CCNA Network Infra Connectivity Questions

Runts result from collisions on half-duplex links or faulty network interface cards, not from CRC errors.

CSMA/CD is only used in half-duplex environments; full-duplex disables collision detection entirely.

Auto-MDIX does not participate in speed or duplex negotiation; that function is handled by auto-negotiation (IEEE 802.3u).

Incorrect. 10.24.7.191 is the last address of a larger /26 subnet block, not the /27 block containing 10.24.7.158, so it cannot be the broadcast address here.

Incorrect. 10.24.7.127 is the broadcast address of the previous /27 subnet block (10.24.7.96/27), not the one containing 10.24.7.158.

Incorrect. 10.24.7.160 is the network address of the next /27 subnet block after 10.24.7.128/27, not the broadcast address of the current subnet.

802.11ac supports only up to 256‑QAM; 1024‑QAM is introduced with 802.11ax.

802.11ac operates only in the 5 GHz band, but 802.11ax operates in both 2.4 GHz and 5 GHz.

Both 802.11ac and 802.11ax support a range of channel widths, including 20, 40, 80, and 160 MHz; neither standard restricts to a single channel width.

The subnet mask /24 is too large, causing a mismatch with R2's /30; the default gateway must be R2's IP (203.0.113.2); EUI-64 is not used on G0/0.

The duplicate IPv4 address on G0/1 is not resolved; it still uses 192.168.1.1 which is already assigned to R3.

EUI-64 is required on G0/0, not G0/1; static IPv6 is required on G0/1, not G0/0.

The /26 prefix corresponds to subnet mask 255.255.255.192, which has 64 addresses per subnet, not 32. The mask 255.255.255.224 has 27 network bits, not 26.

The /28 prefix corresponds to subnet mask 255.255.255.240, which provides 16 addresses per subnet (14 usable). The mask 255.255.255.224 has 32 addresses per subnet, so /28 is incorrect.

The /29 prefix corresponds to subnet mask 255.255.255.248, which provides 8 addresses per subnet (6 usable). The mask 255.255.255.224 has 32 addresses, so /29 is incorrect.

The interface shows Full Duplex and 1000 Mbps with no collisions or late collisions, which indicates no duplex mismatch. Duplex mismatch typically causes collisions and late collisions, which are absent here.

The transmit power of -3.5 dBm is within the normal range for SX SFP (-1.0 to -9.5 dBm), so the SFP is transmitting correctly. Low transmit power would not cause CRC errors if it is within specifications.

The SX SFP supports up to 550m on OM3 fiber, and the link is operational, so distance is not the issue. Excessive CRC errors due to distance would typically cause the link to be down or unstable.

Link-local addresses are not routable; they are confined to a single link or network segment. Routers will not forward packets with a link-local source or destination address beyond the local subnet.

While EUI-64 is one method for generating the interface ID, link-local addresses can also use randomly generated identifiers (privacy extensions) or be manually configured. The statement that they always use EUI-64 is incorrect.

The default gateway for IPv6 hosts is typically a global unicast or unique local address, not a link-local address. While routers may send Router Advertisements with a link-local source, the default gateway address learned by hosts is the router's link-local address, but the host uses that link-local address as the next-hop, not as the default gateway address itself. The statement is misleading because the default gateway is often the link-local address of the router, but the host uses it as the next-hop, not as a routable address.

DFS channels are used to avoid radar interference, but the problem described is co-channel interference on channel 36, not radar events. The exhibit shows no radar events, so enabling DFS channels would not address the high channel utilization.

Increasing channel width to 160 MHz would actually increase the likelihood of co-channel interference because fewer non-overlapping channels are available, and it would not solve the existing high utilization on channel 36.

Disabling 2.4 GHz radios would force all clients to 5 GHz, potentially worsening the co-channel interference on channel 36 by adding more clients to an already congested channel. The 2.4 GHz band is not the source of the problem.

UDP is a connectionless protocol that does not use acknowledgments, sequence numbers, or retransmissions; it simply sends datagrams without any guarantee of delivery. Reliable delivery mechanisms like acknowledgments are a feature of TCP, not UDP.

TCP uses port numbers extensively to identify source and destination applications, just like UDP. Port numbers are a fundamental part of both TCP and UDP headers, enabling multiplexing of multiple services on a single host.

