Show Command Output Questions

The access list 'NAT' is incorrect; it should permit only specific hosts, not the entire subnet.

Why wrong: The access list 'NAT' is not the problem. In dynamic NAT, the ACL is used solely to identify which inside addresses are eligible for translation, and permitting the entire internal subnet is standard and correct when all internal hosts should have Internet access. Even if the ACL were narrowed to specific hosts, the router would still fail to translate return packets because the outside interface has not been designated with 'ip nat outside'. The ACL only controls which source addresses are translated, not whether NAT is applied on an interface.

The NAT configuration lacks the 'overload' keyword, so the pool is exhausted quickly.

Why wrong: The lack of the 'overload' keyword is a red herring here. Without overload, the router performs standard dynamic NAT, which still works correctly as long as the number of active inside addresses does not exceed the number of addresses in the pool. In the scenario, there are only 2 active translations and 11 addresses in the pool, so the pool is not exhausted and overload would not change the immediate outcome. The real issue is that the outside interface is not configured for NAT, and adding overload cannot compensate for that missing interface-level command.

The NAT pool 'POOL' has too few addresses; it should be expanded to a /24 subnet.

Why wrong: The NAT pool 'POOL' has 11 addresses, which is more than enough for the two active translations, so pool size is not causing the problem. Expanding the pool to a /24 subnet would only increase the maximum number of concurrent translations; it would not make the router process return traffic on the outside interface. Even with 254 public addresses available, the router would still not translate reply packets because the outside interface is not marked with 'ip nat outside'. The symptom of no replies is a direct result of missing interface NAT designation, not address exhaustion.

The interface is administratively down.

Why wrong: The interface status in the output reads 'up, line protocol is up', indicating that the interface is fully operational at both Layer 1 and Layer 2. An administratively down interface would instead show 'administratively down, line protocol is down' and would not generate any IPv6 traffic, including Router Advertisements. Since the router is actively advertising and the link-local address is present, the interface is clearly not shut down or disabled.

The MAC address of the interface is invalid, preventing EUI-64 from generating a proper address.

Why wrong: The link-local address FE80::21A:2BFF:FE3C:4D5E shown in the output is derived from the interface MAC address using EUI-64, which inserts FFFE in the middle and flips the U/L bit. The presence of this valid link-local address proves the MAC address is not invalid and EUI-64 generation succeeded. An invalid or all-zero MAC would prevent IPv6 from generating any link-local address, but the output clearly displays a properly formed one, so this option cannot be the cause.

The router is not sending Router Advertisements, so hosts cannot autoconfigure.

Why wrong: The output explicitly states 'ND router advertisements are sent every 200 seconds', which confirms the router is actively sending ICMPv6 Router Advertisements on this interface. Hosts would therefore be able to perform stateless address autoconfiguration (SLAAC) and learn the default router. However, the problem is that hosts cannot reach the router's own link-local address, which points to a missing or incorrectly configured IPv6 address on the router itself, not to a lack of RA messages.

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

Why wrong: Checking the DHCP server logs for Discover messages is premature in this scenario. Because the DHCP server resides on a different subnet than the clients, server-side logs will only show Discover packets if the router's VLAN 20 interface has an IP helper-address configured to forward broadcast DHCP requests. If the helper-address is missing, the router drops the broadcast at the VLAN boundary, and the server never sees any Discover traffic—so the absence of such messages in the logs would merely confirm the relay misconfiguration, not diagnose it. This step is valid only after confirming that the relay configuration is correct, and it wastes troubleshooting time by skipping the most fundamental cause.

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

Why wrong: The host's APIPA address (169.254.x.x) is a link-local address. Without a default gateway or a routable IP, pinging an address on a different subnet will not work, even if the DHCP server is reachable. This test does not confirm DHCP functionality and is not a standard troubleshooting step for DHCP failures.

