Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set1 Q1-15
Visit here for our full Cisco 300-410 exam dumps and practice test questions.
Question 1:
A company deploys OSPF across multiple sites connected through a hub-and-spoke topology. The hub router redistributes a set of static routes into OSPF. After a maintenance window, spoke routers report that they only receive a default route from the hub and none of the redistributed networks, even though OSPF neighbors are fully adjacent. What most likely explains this behavior?
A) The hub router is configured as an NSSA ABR, and the spoke routers reside in a totally NSSA area, causing suppression of all external advertisements.
B) The hub router advertises type 1 external LSAs, but the spokes are configured to allow only type 2 external LSAs.
C) The redistributed networks were summarized too broadly, and the summary address no longer includes the specific redistributed subnets.
D) The OSPF cost on the hub interfaces increased, preventing SPF from selecting them as valid next hops for the redistributed routes.
Answer: A)
Explanation:
A) The first explanation involves a hub device operating as a border router between a backbone region and a region defined as not-so-stubby. In an environment where areas are configured with restrictions on which types of information can enter, different types of link-state records are controlled. When a region is configured to prohibit all but a default path, the routers inside that region receive only a single advertisement that directs them to forward all unknown destinations toward the border. If the border device imposes this restriction by declaring the region not just as not-so-stubby but as totally not-so-stubby, then records representing external destinations are deliberately excluded. When a device inside this region expects detailed information about redistributed destinations but receives only a default entry, reachability to distinct external networks becomes impossible. This situation matches what occurs when only a default entry is passed along rather than the full set of external advertisements. Thus, in designs involving not-so-stubby constraints, the total variant ensures that the internal routers do not learn about redistributed external networks, even though adjacency remains healthy.
B) The second scenario references differentiation between the two styles of external records in the protocol. One style sets a cost that does not depend on interior transit values, while the other style incorporates interior costs into route calculation. A router’s ability to understand or prefer either style does not depend on configuration restrictions that would prevent one type from being learned. Devices participating in the protocol naturally accept both styles unless an explicit filter instructs otherwise, and such filters are uncommon in standard deployments because external record styles do not influence acceptability but rather influence path selection. Therefore, the claim that devices would accept one style but not the other does not match typical protocol behavior and does not explain the disappearance of all redistributed destinations.
C) The third idea suggests that aggregation at the border removed specificity to the extent that internal routers were no longer able to match certain destinations. While aggregation might cause more specific entries to be replaced by a broader representation, this broader representation still describes reachable destinations. Therefore, if aggregation occurred, the result would not be the absence of routes but the presence of a generalized route covering them. Even if some destinations fall outside the advertised range, a complete disappearance of all redistributed networks would require severe misconfiguration of the aggregate itself. In the described case, all redistributed entries disappear entirely, suggesting an area-type restriction rather than an aggregation oversight.
D) The final hypothesis proposes that changes in path cost on the hub prevent internal devices from viewing the hub as a viable path. However, even if the cost increased significantly, internal devices would still learn about the redistributed destinations because the hub is their only neighbor connecting them to external networks. The calculation of shortest paths influences the preferred next hop but does not prevent the learning of routes. Since adjacency is intact and devices receive a default route, the cost itself is not preventing advertisement but merely influencing metric calculation. A complete absence of specific external destinations cannot be attributed to an interface cost change.
The condition described—internal routers receiving only a default route while being denied knowledge of redistributed records—matches the behavior introduced when a region is configured to operate in a mode that restricts all external advertisements and summary advertisements, replacing them with a single default. Such a configuration is typical in environments where efficiency and reduced resource consumption are prioritized. In that design, a border device redistributing external destinations into the backbone will not forward these destinations into the restricted region, offering only the default path instead. This fits exactly the behavior described, making the first explanation the most plausible.
Question 2:
A network uses EIGRP as the primary protocol, but a subset of routes is redistributed into OSPF. After configuring route redistribution, OSPF routers receive the redistributed prefixes, but their path selection prefers less optimal internal OSPF paths even when the redistributed EIGRP routes provide a shorter actual data path. What is the most likely cause?
A) The external metric was assigned a low value, making redistributed routes appear less preferred than internal destinations.
B) The external metric type was set incorrectly, making internal destinations appear more attractive despite being inferior.
C) The seed metric applied during redistribution was too high, causing the redistributed routes to have worse cost values than internal alternatives.
D) The OSPF routers filtered inter-area routes, forcing selection of intra-area paths regardless of cost.
Answer: C)
Explanation:
A) The first possibility is that the metric applied to redistributed destinations was too low. A low metric would cause redistributed destinations to appear highly attractive compared to existing internal options. This would lead devices to select the redistributed destinations more frequently, not less. Since the reported behavior indicates that redistributed destinations are being ignored in favor of internal ones, a very low metric contradicts the observed outcome. Therefore, this cannot reasonably explain why routers avoid paths learned from redistribution.
B) The second suggestion is that the type assigned to external records caused internal destinations to appear more attractive. External records come in two varieties, where one ignores internal transit cost and the other adds it. Neither variety is inherently inferior to internal destinations; rather, they use different logic to compute overall desirability. Internal destinations normally win only if the external metric is high, not because of the external record’s style alone. Therefore, simply choosing one external record style does not inherently force internal destinations to be selected when a redistributed route offers a better metric.