While UDP has lower overhead and can be faster in some scenarios, it is not 'always faster' because network congestion, packet loss, and application behavior can affect performance. Additionally, UDP does not inherently avoid congestion; it simply does not implement congestion control, which can lead to network congestion if used aggressively.

172.31.9.192 is the network address of the subnet, which is reserved and cannot be assigned to any host. In IPv4, the network address is used to identify the subnet itself, not as a host address.

172.31.9.223 is the broadcast address of the subnet, which is reserved for sending traffic to all hosts in the subnet. It cannot be assigned to a host.

172.31.9.201 is a valid host address in the subnet, but it is not the first usable host. The first usable host is .193, which comes after the network address .192.

The specific factual error is that 'no shutdown' only brings an interface out of an administratively down state, but the problem here is a line protocol issue, not an administrative shutdown.

The specific factual error is that the IP address is already configured correctly; the problem is at Layer 2, not Layer 3.

The specific factual error is that keepalive settings are not typically the cause of a line protocol down on modern Ethernet interfaces; duplex mismatch is a more common cause.

The specific factual error: Using a /24 mask on a point-to-point link wastes addresses and may cause subnet overlap; also, R2 needs a default route to reach R1's IPv4 address if the mask is /30, but with /24 they are in the same subnet so no default route is needed, but the mask is still wrong.

The specific factual error: R2's IPv4 mask must match R1's mask to ensure both routers agree on the subnet boundary. Using /28 on R2 while R1 uses /30 creates a mismatch.

The specific factual error: Assigning the same IPv6 address to both routers causes a duplicate address conflict, preventing communication.

Low Rx power points to a physical signal issue, not a counterfeit detection problem.

A partially seated SFP would likely prevent any light from entering, not show a measurable but weak signal.

Distance would not cause a 27 dB loss over such a short path, and dispersion is not measured as a reduction in optical power on the DOM readout.

A faulty cable would likely cause CRC errors, frame errors, or interface resets, not just input errors with zero CRC.

MTU size does not affect duplex negotiation or collisions; it is unrelated to the input errors caused by duplex mismatch.

CDP has no impact on duplex negotiation or error counters; it is unrelated to the problem.

Misunderstanding that auto-negotiation always causes duplex mismatches, when in fact a mismatch occurs because one side is manually configured while the other uses auto-negotiation to negotiate an incompatible mode.

Confusing CRC errors with late collisions. Late collisions are a layer-1 timing issue, not a data integrity problem.

Attributing all network performance problems to broadcast storms, ignoring the specific error counter 'late collisions' that points directly to a duplex mismatch.

Candidates assume a speed mismatch must be the problem due to the down/down state, overlooking that a shutdown interface also appears down/down (without the 'administratively' prefix in some outputs) and that the physical check was already done.

The urgency to blame the newly installed hardware leads many to skip the quick-win config check, potentially wasting time on hardware replacement when the fix is a single command.

Candidates often confuse link status with connectivity issues that occur after the link is up, mistakenly targeting a Layer 2 problem for a Layer 1 symptom.

The specific factual error is that manually setting speed and duplex on one side while the other side is set to auto-negotiate can lead to a duplex mismatch because auto-negotiation relies on both ends participating.

The specific factual error is that manually setting speed and duplex after restoring defaults still disables auto-negotiation on the router, which does not resolve the mismatch.

The specific factual error is that setting duplex half on the router while the switch auto-negotiates will likely result in the switch negotiating to full duplex, causing a mismatch.

The subnet 10.10.10.32/27 covers addresses 10.10.10.32 through 10.10.10.63. The host address 10.10.10.94 is not within this range, so this subnet is incorrect.

The subnet 10.10.10.96/27 covers addresses 10.10.10.96 through 10.10.10.127. The host address 10.10.10.94 is below the starting address of this subnet, so it is not included.

The subnet 10.10.10.0/27 covers addresses 10.10.10.0 through 10.10.10.31. The host address 10.10.10.94 is far outside this range, so this subnet is incorrect.

Using a /16 subnet mask (255.255.0.0) does not match the corporate /24 network, leading to incorrect network identification and potential routing issues.

