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 PracticeA network engineer is troubleshooting an OSPF adjacency failure between two directly connected routers, R1 and R2. Both routers are configured with the same OSPF process ID and area. The engineer verifies that the interfaces are up and IP connectivity exists. Which configuration mismatch is most likely causing the adjacency to fail?
Explanation: An OSPF network type mismatch, such as one interface configured as point-to-point and the other as broadcast, prevents OSPF adjacency from forming. Options A, B, and C are incorrect: An MTU mismatch (A) usually allows adjacency to form but causes problems during LSA exchange; a passive interface (B) stops the router from sending hellos, so it wouldn't see the neighbor at all, but the question implies both interfaces are operational; an area ID mismatch (C) would prevent adjacency, but the scenario specifies that both routers are configured in the same area. Therefore, the most likely cause of the adjacency failure when interfaces are up and IP connectivity exists is a network type mismatch.
A network engineer is troubleshooting QoS in a service provider environment. Customer traffic is marked with DSCP AF31 (011010) at the CE. On the PE router, the policy maps trust DSCP and sets the CoS to 4. However, core routers remark the DSCP to 0. What is the most likely cause?
Explanation: The core router is likely applying a policer that marks down traffic exceeding the committed information rate (CIR). When traffic is out-of-contract, the policer can re-mark the DSCP to 0 (best effort), which overrides the trusted DSCP AF31 and the CoS 4 set at the PE. This behavior is common in service provider environments where core routers enforce traffic contracts and use policing to penalize excess traffic.
An SP is troubleshooting an MPLS L2VPN VPLS network where MAC flapping is occurring between two PEs. Which mechanism in VPLS prevents loops and ensures that a broadcast frame from one PE is not reflected back to the originating PE?
Explanation: In a VPLS environment, split horizon is the mechanism that prevents loops by ensuring that a broadcast, multicast, or unknown unicast frame received from a pseudowire (PW) is never forwarded back to any other PW on the same PE. This rule, defined in RFC 4761 and RFC 4762, stops the originating PE from receiving its own reflected frames, thus eliminating the MAC flapping that would occur if the frame were looped through the MPLS core.
A service provider operates a Layer 2 MPLS VPN using VPLS. Recently, two new CE routers were added to the VPLS domain, but they cannot ping each other. The existing CEs can communicate. The network uses BGP autodiscovery and signaling. The new CEs are connected to different PEs. The 'show vfi status' command on the new PEs indicates the VFI is up but the pseudowire to the remote PE is down. The BGP session between the PEs is established and the L2VPN address family routes are exchanged. What is the most probable cause?
Explanation: B is correct because BGP autodiscovery for VPLS relies on route-target (RT) matching to import remote VPLS endpoints into the local VFI. If the RT configured on the new PEs does not match the RT used by the existing VPLS context, the remote pseudowire will not be created even though BGP sessions are up and L2VPN routes are exchanged. The 'show vfi status' showing VFI up but pseudowire down is a classic symptom of RT mismatch, as the local PE cannot associate the received BGP routes with the correct VPLS instance.
An engineer is troubleshooting MPLS LSP connectivity. The ingress PE router has the label binding for the FEC 10.1.1.0/24, but no LSP is established. Which command should be checked on the P routers to verify the LSP path?
Explanation: The correct command is 'show mpls forwarding-table' because it displays the LFIB (Label Forwarding Information Base) on each P router. Since the ingress PE has a label binding for FEC 10.1.1.0/24 but no LSP is established, the issue likely lies in label propagation or forwarding along the path. Checking the LFIB on P routers reveals whether they have installed the correct outgoing label and interface for that FEC, which is essential for LSP connectivity.
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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 350-501 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 350-501. Practicing each scenario type ensures you're ready for any format.
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