Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 6 Q76-90

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Question 76:

A network engineer configures OSPF on a multi-area network. Routers in area 8 are not receiving inter-area routes from area 0, although the ABR advertises Type-3 summary LSAs. What is the most likely cause?

A) Area 8 is configured as a totally stubbed area.
B) Type-1 LSAs are blocked by access lists.
C) Backbone area 0 is down.
D) OSPF process IDs are mismatched.

Answer: A)

Explanation:

OSPF area types are fundamental to controlling route propagation and optimizing resource usage within a network. A totally stub area is a specific configuration that prevents the installation of external Type-5 LSAs and inter-area Type-3 summary LSAs, except for a default route injected by the ABR. This configuration is particularly beneficial for branch offices or remote sites with limited router memory and CPU capacity because it reduces the size of the routing table and the LSDB while still providing access to networks outside the area. The ABR is responsible for injecting a default route into the totally stubbed area to allow connectivity to destinations beyond the area.

In this scenario, routers in area 8 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. This aligns with the behavior of a totally stub area, where the area type intentionally filters Type-3 LSAs and relies solely on the injected default route for external reachability. If the default route is missing or improperly configured, routers in area 8 cannot access external destinations despite intra-area routing being functional. Network engineers must verify the area type configuration and ensure proper default route injection on the ABR to maintain connectivity. Troubleshooting this situation requires an understanding of the characteristics of totally stub areas, as the ABR may be functioning correctly, but the area itself filters out the summary LSAs.

Type-1 LSAs describe a router’s own links within an area. If Type-1 LSAs were blocked, intra-area routing would fail, and adjacencies would not form, leading to a complete loss of connectivity within the area. Since routers maintain connectivity with Area 8, Type-1 LSAs are being advertised correctly and are not the cause of the missing inter-area routes.

A down backbone area would prevent inter-area routing for all areas connected to it, not just area 8. Because only area 8 is experiencing missing routes, the backbone is operational and is not the cause of the issue.

OSPF process IDs are locally significant identifiers and do not affect LSA propagation across areas. Differences in process IDs do not prevent Type-3 LSA advertisement or default route injection within the same OSPF instance.

The root cause is that area 8 is configured as a totally stubbed area. The ABR is correctly advertising Type-3 summary LSAs, but the area type prevents them from being installed, leaving only the default route as a path to external networks. Network engineers must ensure the ABR is configured to inject a default route using “default-information originate” to provide connectivity. Proper understanding of totally stubbed area behavior is critical when designing OSPF networks because failure to inject a default route can cause the appearance of missing routes despite proper ABR operation. Proper configuration balances the efficiency gains of a reduced LSDB and smaller routing tables with the requirement for external reachability. Misconfigured stub areas or omitted default route injection can isolate routers from external networks. Engineers must verify and document area types and default route configurations to ensure predictable OSPF behavior and reliable network operation while optimizing router resource utilization. Understanding the operational characteristics of totally stub areas helps prevent unexpected connectivity issues, maintains network stability, and allows efficient scaling of OSPF networks without compromising external reachability.

Question 77:

A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic continues to use only one path. What is the most likely cause?

A) The paths are not equal in AS path, origin, or MED attributes.
B) BGP is configured only with iBGP neighbors.
C) One path has an unreachable next-hop.
D) BGP update suppression is enabled.

Answer: A)

Explanation:

BGP multipath allows multiple equal-cost paths to be installed in the forwarding table to increase redundancy and optimize bandwidth utilization. To qualify for multipath, candidate paths must match in several attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. Any discrepancy among these attributes causes BGP to select a single best path, even if multiple paths exist in the BGP table. This behavior ensures deterministic traffic forwarding and prevents inconsistent routing or forwarding loops.

In the described scenario, multiple paths are present in the BGP table, but only one path is being used for forwarding. This indicates that BGP considers the paths unequal due to differences in AS path, origin type, or MED. For instance, one path may include AS path prepending, or the origin type may differ, leading BGP to select a single best path. This strict equality requirement guarantees predictable routing behavior and avoids suboptimal traffic distribution, ensuring network stability. Network engineers must understand the equality requirements for BGP multipath to effectively deploy it in multi-homed environments. Misalignment in AS path, origin, or MED prevents BGP from leveraging multiple available paths, limiting redundancy and traffic distribution.

BGP session type, whether iBGP or eBGP, does not prevent multipath functionality if paths satisfy equality conditions. Both session types support multipath, and the presence of multiple paths in the table confirms that the protocol is functioning correctly.

Next-hop reachability is required for paths to be installed in the forwarding table. Since multiple paths appear in the BGP table, next-hop connectivity is not the limiting factor.