Leaving the default gateway blank prevents the host from reaching any network beyond its own subnet, so internet access is impossible.

Setting the default gateway to the host's own IP address (192.168.1.50) would cause traffic to be sent to itself, never reaching the actual gateway.

The specific factual error is that speed mismatch is not the root cause; duplex mismatch is indicated by CRC errors and flapping.

The specific factual error is that 'no shutdown' only brings an interface up administratively; it does not fix duplex or speed issues.

The specific factual error is that auto-negotiation can fail if one side is manually set; the fix is to manually set both sides to the same duplex.

This range includes the network address (.96) and the broadcast address (.111), which cannot be assigned to hosts. Usable host addresses must exclude these two addresses.

This range starts at .98, which excludes the valid host .97, and ends at .111, which includes the broadcast address. The correct usable range is .97 to .110.

This range (192.168.200.81 to .94) belongs to a different subnet. For a /28 subnet starting at .96, the valid host range is .97 to .110. This range is from a previous subnet (e.g., 192.168.200.80/28).

A duplicate IP would cause intermittent or lost connectivity and likely prevent a consistent reply from the gateway.

Being on the wrong VLAN would typically prevent the workstation from receiving an IP in the 192.168.1.0/24 subnet and reaching the gateway at 192.168.1.1.

DNS is only used for name resolution; pinging an IP address directly does not involve DNS, so a DNS misconfiguration cannot cause the ping failure to 8.8.8.8.

The specific factual error is that 1000BASE-T SFPs use copper cabling and cannot interface with fiber SFPs; they are incompatible.

The specific factual error is that auto-negotiation must be disabled on fiber SFPs; leaving it enabled can prevent the link from coming up.

The specific factual error is that disabling auto-negotiation does not automatically set speed and duplex; they must be configured manually.

Sticky MAC learning is a feature of port security and cannot function without port security being enabled on the interface. The command 'switchport port-security' must be configured first, and then 'switchport port-security mac-address sticky' enables sticky learning.

Sticky MAC learning is a port security feature that operates on access ports and does not affect the interface's operational mode. Trunk ports are configured separately using 'switchport mode trunk' and are used for carrying multiple VLANs, which is unrelated to MAC address learning behavior.

Sticky MAC learning does not override the maximum secure MAC address limit configured with 'switchport port-security maximum'. If the number of learned sticky addresses reaches the limit, additional MAC addresses will trigger a security violation, just like with dynamically learned addresses.

Output queue drops occur when the transmit queue is full due to congestion, typically at Layer 3 (IP) or Layer 2 (switching). They are not caused by physical layer issues but by traffic overload or insufficient buffer space.

Input errors is a catch-all counter that includes CRC, runts, giants, and framing errors. While it can indicate Layer 1 issues, it is not specific to Layer 1 because it also includes errors from higher layers (e.g., alignment errors). The question asks for counters that indicate a Layer 1 issue, and input errors is too broad.

Ignored packets are dropped due to buffer overflow, often from high traffic or hardware limitations, not specifically a Layer 1 error. They are typically caused by congestion at Layer 2 or Layer 3, not physical layer faults.

The address 172.16.10.31 is the broadcast address for the /27 subnet 172.16.10.0–31, not for the subnet containing host 172.16.10.62. Since the host's IP is in the 32–63 range, the broadcast is .63, not .31.

172.16.10.32 is the network address (subnet ID) of the /27 subnet that includes hosts 32–63. It is not the broadcast address; the broadcast is the last address in the subnet, which is .63.

172.16.10.64 is the network address of the next /27 subnet (64–95), not the broadcast address for the subnet containing .62. The broadcast for the subnet containing .62 is .63.

The subnet mask 255.255.255.192 is /26, which provides 64 total addresses per subnet. Subtracting the network and broadcast addresses leaves 62 usable hosts, not 30. 30 usable hosts corresponds to a /27 subnet mask (255.255.255.224).

126 usable hosts would require a /25 subnet mask (255.255.255.128), which provides 128 total addresses. The given mask /26 provides only 64 total addresses, so 126 is incorrect.

254 usable hosts corresponds to a /24 subnet mask (255.255.255.0), which provides 256 total addresses. The mask 255.255.255.192 is /26, which is two bits longer, resulting in only 64 total addresses.

