Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 8 Q106-120

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Question 106:

A network engineer is configuring OSPF on a multi-area network. Routers in area 18 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. What is the most likely cause?

A) Area 18 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 essential for optimizing routing efficiency, managing router resource usage, and controlling LSA propagation. A totally stub area is a specialized configuration designed to block both external Type-5 LSAs and inter-area Type-3 summary LSAs while still allowing a default route injected by the ABR. This type of area is typically deployed in branch offices or remote sites where routers have limited CPU or memory resources. By filtering summary LSAs and external LSAs, the LSDB size is reduced, and the routing table is simplified, making routing more efficient. The ABR is responsible for injecting a default route to provide connectivity to external networks, allowing routers in the totally stub area to access destinations outside the area despite the filtered LSAs.

In this scenario, routers in area 18 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. This aligns with the expected behavior of a totally stubbed area. By design, Type-3 LSAs are filtered, leaving only the default route to external networks. If the default route is missing or misconfigured, routers will lose connectivity to external destinations, 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 using “default-information originate.” Understanding totally stub areas is critical in multi-area OSPF designs, as failure to inject a default route can result in routers losing external connectivity despite proper ABR operation. Proper planning reduces LSDB size, memory usage, and processing overhead while maintaining essential connectivity.

Type-1 LSAs describe a router’s own links within an area. Blocking Type-1 LSAs prevents adjacency formation and intra-area routing. Since routers in area 18 maintain intra-area connectivity, Type-1 LSAs are functioning correctly and are not causing missing inter-area routes.

A down backbone area would prevent inter-area routing for all connected areas. Since only area 18 is affected, the backbone 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 18 is configured as a totally stubbed area. Even though the ABR advertises Type-3 LSAs, the area type prevents them from being installed in the routing table, leaving only the default route for external networks. Network engineers must ensure the ABR injects a default route using “default-information originate.” Proper configuration balances resource optimization with the need for external connectivity. Misconfigured stub areas or missing default route injection can isolate routers, creating operational and troubleshooting challenges. Verification, documentation, and testing of area types and default route injection are critical for predictable OSPF behavior, stable network operation, and efficient scaling. Correct configuration reduces LSDB size, memory usage, and ensures external reachability through the injected default route while preventing unexpected connectivity failures.

Question 107:

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, which improves redundancy, load balancing, and bandwidth utilization. For multipath to function correctly, candidate paths must match in several attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. Any differences in these attributes cause BGP to select a single best path even if multiple paths are available in the BGP table. This behavior ensures predictable routing, avoids forwarding loops, and provides consistent traffic distribution.

In the scenario described, multiple paths appear in the BGP table, but traffic continues to use only one path. This indicates that the paths are unequal in AS path, origin type, or MED attributes. For example, one path may have AS path prepending, or the origin type may differ, which causes BGP to select a single best path and ignore other viable paths for forwarding. Understanding these equality requirements is critical when deploying multipath in multi-homed networks. Failure to align attributes prevents full utilization of available paths, reduces redundancy, and may cause congestion on a single link, affecting overall 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 is functioning properly.

Next-hop reachability is required 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 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 bandwidth utilization. Understanding and adhering to multipath equality requirements is essential for leveraging all available paths, enhancing network performance, and preventing 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, improves network resilience, and ensures efficient utilization of all paths. Planning, attribute management, and operational verification are critical for successful deployment in multi-homed BGP networks.

Question 108:

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 enables operators to establish explicit paths through the network 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 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 and alerts, but do not prevent CSPF from attempting path computation.

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 both bandwidth and link attributes when computing a TE path. Even a path with the lowest IGP metric will be rejected if TE constraints are unmet, such as mismatched TE colors or administrative groups.

The root cause is mismatched link attributes preventing CSPF from finding a feasible path. Engineers must verify all links along the intended path for 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 resource utilization. Correct configuration ensures reliable tunnel establishment, predictable network performance, and consistent operation, avoiding failures caused by 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.

