Cisco Certified Network Associate (CCNA) 200-301 Practice Test 41
Question 400
Four routers share a broadcast Ethernet OSPFv2 segment. A and B have been elected DR and BDR; C and D are DROTHERs. `show ip ospf neighbor` on C reports D in 2-WAY state, while C has FULL adjacency with the DR and BDR. How should the C–D state be interpreted?
- An OSPF area mismatch always prevents the two DROTHERs from receiving any Hello packets.
- Normal OSPF broadcast-network behavior: two DROTHERs can remain 2-WAY while forming FULL adjacencies with DR/BDR.
- A failed Ethernet interface, because 2-WAY means both devices have lost link carrier.
- A mandatory MTU mismatch requiring `ip ospf mtu-ignore` whenever 2-WAY appears.
Explanation
Correct Answer
B:On a multiaccess OSPF broadcast network, routers elect a designated router and backup designated router to reduce adjacency requirements. Two DROTHER routers generally discover each other and reach 2-WAY state without forming a full database-exchange adjacency between themselves. They form full adjacencies with the DR and BDR instead. The stated pattern of FULL toward DR/BDR and 2-WAY toward another DROTHER is therefore expected, not automatically a fault. Check the OSPF interface network type and peer roles before changing any settings; on a genuine point-to-point network, full adjacency expectations differ.
Incorrect Answers
A:An OSPF area mismatch typically prevents compatible neighbor formation, rather than yielding a stable expected 2-WAY relationship between two DROTHERs. The scenario already states that both routers appear as neighbors on a functioning broadcast segment with elected DR/BDR roles. Their 2-WAY state is part of normal topology, not proof of an area mismatch.
C:The 2-WAY state means each peer has received Hello information acknowledging the other's presence; it does not mean carrier is down. A failed physical link would more likely eliminate the neighbor or leave it Down. Inspect interface counters separately if actual Layer 1 symptoms exist.
D:An MTU mismatch commonly affects database exchange states such as ExStart or Exchange, rather than a deliberately stable 2-WAY relationship between DROTHERs on broadcast media. The command `ip ospf mtu-ignore` bypasses one check but is not appropriate solely because two non-DR peers remain 2-WAY. Verify roles first.
Question 401
OSPF chooses lower summed link cost toward a prefix when comparing eligible paths from the same process. The team needs a solution consistent with this requirement. Which option is the most appropriate design or configuration decision?
- Choose the higher path cost because each OSPF hop adds reliability points.
- Choose the most recently refreshed route solely from its age field, disregarding OSPF cost.
- Prefer the eligible OSPF path with lower accumulated link cost to the same destination prefix, subject to equal-cost load-sharing when costs tie.
- Choose the lowest destination host IP as the next hop even though a different path has lower cost.
Explanation
Correct Answer
C:OSPF computes shortest paths from interface costs and link-state topology. Default reference bandwidth and per-interface cost settings influence the result.
Incorrect Answers
A:OSPF generally prefers the minimum accumulated cost, not maximum.
B:Route age does not replace SPF cost comparison.
D:IP number ordering is not the OSPF metric selection rule.
Question 402
R1 and R2 have matching area ID, authentication, router IDs and OSPF Hello timers. Their neighbor state oscillates between ExStart and Exchange. `show interfaces` reveals MTU 1500 on R1 and 1400 on R2. What is the best initial remediation?
- Configure a new default route to 0.0.0.0/0 on both routers to satisfy OSPF exchange.
- Align supported interface MTUs on both sides, then recheck the OSPF database-exchange process and neighbor state.
- Make the OSPF router IDs identical so the routers negotiate the same database identifier.
- Ignore the MTU mismatch permanently and change the DR election priority on the point-to-point link.