C) The third scenario involves the initial metric assigned during redistribution. When destinations from another protocol enter the domain, they require a metric that ensures proper comparison with native destinations. If that assigned metric is excessively large, even inferior native paths appear more attractive because the cost is naturally interpreted as representing greater distance. In such a case, devices compute shortest paths and find that internal destinations provide a smaller calculated distance than those offered through redistribution. As a result, routers consistently choose longer real-world paths because the redistribution metric mistakenly suggests that the redistributed path is less desirable. This precisely matches the observed situation in which routers ignore redistributed paths even when those paths are physically superior.
D) The last idea concerns suppression of routes coming from other regions. When inter-area information is filtered, routers rely on internal destinations or defaults. However, this process affects information between regions, not redistributed external entries. Since redistributed external entries are considered separate from inter-area ones, filtering those types of destinations does not directly eliminate or deprioritize redistributed ones. Thus, such filtering does not cause the behavior of consistently preferring internal routes when redistributed ones should be better.
The core of the issue lies in how redistributed destinations receive their cost. If the metric assigned incorrectly inflates the perceived distance, the devices naturally choose internal paths that appear shorter based on the calculated values. Given the described symptoms, this aligns best with the effect of a misconfigured seed metric, making the third explanation the most accurate.
Question 3:
A multi-area OSPF deployment includes a backbone and several regular areas. One area hosts multiple routers exchanging large volumes of LSAs, causing CPU strain. The engineer decides to convert that area into a stub area. After the change, routers report fewer LSAs, but external destinations beyond the OSPF domain become unreachable. What most likely caused this loss of reachability?
A) External destinations are not advertised inside a restricted region, and only intra-domain routes are permitted.
B) The backbone no longer forwards inter-area destinations into that region, isolating it from the rest of the domain.
C) The summarization performed at the border unintentionally removed routes needed for external reachability.
D) The adjacent neighbors reset their LSDBs incorrectly, resulting in temporary convergence loss.
Answer: A)
Explanation:
A) The first explanation stems from the nature of a region designed to reduce the volume of information devices must maintain. When such a region is used, devices inside it receive information about destinations inside the larger domain but not about destinations outside it. The mechanism eliminates external records to conserve resources. When destinations outside the domain rely on external records, eliminating those records removes knowledge of those destinations. Thus, while intra-domain learning remains intact, any location requiring awareness of destinations introduced from other protocols becomes unreachable. This fits the condition described: converting a region to this restricted type reduces traffic but removes awareness of externally introduced destinations.
B) The second explanation involves withholding inter-area information. Even in a restricted region, inter-area information is permitted unless further restrictions are applied. Devices in regions designed for reduced record processing still allow inter-area information from the backbone. Only external information is suppressed. Therefore, inter-area suppression does not occur simply by enabling such a region, and reachability to internal remote destinations remains intact. Because the described issue affects only external reachability, not inter-area reachability, the second explanation does not match the behavior.
C) The third idea is that summarization removed necessary information. When summarization is applied, broader representations replace specific ones. However, summarization does not remove all awareness of external destinations; it merely aggregates them. If summarization inadvertently excludes some destinations, the loss would apply to only those destinations not covered by the summary, not all of them. In the described scenario, the loss of reachability applies to all external destinations, indicating a more comprehensive suppression consistent with area-type restrictions.
D) The fourth explanation describes a temporary disruption. While updates and synchronization resets can cause brief loss of reachability, the described issue implies a consistent and ongoing loss. Temporary mismatch of records would resolve on its own once the routers converge. Since the problem persists, it cannot be attributed to a temporary synchronization issue.
The primary factor in the described behavior is the restriction inherent in that type of region. Converting an area into that restricted form significantly reduces its awareness of external destinations, allowing routers to maintain fewer records. However, the trade-off is the elimination of information representing destinations outside the protocol’s own domain. As soon as this region type was activated, devices ceased receiving external records. This directly explains why external reachability disappeared, making the first explanation the most consistent and accurate.
Question 4:
A network engineer configures mutual redistribution between OSPF and EIGRP on a border router. After implementation, several routers running OSPF display routing loops when attempting to reach networks originally learned from EIGRP. Investigation shows that the redistributed routes appear in OSPF with overly attractive metrics. What is the most likely cause of this issue?
A) The OSPF external metric type was configured as the type that ignores internal path cost, making redistributed routes appear deceptively efficient.
B) The route maps used during redistribution applied a wide-area community tag that caused OSPF to treat the routes as intra-area instead of external.
C) The seed metric used when redistributing EIGRP into OSPF was set too low, causing OSPF routers to prefer the redistributed routes over their legitimate internal paths.
D) The administrative distance of OSPF external routes was increased, reducing preference for EIGRP-learned paths.
Answer: C)
Explanation:
A) The first explanation relates to a style of external representation that disregards interior transit metrics. Choosing this style means that the total distance to a destination is determined solely by the value assigned at the redistribution point. Although this approach may simplify metric calculation, using this style alone does not automatically create an unrealistically attractive route unless the value itself is artificially low. The style defines whether internal costs are added, but not whether the route becomes deceptively superior. Therefore, even if this style is used, it cannot, in isolation, produce the pronounced disparity that results in routing loops. For loops to emerge, redistributed destinations must seem drastically superior, which requires manipulation of assigned values rather than record representation style alone.
