CCNA Mock Exam 79 — 25 Questions

Question 1

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 2

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 3

You are connected to the console of SW1. The network administrator reports that a workstation connected to interface FastEthernet0/1 cannot communicate with the rest of the network. The workstation is configured for VLAN 10, but the interface is in VLAN 1.

Accepted Answer

Configure the interface as an access port and assign it to VLAN 10.. The port was in VLAN 1 instead of VLAN 10. By configuring 'switchport access vlan 10', the workstation is now in the correct VLAN and can communicate with the rest of the network.

Answer

Configure the interface as a trunk port.

Answer

Configure the interface as a trunk port and allow VLAN 10.

Answer

Configure the interface as a dynamic desirable port.

Question 4

You are connected to R1 via console. R1 and R2 are configured with EIGRP AS 100. R1's loopback0 (1.1.1.1/32) should be advertised into EIGRP. However, after configuration, R2 does not have a route to 1.1.1.1/32. You need to verify the EIGRP configuration on R1 and R2 to determine why the route is missing. Use show commands to identify the issue.

Accepted Answer

Use 'show ip eigrp interfaces' on R1 to verify that EIGRP is enabled on the interface facing R2 and that it is not passive.. The root cause is that R1's interface facing R2 is passive, preventing EIGRP neighbor adjacency. 'show ip eigrp interfaces' verifies the passive state, identifying why no routes are exchanged. Option B is incorrect because the local topology table may contain the route, but without an active neighbor, it will not be advertised; the topology check alone is insufficient. Option C is incorrect because the routing table may also show the locally connected route, but that does not explain why R2 lacks it. Option D is incorrect because checking R2's EIGRP configuration does not reveal R1's passive interface, which is the actual problem.

Answer

Use 'show ip eigrp topology' on R1 to confirm that the 1.1.1.1/32 route is in the EIGRP topology table.

Answer

Use 'show ip route eigrp' on R1 to verify that the 1.1.1.1/32 route is in the routing table.

Answer

Use 'show ip protocols' on R2 to verify that EIGRP AS 100 is configured and that the network statement includes the subnet of the interface facing R1.

Question 5

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 6

A switch port connected to an end host is configured with both PortFast and BPDU Guard. What is the most likely outcome if a small switch is connected there and starts sending BPDUs?

Accepted Answer

The port is error-disabled by BPDU Guard.. BPDU Guard places the port into an error-disabled state upon receiving a BPDU, because PortFast defines the port as an edge port that should never receive BPDUs. Option B is incorrect because receiving a BPDU does not automatically make a port a root port; root port selection depends on bridge ID and path cost, and BPDU Guard prevents further STP processing by disabling the port. Option C is incorrect because a port cannot convert to a trunk solely by receiving a BPDU; trunking requires manual configuration or Dynamic Trunking Protocol (DTP). Option D is incorrect because PortFast does not disable STP entirely; it only speeds up initial convergence, and BPDU Guard actively responds to BPDUs by error-disabling the port.

Answer

The port automatically becomes the root port.

Answer

The port converts into a trunk for the attached switch.

Answer

The port ignores the BPDU because PortFast disables STP entirely.

Question 7

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 8

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 9

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 10

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 11

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 12

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

Accepted Answer

1000BASE-SX SFP transceivers are designed for multimode fiber and typically support distances up to 550 meters.. 1000BASE-SX SFP transceivers are designed for multimode fiber (MMF) and use short-wavelength lasers (850 nm). Over 50/125 µm MMF, they support up to 550 meters. Option D is correct because 1000BASE-LX SFP transceivers are designed for single-mode fiber (SMF) and support distances up to 5 km according to the IEEE 802.3 standard (not 10 km, which is typically a vendor-specific LH extension). Option A is false because SFPs are hot-swappable pluggable modules, not permanently installed. Option C is false because multimode fiber typically uses LED or VCSEL transmitters for short distances, not lasers for long-haul. Option E is false because single-mode fiber has a smaller core diameter (8-10 µm) than multimode fiber (50-62.5 µm), which reduces modal dispersion and allows longer distances.

Answer

SFP transceivers are permanently installed on the switch motherboard and cannot be replaced.

Answer

Multimode fiber typically uses laser-based transmitters for long-distance transmission.

Answer

Single-mode fiber has a larger core diameter than multimode fiber, allowing longer distances.

Question 13

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

Question 14

A user reports that they cannot access the company's internal web server at 'intranet.company.local' from their workstation. The workstation can ping the web server's IP address 192.168.10.50 successfully, and other internal services like email (mail.company.local) are reachable. Which DNS record issue is most likely causing this problem?