Explanation
Correct Answer
B:An MTU disagreement can prevent OSPF neighbors from progressing through database description exchange, even after their Hello parameters and basic neighbor discovery have succeeded. The ExStart/Exchange symptom together with explicit MTU values 1500 and 1400 provides a direct diagnostic clue. Configure a compatible supported MTU end to end, account for transit overhead where relevant, and verify with interface output and `show ip ospf neighbor` that the peer reaches FULL. Cisco documents `ip ospf mtu-ignore` as a bypass, but correcting the actual mismatch is the preferred first action. An adjacency alone does not guarantee the data plane can carry the intended packet sizes.
Incorrect Answers
A:A default route is a forwarding fallback for destinations without a more-specific route. OSPF database description packets exchanged on directly attached interfaces do not require an Internet gateway. The failure occurs at the adjacency synchronization stage, and changing default routing would not align the neighbors' MTU expectations.
C:OSPF routers need distinct router identifiers for unambiguous link-state database operation. Making them identical could create a duplicate-router-ID defect, not solve the mismatch. The routers already recognize one another and have reached database exchange; the explicitly inconsistent MTUs are the more relevant evidence.
D:DR election applies to certain multiaccess OSPF network types, while the problem identifies ExStart/Exchange MTU disagreement. Changing priority does not make 1500-byte database packets compatible with a 1400-byte interface. Bypassing an MTU check also leaves the underlying data-plane mismatch unresolved. Align physical and logical MTUs.
Question 403
A proposed network design has this requirement: A router's transit interface must be administratively and physically up for OSPF to send/receive neighbor hellos. Which choice is technically appropriate for that deployment?
- Remove `shutdown`, correct the physical link as needed and verify the OSPF transit interface is operational up/up and sends/receives hellos.
- Leave Gi0/0 administratively down and assume its OSPF network statement forces hellos out anyway.
- Change only the router ID while keeping the connected transit electrically down.
- Configure an unrelated client VLAN DNS address instead of correcting the OSPF link.
Explanation
Correct Answer
A:An administratively down or physically down router-to-router interface cannot maintain the OSPF hello exchange required for neighbor formation. OSPF process configuration cannot override disabled link state.
Incorrect Answers
B:The interface must be active to send and receive OSPF control packets.
C:Router identity has no ability to restore failed physical carrier.
D:DNS configuration does not bring up an administratively shutdown router port.
Question 404
A branch routing engineer collects this evidence: OSPF selected an unexpected loopback address as router ID because no explicit router-id was configured at process startup. Which root-cause interpretation best fits the observations?
- The router always chooses the lowest configured interface IP as its router ID.
- An OSPF process number of 1 forces router ID 0.0.0.1 irrespective of interface addresses.
- With no explicit router ID at OSPF process start, IOS selected the highest eligible loopback IPv4 address as router ID, which may not be the intended identity.
- The router ID is always the MAC address of the highest-bandwidth Ethernet interface.
Explanation
Correct Answer
C:IOS router ID election generally prefers an explicitly configured ID; otherwise the highest loopback IP, then the highest eligible active physical IPv4 IP. A selected router ID can persist until an OSPF process restart.
Incorrect Answers
A:The usual automatic selection favors the highest loopback, not lowest.
B:Process ID is locally significant and does not encode the router ID.
D:The OSPF router ID is an IPv4-format 32-bit identifier, not an Ethernet MAC.
Question 405
Troubleshooting report: A network statement wildcard excludes one transit interface, so show ip ospf interface does not list it and no neighbor forms. Which finding best accounts for the behavior?
- The interface will run OSPF automatically just because its connected subnet appears in a static route.
- A wildcard that excludes the transit IP still guarantees hellos on every active router interface.
- The OSPF router ID is too high numerically and therefore disables only that interface.
- The OSPF `network` statement wildcard does not include the transit interface IPv4 address, so that interface never participates in OSPF.
Explanation
Correct Answer
D:An IOS OSPF network statement selects local interfaces whose addresses match its IP/wildcard. Excluding a transit suppresses hello exchange there; fix the match or enable OSPF directly under interface.
Incorrect Answers
A:Static routing and OSPF interface selection are independent.
B:The configured match restricts where OSPF is enabled.