B) The second explanation describes classification changes due to tagging. In this context, tagging helps in policy control but does not alter the fundamental category of records within the protocol. External destinations learned from a different protocol remain external regardless of the tags applied. Tags may assist in filtering decisions or redistribution policies, but they do not cause the protocol to treat an external destination as though it were a native internal destination. Because the reported situation involves external destinations that appear overly attractive due to metric issues, tag manipulation does not align with the observed behavior. Reclassification into intra-area form is not possible solely through tagging and cannot account for the routing loops.
C) The third explanation concerns the initial metric assigned during redistribution. This value is crucial because the protocol compares distances to determine the preferred forwarding path. If this value is configured very low, devices throughout the protocol’s domain interpret the redistributed paths as exceptionally close, regardless of their actual network distance. When the protocol’s internal devices compare possible routes to a destination, the artificially low value causes them to choose the external redistributed version over legitimate internal paths. This can trigger loops, especially when mutual redistribution feeds information back in the reverse direction. Such loops occur because a destination learned from one protocol re-enters the other with a misleading metric, causing devices to forward traffic along an unintended path. This effect perfectly matches the described symptom and is a well-known consequence of improper seed metric configuration in mutual redistribution environments.
D) The fourth explanation concerns adjustments to administrative distance. While administrative distance influences route selection between different protocols, it does not control interactions within a single protocol’s internal calculation. Since OSPF routers are selecting between external and internal destinations within OSPF itself, administrative distance does not apply; it only applies when selecting between protocols such as EIGRP versus OSPF directly. Therefore, even if administrative distance was changed, it would not cause OSPF to prefer its own external routes over its internal ones. Because the reported issue involves internal OSPF decisions, not inter-protocol decisions, administrative distance cannot explain the behavior.
The true source of the problem lies in the configuration of redistributed destinations. Improper assignment of the initial metric creates conditions in which the protocol misjudges actual distances. When mutual redistribution is involved, the mismatch becomes especially dangerous because networks learned from one side reappear on the other side with altered characteristics. A very small initial value makes these networks appear more attractive than genuine internal paths, leading devices to send traffic toward the border inappropriately. Once the traffic returns via the other protocol with a different metric, a looping pattern can form. This exact dynamic aligns with the problem described, making the third explanation the most accurate and comprehensive.
Question 5:
A company implements BIDIR-PIM to support large-scale multicast services. After deployment, receivers in multiple subnets successfully join the multicast group, but traffic fails to flow from sources. Troubleshooting reveals that rendezvous point information is correct and group-state entries exist, yet no actual data reaches receivers. What is the most plausible explanation for this behavior?
A) Sources are sending traffic into an interface that is not mapped to the BIDIR-PIM routing domain, preventing the traffic from entering the multicast tree.
B) The RP address was advertised using sparse-mode operations, which are incompatible with BIDIR-PIM’s shared-tree forwarding requirements.
C) The designated forwarder election on one or more segments failed, preventing upstream forwarding onto the shared tree.
D) The RP mapping agent advertised conflicting RP information, causing receivers to join different RPs simultaneously.
Answer: C)
Explanation:
A) The first explanation proposes that traffic originates on an interface not associated with the domain. While multicast forwarding requires appropriate upstream and downstream interfaces to be part of the domain, the absence of domain-enabled interfaces typically results in a complete lack of group-state formation for sources. In the scenario described, group-state entries are present and rendezvous point mapping is correct, indicating that the domain is functioning. Therefore, source-interface association issues would have prevented state creation entirely, not merely prevented traffic flow. Because state already exists, the failure cannot be attributed to interface mapping issues.
B) The second idea suggests reliance on a mechanism designed for a different multicast forwarding mode. While sparse-mode operations differ fundamentally from bidirectional mode, both can coexist and interoperate in network deployments. The identification of a rendezvous point can be learned using a variety of signaling methods without creating compatibility problems. The method of advertisement does not disrupt forwarding behavior in bidirectional mode. Therefore, the presence of sparse-mode advertisement mechanisms does not inherently inhibit bidirectional operations or prevent traffic from flowing.
C) The third explanation concerns election of a critical role on each link participating in the multicast domain. This role ensures that only one device forwards upstream traffic to avoid duplication and loops. In bidirectional multicast, unlike other modes, a unique device must serve as the forwarding representative for each segment because traffic always flows toward the rendezvous point along the shared tree. If this role is not successfully assigned or if the devices disagree on which device holds the role, the forwarding responsibility becomes undefined. As a result, traffic originating from sources is not forwarded upstream even though receivers have joined the group. This precisely matches the observed condition: group-state exists, rendezvous point information is correct, but traffic never progresses from sources to receivers because the necessary upstream forwarding role is absent. This is a classic symptom of forwarder election failure in bidirectional multicast.
D) The fourth explanation suggests inconsistent mapping of rendezvous points. Inconsistent mappings could cause joins to disperse to different points, leading to unexpected behavior. However, such inconsistencies typically result in receivers failing to join the correct distribution tree or forming multiple trees, not in a situation where state is correct but forwarding does not occur. The presence of correct rendezvous point information excludes conflicting mappings as a cause.
