R1. The network has a point-to-point serial link between R1 and R2. The link is down and the line protocol is down. The cable is a DCE/DTE crossover, and R1 is the DCE. The initial configuration shows the interface with an IP address but no clock rate set.

Accepted Answer

Configure the clock rate on R1's serial interface.. On a serial DCE cable, the DCE end must configure a clock rate to provide timing. Without it, the line protocol stays down. Setting 'clock rate 128000' on R1 resolved the issue.

Answer

Configure the clock rate on R2's serial interface.

Answer

Replace the serial cable with a straight-through cable.

Answer

Set the encapsulation on both ends to PPP.

Question 3

Which TWO statements about fiber optic cables and SFP/SFP+ transceivers are correct?

Accepted Answer

Single-mode fiber (SMF) uses a smaller core diameter than multimode fiber (MMF).. Single-mode fiber (SMF) has a core diameter of about 9 microns, which is smaller than multimode fiber (MMF) cores of 50 or 62.5 microns. Option C is correct because SFP+ transceivers are indeed designed for 10 Gigabit Ethernet, supporting data rates up to 10 Gbps. Option B is wrong because multimode fiber is used for shorter distances (up to a few hundred meters) due to higher modal dispersion, while single-mode fiber supports longer distances (tens of kilometers). Option D is wrong because standard SFP modules support up to 1 Gbps, not 10 Gbps; SFP+ modules handle 10 Gbps. Option E is wrong because a 9-micron core is characteristic of single-mode fiber, not multimode fiber.

Answer

Multimode fiber supports longer distances than single-mode fiber.

Answer

SFP modules support data rates up to 10 Gbps.

Answer

Multimode fiber typically has a core diameter of 9 microns.

Question 4

You are connected to SW1 via the console. SW1 is a Layer 2 switch with two redundant links to SW2 (G0/1 and G0/2). The network administrator wants to use both links for load balancing and redundancy by configuring EtherChannel. You need to configure a Layer 2 EtherChannel using LACP on both switches. The port-channel should be in VLAN 1.

Accepted Answer

interface port-channel 1
switchport mode access
switchport access vlan 1
interface range GigabitEthernet0/1-2
channel-group 1 mode active. EtherChannel bundles multiple physical links into a single logical link for load balancing and redundancy. LACP (mode active) negotiates the bundle automatically. The port-channel interface must be configured with the desired switchport settings.

Answer

interface port-channel 1
switchport mode trunk
switchport trunk allowed vlan 1
interface range g0/1-2
channel-group 1 mode desirable

Answer

interface port-channel 1
switchport mode access
switchport access vlan 1
interface range g0/1-2
channel-group 1 mode passive

Answer

interface port-channel 1
switchport mode access
switchport access vlan 1
interface g0/1
channel-group 1 mode active
interface g0/2
channel-group 2 mode active

Question 5

A network requires at least 500 usable host addresses in one IPv4 subnet. Which prefix is the smallest that meets the requirement?

Accepted Answer

/23. To support at least 500 usable hosts, the subnet must provide at least 502 total addresses when the network and broadcast addresses are included. In plain language, that means 256 total addresses in a /24 are not enough, so the next larger power-of-two block is required. A /23 provides 512 total addresses and 510 usable host addresses, which satisfies the requirement while remaining the smallest valid option.

This is a classic host-capacity question because it checks whether you can work backward from a required usable host count and choose the smallest prefix that works without wasting more space than necessary.

Answer

/24

Answer

/22

Answer

/25

Question 6

You are connected to R1. Configure DHCP server on R1 to assign addresses from 192.168.50.0/24 to hosts on VLAN 50, excluding 192.168.50.1-192.168.50.20, with default-router 192.168.50.1 and DNS server 8.8.8.8. On switch SW1, configure DHCP snooping globally and on VLAN 50, and enable trusted ports on the uplink to R1. Then, a host on VLAN 50 reports it received an incorrect IP address; troubleshoot and fix the issue: the wrong helper-address is configured on SW1, the excluded range is too large, and a rogue DHCP server is present on port Fa0/5.