BGP update suppression limits the rate at which updates are sent to neighbors to conserve control-plane resources, but it does not influence local path selection. Therefore, update suppression is not the cause of single-path forwarding in this scenario.

The root cause is that the paths are unequal in AS path, origin, or MED attributes. Network engineers must align these attributes to enable multipath forwarding. Proper configuration ensures effective traffic distribution, redundancy, and optimal bandwidth utilization. Understanding the strict equality requirements for BGP multipath is essential in multi-homed networks to fully leverage available paths and prevent congestion on a single link. Engineers must monitor BGP attributes, adjust configurations as needed, and verify multipath operation to maintain predictable and balanced forwarding. Properly configured multipath forwarding maximizes the utilization of all available paths, enhances network resilience, and improves efficiency, ensuring that multi-homed networks achieve both redundancy and optimal performance. Careful planning, attribute management, and operational verification are critical for successful BGP multipath deployment.

Question 78:

A network engineer deploys RSVP-TE tunnels in an MPLS network. Tunnels fail to establish, even though all links report sufficient bandwidth. What is the most likely cause?

A) Link attribute constraints, such as TE colors, prevent CSPF from finding a feasible path.
B) RSVP authentication is mismatched.
C) RSVP soft-state refresh timers are too long.
D) The IGP metric of the path is misconfigured.

Answer: A)

Explanation:

MPLS Traffic Engineering allows operators to create explicit paths based on link bandwidth, administrative constraints, and link attributes. RSVP-TE is the signaling protocol used to reserve bandwidth along these explicit paths. Constrained Shortest Path First (CSPF) evaluates all candidate paths to ensure that they meet the requested TE constraints, including bandwidth, TE colors, and administrative groups. Even when all links report sufficient bandwidth, tunnels may fail to establish if CSPF cannot find a feasible path that satisfies all constraints. Mismatched link attributes, such as incorrect TE colors, missing administrative group markings, or inconsistent configurations along the path, are the most common causes of RSVP-TE tunnel failures when bandwidth is adequate.

RSVP authentication ensures that only authorized routers can establish reservations. Mismatched authentication prevents tunnel establishment but does not interfere with CSPF path computation. Authentication failures generate log messages but do not directly prevent path feasibility evaluation.

Soft-state refresh timers maintain RSVP tunnel state. If timers are too long, the tunnel state may expire prematurely, but this does not prevent CSPF from attempting to compute a feasible path.

IGP metrics affect unconstrained SPF computations, but CSPF considers both bandwidth and link attributes when computing a path for TE tunnels. Even if a path has the lowest IGP metric, CSPF will reject it if the requested TE constraints are not satisfied, such as unavailable TE colors or incompatible administrative groups.

The root cause is mismatched link attributes preventing CSPF from finding a feasible path. Network engineers must verify that all links along the tunnel path have attributes compatible with the requested TE constraints, including TE colors, administrative groups, and bandwidth availability. Proper alignment allows CSPF to compute a valid path, enabling RSVP-TE tunnels to establish successfully. Understanding the interaction between link attributes, constraints, and CSPF computation is essential for predictable MPLS TE operation, efficient traffic engineering, and optimal utilization of network resources. Accurate configuration ensures reliable tunnel establishment, optimal performance, and predictable network operation, avoiding failures caused by attribute mismatches. Verification and proper planning of TE attributes prevent unexpected tunnel establishment failures and ensure that MPLS TE tunnels function as intended, supporting efficient and predictable network operation.

Question 79:

A network engineer deploys OSPF on a multi-area network. Routers in area 9 are not receiving inter-area routes from area 0, although the ABR advertises Type-3 summary LSAs. What is the most likely cause?

A) Area 9 is configured as a totally stubbed area.
B) Type-1 LSAs are blocked by access lists.
C) Backbone area 0 is down.
D) OSPF process IDs are mismatched.

Answer: A)

Explanation:

OSPF area types are used to optimize routing efficiency and control the amount of routing information that must be maintained by routers. A totally stub area is a specific configuration that blocks external Type-5 LSAs and inter-area Type-3 summary LSAs, leaving only a default route injected by the ABR for connectivity to external networks. This area type is particularly beneficial for remote sites or branch offices where router resources are limited, as it reduces the size of the LSDB and the routing table while still providing access to destinations outside the area. The ABR is responsible for injecting the default route into the totally stub area so that routers can reach external destinations.

In the scenario described, routers in area 9 are not receiving inter-area routes from area 0 despite the ABR advertising Type-3 summary LSAs. This is consistent with the behavior of a totally stub area, where the area type filters out Type-3 LSAs to reduce routing table size and router resource utilization. Routers in area 9 rely solely on the default route for reaching external networks. If the default route is not present or incorrectly configured, routers will not have connectivity to destinations outside the area, even though intra-area routes function normally and the ABR is operating correctly. Troubleshooting requires verifying the area type and ensuring that the ABR is injecting the default route to maintain connectivity. Understanding the behavior of totally stub areas is essential when designing multi-area OSPF networks, as failure to inject a default route can result in missing external routes despite correct ABR operation.