Question 109:

A network engineer configures OSPF in a multi-area network. Routers in area 19 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 19 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 play a vital role in controlling the propagation of routing information, reducing resource usage, and simplifying the LSDB. A totally stub area is a special area configuration designed to block the installation of both external Type-5 LSAs and inter-area Type-3 summary LSAs in the routing table. Despite this restriction, a default route injected by the ABR is allowed, enabling routers to reach destinations outside the area. Totally stub areas are often deployed in branch offices or remote sites where routers have limited CPU and memory resources. By filtering summary LSAs and external LSAs, the LSDB size is reduced, and routing tables are simplified, allowing routers to operate more efficiently while still maintaining access to external networks.

In this scenario, routers in area 19 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. This behavior aligns with the operational characteristics of a totally stubbed area. Type-3 LSAs are filtered by design, leaving only the default route for reaching external networks. If the default route is missing or not properly configured on the ABR, routers in area 19 will lose connectivity to destinations outside the area despite intra-area routing functioning correctly. Troubleshooting requires verification of the area type configuration and ensuring that the ABR injects a default route using the “default-information originate” command. Understanding totally stub areas is essential when designing multi-area OSPF networks because failure to inject a default route can result in routers being isolated from external networks, even if the ABR is operational. Proper planning reduces LSDB size, minimizes memory usage, and ensures predictable routing behavior while maintaining essential connectivity.

Type-1 LSAs describe a router’s links within an area. Blocking Type-1 LSAs would prevent adjacency formation and intra-area routing. Since routers in area 19 maintain intra-area connectivity, Type-1 LSAs are functioning correctly and are not the cause of missing inter-area routes.

A down backbone area would prevent inter-area routing for all connected areas. Since only area 19 is affected, the backbone is operational, making this an unlikely cause.

OSPF process IDs are locally significant 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 default route injection.

The root cause is that area 19 is configured as a totally stubbed area. Even though the ABR advertises Type-3 LSAs, the area type prevents these LSAs 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 injects a default route using “default-information originate.” Proper configuration balances resource optimization with external connectivity requirements. Misconfigured stub areas or missing default route injection can isolate routers, creating operational and troubleshooting challenges. Verification, documentation, and testing of area types and default route injection are critical for predictable OSPF behavior, stable network operation, and efficient scaling. Correct configuration reduces LSDB size, conserves memory, and ensures external reachability while preventing unexpected connectivity failures.

Question 110:

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 enables multiple equal-cost paths to be installed in the forwarding table, improving redundancy, load balancing, and overall bandwidth utilization. For multipath to work, candidate paths must match in multiple attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. Any differences among these attributes 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 maintains consistent traffic distribution across the network.

In this scenario, multiple paths appear in the BGP table, but traffic continues to use only one path. This indicates that the paths are unequal in AS path, origin type, or MED attributes. For example, one path might have AS path prepending, or the origin type might differ, causing BGP to select a single best path and ignore other viable paths. Understanding these strict equality requirements is essential when deploying multipath in multi-homed environments. Failure to align attributes prevents full utilization of all available paths, reduces redundancy, and can lead to congestion on a single path, negatively affecting network performance.

Next-hop reachability is required 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. Adhering to multipath equality requirements is essential for leveraging all available paths, enhancing network performance, and preventing congestion. Engineers must monitor BGP attributes, adjust configurations as needed, and verify multipath operation to maintain predictable and balanced traffic forwarding. Correct multipath configuration maximizes redundancy, improves network resilience, and ensures efficient utilization of all paths. Planning, attribute management, and operational verification are critical for successful deployment in multi-homed BGP networks.

Question 111:

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 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 available bandwidth, TE colors, and administrative groups. Even if 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 configuration along the intended path, are the most common cause of RSVP-TE tunnel failures in such scenarios.

RSVP authentication ensures only authorized routers can reserve bandwidth. Mismatched authentication prevents tunnel establishment but does not affect CSPF path computation. Authentication failures generate log messages and alerts, but do not prevent CSPF from attempting path computation.