The /30 subnet provides a total of 4 addresses, but one is the network address and one is the broadcast address, leaving only 2 usable host addresses. Saying 4 is incorrect because it counts the network and broadcast addresses as usable.

A /30 subnet has only 2 bits for host addresses, yielding 2^2 = 4 total addresses, of which 2 are usable. 6 usable hosts would require at least 3 host bits (2^3 - 2 = 6), which corresponds to a /29 subnet.

14 usable hosts correspond to a /28 subnet (255.255.255.240), which has 4 host bits (2^4 - 2 = 14). A /30 subnet has only 2 host bits, so it cannot provide 14 usable hosts.

A /26 prefix provides 2^(32-26) - 2 = 64 - 2 = 62 usable host addresses, which is insufficient for the requirement of at least 126 usable hosts.

A /24 prefix provides 2^(32-24) - 2 = 256 - 2 = 254 usable host addresses, which is more than required. While it works, it is not the smallest prefix that meets the requirement, wasting IP address space.

A /27 prefix provides 2^(32-27) - 2 = 32 - 2 = 30 usable host addresses, far below the required 126. This is insufficient for the subnet.

The subnet mask 255.255.255.240 corresponds to a /28 prefix length, not /29. This mask has 28 network bits, leaving 4 host bits, which yields 14 usable hosts per subnet.

The subnet mask 255.255.255.252 corresponds to a /30 prefix length, not /29. A /30 mask has 30 network bits and only 2 host bits, providing 2 usable addresses, typically used for point-to-point links.

The subnet mask 255.255.255.224 corresponds to a /27 prefix length, not /29. A /27 mask has 27 network bits and 5 host bits, providing 30 usable hosts per subnet.

172.16.100.191 is the broadcast address of the previous /27 subnet (172.16.100.160/27), not the subnet containing .222. The host .222 is in the 172.16.100.192/27 subnet, so its broadcast is .223.

172.16.100.224 is the network address of the next /27 subnet (172.16.100.224/27), not a broadcast address. Broadcast addresses are always the last address in a subnet, not the first.

172.16.100.255 is the broadcast address of the entire /24 subnet (172.16.100.0/24), not the /27 subnet containing .222. The /27 subnet has a smaller range, so its broadcast is .223.

ping 127.0.0.1 only tests the local TCP/IP stack and does not verify DHCP address assignment or DNS resolution.

tracert 8.8.8.8 uses an IP address directly and does not test domain-name resolution.

arp -a displays the ARP cache, which is unrelated to DHCP or DNS.

The network statement is already correct; changing it does not resolve the incorrect default gateway.

No helper address is needed because the DHCP server (the router itself) is on the same subnet as the PC.

Setting the interface IP to 192.168.1.254 would create an address mismatch with the DHCP pool’s gateway and break the subnet.

ICMP does not use TCP; it is encapsulated directly in IP. The symptom would be unrelated to TCP.

A missing route would prevent the IP layer from even attempting to send the packet, not result in a generated-but-not-transmitted error.

Port security would have blocked the incoming request if Host A's MAC violated the policy, but the request was received, so this cannot explain the missing reply.

The /25 prefix length corresponds to subnet mask 255.255.255.128, not 255.255.255.192. The mask 255.255.255.128 has 128 in the last octet, while 255.255.255.192 has 192, indicating a different number of host bits.

The /27 prefix length corresponds to subnet mask 255.255.255.224, not 255.255.255.192. The mask 255.255.255.224 has 224 in the last octet, which provides 30 usable hosts per subnet, whereas 255.255.255.192 provides 62 usable hosts.

The /28 prefix length corresponds to subnet mask 255.255.255.240, not 255.255.255.192. The mask 255.255.255.240 has 240 in the last octet, which supports 14 usable hosts, while 255.255.255.192 supports 62 usable hosts.

A duplicate IP address conflict would generate an error message and the workstation would still attempt to use the conflicting IP, not fall back to APIPA. The workstation would retain its DHCP-assigned address and display a conflict notification.

DNS misconfiguration would affect name resolution, but the workstation would still obtain a valid IP address from DHCP, not an APIPA address. APIPA is only triggered when DHCP discovery fails entirely.