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

Why wrong: Restarting the DHCP service is a disruptive action and should not be the first step, especially when the root cause likely lies in the network configuration (missing relay). It does not address the underlying issue and may cause unnecessary downtime.

interface GigabitEthernet1/0/1 switchport mode trunk switchport trunk allowed vlan 10,110 power inline auto interface GigabitEthernet1/0/2 switchport mode access switchport access vlan 20

Why wrong: This is incorrect because Gi1/0/1 is configured as a trunk port instead of an access port. While a trunk can carry multiple VLANs, the requirement is for an access port with a voice VLAN. The 'switchport mode trunk' command changes the port to trunk mode, which is not desired.

interface GigabitEthernet1/0/1 switchport mode access switchport access vlan 10 switchport voice vlan 110 power inline never interface GigabitEthernet1/0/2 switchport mode access switchport access vlan 20

Why wrong: This is incorrect because 'power inline never' disables PoE on Gi1/0/1, preventing the IP phone from receiving power. The requirement explicitly states to enable PoE. The correct command is 'power inline auto' (or no command, as auto is default).

interface GigabitEthernet1/0/1 switchport mode access switchport access vlan 110 switchport voice vlan 10 power inline auto interface GigabitEthernet1/0/2 switchport mode access switchport access vlan 20

Why wrong: This is incorrect because the access VLAN and voice VLAN are swapped. The access VLAN should be 10 (data) and the voice VLAN should be 110 (voice). This misconfiguration would place the IP phone's data traffic in VLAN 110 and voice traffic in VLAN 10, which is not the requirement.

Generate a new RSA key pair and restart the SSH service.

Why wrong: Generating a new RSA key pair and restarting the SSH service is unrelated to the actual failure. The 'connection refused' occurs at the TCP/VTY layer because the VTY lines only permit Telnet ('transport input telnet'), so the router rejects the SSH handshake before any key-based authentication or encryption is negotiated. Even if the key pair is absent or stale, SSH would fail differently (e.g., 'no host key' or authentication errors), not with a transport-level refusal; regenerating keys does not modify the line's transport input policy.

Configure 'ip ssh version 1' to enable backward compatibility.

Why wrong: The router already runs SSH version 2, and version 1 is less secure and not required for compatibility. Changing the version does not override the VTY transport input restriction, so SSH connections will still be refused.

Configure AAA authentication on the VTY lines.

Why wrong: AAA authentication is not required for SSH to function; the router is already using local authentication (implied by the working Telnet). The refusal occurs before authentication, at the transport protocol layer, so adding AAA would not resolve the issue.

snmp-server community public snmp-server community private snmp-server user admin v3 auth md5 Cisco123 priv des56 Cisco456 snmp-server enable traps snmp-server host 203.0.113.10 version 2c public snmp-server host 203.0.113.10 version 3 auth admin ip flow-export destination 203.0.113.20 2055 ip flow-export version 9

Why wrong: This configuration is incorrect because it uses MD5 instead of SHA for SNMPv3 authentication and DES instead of AES 128 for encryption. Additionally, the community strings lack the RO and RW keywords, which are required to specify access levels.

snmp-server community public RO snmp-server community private RW snmp-server user admin v3 auth sha Cisco123 priv aes 128 Cisco456 snmp-server enable traps snmp-server host 203.0.113.10 version 2c private snmp-server host 203.0.113.10 version 3 auth admin ip flow-export destination 203.0.113.20 2055 ip flow-export version 9

Why wrong: This configuration fails because the SNMPv2c trap receiver at 203.0.113.10 is bound to the 'private' community string, which is the read-write (RW) community. The question explicitly requires a v2c trap host using the read-only (RO) community 'public' to match the monitoring station's read-only access. Sending traps with the RW community violates the least-privilege principle and would be rejected by a properly configured NMS that expects traps from the 'public' community. Additionally, specifying the v2c host with the wrong community string means the NMS may not authenticate the trap, so no alarms would be received.

snmp-server community public RO snmp-server community private RW snmp-server user admin v3 auth sha Cisco123 priv aes 128 Cisco456 snmp-server enable traps snmp-server host 203.0.113.10 version 2c public snmp-server host 203.0.113.10 version 3 auth admin ip flow-export destination 203.0.113.20 2055 ip flow-export version 5