Type-1 LSAs describe a router’s own links within an area. Blocking these LSAs would prevent adjacency formation and intra-area route propagation. Since routers in area 9 are maintaining intra-area connectivity, Type-1 LSAs are not blocked and are functioning correctly.

A down backbone area would prevent inter-area routing for all areas connected to it. In this scenario, only area 9 is affected, indicating that the backbone is operational and not the cause of the problem.

OSPF process IDs are locally significant and do not affect LSA propagation across areas within the same OSPF instance. Mismatched process IDs would not prevent Type-3 LSAs from being advertised or the default route from being injected.

The root cause is that area 9 is configured as a totally stubbed area. The ABR advertises Type-3 LSAs, but the area type prevents them from being installed, leaving only the default route for external destinations. Engineers must ensure that the ABR injects a default route using the “default-information originate” command. Properly understanding and configuring totally stub areas balances the benefits of reduced router resource usage with the need for external network connectivity. Failure to inject default routes can isolate routers, creating troubleshooting challenges and reducing network reliability. Proper documentation and verification of area types and default route injection are essential for predictable OSPF behavior, network stability, and efficient scaling without sacrificing connectivity to external networks. Network engineers must carefully design area types and verify default route injection to maintain connectivity while optimizing router performance and memory usage.

Question 80:

A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic continues to use only one path. What is the most likely cause?

A) The paths are not equal in AS path, origin, or MED attributes.
B) BGP is configured only with iBGP neighbors.
C) One path has an unreachable next-hop.
D) BGP update suppression is enabled.

Answer: A)

Explanation:

BGP multipath functionality allows multiple equal-cost paths to be installed in the forwarding table to improve redundancy, increase bandwidth utilization, and provide load balancing across links. However, BGP imposes strict equality requirements for multipath to operate. Candidate paths must match in attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. If any of these attributes differ, BGP will select a single best path based on its decision process, even if multiple paths exist in the BGP table. This strict behavior ensures deterministic routing, avoids forwarding loops, and guarantees predictable traffic flow across the network.

In the scenario, multiple paths are present in the BGP table, but traffic only uses a single path. This suggests that the paths are unequal in AS path, origin type, or MED. For example, one path may include AS path prepending, or the origin type may differ, causing BGP to select a single best path and ignore the other paths for forwarding purposes. Understanding the equality requirements is crucial for engineers deploying multipath in multi-homed environments. Failure to align attributes prevents traffic from utilizing all available paths, limiting redundancy and potentially leading to congestion on a single link.

BGP session type, whether iBGP or eBGP, does not prevent multipath functionality if all equality conditions are met. Both session types support multipath forwarding, and the presence of multiple paths in the table indicates that BGP is receiving updates correctly.

Next-hop reachability is necessary for a path to be installed in the forwarding table. Since multiple paths appear in the table, next-hop connectivity is not the limiting factor.

BGP update suppression limits the frequency of updates sent to neighbors to reduce control-plane load but does not influence the local path selection process. Therefore, update suppression is not the cause of single-path forwarding in this scenario.

The root cause is that the paths are unequal in AS path, origin, or MED attributes. Network engineers must align these attributes to enable BGP multipath. Proper configuration ensures effective traffic distribution, redundancy, and optimal utilization of available bandwidth. Understanding the strict equality requirements for BGP multipath is essential for multi-homed networks to fully utilize available paths and prevent congestion on a single link. Engineers must monitor BGP attributes, adjust configurations as necessary, and verify multipath operation to ensure balanced and predictable traffic forwarding. Correctly configured multipath forwarding maximizes redundancy, enhances network resilience, and ensures efficient utilization of all available paths. Proper planning, attribute management, and operational verification are key to successful multipath deployment in complex network environments.

Question 81:

A network engineer deploys RSVP-TE tunnels in an MPLS network. Tunnels fail to establish, even though all links report sufficient bandwidth. What is the most likely cause?

Answer: A)

Explanation:

MPLS Traffic Engineering provides the ability to establish explicit paths through the network based on bandwidth, administrative constraints, and link attributes. RSVP-TE is the signaling protocol responsible for reserving bandwidth along these paths. Constrained Shortest Path First (CSPF) computes the feasible path by evaluating candidate paths against the requested TE constraints, which include available bandwidth, TE colors, and administrative groups. Even when all links report sufficient bandwidth, tunnels may fail to establish if CSPF cannot find a path that satisfies all constraints. Mismatched link attributes, such as incorrect TE colors, missing administrative group markings, or inconsistent configuration along the path, are the most common cause of RSVP-TE tunnel failures under these conditions.