Accepted Answer

Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, correct the excluded range on R1 to 192.168.50.1-192.168.50.20, and change the helper-address on SW1's G0/1 to 192.168.50.1.. The host received a wrong IP because a rogue DHCP server on Fa0/5 was responding. First, enable DHCP snooping globally with 'ip dhcp snooping' and on VLAN 50 with 'ip dhcp snooping vlan 50'. Then, trust the uplink port to R1 (G0/1) with 'ip dhcp snooping trust'. Next, fix the excluded range on R1: change it to exclude only the first 20 addresses (192.168.50.1-192.168.50.20) so that hosts can obtain other addresses. Finally, correct the helper-address on SW1's G0/1: change 'ip helper-address 10.0.0.1' to 'ip helper-address 192.168.50.1' because the DHCP server is on the VLAN 50 SVI, not on the point-to-point link.

Answer

Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and change the helper-address on SW1's G0/1 to 10.0.0.1.

Answer

Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and correct the excluded range on R1 to 192.168.50.1-192.168.50.20.

Answer

Enable DHCP snooping globally and on VLAN 50, trust the uplink port to R1, and change the helper-address on SW1's G0/1 to 192.168.50.1.

Question 7

If a host has a valid IP address and subnet mask but no default gateway, what is the most likely result?

Accepted Answer

The host can usually reach only local-subnet destinations and not remote networks.. The host will normally reach local destinations but fail to reach remote networks. In practical terms, the subnet mask still lets the host identify what is local, but without a default gateway it has no next hop for off-subnet traffic. That means local ARP-based communication can still work, while remote communication usually fails.

This is a core host-configuration concept and a very common certification question. The missing gateway does not break all communication — it breaks off-subnet communication.

Answer

The host cannot use ARP at all.

Answer

The host automatically joins every subnet in the LAN.

Answer

The host becomes the default gateway for other devices.

Question 8

An AP broadcasts the correct SSID, but many clients on one floor experience poor performance while the same SSID works well on another floor. Which category of issue is most strongly suggested first?

Accepted Answer

A radio-frequency or local wireless environment issue on that floor. The issue is location-specific, with performance problems only on one floor. This strongly suggests a local radio frequency (RF) or wireless environment issue such as interference, signal attenuation, or channel congestion on that floor. The SSID is correctly broadcast because clients on other floors connect successfully, so option B (misspelling) is not plausible. Options C and D are unrelated to wireless performance: BGP is a routing protocol not used in basic WLAN deployments, and IPv6 loopback addressing does not affect client connectivity or throughput. Therefore, the most direct and likely first suspect is an RF or environmental issue on that specific floor.

Answer

The SSID name must be misspelled only on that floor

Answer

BGP autonomous system mismatch

Answer

IPv6 loopback addressing on the clients

Question 9

A switchport connected to another switch is configured with `switchport mode dynamic auto` on both ends. What is the most likely outcome if neither side actively negotiates trunking?

Accepted Answer

The link is likely to remain non-trunking because both sides are waiting passively.. If both ends are set to dynamic auto, the most likely outcome is that the link does not become a trunk automatically. In plain language, both interfaces are waiting passively for the other side to initiate the negotiation. Since neither side is actively trying to form the trunk, the link typically remains non-trunking unless one side is changed to a more active mode or trunk is configured directly.

This is a classic DTP behavior question because it tests whether you understand the difference between active and passive negotiation roles. The correct answer is the one that reflects the passive nature of dynamic auto on both sides.

Answer

The link always becomes a trunk immediately.

Answer

The link becomes a routed Layer 3 link.

Answer

All VLANs are deleted from both switches.

Question 10

You are connected to R1. Configure OSPFv3 for IPv6 so that R1 and R2 can exchange IPv6 routes over their directly connected link. Enable IPv6 routing, assign OSPFv3 process and area on the interface, and verify that the neighbor adjacency forms and routes appear in the IPv6 routing table.

Accepted Answer

Enable IPv6 routing with 'ipv6 unicast-routing', configure OSPFv3 on the interface with 'ipv6 ospf 1 area 0', and verify with 'show ospfv3 neighbor' and 'show ipv6 route ospf'.. It includes enabling IPv6 routing with 'ipv6 unicast-routing', applying OSPFv3 to the interface using 'ipv6 ospf 1 area 0', and verifying with the correct OSPFv3-specific commands 'show ospfv3 neighbor' and 'show ipv6 route ospf'. Option B is incorrect because it adds a global 'router ospfv3 1' command, which is unnecessary; OSPFv3 can be configured directly on the interface without a global process. Option C is incorrect because it uses IPv4 OSPF verification commands 'show ip ospf neighbor' and 'show ip route ospf', which are not valid for OSPFv3. Option D is incorrect because although it uses the correct 'show ospfv3 neighbor', the 'show ipv6 route' command does not filter to OSPF-learned routes, so it displays all IPv6 routes rather than just OSPF routes.