Why wrong: The entire SNMP configuration is correct, including the v2c public trap host, v3 user 'admin' with SHA/AES-128, and the trap hosts, but NetFlow export is misconfigured by using version 5 instead of the required version 9. NetFlow version 5 does not support templates, so it cannot carry flexible field definitions or accommodate new flow record formats; version 9 is template-based and allows extensible, structured data export. The question explicitly mandates version 9 for the flow exporter, making version 5 an incorrect choice. Even though the destination 203.0.113.20 and port 2055 are correct, the wrong version breaks the expected flow data encoding and may be unreadable by the collector.

snmp-server community publicRW ro snmp-server user admin admin v3 auth md5 cisco123 priv des cisco456 snmp-server enable traps snmp linkdown linkup snmp-server host 192.0.2.100 traps version 2c publicRW ip flow-export destination 192.0.2.200 2055 ip flow-export version 9

Why wrong: This configuration is missing the 'ip flow ingress' command on an interface, which is required to enable NetFlow data collection. Without it, no flow records will be exported, and 'show ip cache flow' will show no entries.

snmp-server community publicRW ro snmp-server user admin admin v3 auth md5 cisco123 priv des cisco456 snmp-server enable traps snmp linkdown linkup snmp-server host 192.0.2.100 traps version 2c publicRW ip flow-export destination 192.0.2.200 2055 ip flow-export version 9 interface GigabitEthernet0/0 ip flow egress

Why wrong: This configuration uses 'ip flow egress' instead of 'ip flow ingress'. While egress flow monitoring is valid, the task does not specify direction; however, ingress is more commonly used and the verification command 'show ip cache flow' expects ingress flows by default. More importantly, the task requires NetFlow export to work, and using egress may not capture the desired traffic.

snmp-server community publicRW ro snmp-server user admin admin v3 auth md5 cisco123 priv des cisco456 snmp-server enable traps snmp linkdown linkup snmp-server host 192.0.2.100 traps version 2c publicRW ip flow-export destination 192.0.2.200 2055 ip flow-export version 9 interface GigabitEthernet0/0 ip flow monitor FLOW-MONITOR input

Why wrong: This configuration uses 'ip flow monitor' which is a newer Flexible NetFlow command, but the task requires traditional NetFlow (version 9). The 'ip flow monitor' command requires a flow monitor configuration that is not provided, and it does not work with 'ip flow-export version 9' in the same way. The correct command for traditional NetFlow is 'ip flow ingress'.

Because static routes cannot reference exit interfaces.

Why wrong: Static routes may legitimately reference an exit interface, such as ip route 10.200.0.0 255.255.0.0 GigabitEthernet0/0, which is a common and valid configuration for point-to-point links. The problem in the exhibit is not that the configuration uses an exit interface, but that the referenced interface has been administratively shut down. When the interface is down, the route is removed from the routing table, whereas a healthy interface would allow the static route to appear.

Because the destination must be a /24, not a /16.

Why wrong: The prefix length /16 is perfectly valid for a static route, and there is no Cisco IOS requirement that a destination be a /24. Static routes accept any legitimate IPv4 prefix, and subnetted networks can be summarized into larger blocks like /16. The exhibit shows the route missing because the outgoing interface is down, not because the mask is too short or unsupported.

Because static routes require NTP before installation.

Why wrong: Network Time Protocol (NTP) is used for clock synchronization and does not affect the Routing Information Base or the installation of static routes. A static route is evaluated based on the administrative distance, metric, and the reachability of the configured next hop or exit interface, not on any time-based condition. NTP misconfiguration could impact logging or certificate validation but will never prevent a valid static route from being installed.

Configure the interface with 'speed 100' to force the link speed to 100 Mbps.

Why wrong: Forcing 'speed 100' only corrects a speed mismatch, but the interface is already running at the same speed; the real issue is that R1 is set to full-duplex while R2 is half-duplex. A duplex mismatch causes hidden collisions and input errors because the half-duplex side cannot detect an idle medium before transmitting, so changing only the speed leaves the underlying fault untouched. You must align the duplex mode, not just the speed.

Replace the Ethernet cable with a new one to fix the physical layer issue.