The essential element of the failure is that bidirectional multicast relies heavily on a special upstream forwarding role on each segment. Without successful determination of this role, traffic cannot move upstream toward the rendezvous point. Because receivers already have state but no actual traffic flows, the failure precisely matches the known failure mode caused by unsuccessful election of the segment’s forwarding representative. Hence, the third explanation is the proper conclusion.
Question 6:
An engineer configures VRF-aware IPsec on a router to support secure communication between isolated customer networks. After setup, the tunnel forms successfully, but traffic inside the VRF fails to encrypt correctly. Instead, packets are forwarded unencrypted. Routing tables appear correct, and next-hop resolution points toward the IPsec tunnel. What is the most likely cause?
A) The IPsec profile was not bound to the correct VRF interface, causing packets to bypass the encryption policy.
B) The transform set used for encryption was configured with an incompatible authentication method.
C) The VRF’s route-target import and export settings prevented the establishment of a valid security association.
D) The IPsec tunnel was established using a wildcard traffic selector that applies only to the global routing table.
Answer: A)
Explanation:
A) The first explanation focuses on binding encryption policy to the appropriate interface. In virtual routing environments, each virtual domain maintains its own set of routing and forwarding policies. For encryption to occur, the traffic must traverse an interface or logical construct associated with the encryption mechanism. If the encryption policy is applied to an interface belonging to the global forwarding domain instead of the intended virtual domain, traffic matching the customer’s prefixes will not encounter the policy as it travels. This results in packets being forwarded normally without being encrypted, even though the tunnel relationship itself is active. This aligns precisely with the described behavior, in which the tunnel is present but data fails to undergo protection as expected.
B) The second idea suggests that the encryption method chosen is incompatible. Incompatibility in authentication or cryptographic transforms prevents establishment of the secure relationship. When a mismatch of encryption parameters occurs, the secure session does not form successfully. In this case, the tunnel is functioning, indicating that all negotiation steps succeeded. As such, any transform mismatch would have prevented the tunnel from forming at all. Because the session exists, transform incompatibility cannot be the cause of unencrypted forwarding.
C) The third scenario concerns the import and export mechanisms of virtual domains. Improperly configured import and export values can prevent dissemination of routing updates between virtual domains or to remote devices. However, such issues would manifest as absent routing entries or unreachable destinations. In the described case, all routing information is correct and next-hop resolution points to the secure path. Since the route is correct and reachability is established, route-target configuration is not the cause of the encryption failure.
D) The last explanation proposes that the secure relationship uses general rules that apply only in the global domain. Wildcard traffic definitions in secure configuration may limit the applicability of security rules to a broad set of traffic but will not override virtual domain-specific forwarding behavior. If selector definitions applied exclusively to the global domain, the secure session would not handle traffic originating from the virtual domain. However, this condition would not allow the secure relationship to form successfully because selectors define the set of traffic that must be protected. If selectors omitted virtual domain traffic, the session would still form, but packets would follow the policies associated with their domain. Yet selector scope does not explain why the route resolves to the tunnel yet encryption does not occur.
The determining factor is that encryption must be applied at the point where traffic leaves the virtual domain and enters the secure tunnel. If the encryption policy is attached to an interface in the global domain rather than the virtual domain, traffic within the virtual domain bypasses the enforcement mechanism entirely. The secure relationship appears healthy because its control-plane signaling is independent of data-plane forwarding, but actual data packets never encounter the required policy because the wrong interface handles them. This exactly matches the described symptoms, making the first explanation the correct one.
Question 7:
A large enterprise deploys DMVPN Phase 3 to interconnect remote branch sites. After migration, spokes successfully establish NHRP registrations with the hub, and spoke-to-spoke tunnels form as expected. However, certain spokes still forward traffic through the hub instead of using direct tunnels, even though NHRP shortcuts are installed. What is the most likely reason for this suboptimal behavior?
A) The hubs are configured with a lower EIGRP delay value, attracting spoke routes toward the hub instead of encouraging spoke-to-spoke path selection.
B) The spokes have disabled next-hop rewriting, causing them to treat the hub as the default next hop even when shortcuts exist.
C) The routing protocol running over DMVPN has unequal-cost multipath disabled, preventing proper installation of multiple next hops.
D) The NHRP hold time on the spokes is too short, causing frequent expiration of peer entries and fallback to hub forwarding.
Answer: B)
Explanation:
A) The idea that the hub has a lower delay value implies that metrics attract routes through the hub. In a scenario where path selection is fully dependent on routing metrics, a significantly lower delay could influence route preference. However, DMVPN Phase 3 introduces a mechanism that overrides normal path selection by enabling dynamic next-hop resolution through control-plane signaling. Even if the hub possesses metrics that make it attractive, once shortcut information arrives, the spokes should update the next-hop attribute in their forwarding tables. The presence of shortcuts indicates that dynamic signaling is working. Therefore, path selection should favor direct tunnels regardless of interface delay values. Since shortcuts are already installed, this factor does not adequately explain the persistent reliance on the hub.
