These questions describe a network symptom and ask you to identify the root cause or the correct fix. They appear across all certification exams and reward systematic thinking over memorisation. The best candidates follow a consistent troubleshooting framework even under time pressure.
Start Scenario PracticeAn engineer is troubleshooting a problem where a trunk link between two Cisco switches is not passing traffic for VLAN 10, but other VLANs are working. The trunk is configured with switchport mode trunk on both sides. The engineer checks the allowed VLAN list and sees VLAN 10 is included. The native VLAN is set to 1 on both sides. What is the most likely cause?
Explanation: The most likely cause is that VLAN 10 is not created in the VLAN database on one of the switches. Even if VLAN 10 is included in the allowed VLAN list on the trunk, a switch will not forward traffic for a VLAN that does not exist in its local VLAN database. The trunk interface will be operationally down for that specific VLAN, preventing traffic from passing.
A network engineer is troubleshooting a VRF-lite deployment on a Cisco Nexus 9000 switch. Two VRFs, PROD and DEV, are configured. The switch has an SVI for VLAN 10 in VRF PROD and VLAN 20 in VRF DEV. A firewall is connected to a Layer 3 port in VRF PROD for internet access. The engineer needs to allow the DEV VRF to reach the internet through the same firewall, but without using a separate physical interface. What should the engineer configure?
Explanation: VRF-lite does not support direct route leaking between VRFs without an external mechanism. By configuring a static route in VRF DEV pointing to the firewall's IP address (which resides in VRF PROD) and using a route-map to leak the route, the engineer enables inter-VRF routing. This allows DEV traffic to reach the firewall's interface in PROD without requiring a separate physical interface, as the route-map controls which prefixes are shared between VRFs.
A network engineer is troubleshooting an EtherChannel between two Cisco switches. The show etherchannel 1 port-channel command shows the port-channel is up, but traffic is not load-balanced evenly. The engineer notices that all traffic is using only one link. The physical ports are all configured identically. What is the most likely cause?
Explanation: When the load-balancing method is set to src-dst-ip, the hash is computed from the source and destination IP addresses. If all traffic flows between the same two IP addresses, the hash result will always be identical, causing all frames to be forwarded over the same physical link. This explains why the port-channel is up but only one link carries all traffic.
A network administrator is troubleshooting a BGP routing issue where routes from an eBGP neighbor are not being installed in the routing table. The 'show ip bgp' output shows the routes are received but not valid. What is the most likely cause?
Explanation: For a BGP route to be considered valid and installed in the routing table, the next-hop IP address must be reachable via an IGP or a static route. If the next hop is not reachable, the route will appear in the 'show ip bgp' output but will be marked as not valid (often with a 'r' for received but not valid). This is the most common cause when routes are received from an eBGP neighbor but not installed.
A network engineer is troubleshooting a problem where Cisco DNA Center is not receiving syslog messages from a critical core switch. The switch is configured to send syslog to the DNA Center's IP address. The engineer checks the DNA Center syslog collector and finds that it is enabled. What should the engineer check next?
Explanation: Cisco DNA Center's syslog collector listens on specific ports (default UDP 514). If the switch is configured to send syslog but DNA Center is not receiving it, a common issue is that the syslog port is blocked by a firewall or ACL between the switch and DNA Center. The engineer should verify network connectivity and port accessibility.
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Practice all Troubleshooting Scenario QuestionsThese questions describe a network symptom and ask you to identify the root cause or the correct fix. They appear across all certification exams and reward systematic thinking over memorisation. The best candidates follow a consistent troubleshooting framework even under time pressure. These appear throughout the 350-401 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 350-401. Practicing each scenario type ensures you're ready for any format.
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