Why wrong: A faulty Ethernet cable typically produces CRC errors, alignment errors, runts, or giants due to electrical signal degradation or defective pins, not the pattern of high input errors with a healthy CRC counter. In this scenario, the error signature—where frames are discarded on the full-duplex side because the half-duplex side aborts them after detecting collisions—is the classic duplex mismatch. Replacing the cable would not alter the fact that R2 remains half-duplex while R1 stays full-duplex, so the errors would persist.

Configure the interface with 'no shutdown' to bring the interface up.

Why wrong: The interface output shows the port is administratively up and line protocol is up, so 'no shutdown' is unnecessary and would have no impact on the rising input error counters. A duplex mismatch is a logical configuration issue that disrupts data transfer while the link itself remains physically active; this is not an interface-shutdown problem. The correct resolution is to reconfigure the duplex setting, not to toggle the administrative state.

Assign access ports, create VLANs, configure 802.1Q trunks, set native VLAN, verify with 'show vlan brief' and 'show interfaces trunk'

Why wrong: Attempting to assign access ports before creating the VLANs violates the fundamental dependency that the VLAN must exist in the switch's VLAN database for the port assignment to be valid. If the VLAN is not yet defined, the switch either rejects the 'switchport access vlan' command or creates an implicit VLAN dynamically, leading to inconsistent configuration that may not match the design intent. The correct sequence is to first create the VLANs globally, then assign access ports, and only after those steps should trunking and native VLAN configuration be considered.

Create VLANs, assign access ports, set native VLAN, configure 802.1Q trunks, verify with 'show vlan brief' and 'show interfaces trunk'

Why wrong: Setting the native VLAN before enabling 802.1Q trunking is logically flawed because the native VLAN parameter applies exclusively to trunk interfaces and is either ignored or rejected while the port is in access mode. On most Cisco switches, the command may be accepted but silently stored without taking effect until the interface is changed to trunk mode, potentially causing confusion during troubleshooting. Therefore, the native VLAN must be configured after the trunk has been established, ensuring the trunk can properly handle untagged frames on the correct VLAN.

Assign access ports, configure 802.1Q trunks, create VLANs, set native VLAN, verify with 'show vlan brief' and 'show interfaces trunk'

Why wrong: This ordering errs twice: it assigns access ports and configures trunking before VLANs exist, and it also creates VLANs after trunk configuration, which is far too late for the earlier commands to remain valid. Without a populated VLAN database, the access ports cannot be correctly mapped to their intended VLANs, and trunks have no valid VLANs to carry across the link. The proper process is to create VLANs first, then assign access ports, then configure 802.1Q trunks, and finally set the native VLAN, with verification performed after all steps to ensure the configuration matches the intended design.

The trunk port is using ISL encapsulation, which does not support VLAN 40.

Why wrong: ISL encapsulation supports VLANs 1–1005, so VLAN 40 is within the valid range. Modern Cisco switches default to 802.1Q, and if encapsulation were mismatched the trunk would likely be down. The root cause is not encapsulation.

The technician omitted the 'add' keyword when adding VLAN 40 to the allowed list, so the trunk no longer permits VLAN 40.

Why wrong: When the 'add' keyword is omitted, the allowed list is overwritten and all previously allowed VLANs are removed. In that case, the show interfaces trunk command would not show VLAN 40 in the allowed list at all, contradicting the symptom of it being allowed but not in forwarding state.

VTP pruning is enabled, and VLAN 40 is not needed by any downstream neighbor, so it is pruned from this trunk.

Why wrong: VTP pruning removes VLAN traffic from trunks when no downstream switch has ports in that VLAN, but it requires a VTP domain and is a separate feature. Without any indication of VTP configuration, the most likely reason a single VLAN is missing from the forwarding list after being allowed is that the VLAN itself has not been created.

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

Why wrong: An access list could block ICMPv6 Router Advertisements, preventing hosts from obtaining prefixes and global unicast addresses. However, this does not explain why the router's own interface lacks a global unicast address when statically configured or when the router itself should be generating RAs. The missing ipv6 unicast-routing command is a more fundamental cause.