B) The second explanation focuses on the operational enhancement introduced with this phase of the technology. In the earlier phases, the hub remained the central point of forwarding because next-hop values were not dynamically rewritten to reflect the true forwarding path. In the latest phase, the hub rewrites the next-hop to the real destination spoke so that routing protocols interpret the spoke as the next hop. If a spoke disables this rewriting behavior, either through configuration or through implicit suppression, routing decisions continue to rely on the hub because the next-hop attribute still shows the hub. Even with dynamic shortcuts present, the forwarding table prioritizes the next-hop indicated by the routing protocol rather than the overlay tunnel discovered through dynamic signaling. The result is that traffic continues to follow the hub even though the correct tunnel is available. This is highly consistent with the reported symptoms and is a known cause of suboptimal behavior in deployments of this phase.
C) The third explanation relates to unequal-cost path processing. In many scenarios, disabling such multipath capabilities limits a router’s ability to install multiple feasible next hops. However, the presence or absence of this mechanism does not prevent the use of shortcuts because the technology relies on rewriting next hops rather than introducing additional next hops. Only one next hop is required to enable direct communication. Therefore, unequal-cost multipath configuration does not influence shortcut operation. The failure of shortcut forwarding is not related to multiple next-hop installation but rather the correctness of the single next hop.
D) The fourth explanation concerns frequent expiration of peer entries causing fallback to hub forwarding. While short expiration periods may cause temporary reliance on the hub, shortcuts would be removed quickly and then reinstalled whenever needed through dynamic signaling. The scenario describes persistent behavior rather than periodic fallback. Since the shortcuts remain available, frequent expiration does not fit the observed condition.
The core requirement for optimal forwarding in this phase is successful modification of next-hop attributes so that routing tables point directly toward the intended destination. When this modification is inhibited, even though control-plane shortcut entries exist, the data-plane forwarding continues to follow the default path toward the hub. This perfectly aligns with the described behavior, making the second explanation the most likely.
Question 8:
A service provider configures MPLS Layer 3 VPNs to support multiple customers. After provisioning, one customer reports that certain routes in its VRF fail to propagate to remote sites. The provider verifies that MP-BGP neighbors are established and that route targets are correctly configured. However, the missing routes appear in the local VRF but not in the MP-BGP table. What most likely explains the missing propagation?
A) The customer routes lack a proper next-hop that resolves in the provider’s global routing table, preventing advertisement into MP-BGP.
B) The provider configured the VRF interfaces with an incompatible encapsulation type that strips MPLS labels prematurely.
C) The route distinguisher value is duplicated across two different VRFs, causing MP-BGP to suppress routes for consistency.
D) The customer configured an overlapping prefix that triggered the CE router to prefer an internal route instead of exporting it.
Answer: A)
Explanation:
A) The first explanation relates to next-hop resolution requirements for advertisement into the provider’s core protocol. To place a VRF route into the provider’s core protocol, the originating router must attach extended information, including a next-hop value that is reachable inside the provider’s core domain. If this next-hop does not resolve in the global context, the provider’s router cannot advertise the VRF route because it lacks assurance that the route’s traffic can be transported through the backbone. When such a resolution fails, the route remains locally installed in the VRF but is not exported into the provider’s core protocol. This outcome matches precisely with the reported symptom: the route appears in the VRF but not in the core protocol table.
B) The second explanation concerns encapsulation problems on VRF interfaces. If an incompatible encapsulation were used, packets entering or leaving the interface might be mishandled. However, encapsulation issues do not prevent the provider’s router from inserting a VRF route into the core protocol; they affect data forwarding, not control-plane advertisement. Since the problem affects propagation into the core protocol rather than data-plane behavior, encapsulation cannot explain the absence of routes in the core protocol.
C) The third idea suggests duplication of identification values across VRFs. While identification values must be unique per VRF, duplication does not cause the core protocol to suppress routes. Identification values are used only to distinguish routing information belonging to different customers. Even if duplication occurred, the core protocol stores separate entries because the VRFs are distinct entities. Thus, the absence of routes in the core protocol cannot be attributed to identification duplication.
D) The fourth explanation suggests that the customer chose a prefix that overlapped with an internal route, causing the customer’s router to prefer one over the other. While overlapping prefixes can confuse within a customer’s network, they do not affect the provider’s control-plane advertisement behavior. The routes the provider receives from the customer are either accepted or rejected based solely on configuration and next-hop resolution. Overlapping prefixes on the customer’s side do not cause the provider to withhold advertisement in the core protocol.
The fundamental issue lies in the requirement that all VRF routes must be resolvable in the provider’s global routing domain. If the provider’s router cannot map the next hop to a reachable core destination, the route cannot be exported. This is the precise behavior described and is widely recognized as a common configuration pitfall in provider environments. Therefore, the first explanation is the most plausible.
Question 9:
A network uses advanced route filtering to control the propagation of BGP routes between autonomous systems. After implementing a new inbound route policy, the engineer notices that several expected prefixes disappear from the routing table even though they appear in the BGP received table. Further analysis shows that the prefixes have attributes that differ from the expected format. What is the most likely cause?
A) The inbound policy applied a weight value that caused the prefixes to be deprioritized below the router’s local preferences.
B) The inbound policy modified route attributes in a way that invalidated the best-path selection process, causing the prefixes to be rejected.
C) The inbound policy reduced the AS-path length artificially, making the prefixes appear to originate from the local AS.
D) The inbound policy replaced the next-hop attribute with a private address, causing next-hop reachability to fail.