Answer

Enable IPv6 routing with 'ipv6 unicast-routing', configure OSPFv3 globally with 'router ospfv3 1' and 'router-id 1.1.1.1', then assign the interface to area 0 with 'ipv6 ospf 1 area 0'.

Answer

Enable IPv6 routing with 'ipv6 unicast-routing', configure OSPFv3 on the interface with 'ipv6 ospf 1 area 0', and verify with 'show ip ospf neighbor' and 'show ip route ospf'.

Answer

Enable IPv6 routing with 'ipv6 unicast-routing', configure OSPFv3 on the interface with 'ipv6 ospf 1 area 0', and verify with 'show ospfv3 neighbor' and 'show ipv6 route'.

Question 11

Refer to the exhibit. A network engineer is troubleshooting intermittent connectivity between a branch router and the upstream switch. The switch port is manually configured for full-duplex, and the Ethernet cable has been tested and is working properly. The engineer runs the show interfaces GigabitEthernet0/0 command on the router and receives the output shown. Based on the output, what is the most likely cause of the problem?

Accepted Answer

The router interface is operating in half-duplex while the switch port is full-duplex, causing a duplex mismatch.. The output shows the router interface is operating in half-duplex (as indicated by 'Half-duplex' in the show interfaces output), while the switch port is manually configured for full-duplex. This duplex mismatch causes the router to use CSMA/CD and detect collisions, while the switch transmits without collision detection, leading to late collisions, CRC errors, and intermittent connectivity. The correct fix is to set both sides to the same duplex setting, ideally full-duplex.

Answer

The switch port is configured for half-duplex instead of the expected full-duplex.

Answer

The cable is faulty, causing excessive late collisions and CRC errors.

Answer

The excessive input errors are a result of a broadcast storm on the network.

Question 12

You are connected to R1. The network has R1, R2, and a multilayer switch MLS1. Configure IPv4 and IPv6 addressing on R1's interfaces so that R1 can ping both R2 (198.51.100.2) and MLS1 (203.0.113.2) via IPv4. Additionally, configure IPv6 on G0/1 using EUI-64 with prefix 2001:db8:1::/64 and verify that R1 can ping the IPv6 address of MLS1 (2001:db8:1::2). The current configuration has incorrect subnet masks and missing IPv6 settings, causing reachability failures.

Accepted Answer

Change the subnet mask on G0/0 to 255.255.255.252, change G0/1 to 255.255.255.252, then configure IPv6 on G0/1 with the EUI-64 address using the prefix 2001:db8:1::/64.. The interfaces on R1 were configured with subnet masks that were not /30, which is required for these point-to-point links. With an incorrect mask, R1 does not consider the neighboring IPs (198.51.100.2 and 203.0.113.2) as directly connected, preventing ARP resolution and IPv4 reachability. Additionally, IPv6 was missing on G0/1. To fix, change the subnet mask on G0/0 to 255.255.255.252, change G0/1 to 255.255.255.252, then configure IPv6 on G0/1 with the EUI-64 address using the prefix 2001:db8:1::/64. After these changes, pings succeed.

Answer

Change the subnet mask on G0/0 to 255.255.255.0, change G0/1 to 255.255.255.0, then configure IPv6 on G0/1 with the EUI-64 address using the prefix 2001:db8:1::/64.

Answer

Change the subnet mask on G0/0 to 255.255.255.252, change G0/1 to 255.255.255.252, then configure IPv6 on G0/1 with the static address 2001:db8:1::1/64.

Answer

Change the subnet mask on G0/0 to 255.255.255.252, change G0/1 to 255.255.255.252, then configure IPv6 on G0/1 with the EUI-64 address using the prefix 2001:db8:1::/32.

