These questions present the output of IOS show commands — show ip route, show interfaces, show ip ospf neighbor, show vlan brief — and ask you to interpret what they reveal about the network state. Reading IOS output accurately is one of the highest-value skills on the CCNA.
Start Scenario PracticeA 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 network engineer runs the following command on Router R1: R1# show ipv6 interface tunnel 0 Tunnel0 is up, line protocol is up IPv6 is enabled, link-local address is FE80::1 Global unicast address(es): 2001:DB8:2::1, subnet is 2001:DB8:2::/64 Joined group address(es): FF02::1 FF02::2 ICMP redirects are enabled ICMP unreachables are enabled ND DAD is enabled, number of DAD attempts: 1 ND reachable time is 30000 milliseconds IPv6 uRPF: loose mode (allow default route) Based on this output, what is the uRPF configuration on this interface?
Explanation: The output explicitly shows 'IPv6 uRPF: loose mode (allow default route)', which confirms that unicast Reverse Path Forwarding (uRPF) is enabled in loose mode. In loose mode, the router checks that the source address of an incoming packet has a matching entry in the routing table, but it does not require the incoming interface to match the best return path. This is distinct from strict mode, which requires both a routing table entry and that the incoming interface is the same as the outgoing interface for the return route.
An engineer is troubleshooting a DMVPN phase 3 network where spoke-to-spoke tunnels are established, but traffic between spokes is taking a suboptimal path through the hub. The engineer checks 'show ip nhrp shortcut' on the spoke and sees no shortcut entries. The hub has 'ip nhrp redirect' enabled, and the spoke has 'ip nhrp shortcut' enabled. The engineer also verifies that the spoke's routing table has a route to the remote spoke's LAN via the hub. What is the most likely cause?
Explanation: In DMVPN phase 3, for spoke-to-spoke shortcuts to be installed, the spoke must receive an NHRP redirect from the hub. If the hub does not send a redirect, the spoke will not create a shortcut. The hub sends a redirect only if it has a route to the destination subnet. If the hub does not have a route to the remote spoke's LAN, it will not send a redirect.
A network engineer runs the following command to verify IPv6 uRPF operation: R1# show ipv6 interface GigabitEthernet0/0 | include verify IPv6 verify source: strict What does this output indicate?
Explanation: The output 'IPv6 verify source: strict' indicates that strict unicast Reverse Path Forwarding (uRPF) is enabled on the interface. Strict uRPF verifies that the source IPv6 address of an incoming packet is reachable via the routing table AND that the best return path to that source uses the same interface on which the packet was received. If either condition fails, the packet is dropped. This matches option A exactly.
An engineer redistributes OSPF routes into EIGRP. The OSPF routes have a metric of 20. After redistribution, the EIGRP topology table shows the routes but they are not installed in the routing table. The 'show ip eigrp topology' shows the route in active state. Which is the most likely explanation?
Explanation: When redistributing into EIGRP, a seed metric must be specified. If not, the routes are redistributed with an infinite metric (e.g., 4294967295) and are not installed. However, the question says the route is in active state, which indicates a query is pending. This is an edge case where the route is redistributed with a valid metric but the EIGRP process is stuck-in-active due to a unidirectional link or a neighbor not replying to queries.
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Practice all Show Command Output QuestionsThese questions present the output of IOS show commands — show ip route, show interfaces, show ip ospf neighbor, show vlan brief — and ask you to interpret what they reveal about the network state. Reading IOS output accurately is one of the highest-value skills on the CCNA. 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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