Answer: B)
Explanation:
A) The first explanation concerns the local selection mechanism that influences outbound traffic forwarding. Weight is a locally significant attribute that dictates how the router chooses among multiple paths but does not prevent a prefix from being installed into the routing table. Regardless of how high or low the value is, the prefix remains valid and is placed in the routing table if it passes all validation rules. Therefore, even an extremely low value does not remove the prefix; it simply makes the router choose other paths when available. Because the described behavior involves the disappearance of prefixes entirely, weight cannot be the cause.
B) The second explanation centers around the effect of altering critical attributes during inbound processing. When the router modifies certain values such as the originating prefix’s validity indicators, the origin attribute, or critical flags related to the path for selection, the router can inadvertently make the prefix noncompliant with the protocol’s requirements. For instance, changes that produce invalid combinations or unexpected attribute structures can cause the router to deem the path unusable. Even though the prefix exists in the received table, it will not pass validation and thus will not be placed into the routing table. This directly corresponds with the described behavior in which inbound modifications rendered the prefixes incompatible with the selection process. Attribute changes made without regard to acceptable structures can easily cause such issues.
C) The third idea suggests artificial shortening of the path length. While this change can influence selection by making a route appear more attractive, it does not invalidate the prefix. Even if the path becomes unusually short, the router still accepts the prefix as legitimate. This modification can cause suboptimal behavior but does not remove routes from the routing table. Therefore, artificial path reduction cannot explain the disappearance.
D) The fourth explanation proposes that the next-hop was replaced with an unreachable address. While next-hop reachability is required for installation of a route, unreachable next hops typically result in prefixes being marked as having unresolved next-hop issues. However, such prefixes still appear in the local list of candidate routes and do not disappear from visibility. Moreover, if they appear in the received table but disappear from the routing table, it suggests that validation rules were not met rather than reachability rules alone. Next-hop issues create forwarding failures, not prefix disappearance from the selection process.
The root cause lies in the fact that inbound modifications can alter attributes in ways that make the path fail validation. If the attributes do not conform to expected patterns, the router excludes them from consideration. This fits the exact symptoms and explains why the prefixes appear in the received list but not in the installed list. Therefore, the second explanation is the correct conclusion.
Question 10:
In an EIGRP deployment across multiple remote sites, an engineer notices that certain remote routers occasionally lose reachability to core routes even though the WAN links remain active. Debugs show that the affected routers fail to receive updates during moments of high CPU utilization. Which mechanism is most likely causing these gaps in route propagation?
A) Split horizon is preventing certain routes from being advertised correctly
B) EIGRP hello packets are being filtered by a route map applied outbound
C) EIGRP Stuck-In-Active events caused neighbor resets under heavy processing load
D) Unequal-cost load balancing is causing incorrect installation of successor routes
Answer: C)
Explanation:
A) The first possibility involves the mechanism commonly used to prevent routing loops on interfaces by stopping the advertisement of information back out of the interface on which it was learned. This behavior is essential for preventing count-to-infinity scenarios. However, this mechanism does not trigger the type of intermittent loss of reachability described. When it is enabled, its effects are consistent and predictable, and it would not cause sporadic failures only during high processor usage. The absence of updates during periods of heavy computation aligns poorly with this explanation because this mechanism is independent of CPU load, and its operational impact remains steady regardless of traffic or device strain.
B) The second possibility concerns the presence of a filtering policy applied in the outbound direction. These policies can change the behavior of update transmission, potentially removing attributes or suppressing certain networks. However, their application leads to consistent results. A route map does not fluctuate based on system load, and routers would exhibit continuous, not intermittent, loss of certain routes. The scenario mentions that reachability issues occur during heavy processor utilization. A filtering policy does not become more restrictive under high system strain, making it an unlikely explanation for a problem that appears only when resources are limited.
C) The third explanation relates to the mechanism triggered when an entry in the topology table remains in a transitional state for too long. This usually occurs when a router queries its neighbors to find an alternative path but does not receive a timely response. When a device is experiencing significant processor load, it may be unable to respond in the expected time window, causing neighbors to mark routes as problematic and potentially reset adjacencies. This behavior would directly result in intermittent reachability loss as the relationship between peers becomes unstable. The debug messages noting the failure to receive timely updates align closely with this event. The description of missed communication and reestablishment of adjacencies points strongly to this behavior, making it the closest match to the symptoms described.
D) The final possibility suggests that differences in metric calculation across available paths lead to the installation of entries that are not ideal. This situation would normally cause inefficient path forwarding, traffic oscillation, or unusual load-balancing behavior. However, it does not generally cause the disappearance of routing information or adjacency resets. The inconsistent loss of routes under heavy processor usage cannot be attributed to load balancing decisions because those rely strictly on the stable computation of metrics rather than system resource availability.
The most accurate explanation is the behavior associated with transitions remaining unresolved due to delayed query responses during periods of high processor usage. When a router cannot adequately support rapid communication due to system strain, it may fail to return timely responses. The device that issued the query then considers the path unavailable and may eventually drop adjacency, leading to gaps in available routes. This is precisely the pattern described and fully explains why the WAN remains active but the routes periodically disappear.