RSVP authentication ensures that only authorized routers can reserve bandwidth. Mismatched authentication prevents tunnel establishment but does not affect CSPF path computation. Authentication failures generate log messages but do not prevent the feasibility evaluation itself.

Soft-state refresh timers maintain RSVP tunnel state. If refresh intervals are too long, the tunnel state may expire prematurely, but this does not prevent CSPF from computing a feasible path initially.

IGP metrics affect unconstrained SPF computation, but CSPF evaluates both bandwidth and link attributes when determining a valid path for TE tunnels. Even a path with the lowest IGP metric will be rejected if TE constraints are not satisfied, such as incorrect TE colors or administrative group restrictions.

The root cause is mismatched link attributes preventing CSPF from finding a feasible path. Engineers must verify that all links along the intended tunnel path have compatible attributes, including TE colors, administrative groups, and bandwidth availability. Proper alignment enables CSPF to compute a valid path and allows RSVP-TE tunnels to establish successfully. Understanding the relationship between link attributes, constraints, and CSPF computation is critical for predictable MPLS TE behavior, efficient traffic engineering, and optimal utilization of network resources. Accurate configuration ensures reliable tunnel establishment, predictable network performance, and consistent operation, avoiding failures caused by attribute mismatches. Verification and proper planning of TE attributes prevent unexpected tunnel establishment failures and ensure that MPLS TE tunnels function as intended, supporting efficient and predictable network behavior across the entire MPLS network.

Question 82:

A network engineer configures OSPF on a multi-area network. Routers in area 10 are not receiving inter-area routes from area 0, although the ABR advertises Type-3 summary LSAs. What is the most likely cause?

A) Area 10 is configured as a totally stubbed area.
B) Type-1 LSAs are blocked by access lists.
C) Backbone area 0 is down.
D) OSPF process IDs are mismatched.

Answer: A)

Explanation:

OSPF area types are used to optimize routing efficiency, reduce router memory usage, and control the amount of routing information distributed to different parts of the network. A totally stub area is a specific configuration that filters both external Type-5 LSAs and inter-area Type-3 summary LSAs, leaving only a default route injected by the ABR to provide connectivity to destinations outside the area. This configuration is commonly used in branch offices or remote sites where routers have limited resources, as it reduces the size of the LSDB and routing table while still allowing access to networks beyond the area. The ABR plays a crucial role in injecting the default route to ensure that routers in the totally stub area can reach external networks.

In this scenario, routers in area 10 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. This behavior is consistent with a totally stub area, where the area type intentionally filters Type-3 LSAs to reduce routing table complexity and optimize router resource usage. Routers in area 10 rely solely on the default route for reaching destinations outside the area. If the default route is missing or misconfigured, routers will not have connectivity to external networks, even though intra-area routes are functional and the ABR is operational. Troubleshooting requires verification of the area type configuration and ensuring proper default route injection on the ABR. Understanding the behavior of totally stub areas is critical when designing multi-area OSPF networks because failure to inject a default route can cause external destinations to become unreachable despite correct ABR functionality.

Type-1 LSAs describe a router’s own links within an area. Blocking Type-1 LSAs would prevent adjacency formation and intra-area route propagation. Since routers in area 10 maintain intra-area connectivity, Type-1 LSAs are functioning correctly and are not the cause of the missing inter-area routes.

A down backbone area would prevent inter-area routing for all areas connected to it. Since only area 10 is affected in this scenario, the backbone is operational, making this an unlikely cause.

OSPF process IDs are locally significant identifiers and do not affect LSA propagation across areas within the same OSPF instance. Differences in process IDs would not prevent Type-3 LSA advertisement or the injection of a default route.

The root cause is that area 10 is configured as a totally stubbed area. Although the ABR advertises Type-3 LSAs, the area type prevents them from being installed, leaving only a default route as the path to external networks. Network engineers must ensure the ABR is configured to inject the default route using the “default-information originate” command. Proper understanding and configuration of totally stub areas balances reduced resource usage with the need for external reachability. Misconfigured stub areas or omitted default route injection can isolate routers from external networks, leading to operational challenges and troubleshooting complexity. Proper documentation, verification, and testing of area types and default route injection are essential for predictable OSPF behavior, stable network operation, and efficient scaling without compromising access to external networks. Network engineers must carefully design area types, validate default route injection, and ensure consistent behavior across all areas for optimal OSPF performance and network reliability.

Question 83:

A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic continues to use only one path. What is the most likely cause?

A) The paths are not equal in AS path, origin, or MED attributes.
B) BGP is configured only with iBGP neighbors.
C) One path has an unreachable next-hop.
D) BGP update suppression is enabled.

