Cisco Certified Network Associate (CCNA) 200-301 Practice Test 42
Question 410
R1 and R2 have an up/up Ethernet link in OSPF area 0 and agree on hello timers and authentication. `show ip ospf interface Gi0/1` on R1 indicates the transit port is passive; neither side forms an adjacency. Which diagnosis is supported?
- An interface reference-bandwidth change caused a route to have higher cost while the neighbor remains FULL.
- A physical down/down interface prevents all OSPF operation despite the observed up/up link.
- R1 is suppressing OSPF Hellos on the transit interface because it is passive; remove passive on that neighbor-facing link.
- A different OSPF area ID on R2 is the only possible explanation, even though both sides report area 0.
Explanation
Correct Answer
C:A passive OSPF interface may still advertise its connected network from another active adjacency, but it does not send Hellos and cannot form a neighbor on that interface. Correct the `passive-interface` setting for the transit port and confirm neighbor formation.
Incorrect Answers
A:OSPF interface cost affects path selection after topology information is learned; it does not by itself stop the neighbor from exchanging Hellos on an operating link.
B:A physically down port would explain absent Hellos but conflicts with the confirmed Ethernet up/up state. The directly reported passive status is a stronger, specific cause.
D:Area mismatch can block adjacencies, but the observed interfaces are both in area 0. The passive flag provides direct mechanism-based evidence of why R1 never sends Hellos.
Question 411
A branch routing engineer must explain an outage. The interface still reports Broadcast network type and elects a DR/BDR even though only two routers share the link. Which diagnosis follows from these observations?
- DR/BDR elections are impossible on any two-router broadcast Ethernet segment.
- The Ethernet interface remains OSPF broadcast type, so eligible routers elect DR/BDR even when the segment has only two routers; point-to-point can be set explicitly if desired.
- Only an OSPF router-ID collision can explain a displayed DR/BDR when no third router exists.
- A DHCP server automatically chooses the OSPF DR and BDR roles.
Explanation
Correct Answer
B:OSPF network type governs designated-router election. Two endpoints do not automatically convert an Ethernet broadcast interface into a point-to-point OSPF link type.
Incorrect Answers
A:Broadcast OSPF can elect DR and BDR with two eligible routers.
C:The observed behavior is normal for the broadcast network type.
D:OSPF priority/router ID election is independent of DHCP.
Question 412
On an Ethernet OSPF broadcast segment, R1 is elected DR when all routers start. Later R2's interface priority is raised from 1 to 200 while both routers remain up. R1 continues to be DR. Which property best explains the outcome?
- OSPF chooses the DR by the lowest priority and R1 therefore remains preferred.
- OSPF DR/BDR election is nonpreemptive; a higher priority on an already-running router does not automatically replace the current DR.
- The OSPF Dead timer must be increased above 200 seconds before a DR change can occur.
- A higher router ID on R2 always replaces the current DR immediately, without an election event.
Explanation
Correct Answer
B:OSPF broadcast-network DR and BDR roles are elected during neighbor and interface election events and do not continuously preempt an existing DR solely because another router's priority is increased. R2's new priority may matter at a future appropriate election, but R1 can legitimately remain DR while the current topology and state are stable. Verify roles and interface priority using `show ip ospf interface` and neighbor output. Do not force an avoidable adjacency restart in production merely to satisfy a changed number; plan a controlled election if a different DR is required. Priority zero, in contrast, makes a router ineligible.
Incorrect Answers
A:Higher nonzero OSPF interface priority generally improves a router's candidacy during an election. The lowest priority is not the selection rule. The example is about changing priority after R1 has already been elected, when election nonpreemption explains the persistent DR.
C:Hello and Dead timers help neighbors recognize one another and detect failures. They are not a threshold that must exceed the numerical router priority value before a DR can change. Arbitrarily raising dead timers could slow fault detection while doing nothing to cause preemption.
D:Router ID can break ties among otherwise eligible election candidates, but a higher router ID does not automatically evict an existing DR when the network remains stable. The election behavior is nonpreemptive. Inspect current roles instead of treating every priority or ID change as an immediate election trigger.
Question 413
Troubleshooting report: One healthy router remains DROTHER and never wins DR election because its interface priority is zero. Which finding best accounts for the behavior?
- A priority-zero OSPF interface is intentionally ineligible for DR/BDR and can remain DROTHER while forming appropriate adjacencies with the elected DR/BDR.
