These are the questions most candidates get wrong. They require connecting multiple concepts, reading tricky output, or knowing edge-case behaviour that isn't on most study cards. Practising them trains you to operate under uncertainty — a necessary skill on the real exam.
Start Scenario PracticeOn a DMVPN Phase 2 hub-and-spoke network, an engineer applies an inbound IPv4 ACL on the tunnel interface of a spoke router to permit only traffic from specific spoke IPs. After the ACL is applied, the spoke cannot establish a direct spoke-to-spoke tunnel with another spoke, even though NHRP resolution succeeds. What is the most likely explanation?
Explanation: In a DMVPN Phase 2 network, spoke-to-spoke traffic is encapsulated in a new IP header with the destination IP set to the target spoke's public address. When an inbound ACL is applied to the spoke's tunnel interface, it filters traffic after decapsulation, meaning it inspects the inner (original) IP header. However, the ACL is applied on the physical interface or the tunnel interface in a way that inadvertently filters the outer IP header of the mGRE tunnel, blocking the encapsulated spoke-to-spoke packets before they can be processed. Since NHRP resolution succeeds, the spoke knows the peer's address, but the actual data traffic is dropped because the ACL denies the outer encapsulation.
A DMVPN network with hub R1 and spokes R2 and R3 is configured with mGRE and NHRP. Spoke-to-spoke tunnels fail to form. R1 configuration: interface Tunnel0, ip address 10.0.0.1 255.255.255.0, tunnel source GigabitEthernet0/0, tunnel mode gre multipoint, ip nhrp network-id 1, ip nhrp map multicast dynamic. R2 shows: 'show dmvpn' shows no dynamic sessions. R3 shows: 'show ip nhrp' shows no entries for R2. What is the root cause?
Explanation: In a DMVPN network, spokes must use the 'ip nhrp nhs' command to register their real (physical) IP addresses with the hub (NHS). Without this command, the hub never learns the spokes' addresses, so it cannot provide NHRP resolution replies to other spokes attempting to form spoke-to-spoke tunnels. The absence of dynamic sessions on R2 and no NHRP entries for R2 on R3 directly indicate that R2 never registered with the hub.
A switch is configured with RSPAN to monitor traffic from VLAN 50 to a remote switch via VLAN 200. The source switch has: monitor session 1 source vlan 50 rx monitor session 1 destination remote vlan 200. The remote switch has: monitor session 2 source remote vlan 200 monitor session 2 destination interface Gi0/2. The intermediate switches have VLAN 200 configured with 'remote-span'. The network uses VTP transparent mode. The analyzer connected to Gi0/2 sees intermittent traffic. The RSPAN VLAN 200 is also used as a native VLAN on some trunk ports. What is the likely cause of intermittent traffic?
Explanation: When the RSPAN VLAN is also configured as the native VLAN on a trunk, mirrored frames are sent untagged on that trunk. Intermediate switches may treat untagged frames as belonging to the native VLAN rather than the RSPAN VLAN, causing them to be forwarded incorrectly or dropped, resulting in intermittent traffic at the destination.
An engineer configures CoPP on a router with the following policy: class-map match-any PROTECT, match protocol ospf, police 1000 pps; class class-default, police 500 pps. After applying, OSPF neighbors form, but the router's CPU utilization remains high. Which is the most likely explanation?
Explanation: The class-default police rate of 500 pps is too low to handle essential control-plane traffic like ARP, which falls into class-default. When ARP packets are dropped, the router must retry ARP resolution, generating additional CPU overhead from policing and retransmissions, keeping CPU utilization high. OSPF neighbors form because OSPF traffic is explicitly protected in the PROTECT class with a 1000 pps policer, but the underlying ARP starvation causes CPU strain.
An engineer configures OSPF on a link with an MTU of 1500 on one side and 1400 on the other. The OSPF neighbor state is stuck in EXSTART. What is the most likely explanation?
Explanation: OSPF uses the MTU of the interface to determine the maximum size of Database Description (DBD) packets. If the MTU mismatches, the router with the smaller MTU will reject DBD packets that exceed its MTU, causing the neighbor to remain in EXSTART.
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Practice all Hard Difficulty QuestionsThese are the questions most candidates get wrong. They require connecting multiple concepts, reading tricky output, or knowing edge-case behaviour that isn't on most study cards. Practising them trains you to operate under uncertainty — a necessary skill on the real exam. These appear throughout the 300-410 and require you to apply your knowledge, not just recall facts.
Cisco doesn't publish an exact breakdown, but scenario-based questions (especially exhibit and command-output formats) make up a significant portion of the 300-410. Practicing each scenario type ensures you're ready for any format.
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