Answer: A)

Explanation:

BGP multipath allows multiple equal-cost paths to be installed in the forwarding table, enabling better load balancing, redundancy, and utilization of available bandwidth. However, BGP imposes strict equality rules for multipath operation. To qualify for multipath, candidate paths must match in several attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. Any difference in these attributes causes BGP to select a single best path, even if multiple paths exist in the BGP table. This ensures deterministic routing, prevents forwarding loops, and guarantees predictable traffic flow.

In the scenario described, multiple paths appear in the BGP table, but traffic continues to use a single path. This suggests that the paths are unequal in AS path, origin type, or MED attributes. For instance, one path might include AS path prepending, or the origin type could differ, leading BGP to select a single best path and ignore the other viable paths for forwarding. Understanding the strict equality requirements for BGP multipath is essential when designing multi-homed networks. Misaligned attributes prevent traffic from utilizing all available paths, which reduces redundancy and can cause congestion on a single link, diminishing overall network performance.

BGP session type, whether iBGP or eBGP, does not prevent multipath if the equality conditions are met. Both session types support multipath forwarding, and the presence of multiple paths in the table indicates that BGP updates are being received correctly.

Next-hop reachability is necessary for a path to be installed in the forwarding table. Since multiple paths appear in the BGP table, next-hop connectivity is not the limiting factor.

BGP update suppression reduces the frequency of updates sent to neighbors to conserve control-plane resources, but it does not affect local path selection. Therefore, update suppression is not the cause of single-path forwarding in this scenario.

The root cause is that the paths are not equal in AS path, origin, or MED attributes. Engineers must align these attributes to enable BGP multipath functionality. Proper configuration ensures effective traffic distribution, redundancy, and optimal bandwidth utilization. Understanding the strict equality requirements for multipath is crucial to fully leveraging all available paths in multi-homed networks. Engineers must monitor BGP attributes, adjust configurations as needed, and verify multipath operation to maintain predictable, balanced traffic forwarding. Correctly configured multipath forwarding enhances redundancy, network resilience, and efficient utilization of resources. Careful planning, attribute management, and operational verification are key to successful multipath deployment in complex networks, ensuring that all available paths are effectively leveraged.

Question 84:

A network engineer deploys RSVP-TE tunnels in an MPLS network. Tunnels fail to establish, even though all links report sufficient bandwidth. What is the most likely cause?

Answer: A)

Explanation:

MPLS Traffic Engineering allows operators to create explicit paths based on bandwidth availability, administrative constraints, and link attributes. RSVP-TE is the signaling protocol responsible for reserving bandwidth along these paths. Constrained Shortest Path First (CSPF) computes a feasible path by evaluating candidate paths against the requested TE constraints, including available bandwidth, TE colors, and administrative groups. Even when all links report sufficient bandwidth, tunnels may fail to establish if CSPF cannot find a path that satisfies all constraints. Mismatched link attributes, such as incorrect TE colors, missing administrative group markings, or inconsistent configurations along the path, are the most common causes of RSVP-TE tunnel failures in this scenario.

Soft-state refresh timers maintain RSVP tunnel state. If timers are too long, the tunnel state may expire prematurely, but this does not prevent CSPF from computing a feasible path initially.

IGP metrics influence unconstrained SPF calculations, but CSPF considers both bandwidth and link attributes when computing a TE path. Even a path with the lowest IGP metric will be rejected if it does not satisfy TE constraints, such as TE colors or administrative groups.

The root cause is mismatched link attributes preventing CSPF from finding a feasible path. Engineers must verify that all links along the intended path have compatible attributes, including TE colors, administrative groups, and available bandwidth. Proper alignment enables CSPF to compute a valid path and allows RSVP-TE tunnels to establish successfully. Understanding the relationship between link attributes, TE constraints, and CSPF computation is critical for predictable MPLS TE operation, efficient traffic engineering, and optimal network resource utilization. Accurate configuration ensures reliable tunnel establishment, optimal performance, and predictable network behavior, avoiding failures caused by attribute mismatches. Verification and proper planning of TE attributes prevent unexpected tunnel establishment failures and ensure that MPLS TE tunnels function as intended, supporting efficient and reliable network operation across the MPLS network.

Question 85:

A network engineer configures OSPF on a multi-area network. Routers in area 11 are not receiving inter-area routes from area 0, although the ABR advertises Type-3 summary LSAs. What is the most likely cause?

A) Area 11 is configured as a totally stubbed area.
B) Type-1 LSAs are blocked by access lists.
C) Backbone area 0 is down.
D) OSPF process IDs are mismatched.