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 both bandwidth and link attributes when computing a TE path. Even a path with the lowest IGP metric will be rejected if TE constraints are not met, such as incorrect TE colors or administrative group mismatches.

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 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 resource utilization. Correct configuration ensures reliable tunnel establishment, predictable network performance, and consistent operation, avoiding failures caused by attribute mismatches. Verification and careful planning prevent unexpected tunnel failures and ensure RSVP-TE tunnels function as intended, supporting efficient and predictable MPLS network operation.

Question 112:

A network engineer configures OSPF in a multi-area network. Routers in area 20 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. What is the most likely cause?

A) Area 20 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 essential for controlling routing information propagation, optimizing resource usage, and simplifying LSDB management. A totally stub area is a specialized OSPF area type designed to prevent the installation of both external Type-5 LSAs and inter-area Type-3 summary LSAs in the routing table. This configuration still allows a default route injected by the ABR, enabling routers within the totally stub area to access destinations outside the area. Totally stub areas are typically deployed in branch offices or remote sites where router resources, such as CPU and memory, are limited. Filtering summary LSAs and external LSAs reduces the LSDB size and routing table complexity, allowing routers to operate more efficiently while still maintaining essential connectivity to external networks.

In this scenario, routers in area 20 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. By design, Type-3 LSAs are filtered in a totally stub area, leaving only the default route for connectivity to external networks. If the default route is missing or misconfigured on the ABR, routers in area 20 will lose connectivity to destinations outside the area, even though intra-area routes are functional and the ABR is operational. Troubleshooting requires verification of the area type configuration and ensuring that the ABR injects a default route using the “default-information originate” command. Understanding the function of totally stubbed areas is crucial when designing multi-area OSPF networks. Failure to inject a default route can isolate routers from external networks despite proper ABR operation. Proper planning reduces LSDB size, conserves memory, and ensures predictable routing behavior while maintaining connectivity.

Type-1 LSAs describe a router’s own links within an area. Blocking Type-1 LSAs would prevent adjacency formation and intra-area routing. Since routers in area 20 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 connected areas. Since only area 20 is affected, the backbone area is operational, making this cause unlikely.

The root cause is that area 20 is configured as a totally stubbed area. Even though the ABR advertises Type-3 LSAs, the area type prevents their installation in the routing table, leaving only the default route for external networks. Network engineers must ensure that the ABR injects a default route using “default-information originate.” Proper configuration balances optimized resource usage with external connectivity requirements. Misconfigured stub areas or missing default route injection can isolate routers, creating operational challenges. Verification, documentation, and testing of area types and default route injection are critical for predictable OSPF behavior, stable network operation, and efficient scaling. Correct configuration reduces LSDB size, conserves memory, and ensures external reachability while preventing unexpected connectivity failures.

Question 113:

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, load balancing, and utilization of available bandwidth. To function properly, candidate paths must match in several attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. Any differences among these attributes result in BGP selecting a single best path even if multiple paths exist in the BGP table. This deterministic behavior ensures predictable routing, avoids forwarding loops, and maintains consistent traffic distribution across the network.

In this scenario, multiple paths appear in the BGP table, but traffic continues to use a single path. This indicates that the paths are unequal in AS path, origin type, or MED attributes. For example, one path might include AS path prepending, or the origin type might differ, causing BGP to select a single best path while ignoring other viable paths. Understanding the strict equality requirements for multipath is critical when deploying it in multi-homed networks. Failure to align attributes prevents full utilization of available paths, reduces redundancy, and may cause congestion on a single path, affecting overall network performance.

Next-hop reachability is required 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.

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 and adhering to multipath equality requirements is essential for leveraging all available paths, enhancing network performance, and preventing congestion. Engineers must monitor BGP attributes, adjust configurations as needed, and verify multipath operation to maintain predictable and balanced traffic forwarding. Correct multipath configuration maximizes redundancy, improves resilience, and ensures efficient utilization of all paths. Planning, attribute management, and operational verification are critical for successful deployment in multi-homed BGP networks.