Question 11:
A network uses a DMVPN Phase 3 design with multiple spoke routers. Some spokes report suboptimal routing when communicating with each other, with traffic traversing the hub unnecessarily. What configuration oversight most commonly leads to this condition?
A) Missing NHRP redirect messages on the hub router
B) Incorrect EIGRP variance configuration on the hub
C) GRE keepalives disabled on spoke tunnel interfaces
D) An MTU mismatch between hub and spoke causing fragmentation
Answer: A)
Explanation:
A) The first possibility relates to the feature responsible for informing spokes that they can establish a more efficient communication path with one another. These informational messages, generated by the central router, signal that a direct path should be formed instead of forcing all flows to pass through a central point. If these messages are not present, spokes will be unaware of the available improved path and will continue forwarding their traffic through the main device even though the architecture supports direct communication. This aligns well with the described behavior in which the spokes appear to rely unnecessarily on the central router.
B) The second possibility refers to a configuration that manages how multiple potential paths may be used simultaneously. This mechanism is generally used for balancing traffic over unequal routes. However, the issue described in the question refers not to load balancing preference but to the absence of direct tunnel establishment between remote devices. Even if the parameters for traffic distribution are misconfigured, the absence of direct spoke-to-spoke paths cannot be explained by the presence or absence of unequal-cost balancing.
C) The third explanation involves the heartbeat mechanism used by devices exchanging encapsulated packets. This mechanism ensures that endpoints are reachable. However, its absence typically results in failed tunnels or tunnels remaining operational without periodic verification. What it does not cause is the absence of optimized communication between remote devices. Even without these heartbeat messages, traffic can still flow, and remote devices can form direct tunnels as long as the signaling necessary for dynamic establishment exists.
D) The fourth possibility concerns the discrepancy in the frame size supported by different devices. While fragmentation can influence path performance and occasionally cause sessions to degrade, it does not influence the routing decisions that determine whether remote devices communicate directly. The problem at hand involves route optimization rather than packet fragmentation or tunnel instability.
The issue described is directly caused by the absence of the signaling mechanism responsible for informing remote nodes of better paths. In an environment using this specific implementation of dynamic multipoint tunnels, the central device must supply informational messages to direct remote devices to build direct tunnels with each other. Without those messages, remote devices are unaware that direct communication is permissible or optimal. This oversight yields behavior consistent with the symptoms described: unnecessary traversal of the central device and the absence of optimized pathways.
Question 12:
An engineer is troubleshooting a scenario where a route learned via OSPF is present in the database but fails to appear in the routing table. Which situation most likely explains this discrepancy?
A) The route has a higher administrative distance than an existing entry
B) The interface MTU on the neighboring router is oversized
C) The router is configured as an ASBR and has suppressed type 5 LSAs
D) SPF computation is disabled on the router due to throttling timers
Answer: A)
Explanation:
A) The first possibility concerns the numerical preference a device assigns when multiple sources attempt to contribute a destination. When various protocols attempt to assert different paths, the device selects the entry with the lowest preference value. In situations where an entry from one protocol is already present in the forwarding information base, another entry may exist in the database but will not be installed because it has a worse preference value. This scenario is consistent with the behavior described, where the routing database contains the entry but the forwarding information base does not reflect it.
B) The second possibility relates to a mismatch in the maximum segment size recognized by endpoints. Such a mismatch can cause an adjacency not to form properly, because the protocol requires matching values during the exchange process. However, in such cases, the entry typically would not appear in the database at all because the adjacency would not reach a stable state. The question states that the entry exists in the database, which implies that the adjacency has already formed. Therefore, this explanation does not align with the observed behavior.
C) The third explanation concerns the suppression behavior of routers performing redistribution. This mechanism applies to how external entries propagate across areas. It does not affect the installation of internal or inter-area entries into the forwarding information base. Even if external entries were suppressed, those entries would not appear in the database, and the question explicitly mentions the presence of the entry in the database. This contradicts the effects of suppression.
D) The final possibility refers to the delay mechanisms meant to prevent repetitive recalculation. These mechanisms may schedule recalculations or delay them, but they do not disable the computation entirely. Even during extended delay periods, the presence of the entry in the database indicates that the protocol has processed the information and determined available paths. The absence of the entry from the forwarding table cannot be attributed to the timing mechanism.
The most likely explanation is that the path found in the database competes with another in the forwarding information base that has a better preference value. When multiple sources provide different entries for the same destination, the device compares their values and selects the one with the lowest numerical preference. As a result, even though the protocol recognizes the entry and stores it in the database, it does not install the entry because the forwarding table already contains a more preferred source. This explains the discrepancy between what the database shows and what the forwarding table installs.
Question 13:
A router participating in an MPLS Layer 3 VPN is failing to install VPNv4 routes from the provider edge router. The BGP session is established, but the VRF contains no remote prefixes. What condition most likely leads to this symptom?
A) Route targets are not imported into the VRF
B) MPLS TTL propagation is disabled across the core
C) The PE router is sending unlabeled IPv4 routes
D) The CE router is not redistributing connected routes
Answer: A)
Explanation:
A) The first possibility involves the identifiers used in multi-tenant environments to define what information each virtual instance is permitted to accept. These identifiers enable the separation of routing information for different customers. If these identifiers are not configured correctly, the device may reject incoming information even though the control-plane relationship is functioning. This behavior matches the symptom where the control-plane is active, but no relevant entries populate the forwarding instance.