Answer: A)

Explanation:

OSPF area types are used to optimize routing efficiency and manage the size of the routing table and link-state database on routers. A totally stub area is a special configuration that prevents the installation of external Type-5 LSAs and inter-area Type-3 summary LSAs, except for a default route injected by the ABR. This type of area is typically deployed in remote or branch locations where routers have limited memory or CPU capacity, allowing the network to minimize resource usage while maintaining connectivity to external destinations. The ABR is responsible for injecting the default route to provide access to networks outside the area, ensuring that routers can reach external networks even when summary LSAs are filtered.

In the described scenario, routers in area 11 are not receiving inter-area routes from area 0, despite the ABR advertising Type-3 summary LSAs. This is consistent with the behavior of a totally stub area, where Type-3 LSAs are intentionally filtered, leaving only the default route for external connectivity. If the default route is missing or misconfigured, routers in area 11 will not be able to access external networks, even though intra-area routes are functional and the ABR is operating correctly. Troubleshooting this issue requires verification of the area type and proper default route injection on the ABR. Understanding the behavior of totally stub areas is essential in multi-area OSPF network design because failure to inject a default route can lead to missing external routes despite correct ABR functionality.

Type-1 LSAs describe a router’s own links within an area. Blocking these LSAs would prevent adjacency formation and intra-area route propagation. Since routers in area 11 maintain connectivity within the area, Type-1 LSAs are functioning correctly and are not the cause of the missing inter-area routes.

A down backbone area would prevent inter-area routing for all areas connected to it. Since only area 11 is affected in this case, the backbone is operational, making this an unlikely cause.

The root cause is that area 11 is configured as a totally stubbed area. Although the ABR advertises Type-3 LSAs, the area type prevents them from being installed, leaving only a default route as the path to external networks. Engineers must ensure that the ABR is configured to inject a default route using the “default-information originate” command. Proper understanding and configuration of totally stub areas balances reduced resource usage with external connectivity requirements. Misconfigured stub areas or missing default route injection can isolate routers, creating operational and troubleshooting challenges. Proper documentation, verification, and testing of area types and default route injection are essential for predictable OSPF behavior, stable network operation, and efficient scaling without compromising access to external networks.

Question 86:

A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic continues to use only one path. What is the most likely cause?

A) The paths are not equal in AS path, origin, or MED attributes.
B) BGP is configured only with iBGP neighbors.
C) One path has an unreachable next-hop.
D) BGP update suppression is enabled.

Answer: A)

Explanation:

BGP multipath allows multiple equal-cost paths to be installed in the forwarding table, improving redundancy, traffic distribution, and utilization of available bandwidth. However, BGP applies strict equality requirements for multipath operation. Candidate paths must match in several attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. Any difference in these attributes will cause BGP to select a single best path, even if multiple paths exist in the BGP table. This deterministic behavior ensures predictable routing, prevents forwarding loops, and guarantees consistent traffic flow across the network.

In the scenario described, multiple paths appear in the BGP table, but traffic continues to use only a single path. This indicates that the paths are not equal in AS path, origin type, or MED attributes. For example, one path may include AS path prepending, or the origin type may differ, causing BGP to select a single best path and ignore the other viable paths for forwarding. Understanding these strict equality requirements is essential when designing multi-homed networks with BGP multipath. Failure to align attributes prevents traffic from utilizing all available paths, reducing redundancy and potentially causing congestion on a single link, which limits overall network efficiency and resilience.

BGP session type, whether iBGP or eBGP, does not prevent multipath if equality conditions are met. Both session types support multipath forwarding, and the presence of multiple paths in the BGP table indicates that BGP updates are being received correctly from neighbors.

Next-hop reachability is necessary for a path to be installed in the forwarding table. Since multiple paths appear in the BGP table, next-hop connectivity is not the limiting factor.

BGP update suppression reduces the rate at which updates are sent to neighbors to conserve control-plane resources, but it does not affect local path selection. Therefore, update suppression is not the cause of single-path forwarding in this scenario.

The root cause is that the paths are not equal in AS path, origin, or MED attributes. Engineers must align these attributes to enable multipath forwarding. Proper configuration ensures effective traffic distribution, redundancy, and optimal utilization of available bandwidth. Understanding and adhering to multipath equality requirements is essential for fully leveraging all available paths, improving network performance, and preventing congestion on individual links. Engineers must monitor BGP attributes, adjust configurations as needed, and verify multipath operation to maintain predictable and balanced traffic forwarding. Correctly configured multipath forwarding enhances redundancy, network resilience, and efficient utilization of resources. Careful planning, attribute management, and operational verification are key to successful multipath deployment.

Question 87:

A network engineer deploys RSVP-TE tunnels in an MPLS network. Tunnels fail to establish, even though all links report sufficient bandwidth. What is the most likely cause?