Question 114:

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 through the network 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 available bandwidth, TE colors, and administrative groups. Even if 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 causes of RSVP-TE tunnel failures in such scenarios.

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.

Question 115:

A network engineer configures OSPF in a multi-area network. Routers in area 21 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 21 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 essential for controlling routing information propagation, reducing LSDB size, and optimizing router resource utilization. A totally stubbed area is a special configuration designed to prevent the installation of both external Type-5 LSAs and inter-area Type-3 summary LSAs in the routing table. Despite this restriction, a default route injected by the ABR is allowed, enabling routers within the totally stub area to access destinations outside the area. Totally stub areas are typically deployed in branch offices or remote sites with limited CPU and memory resources. By filtering summary LSAs and external LSAs, the LSDB size and routing table complexity are reduced, which allows routers to operate more efficiently while still maintaining essential connectivity.

In this scenario, routers in area 21 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. This behavior aligns with the operational characteristics of a totally stubbed area. Type-3 LSAs are filtered, leaving only the default route to external networks. If the default route is missing or misconfigured, routers in area 21 will lose connectivity to destinations outside the area, despite intra-area routes being functional and the ABR operating properly. Troubleshooting requires verification of the area type configuration and ensuring that the ABR injects a default route using “default-information originate.” Understanding totally stub areas is critical in multi-area OSPF designs because failure to inject a default route can result in routers being isolated from external networks. Proper planning reduces LSDB size, conserves memory, and ensures predictable routing behavior while maintaining connectivity.

Type-1 LSAs describe a router’s links within an area. Blocking Type-1 LSAs would prevent adjacency formation and intra-area routing. Since routers in area 21 maintain intra-area connectivity, Type-1 LSAs are functioning correctly and are not the cause of missing inter-area routes.

A down backbone area would prevent inter-area routing for all connected areas. Since only area 21 is affected, the backbone is operational, making this an unlikely cause.

The root cause is that area 21 is configured as a totally stubbed area. Even though the ABR advertises Type-3 LSAs, the area type prevents their installation in the routing table, leaving only the default route for external networks. Network engineers must ensure the ABR injects a default route using “default-information originate.” Proper configuration balances optimized resource usage with external connectivity requirements. Misconfigured stub areas or missing default route injection can isolate routers, creating operational challenges. Verification, documentation, and testing of area types and default route injection are critical for predictable OSPF behavior, stable network operation, and efficient scaling. Correct configuration reduces LSDB size, conserves memory, and ensures external reachability while preventing unexpected connectivity failures.

Question 116:

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 enables multiple equal-cost paths to be installed in the forwarding table, enhancing redundancy, load balancing, and bandwidth utilization. For multipath to function properly, candidate paths must match in several attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. Any discrepancies in these attributes 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 maintains consistent traffic distribution.

In this scenario, multiple paths appear in the BGP table, but traffic continues to use only one path. This indicates that the paths are unequal in AS path, origin type, or MED attributes. For instance, one path might have AS path prepending, or the origin type could differ, causing BGP to select a single best path and ignore other viable paths. Understanding these equality requirements is crucial when deploying multipath in multi-homed environments. Failure to align attributes prevents full utilization of available paths, reduces redundancy, and can cause congestion on a single path, negatively affecting network performance.

Next-hop reachability is required 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.

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 and adhering to multipath equality requirements is essential for leveraging all available paths, enhancing network performance, and preventing congestion. Engineers must monitor BGP attributes, adjust configurations as needed, and verify multipath operation to maintain predictable and balanced traffic forwarding. Correct multipath configuration maximizes redundancy, improves resilience, and ensures efficient utilization of all paths. Planning, attribute management, and operational verification are critical for successful deployment in multi-homed BGP networks.

Question 117:

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 establish explicit paths through a network 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 available 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 configuration along the intended path, are the most common causes of RSVP-TE tunnel failures under these conditions.