Accepted Answer

Add an A record mapping 'intranet.company.local' to IP address 192.168.10.50. The workstation can ping the web server's IP address (192.168.10.50), confirming network connectivity and that the server is online. However, the user cannot access the server by its hostname 'intranet.company.local', which indicates a DNS resolution failure. Since other internal services like email are reachable, the DNS zone is functioning, but there is no A record that maps the hostname 'intranet' to its IP address. Adding an A record for 'intranet.company.local' pointing to 192.168.10.50 will resolve the issue.

Answer

Add a CNAME record that aliases 'intranet.company.local' to 'webserver.company.local'

Answer

Verify the PTR record for 192.168.10.50 points to 'intranet.company.local'

Answer

Check the NS records for the 'company.local' zone to ensure proper delegation

Question 15

A network administrator configures OSPF on two routers, R1 and R2, connected via their Serial0/0/0 interfaces (IP addresses 10.1.1.1/30 and 10.1.1.2/30). They verify that both routers use the same OSPF process ID and area 0, but R1's 'show ip ospf neighbor' shows no adjacencies. Given the partial exhibit from R1, what is the most likely cause of the adjacency failure and its correct solution?

Accepted Answer

Configure 'no passive-interface Serial0/0/0' under router ospf 1 on R1.. The most likely cause is that R1's Serial0/0/0 interface is configured as a passive interface under OSPF. When an interface is set as passive, OSPF does not send Hello packets out of it, preventing neighbor discovery and adjacency formation. The solution is to use the 'no passive-interface Serial0/0/0' command under router ospf 1 on R1, which allows Hello packets to be transmitted and the adjacency to establish.

Answer

Replace the network statement with 'network 10.1.1.0 0.0.0.255 area 0' to cover a larger range.

Answer

Change the OSPF process ID on R1 to match R2, using 'router ospf 100' and re-entering the network command.

Answer

Issue 'clear ip ospf process' on R1 to restart OSPF and reattempt neighbor discovery.

Question 16

A switch displays this output:

Port      Name               Status    Vlan
Fa0/1                        connected 10
Fa0/2                        connected 10
Fa0/24                       connected trunk

Which port should be checked first if a user in VLAN 20 cannot reach the distribution switch over the uplink?

Accepted Answer

Fa0/24. If users in VLAN 20 must cross the uplink, the trunk port is the first place to verify allowed VLANs and tagging.

Answer

Fa0/1

Answer

Fa0/2

Answer

Any access port in VLAN 1

Question 17

You are connected to R1. The inside network 192.168.1.0/24 must be able to access the internet using PAT (NAT overload) with the outside interface G0/1 IP 203.0.113.1. Additionally, the internal server at 192.168.1.10 must be reachable from the internet via static NAT to 203.0.113.10. The current configuration is incomplete and contains errors. Identify and fix all issues so that both PAT and static NAT work correctly.

Accepted Answer

Swap ip nat inside/outside on interfaces, add 'overload' to the dynamic NAT rule, and correct ACL 1 to permit 192.168.1.0/24.. The configuration had multiple faults: 1) Inside/outside interfaces were swapped – G0/0 (LAN) should be inside, G0/1 (WAN) should be outside. 2) The PAT command was missing the 'overload' keyword. 3) ACL 1 permitted the wrong subnet (192.168.2.0/24 instead of 192.168.1.0/24). 4) The static NAT configuration was correct, but the interface misconfiguration prevented it from working. Fixes: swap ip nat inside/outside on the interfaces, add 'overload' to the dynamic NAT rule, and correct ACL 1 to permit 192.168.1.0/24.

Answer

Change the static NAT inside address to 192.168.1.1 and add 'overload' to the dynamic NAT rule.

Answer

Remove the static NAT configuration and rely solely on PAT for the server.

Answer

Add the 'overload' keyword to the dynamic NAT rule and correct ACL 1 to permit 192.168.1.0/24.

Question 18

You are connected to a multilayer switch MLS1. Configure a static default route for IPv4 that points to next-hop 192.0.2.2, but also configure a floating static default route with an administrative distance of 10 that uses next-hop 198.51.100.2. Additionally, configure a static host route for IPv6 host 2001:db8:1::10/128 via next-hop 2001:db8:1::1. The current configuration has a recursive routing failure for the IPv4 default route because the next-hop 192.0.2.2 is not reachable; you must first fix that by adding a directly connected static route. Ensure the floating route becomes active only when the primary route fails.