Answer: A)

Explanation:

MPLS Traffic Engineering enables network operators to establish explicit paths based on bandwidth availability, administrative constraints, and link attributes. RSVP-TE is the signaling protocol responsible for reserving bandwidth along these paths. Constrained Shortest Path First (CSPF) computes a feasible path by evaluating candidate paths against the requested TE constraints, including bandwidth availability, TE colors, and administrative groups. Even when all links report sufficient bandwidth, tunnels may fail to establish if CSPF cannot find a path that satisfies all constraints. Mismatched link attributes, such as incorrect TE colors, missing administrative group markings, or inconsistent configurations along the path, are the most common cause of RSVP-TE tunnel failures in such scenarios.

RSVP authentication ensures that only authorized routers can reserve bandwidth. Mismatched authentication prevents tunnel establishment but does not affect CSPF path computation. Authentication failures generate log messages and alarms but do not prevent CSPF from attempting to compute a feasible path.

Soft-state refresh timers maintain RSVP tunnel state. If the refresh intervals are too long, the tunnel state may expire prematurely, but this does not prevent CSPF from computing a feasible path initially.

IGP metrics influence unconstrained SPF computation, but CSPF evaluates both bandwidth and link attributes when determining a TE path. Even a path with the lowest IGP metric will be rejected if TE constraints are not satisfied, such as TE colors or administrative group mismatches.

The root cause is mismatched link attributes preventing CSPF from computing a feasible path. Engineers must verify that all links along the intended path have compatible attributes, including TE colors, administrative groups, and available bandwidth. Proper alignment allows CSPF to compute a valid path and enables RSVP-TE tunnels to establish successfully. Understanding the relationship between link attributes, TE constraints, and CSPF computation is critical for predictable MPLS TE operation, efficient traffic engineering, and optimal network resource utilization. Accurate configuration ensures reliable tunnel establishment, predictable network performance, and consistent operation, avoiding failures caused by attribute mismatches. Verification and proper planning of TE attributes prevent unexpected tunnel establishment failures and ensure that MPLS TE tunnels function as intended, supporting efficient and reliable network behavior across the MPLS network.

Question 88:

A network engineer configures OSPF on a multi-area network. Routers in area 12 are not receiving inter-area routes from area 0, although the ABR advertises Type-3 summary LSAs. What is the most likely cause?

A) Area 12 is configured as a totally stubbed area.
B) Type-1 LSAs are blocked by access lists.
C) Backbone area 0 is down.
D) OSPF process IDs are mismatched.

Answer: A)

Explanation:

OSPF area types are crucial for controlling the distribution of routing information and optimizing resource usage within a network. A totally stub area is a specific OSPF configuration that blocks both external Type-5 LSAs and inter-area Type-3 summary LSAs. This type of area relies solely on a default route injected by the ABR to reach destinations outside the area. Totally stub areas are particularly suitable for branch offices or remote locations where routers have limited memory or processing capacity. By filtering summary LSAs, the LSDB size is reduced, and routing tables are simplified, providing operational efficiency without sacrificing essential external connectivity. The ABR is responsible for injecting the default route, ensuring that routers in the totally stubby area maintain access to external destinations.

In this scenario, routers in area 12 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. This behavior is consistent with a totally stubbed area, where Type-3 LSAs are intentionally filtered. Routers rely exclusively on the injected default route for external connectivity. If the default route is missing or improperly configured, routers in area 12 will lose connectivity to networks outside the area, even though intra-area routing is operational and the ABR is functioning correctly. Troubleshooting this situation requires verifying the area type and ensuring that the ABR injects the default route properly. Understanding the behavior of totally stub areas is essential for designing OSPF networks, as failure to inject a default route can result in missing external routes despite a functioning ABR.

Type-1 LSAs describe a router’s own links within an area. If Type-1 LSAs were blocked, adjacency formation and intra-area routing would fail. Since routers in area 12 maintain intra-area connectivity, Type-1 LSAs are functioning correctly and are not causing the missing inter-area routes.

A down backbone area would prevent inter-area routing for all areas connected to it. Since only area 12 is affected, the backbone area is operational, making this an unlikely cause.

OSPF process IDs are locally significant and do not affect LSA propagation across areas in the same OSPF instance. Differences in process IDs would not prevent Type-3 LSA advertisement or default route injection.

The root cause is that area 12 is configured as a totally stubbed area. Although the ABR advertises Type-3 LSAs, the area type prevents them from being installed in the routing table, leaving only the default route as the path to external networks. Network engineers must ensure that the ABR is configured to inject the default route using “default-information originate.” Proper understanding and configuration of totally stubbed areas balances resource optimization with external connectivity. Misconfigured stub areas or omitted default route injection can isolate routers, causing operational and troubleshooting challenges. Verification and documentation of area types and default route injection are essential for predictable OSPF behavior, stable network operation, and efficient scaling without compromising access to external networks. Network engineers must carefully plan area types, validate default route injection, and test network behavior to ensure optimal OSPF performance and reliable connectivity.