- A priority-zero router must always preempt the current DR when its router ID is higher.
- All DROTHER routers must be down because only DR/BDR can exchange OSPF traffic.
- The DROTHER role is caused solely by having the lowest Ethernet speed.
Explanation
Correct Answer
A:The interface can participate in OSPF without becoming a designated router. Check expected DROTHER and neighbor states rather than falsely treating priority zero as link failure.
Incorrect Answers
B:Priority zero makes it ineligible to become DR/BDR.
C:DROTHERs can form Full adjacencies with DR/BDR and exchange routes.
D:Interface OSPF priority and election rules determine the designated-router role.
Question 414
Two DROTHERs show 2-WAY but route exchange through the DR works; someone incorrectly treats the stable state as failure. The team should investigate which specific technical cause?
- Every pair of DROTHER routers must reach Full with one another for the area LSDB to function.
- Two DROTHER routers on the same OSPF broadcast segment can remain in 2-WAY with each other while both form Full adjacencies with the DR/BDR.
- Any 2-WAY neighbor state proves mismatched OSPF authentication and failed Hello reception.
- A single DROTHER automatically acts as the DR when the other DROTHER sends traffic.
Explanation
Correct Answer
B:Broadcast OSPF reduces the number of complete database adjacencies by centering on the DR/BDR. A 2-WAY neighbor state between two DROTHERs is normally expected, not a routing outage.
Incorrect Answers
A:2-WAY between DROTHER peers can be stable and normal.
C:2-WAY shows bidirectional Hello communication; the role relationship explains absent Full.
D:DR/BDR election is controlled by protocol rules, not ad hoc traffic forwarding.
Question 415
Two parallel routes from R1 to a remote network use 1-Gbps and 10-Gbps Ethernet links. IOS OSPF auto-cost still uses the default reference bandwidth of 100 Mbps. The interface cost display reports 1 for both links even though the links have different speeds, and unexpected ECMP occurs. Which explanation and correction are technically justified?
- OSPF prefers a higher cost, so change the 10-Gbps link to cost 100 to ensure that it wins.
- Both rates exceed the 100-Mbps default reference; automatic cost bottoms out at 1. Configure an appropriate larger reference bandwidth consistently on OSPF routers or explicit interface costs.
- Increase the DNS cache TTL so SPF can differentiate the interface link speeds.
- OSPF counts link speed only when there are no equal-cost paths; a 10-Gbps interface always has the same hardcoded metric regardless of reference-bandwidth configuration.
Explanation
Correct Answer
B:IOS normally uses reference bandwidth divided by interface bandwidth to derive cost, with a minimum cost of 1. Its default 100-Mbps reference makes 1-Gbps and 10-Gbps links both cost 1, creating a possible equal-cost choice rather than proving 10-Gbps is better. Choose an appropriately higher common reference such as 10000 Mbps (giving approximately 10 for 1 Gbps and 1 for 10 Gbps), configure it consistently on the OSPF routers, or use explicit interface costs. The path calculation then reflects the intended preference.
Incorrect Answers
A:OSPF selects the lower accumulated path cost, not the higher one. Assigning cost 100 to the 10-Gbps interface would make it less attractive than cost 1, so it fails the intended speed preference.
C:DNS cache TTL affects resolver record freshness and has no role in OSPF cost computation or link-state SPF path selection. Modifying DNS cannot solve the equal-cost route decision.
D:Interface bandwidth and the configured OSPF reference bandwidth determine the default OSPF cost. High-speed links are not permanently tied to cost 1; raising the reference bandwidth or configuring ip ospf cost changes the values considered by SPF.
Question 416
A branch routing engineer collects this evidence: Hellos succeed, but neighbor state stays ExStart with database-description retransmits; the two interfaces have different MTUs. Which root-cause interpretation best fits the observations?
- A healthy OSPF Full adjacency is guaranteed by successful Hello reception even if DBD exchange repeatedly fails.
- The problem must be an unmatched router-ID because the interface MTUs are explicitly unequal.
- Increasing the static route administrative distance fixes an OSPF DBD MTU discrepancy.
- The peers have mismatched interface MTUs, causing OSPF database-description negotiation to fail and remain in ExStart/Exchange.