Accepted Answer

ip route 0.0.0.0 0.0.0.0 192.0.2.2
ip route 0.0.0.0 0.0.0.0 198.51.100.2 10
ip route 192.0.2.0 255.255.255.252 GigabitEthernet0/0 192.0.2.2
ipv6 route 2001:db8:1::10/128 2001:db8:1::1. The IPv4 primary default route fails because next-hop 192.0.2.2 is not directly connected and no recursive route exists to it. The floating static route with next-hop 198.51.100.2 is in the directly connected Vlan20 subnet (198.51.100.1/30), so it installs without issue. To fix the primary default route, add a directly connected static route for the network containing 192.0.2.2, e.g., ip route 192.0.2.0 255.255.255.252 GigabitEthernet0/0 192.0.2.2. The IPv6 host route is correct if next-hop 2001:db8:1::1 is reachable.

Answer

ip route 0.0.0.0 0.0.0.0 192.0.2.2
ip route 0.0.0.0 0.0.0.0 198.51.100.2 10
ip route 192.0.2.0 255.255.255.0 GigabitEthernet0/0
ipv6 route 2001:db8:1::10/128 2001:db8:1::1

Answer

ip route 0.0.0.0 0.0.0.0 192.0.2.2
ip route 0.0.0.0 0.0.0.0 198.51.100.2 10
ip route 192.0.2.2 255.255.255.255 GigabitEthernet0/0
ipv6 route 2001:db8:1::10/128 2001:db8:1::1

Answer

ip route 0.0.0.0 0.0.0.0 192.0.2.2
ip route 0.0.0.0 0.0.0.0 198.51.100.2 10
ip route 192.0.2.0 255.255.255.252 198.51.100.2
ipv6 route 2001:db8:1::10/128 2001:db8:1::1

Question 19

A host is configured as 172.16.20.190/26. Which range contains the usable host addresses for that subnet?

Accepted Answer

172.16.20.129 to 172.16.20.190. A /26 uses blocks of 64 addresses. In practical terms, the fourth-octet ranges are 0–63, 64–127, 128–191, and 192–255. Since 190 falls inside the 128–191 block, the network address is .128 and the broadcast address is .191. That leaves .129 through .190 as the usable range.

This is a strong test of whether you can identify the correct block and then exclude the reserved boundary addresses correctly.

Answer

172.16.20.128 to 172.16.20.191

Answer

172.16.20.130 to 172.16.20.191

Answer

172.16.20.193 to 172.16.20.254

Question 20

You are connected to R1. Configure R1 as a DHCP server for VLAN 20 clients (192.168.20.0/24) with DNS server 203.0.113.10 and default gateway 192.168.20.1. On switch SW1, enable DHCP snooping globally and on VLAN 20, configure the uplink to R1 as trusted, and ensure that the DHCP server is reachable via ip helper-address on the VLAN 20 SVI. Currently, clients are not receiving IP addresses because of misconfigurations: the excluded-address range on R1 is too large (excluding the entire subnet), the helper-address on SW1 points to a wrong IP (192.0.2.99), and a rogue DHCP server is connected to port Fa0/3 on SW1. Fix all issues so that clients can get addresses securely.

Accepted Answer

On R1, change the excluded-address range to 192.168.20.1 192.168.20.1; on SW1, configure ip helper-address 10.0.0.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.. The DHCP server had an excluded-address range covering the entire subnet (192.168.20.0–255), which prevented any IP from being assigned. Fix: change the excluded range to only include the gateway (192.168.20.1) and optionally other reserved addresses. The helper-address on SW1 pointed to 192.0.2.99 (nonexistent) instead of the DHCP server at 10.0.0.1; correct it. DHCP snooping was disabled; enable it globally and on VLAN 20, then configure the uplink to R1 (port connecting to R1) as trusted and the port connecting the rogue server (Fa0/3) as untrusted (or simply leave it untrusted by default, but the rogue server will be blocked).

Answer

On R1, remove the excluded-address range entirely; on SW1, configure ip helper-address 192.168.20.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.

Answer

On R1, change the excluded-address range to 192.168.20.1 192.168.20.254; on SW1, configure ip helper-address 192.0.2.99 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, and configure the uplink to R1 as trusted.

Answer

On R1, change the excluded-address range to 192.168.20.1 192.168.20.1; on SW1, configure ip helper-address 10.0.0.1 under interface Vlan20; enable DHCP snooping globally and on VLAN 20, but do not configure any trusted ports.

CCNA Mock Exam 79 — 25 Questions

Exhibit

CCNA Mock Exam 79 — 25 Questions

Exhibit