Question 89:

A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic continues to use only one path. What is the most likely cause?

A) The paths are not equal in AS path, origin, or MED attributes.
B) BGP is configured only with iBGP neighbors.
C) One path has an unreachable next-hop.
D) BGP update suppression is enabled.

Answer: A)

Explanation:

BGP multipath allows multiple equal-cost paths to be installed in the forwarding table, enhancing redundancy, traffic distribution, and bandwidth utilization. For multipath to operate, paths must match in multiple attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. Any difference among these attributes will result in BGP selecting a single best path, even if multiple paths exist in the BGP table. This deterministic behavior ensures predictable routing, prevents forwarding loops, and guarantees consistent traffic flow.

In this scenario, multiple paths appear in the BGP table, but traffic only uses one path. This indicates that the paths are unequal in AS path, origin type, or MED attributes. For instance, one path may have AS path prepending, or the origin type may differ, causing BGP to choose a single best path and ignore other viable paths. Understanding these strict equality requirements is essential for deploying multipath in multi-homed networks. Misalignment prevents traffic from utilizing all available paths, reduces redundancy, and may lead to congestion on a single link, impacting network performance.

BGP session type, whether iBGP or eBGP, does not prevent multipath if equality conditions are satisfied. Both session types support multipath, and the presence of multiple paths in the BGP table indicates that BGP updates are being correctly received.

Next-hop reachability is necessary for a path to be installed in the forwarding table. Since multiple paths appear in the BGP table, next-hop connectivity is not the limiting factor.

BGP update suppression reduces the frequency of updates sent to neighbors to conserve control-plane resources but does not influence local path selection. Therefore, update suppression is not the cause of single-path forwarding in this scenario.

The root cause is that the paths are not equal in AS path, origin, or MED attributes. Engineers must align these attributes to enable multipath forwarding. Proper configuration ensures effective traffic distribution, redundancy, and optimal bandwidth utilization. Understanding multipath equality requirements is essential to fully leverage all available paths, enhance network resilience, and prevent congestion. Engineers must monitor BGP attributes, adjust configurations as necessary, and verify multipath operation to maintain predictable and balanced traffic forwarding. Correct multipath configuration maximizes redundancy, enhances network resilience, and improves overall utilization of available paths. Planning, attribute management, and operational verification are crucial for successful deployment in complex multi-homed networks.

Question 90:

A network engineer deploys RSVP-TE tunnels in an MPLS network. Tunnels fail to establish, even though all links report sufficient bandwidth. What is the most likely cause?

Answer: A)

Explanation:

MPLS Traffic Engineering enables operators to establish explicit paths based on available bandwidth, administrative constraints, and link attributes. RSVP-TE is the signaling protocol used to reserve bandwidth along these paths. Constrained Shortest Path First (CSPF) computes a feasible path by evaluating candidate paths against requested TE constraints, including bandwidth, TE colors, and administrative groups. Even when all links report sufficient bandwidth, tunnels may fail if CSPF cannot find a path that satisfies all constraints. Mismatched link attributes, such as incorrect TE colors, missing administrative group markings, or inconsistent configurations along the path, are the most common causes of RSVP-TE tunnel failures under these conditions.

RSVP authentication ensures only authorized routers can establish reservations. Mismatched authentication prevents tunnel setup but does not affect CSPF path computation. Authentication failures generate logs and alarms, but do not stop feasibility evaluation.

Soft-state refresh timers maintain RSVP tunnel state. If refresh intervals are too long, tunnel state may expire prematurely, but this does not prevent CSPF from computing a feasible path initially.

IGP metrics influence unconstrained SPF computation, but CSPF evaluates bandwidth and link attributes when determining a TE path. Even a path with the lowest IGP metric will be rejected if TE constraints are unmet, such as incorrect TE colors or mismatched administrative groups.

The root cause is mismatched link attributes preventing CSPF from finding a feasible path. Engineers must verify all links along the intended tunnel path for compatible attributes, including TE colors, administrative groups, and bandwidth. Proper alignment allows CSPF to compute a valid path, enabling RSVP-TE tunnels to establish successfully. Understanding the interaction between link attributes, constraints, and CSPF computation is critical for predictable MPLS TE operation, efficient traffic engineering, and optimal resource utilization. Correct configuration ensures reliable tunnel establishment, predictable network performance, and consistent operation, avoiding failures due to attribute mismatches. Verification and careful planning prevent unexpected tunnel establishment failures and ensure RSVP-TE tunnels function as intended, supporting efficient and predictable MPLS network operation.

Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 6 Q76-90

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