Explanation
Correct Answer
D:OSPF can reject DBD packets with incompatible interface MTU fields, leaving hellos up while database synchronization stalls. Align actual path MTU rather than suppressing checks without reason.
Incorrect Answers
A:Neighbor discovery precedes full LSDB synchronization; ExStart is not Full.
B:The stated DBD retransmits and unequal MTUs point directly to an MTU negotiation issue.
C:Static route preference does not change packet MTU fields or OSPF neighbor database exchange.
Question 417
Troubleshooting report: show ip interface brief lists Gi0/0 administratively down; no OSPF hellos leave even though the process is configured. Which finding best accounts for the behavior?
- The configured OSPF process forces hellos out of an administratively shutdown port.
- The issue is necessarily a remote OSPF DR priority zero even though the local transmit interface is disabled.
- A route age 00:00:10 is the only cause of missing OSPF hellos on a down interface.
- Gi0/0 is administratively shutdown, preventing OSPF hellos from leaving even though an OSPF process and network match are configured.
Explanation
Correct Answer
D:An OSPF process cannot send control packets across an interface that is down. Restore link and verify `show ip interface brief` before investigating complex LSDB mismatch.
Incorrect Answers
A:Interface administrative state blocks ordinary traffic and protocol control emission.
B:No local hello transport exists until the interface itself comes up.
C:Route age is unrelated to physical/admin interface availability.
Question 418
A branch routing engineer has repaired a network problem. OSPF selected an unexpected loopback address as router ID because no explicit router-id was configured at process startup. Which device or protocol check is the strongest evidence that this particular failure is gone?
- `show ip ospf` reports the deliberately configured router ID, and the running OSPF process/neighbor state uses that identity after any required process restart.
- A `router-id` command appears in configuration but operational `show ip ospf` still shows the unexpected old loopback ID.
- The highest physical Ethernet IP was changed while the old process still advertises the unwanted router ID.
- A `show cdp neighbors` display shows the desired hostname but OSPF still uses the wrong router ID.
Explanation
Correct Answer
A:An explicit `router-id` overrides automatic highest-loopback selection, but an already-running OSPF process may require a safe process restart for the new ID to take effect. Verify actual operational ID, not configuration text alone.
Incorrect Answers
B:The runtime OSPF process identity has not yet switched to the intended ID.
C:Automatic router-ID selection is generally stable during the process lifetime.
D:CDP device names are not OSPF router-ID verification.
Question 419
A network acceptance test follows this reported failure: A network statement wildcard excludes one transit interface, so show ip ospf interface does not list it and no neighbor forms. Which verification most specifically demonstrates the repaired function rather than checking an unrelated feature?
- The router’s OSPF process exists but Gi0/0 remains absent from the OSPF interface list.
- `show ip ospf interface Gi0/0` now lists the formerly omitted transit in the correct OSPF area, and `show ip ospf neighbor` reports the expected neighbor.
- An unrelated loopback appears under `show ip ospf interface` while the transit neighbor still fails.
- A local connected C subnet is installed but no hello exchange occurs on the transit.
Explanation
Correct Answer
B:The original wildcard omitted the transit interface. Checking activation on that exact interface and successful hello/neighbor state directly proves the match was repaired.
Incorrect Answers
A:A process-level configuration alone was already present during the fault.
C:The affected router-to-router interface remains excluded.
D:IP interface addressing alone does not establish OSPF neighbor participation.
Question 420
A branch routing engineer is validating a repair. Earlier, r1 uses Hello 10/Dead 40 and R2 Hello 5/Dead 20; hellos arrive but the adjacency never reaches FULL. Which postchange check most directly establishes the specific problem is resolved?
- One end remains 10/40 and the other 5/20 but both ratios are one-to-four.
- A local OSPF router ID appears but the neighbor remains Down or Init due to timer mismatch.
- Physical interface counters rise while authenticated OSPF hellos still disagree on timers.
- Both ends now show identical Hello/Dead timers and the expected neighbor reaches the appropriate Full state; inspect `show ip ospf interface` and neighbor output.
Explanation
Correct Answer
D:The earlier peers used 10/40 and 5/20 intervals. Aligning real interval values, not their ratios, permits compliant hello exchange and adjacency formation.
Incorrect Answers
A:OSPF requires actual timer values to match, not merely proportional intervals.
B:A router ID does not establish working neighbor compatibility.
C:Ethernet link traffic is not proof of Full OSPF adjacency.