B) The second explanation concerns the behavior of time-to-live values across a labeled environment. The propagation of these values affects visibility and hop-count behavior, but has no influence on the control-plane’s ability to accept or reject learned entries. Whether these values propagate or not will not affect the presence of entries inside a virtual instance operating over the labeled infrastructure.
C) The third possibility refers to the difference between labeled and non-labeled forwarding information. In a multi-tenant environment, the provider edges exchange information using a family of entries that include labels. If a router were to send non-labeled entries where labeled entries are expected, the receiving router would likely be unable to process them. However, this situation would typically prevent the formation of the extended control-plane relationship itself, rather than allowing the relationship to form but not populating the instance. Since the session has already formed, this explanation does not align with the behavior described.
D) The fourth explanation addresses a situation where the customer device fails to advertise its internal networks. When this occurs, the provider edge has nothing to send to its distant peers. However, even in such an environment, the absence of entries would manifest in the forwarding instance of the remote provider edge, not necessarily on the device described unless the environment consists of only two endpoints. Moreover, the control-plane relationship described is between the provider edges themselves, and the customer device’s behavior does not prevent the importation of entries if the identifiers used for distinguishing tenants are configured properly.
The most accurate explanation involves the identifiers used to control what information a virtual instance imports. Even though the extended control-plane relationship has formed, the absence of the proper identifiers prevents the information from being accepted into the virtual instance. As a result, no routes appear, and the forwarding instance remains empty. This explanation aligns precisely with the observed situation.
Question 14:
A network engineer observes that BGP routers in a large enterprise are taking several minutes to fully converge after a failure. The routers exchange a massive number of updates during the event. Which mechanism is primarily responsible for improving stability at the cost of increased convergence time?
A) BGP route dampening
B) BGP multipath load balancing
C) BGP next-hop-self
D) BGP synchronization
Answer: A)
Explanation:
A) The first possibility concerns the mechanism designed to reduce the effect of unstable prefixes by penalizing entries that change state too frequently. Once an entry accumulates a sufficient penalty, it is suppressed for a period of time before the device considers it viable again. This behavior reduces the number of updates in the environment and stabilizes the control plane, but at the expense of slowing down how quickly entries become usable again. The symptom of delayed stabilization is consistent with this behavior.
B) The second explanation involves the capability that allows a device to use multiple parallel routes simultaneously. While this mechanism increases the efficiency of forwarding by using multiple available paths, it does not slow down convergence or suppress updates during a failure. Its purpose is not related to stability or update reduction.
C) The third possibility concerns the behavior where a device rewrites the next-hop for routes coming from a neighbor, ensuring proper forwarding paths exist. This mechanism ensures that downstream devices maintain correct addressing, but it does not influence convergence time or suppress unstable entries.
D) The final option refers to a legacy mechanism intended to ensure consistency between the internal and external control-plane behaviors. This mechanism is rarely used in modern environments and is typically disabled, as it slows convergence unnecessarily. However, it does not actively penalize or suppress unstable entries, nor does it match the behavior described, where unstable updates are being reduced.
The mechanism most aligned with the described symptom is the one designed to penalize entries that change frequently, which improves stability but slows the spread of corrected entries. This corresponds exactly with what is observed when devices take longer to stabilize after an event.
Question 15:
An engineer troubleshooting a VTI-based IPsec configuration notices that although tunnels come up, OSPF routes are not being exchanged. The tunnel interface is operational, and hello packets are visible, but no routes populate the database. Which configuration oversight is most likely responsible?
A) The tunnel does not have an OSPF network type compatible with the peer
B) The IPsec profile uses AES instead of 3DES
C) The peer router has a lower OSPF router ID
D) Dead-interval timers are mismatched
Answer: A)
Explanation:
A) The first possibility concerns the mode in which an interface exchanges information with its neighbor. Some modes require the establishment of multiple adjacencies, while others rely on point-to-point communication. When the mode does not match across devices, a relationship may form, but may not advance to full exchange. The appearance of hello messages without the exchange of detailed database information is characteristic of this situation. As a result, the devices may appear connected but fail to share their entries.
B) The second explanation concerns the encryption algorithm used to secure communication across the remote tunnel. This algorithm affects confidentiality and performance, but it does not influence the behavior of the control-plane protocol riding over the encrypted path. Whether the algorithm is of one type or another does not determine whether entries in the routing domain will be exchanged correctly.
C) The third possibility refers to the identifier used to represent the device in the control plane. While differing identifiers can influence tie-break conditions, they do not prevent exchange. Even when devices select different roles, the exchange of entries should still occur. A lower identifier does not prevent information from populating the database.
D) The last explanation concerns the timing values determining when a neighbor is considered unreachable. If these values do not match, the devices will not form a stable adjacency. The question specifies that the devices are exchanging heartbeat messages, indicating that they have formed a relationship. Thus, such mismatches do not explain why entries fail to populate the database.
The most accurate explanation is the mismatch in the mode the devices use when exchanging information. The presence of heartbeat messages indicates that communication is occurring, but the failure to advance to complete exchange suggests that the mode is preventing proper synchronization. Once corrected, the devices will share entries properly.