The IPv4 resource is not configured for NAT64 translation.

Why wrong: While NAT64 is a common method for IPv6-only hosts to access IPv4 resources, the symptom explicitly shows a missing global unicast address on the router interface and a missing ipv6 unicast-routing command. This indicates a basic IPv6 routing failure, not a translation service configuration issue.

The host has an incorrect default gateway for IPv6.

Why wrong: A misconfigured default gateway on the host would prevent it from reaching off-link resources. However, the technician observed a missing global unicast address on the router interface and the absence of ipv6 unicast-routing, indicating that the router itself is not properly configured for IPv6 routing. The host's gateway issue would be a consequence, not the root cause.

Verify that both switches are using the same EtherChannel protocol (LACP or PAgP).

Why wrong: Because both switches were configured with `channel-group 1 mode active`, they are already using LACP unambiguously — `active` is an LACP-specific mode, not a PAgP mode. If the neighboring switch had been set to PAgP, LACPDUs would fail and the ports would remain in the stand-alone 'I' state, but inspecting the protocol now would only confirm what the configuration already proves. A protocol mismatch is not the issue here, since both sides explicitly negotiated LACP; the correct diagnostic path is to check VLAN consistency, which directly affects whether LACP considers the member links eligible for bundling.

Check that the speed and duplex settings match on all member interfaces.

Why wrong: While speed/duplex mismatch is a valid cause of the 'I' state, it is not the most targeted next step after confirming LACP mode. The technician should first rule out VLAN configuration mismatches, which are equally likely and require immediate inspection when trunk ports are involved.

Determine whether Spanning Tree Protocol is blocking one of the ports.

Why wrong: STP blocking state does not affect EtherChannel negotiation. A port that is part of a bundle will be shown in the 'P' (bundled) state regardless of STP status. The 'I' state indicates the port is not even joining the bundle, so STP is not the cause.

GigabitEthernet0/0 is administratively down, which prevents the host from communicating.

Why wrong: Administratively down is a switchport shutdown state, not an STP state. The output shows GigabitEthernet0/0 in the BLK (blocking) state and role Altn (alternate), which can only occur when the port is administratively and operationally up, allowing BPDUs to be received and processed. A shutdown port would display as 'administratively down' in interface status and would either be absent from spanning-tree output or shown as disabled, never as BLK. Therefore, this option misinterprets a normal STP blocking state as a manual administrative configuration.

The port is in the Blocking state because the switch detected a loop and moved the port to error-disabled state.

Why wrong: Error-disabled ports would show a status of 'err-disabled' in spanning-tree output, not 'BLK'. The output clearly shows the port is in the normal blocking state (BLK) as part of STP convergence, not due to an error condition.

The port is blocked because SW3 has a lower bridge priority than the root bridge and should be the designated port for that segment.

Why wrong: The exhibit shows the root bridge is 00d0.97f1.8a00, and SW3's port Gi0/0 has role 'Altn' (alternate), not 'Desg' (designated). A designated port would be forwarding, not blocking. SW3 is not the designated bridge for that segment.

R1 has a mismatched OSPF process ID compared to the ABR.

Why wrong: OSPF process IDs are locally significant and only identify the OSPF instance running on that router itself. There is no requirement for process IDs to match between neighboring routers for adjacency formation or route exchange, so a mismatch would neither prevent a route from being installed nor alter its classification as intra-area (Type-1) or inter-area (Type-3). The routing table entry showing 'inter area' is solely determined by the LSA type generated by the ABR, which depends on the destination network's area membership—not on the process ID.

The network type on GigabitEthernet0/0 is configured as non-broadcast, preventing full adjacency.

Why wrong: A non-broadcast network type could prevent neighbor formation, but the route is present in the routing table, indicating adjacency is established. Even if adjacency exists, the network type does not alter whether a route is intra-area or inter-area.

The metric for the route is too high, causing OSPF to treat it as an inter-area route.

Why wrong: OSPF classifies routes based on the LSA type (Type-1 for intra-area, Type-3 for inter-area), not on the metric value. A high metric would still result in an O route if the network is in the same area.