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.

Question 118:

A network engineer configures OSPF in a multi-area network. Routers in area 22 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. What is the most likely cause?

A) Area 22 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 routing information propagation, reducing LSDB size, and optimizing router performance. A totally stub area is a specialized configuration that prevents the installation of both external Type-5 LSAs and inter-area Type-3 summary LSAs in the routing table. Despite this restriction, a default route injected by the ABR is allowed, enabling routers within the totally stub area to access destinations outside the area. Totally stubbed areas are commonly deployed in branch offices or remote sites with limited CPU or memory resources. By filtering summary LSAs and external LSAs, the LSDB size and routing table complexity are reduced, allowing routers to operate efficiently while maintaining connectivity to essential destinations.

In this scenario, routers in area 22 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. This behavior aligns with the operational characteristics of a totally stubbed area. Type-3 LSAs are filtered by design, leaving only the default route to external networks. If the default route is missing or misconfigured on the ABR, routers in area 22 will lose connectivity to external destinations despite intra-area routes functioning correctly and the ABR being operational. Troubleshooting requires verification of the area type configuration and ensuring that the ABR injects a default route using “default-information originate.” Understanding totally stub areas is essential in multi-area OSPF networks because failure to inject a default route can result in routers being isolated from external networks. Proper planning reduces LSDB size, conserves memory, and ensures predictable routing behavior while maintaining essential connectivity.

Type-1 LSAs describe a router’s own links within an area. Blocking Type-1 LSAs prevents adjacency formation and intra-area routing. Since routers in area 22 maintain intra-area connectivity, Type-1 LSAs are functioning correctly and are not the cause of missing inter-area routes.

A down backbone area would prevent inter-area routing for all connected areas. Since only area 22 is affected, the backbone area is operational, making this an unlikely cause.

The root cause is that area 22 is configured as a totally stubbed area. Even though the ABR advertises Type-3 LSAs, the area type prevents them from being installed in the routing table, leaving only the default route for external networks. Network engineers must ensure the ABR injects a default route using “default-information originate.” Proper configuration balances optimized resource usage with external connectivity requirements. Misconfigured stub areas or missing default route injection can isolate routers, creating operational challenges. Verification, documentation, and testing of area types and default route injection are critical for predictable OSPF behavior, stable network operation, and efficient scaling. Correct configuration reduces LSDB size, conserves memory, and ensures external reachability while preventing unexpected connectivity failures.

Question 119:

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, load balancing, and efficient bandwidth utilization. To operate correctly, candidate paths must match in several attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. Any differences among these attributes cause BGP to select a single best path even if multiple paths exist in the BGP table. This behavior ensures deterministic routing, avoids forwarding loops, and provides consistent traffic distribution across the network.

In this scenario, multiple paths appear in the BGP table, but traffic continues to use a single path. This indicates that the paths are unequal in AS path, origin type, or MED attributes. For example, one path may include AS path prepending, or the origin type might differ, causing BGP to select a single best path while ignoring other viable paths. Understanding these strict equality requirements is essential when deploying multipath in multi-homed environments. Failure to align attributes prevents full utilization of available paths, reduces redundancy, and can lead to congestion on a single path, adversely affecting network performance.

Next-hop reachability is required 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.

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. Adhering to multipath equality requirements is essential for leveraging all available paths, improving network performance, and preventing congestion. Engineers must monitor BGP attributes, adjust configurations, and verify multipath operation to maintain predictable and balanced traffic forwarding. Correct multipath configuration maximizes redundancy, enhances network resilience, and ensures efficient utilization of all available paths. Planning, attribute management, and operational verification are critical for successful deployment in multi-homed BGP networks.

Question 120:

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 network operators to establish explicit paths through the network 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 available bandwidth, TE colors, and administrative groups. Even if 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 configuration along the intended path, are the most common cause of RSVP-TE tunnel failures in such scenarios.

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.

Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 8 Q106-120

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