Being on the wrong VLAN would likely result in an IP from a different subnet, not an APIPA address, unless the DHCP server for that VLAN is unreachable. APIPA occurs only when no DHCP server is reachable at all.

The show ip route command does not provide interface-level details such as encapsulation or clock rate; it only shows routing information.

Ping does not provide detailed interface status or configuration information; it only indicates whether the neighbor is reachable, not why it is not.

The running-config shows the intended configuration but not the current operational state; for example, it won't show if the interface is administratively down unless you check the shutdown command.

The application layer PDU is just data; no transport header is added at this stage.

At the network layer, the PDU is a packet with IP addresses; MAC addresses are added at the data link layer.

The data link layer PDU is a frame, not a packet, and it contains source and destination MAC addresses.

The specific factual error is that 1000BASE-SX cannot support 2 km distances; it is limited to 550 m.

The specific factual error is that Gigabit Ethernet interfaces cannot be set to 100 Mbps; they only support 1000 Mbps or auto-negotiation.

The specific factual error is that manual speed/duplex settings should be removed to allow auto-negotiation; they are not recommended for fiber interfaces.

The specific factual error is that the EUI-64 keyword cannot be used with the double colon (::) abbreviation; the prefix must be fully specified.

The specific factual error is that the command does not use the eui-64 keyword, so it does not meet the requirement to use EUI-64.

The specific factual error is that the 7th bit inversion was not applied; the interface ID should start with 021C, not 001C.

The 'show wlan summary' output explicitly shows the WLAN status as 'Enabled', so the WLAN is not disabled. A disabled WLAN would prevent associations, but that is not the case here.

The output shows PSK is enabled with a passphrase 'Cisco123', so a pre-shared key is configured. Missing PSK would cause authentication failures, but that is not the issue here.

CCKM is a fast roaming method that is optional for client association. Clients can associate without CCKM support; it only affects roaming performance, not initial association.

The ipconfig /all command displays detailed IP configuration, including DNS servers and MAC addresses, but does not show active network connections or sessions. Since the client can ping the gateway, IP configuration is likely correct, and this command does not help identify issues with active sessions to the web server.

The tracert command performs a route trace to the destination, which requires Layer 3 reachability. Since ping to 192.168.1.100 already failed, tracert will likely also fail and does not provide information about active connections or listening ports. It is useful for identifying where packets are dropped along the path, but not for verifying active sessions.

The ping -t command sends continuous ICMP echo requests to test reachability over time, but it does not reveal connection states or listening ports. Since ping already failed, continuous pings will also fail and do not help identify issues with active TCP sessions to the web server.

A duplex mismatch occurs after auto-negotiation completes; no 'auto-negotiation failed' message appears on the interface.

Duplex mismatch causes errors and retransmissions, usually reducing the effective input rate, not increasing it.

Duplex mismatch keeps the line protocol up; it affects data integrity but not the Layer 1/2 link state.

The IPv6 subnet mismatch prevents direct connectivity; R1 and R3 must be on the same subnet for a ping to work without additional routing.

The requirement explicitly states that the IPv6 address must be derived using EUI-64; a manually specified interface ID violates this.

The IPv6 subnet must be the same as R3's for direct connectivity; using a different subnet requires additional routing, which is not configured.

The subnet 192.168.22.128/28 includes addresses 192.168.22.128 to 192.168.22.143. The host address 192.168.22.145 is outside this range, so it does not belong to this subnet.

The subnet 192.168.22.160/28 includes addresses 192.168.22.160 to 192.168.22.175. The host address 192.168.22.145 is below this range, so it cannot be in this subnet.

The subnet 192.168.22.148/28 is not a valid subnet because the network address must be a multiple of the subnet size (16). Valid network addresses for /28 are 0, 16, 32, 48, etc. 148 is not a multiple of 16, so this is not a valid subnet.

10.10.10.0 is the network address of the 10.10.10.0/25 subnet (range 0-127), but the host IP 10.10.10.130 is not in that range. The /25 mask creates two subnets: 10.10.10.0/25 and 10.10.10.128/25, and .130 belongs to the latter.

10.10.10.64 is not a valid network address for any /25 subnet derived from 10.10.10.0/24. A /25 subnet has a block size of 128, so the network addresses are multiples of 128: 0 and 128. 64 is a multiple of 64, which would be a /26 boundary, not /25.