C:Router ID value does not determine wildcard membership of a local interface.
Question 406
R1 uses Hello 10/Dead 40 and R2 Hello 5/Dead 20; hellos arrive but the adjacency never reaches FULL. The team should investigate which specific technical cause?
- The timers already agree because both pairs have a 1:4 ratio, regardless of absolute seconds.
- A matching OSPF process number automatically overrides the advertised hello/dead mismatch.
- The failure proves the switch blocked all OSPF frames, even though hellos arrive at the routers.
- The neighbor Hello/Dead intervals mismatch (10/40 versus 5/20), causing OSPF hello compatibility failure until both sides agree.
Explanation
Correct Answer
D:OSPF peers on a normal link need compatible hello and dead interval values. An arriving Hello is insufficient if its timer fields fail neighbor acceptance.
Incorrect Answers
A:OSPF checks actual interval values, not only the ratio between them.
B:Process IDs are local; neighbor compatibility still checks timing.
C:Received hello packets indicate at least some OSPF control traffic passes the link.
Question 407
A branch routing engineer must explain an outage. One interface is area 0 and the other area 1, so neighbor relationship fails despite identical MTUs and Hello/Dead timers. Which diagnosis follows from these observations?
- R1 area 0 and R2 area 1 form a Full adjacency automatically if Ethernet carrier is present.
- The only failure can be an MTU mismatch, although both MTUs were confirmed identical.
- The two OSPF router interfaces belong to different area IDs (0 and 1), so they reject adjacency on their common link.
- Identical OSPF process IDs would make area mismatch irrelevant.
Explanation
Correct Answer
C:OSPF neighbor hellos carry the area ID. Matching MTU, timers and IP subnet cannot compensate for different area membership on the same adjacency.
Incorrect Answers
A:Area IDs must agree for a normal OSPF neighbor on that link.
B:The observed and stated mismatch is area configuration, not packet size.
D:Process IDs are locally scoped labels and do not change advertised area ID.
Question 408
A branch routing engineer collects this evidence: R1 has 192.0.2.1/24 and R2 has 192.0.2.2/25; network/mask validation fails while the switch is forwarding frames. Which root-cause interpretation best fits the observations?
- The routers use inconsistent IPv4 masks on the shared broadcast segment: 192.0.2.1/24 versus 192.0.2.2/25; align link subnet and mask.
- The different masks must be harmless because both host IPs fall inside 192.0.2.0/25.
- Changing only the OSPF cost on one interface fixes inconsistent hello netmask fields.
- Disabling STP on the access switch is required because OSPF never sends hellos on broadcast Ethernet.
Explanation
Correct Answer
A:OSPF broadcast-network hello validation includes network mask compatibility. Working Ethernet does not remove a Layer 3 subnet-mask mismatch in OSPF packet handling.
Incorrect Answers
B:The OSPF hello network-mask fields can still fail equality checks despite overlapping IP ranges.
C:Cost affects path calculation, not neighbor subnet/mask compatibility.
D:The scenario confirms Ethernet forwarding; OSPF does use hello packets on broadcast links.
Question 409
Troubleshooting report: Hello packets arrive, but packet authentication fails because one router uses a different digest key from its peer. Which finding best accounts for the behavior?
- The neighbor is Full because receiving any Hello guarantees authentication success.
- Changing only interface speed to full duplex makes mismatched authentication keys match.
- A working connected C route proves OSPF keys are identical.
- OSPF authentication mode/key material differs between peers, so received hello packets fail cryptographic/digest validation.
Explanation
Correct Answer
D:OSPF packet authentication must agree in method and key values or key IDs as configured. This prevents the routers from accepting one another’s neighbor control packets even when packets physically arrive.
Incorrect Answers
A:A received but invalidly authenticated Hello is not accepted as a valid neighbor exchange.
B:Physical duplex and cryptographic OSPF keys are independent settings.
C:Connected routes derive from local L3 interface state, not OSPF authentication.
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.