Question 13

You are connected to R1 via console. Configure OSPFv3 for IPv6 on R1 and R2 so that IPv6 loopback interfaces on both routers can communicate. R1's GigabitEthernet0/0 and R2's GigabitEthernet0/1 are directly connected. Ensure OSPFv3 is enabled on the correct interfaces and verify neighbors and routes.

Accepted Answer

Enable OSPFv3 on R1's GigabitEthernet0/0 and Loopback0, and on R2's GigabitEthernet0/1 and Loopback0.. The issue is that OSPFv3 is not enabled on the interfaces. On R1, OSPFv3 must be enabled on GigabitEthernet0/0 (the link to R2) and Loopback0 (to advertise the loopback). On R2, OSPFv3 must be enabled on GigabitEthernet0/1 (the link to R1) and Loopback0. After enabling OSPFv3 on the correct interfaces, the neighbor adjacency forms and routes are exchanged.

Answer

Enable OSPFv3 only on the loopback interfaces of both routers.

Answer

Enable OSPFv3 on R1's GigabitEthernet0/0 and R2's GigabitEthernet0/1 only, without loopbacks.

Answer

Enable OSPFv3 on R1's Loopback0 and R2's GigabitEthernet0/1 only.

Question 14

A junior network engineer configured a floating static route on Router R1 to provide backup connectivity to a remote network 10.10.10.0/24. The primary connection uses OSPF. However, after the primary link fails, hosts on R1 cannot reach the remote network. The OSPF adjacency is down, and the floating static route is not appearing in the routing table. Based on the exhibit, what is the most likely cause of the issue?

Accepted Answer

The static route's next-hop becomes unreachable after the primary OSPF link fails.. The floating static route's next-hop becomes unreachable after the primary OSPF link fails. In the exhibit, the next-hop IP is likely configured to an address that is only reachable via OSPF; when that adjacency drops, the router has no route to the next-hop, so it cannot recursively resolve the static route. As a result, the route does not appear in the routing table. Option A is wrong because the route is present in the configuration (as a floating static route). Option C is wrong because the administrative distance of the floating static route is intentionally higher than OSPF's so that it only installs when OSPF fails; this is correct behavior. Option D is wrong because a default route would not override a more specific static route to 10.10.10.0/24.

Answer

The floating static route is missing from the configuration.

Answer

The administrative distance of the static route is too high.

Answer

The default route is overriding the static route to 10.10.10.0/24.

Question 15

You are connected to SW1, a Layer 2 switch. The network administrator wants to prevent unauthorized switches from being connected to access ports. Port G0/1 is an access port in VLAN 10. You need to configure BPDU Guard on this port to protect against STP loops caused by rogue switches. Additionally, enable PortFast for immediate transition to forwarding.

Accepted Answer

SW1(config-if)# spanning-tree portfast
SW1(config-if)# spanning-tree bpduguard enable. PortFast allows an access port to skip STP listening/learning and transition immediately to forwarding. BPDU Guard protects against STP loops by error-disabling the port if a BPDU is received, which would indicate an unauthorized switch connection.

Answer

SW1(config)# spanning-tree portfast default
SW1(config)# spanning-tree bpduguard default

Answer

SW1(config-if)# spanning-tree portfast
SW1(config-if)# spanning-tree guard root

Answer

SW1(config-if)# spanning-tree portfast
SW1(config-if)# spanning-tree bpduguard disable

Question 16

A network administrator is troubleshooting an issue where hosts in the 192.168.20.0/24 subnet cannot reach the Internet, while hosts in 192.168.10.0/24 can. The router is configured for PAT overload using a dynamic pool on the outside interface. The administrator collects the configuration shown in the exhibit. What is the most likely cause of the connectivity problem for the 192.168.20.0/24 subnet?

Accepted Answer

The wildcard mask in access list 20 is incorrect; it matches only the network address.. The issue is that access list 20, used to define which internal addresses are eligible for NAT, has a wildcard mask of 0.0.0.0. This wildcard mask matches only the exact address 192.168.20.0, not the entire 192.168.20.0/24 subnet. For a /24 subnet, the correct wildcard mask should be 0.0.0.255, which would match all addresses from 192.168.20.1 to 192.168.20.254. Because the ACL matches only the network address (192.168.20.0), no host traffic from that subnet is translated, breaking Internet connectivity.