10.10.10.255 is the broadcast address for the 10.10.10.128/25 subnet, not the network address. The broadcast address is the last address in the subnet (all host bits set to 1), while the network address is the first address (all host bits set to 0).

A default gateway is needed only for management traffic from the switch itself, not for DNS forwarding; the switch already communicates with the router via its default route.

A static default route is already present and unrelated to DNS resolution; adding another route would not solve the name resolution failure.

The 'ip domain-lookup' command only enables the DNS client on the switch itself, not DNS forwarding for clients like PC1.

The trunk port Gi1/0/24 is in 'on' mode and trunking, as shown in 'show interfaces trunk'. Therefore, the trunk is operational and not the cause of the issue.

The 'show vlan brief' output shows that port Gi1/0/12 is assigned to VLAN 10, so the PC is in the correct VLAN. This is not the issue.

A duplicate IP address would cause connectivity issues to the gateway as well, but the PC can ping the gateway successfully. Therefore, duplicate IP is not the problem.

Neighbor Solicitation (NS) messages are used for address resolution (determining the link-layer address of a neighbor) and duplicate address detection, not for advertising default gateway or prefix information.

DHCPv6 Solicit messages are used in stateful DHCPv6 to request configuration parameters like addresses and DNS servers, but SLAAC does not use DHCPv6 for default gateway or prefix information; those are provided by RA messages.

ICMP Echo Reply messages are used for ping responses and do not carry any routing or prefix information; they are not involved in neighbor discovery or address autoconfiguration.

The subnet 10.10.40.48/28 includes addresses 10.10.40.48 through 10.10.40.63. The host address 10.10.40.78 is outside this range, so it does not belong to this subnet.

The subnet 10.10.40.72/28 is not a valid subnet because /28 subnets have boundaries that are multiples of 16. The valid subnet starting addresses for /28 are 0, 16, 32, 48, 64, 80, etc. 72 is not a multiple of 16, so this is not a valid network address.

The subnet 10.10.40.80/28 includes addresses 10.10.40.80 through 10.10.40.95. The host address 10.10.40.78 is below this range, so it does not belong to this subnet.

The /26 subnet mask has a block size of 64, not 32. The network addresses for /26 are 0, 64, 128, and 192. 192.168.60.32 is not a valid network address because it is not a multiple of 64.

192.168.60.64 is the network address of the next /26 subnet (64–127). The host address 192.168.60.33 belongs to the 0–63 range, not the 64–127 range.

192.168.60.63 is the broadcast address of the 192.168.60.0/26 subnet, not the network address. The network address is always the first address in the subnet (all host bits set to 0).

Hiding the SSID is already satisfied; changing broadcast settings would make the SSID visible, contradicting the requirement.

WPA3 is already enabled on the WLAN, so there is no need to configure security. The client associates successfully, proving security is not the issue.

The WLAN is enabled because the client can see the SSID and associates; enabling it again would not fix the VLAN mismatch.

SLAAC does not require NAT because IPv6 has a vast address space, eliminating the need for address translation. NAT is a workaround for IPv4 address exhaustion and is not used in native IPv6 networks. SLAAC relies on router advertisements to provide prefix information, and hosts generate their own addresses without any translation.

DHCPv6 does not replace router advertisements; in fact, router advertisements are still required for hosts to determine the default gateway and other network parameters. Even when DHCPv6 is used, hosts rely on RAs to learn the on-link prefix and to decide whether to use stateful or stateless configuration.

Both SLAAC and DHCPv6 can provide addressing information to hosts. SLAAC allows hosts to form their own addresses from prefix information in RAs, while DHCPv6 can assign addresses and other configuration parameters. Therefore, the statement that neither can provide addressing information is false.

10.10.10.31 is the broadcast address of the previous /27 subnet (10.10.10.0–10.10.10.31), not the subnet containing .33.

10.10.10.32 is the network address (subnet ID) of the subnet containing .33, not the broadcast address. The network address is the first address in the block.

10.10.10.64 is the network address of the next /27 subnet (10.10.10.64–10.10.10.95), not the broadcast address of the subnet containing .33.