Answer

The NAT pool does not have enough IP addresses to support both subnets.

Answer

Interface GigabitEthernet0/2 is missing the ip nat inside command.

Answer

Access list 10 is incorrectly applied to the NAT pool, causing a conflict.

Question 17

A network engineer is troubleshooting a link between two Cisco Catalyst 9300 switches that are connected via a single-mode fiber optic cable. The link is up, but the interface counters show a high number of CRC errors and frame check sequence (FCS) errors. The interface is configured for 1000 Mbps and full duplex on both ends. What is the most likely cause of these errors?

Accepted Answer

Faulty or dirty fiber optic cable or connectors. CRC and FCS errors indicate corruption at the data-link layer, typically caused by physical-layer issues such as signal degradation. Since the link is up and both ends are configured for 1000 Mbps full duplex, a speed/duplex mismatch is ruled out. Dirty or faulty single-mode fiber connectors or the cable itself can introduce optical signal loss or reflections, leading to bit errors that manifest as CRC/FCS errors.

Answer

A speed or duplex mismatch between the two switches

Answer

Incorrect SFP+ module type inserted in the switch

Answer

Excessive cable length beyond the distance limit

Question 18

Match each controller or automation term to its most accurate description.

Accepted Answer

PLC: Industrial computer used for automation of electromechanical processes. PLC (Programmable Logic Controller) is an industrial computer for automating electromechanical processes. SCADA (Supervisory Control and Data Acquisition) is a system for remote monitoring and control. DCS (Distributed Control System) controls production processes within a plant. RTU (Remote Terminal Unit) interfaces with sensors and actuators in remote locations. All given pairings are correct.

Question 19

You are connected to R1. Configure static routes so that R1 can reach the IPv4 network 203.0.113.0/24 and the IPv6 network 2001:db8:acad:1::/64 via R2 (G0/0 10.0.0.2/30). Additionally, configure a floating static default route (IPv4) with an administrative distance of 200 via R2, and a fully specified IPv6 default route via R2. Then, verify that the IPv4 static route to 203.0.113.0/24 is correctly installed by checking the routing table. The current configuration has an incorrect next-hop causing recursive routing failure for the IPv4 static route.

Accepted Answer

Change the IPv4 static route next-hop from 10.0.0.1 to 10.0.0.2, change the IPv6 static route next-hop from 10.0.0.2 to 2001:db8:acad:2::2, and change the IPv6 default route to use next-hop 2001:db8:acad:2::2.. The IPv4 static route to 203.0.113.0/24 incorrectly uses next-hop 10.0.0.1 (R1's own interface) instead of 10.0.0.2 (R2), causing recursive routing failure because R1 tries to reach itself. To fix, change the next-hop to 10.0.0.2. The IPv6 static route to 2001:db8:acad:1::/64 also incorrectly uses an IPv4 next-hop; it must be a fully specified IPv6 next-hop (2001:db8:acad:2::2) or an exit interface. The IPv4 default route has AD 200 which is correct for a floating route, but it is not shown in the routing table because there is no route to the next-hop; after fixing the recursive route, the default route will appear. The IPv6 default route uses an IPv4 next-hop which is invalid; it should be a fully specified IPv6 route (e.g., ipv6 route ::/0 2001:db8:acad:2::2).

Answer

Change the IPv4 static route next-hop from 10.0.0.1 to 10.0.0.2, and change the IPv6 static route to use exit interface G0/0 instead of a next-hop.

Answer

Change the IPv4 static route next-hop from 10.0.0.1 to 10.0.0.2, and change the IPv4 default route administrative distance to 1.

Answer

Change the IPv4 static route next-hop from 10.0.0.1 to 10.0.0.2, and change the IPv6 default route to use exit interface G0/0.

Question 20

An ACL entry reads:

access-list 25 permit 192.168.8.0 0.0.0.15

Which address range does this statement match?

Accepted Answer

192.168.8.0 through 192.168.8.15. A wildcard of 0.0.0.15 means the last 4 bits can vary, which corresponds to a block size of 16 addresses. Starting at 192.168.8.0, the range is 192.168.8.0 through 192.168.8.15.

Answer

192.168.8.0 through 192.168.8.31

Answer

192.168.8.0 through 192.168.8.7

Answer

Only host 192.168.8.15