Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 7 Q91-105

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Question 91:

A network engineer deploys OSPF in a multi-area network. Routers in area 13 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 13 is configured as a totally stub 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 for optimizing routing efficiency, controlling LSA propagation, and managing router resource usage. A totally stub area is a specialized configuration that blocks external Type-5 LSAs as well as inter-area Type-3 summary LSAs, except for a default route injected by the ABR. This area type is commonly used in branch offices or remote locations with routers that have limited CPU or memory capacity, reducing the size of the LSDB and the routing table while still providing access to external networks. The ABR is responsible for injecting the default route into the totally stub area to ensure that routers can reach destinations outside the area.

In the described scenario, routers in area 13 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 stub area. By design, Type-3 summary LSAs are filtered in totally stub areas, leaving only the injected default route as the path to external networks. If the default route is missing or improperly configured, routers in area 13 will lose connectivity to networks outside the area, even though intra-area routes are functional and the ABR is operational. Troubleshooting this situation 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 OSPF multi-area networks because failure to inject a default route can result in missing external routes despite correct ABR operation. Proper area type planning allows network engineers to reduce router memory usage and LSDB size while maintaining essential connectivity.

Type-1 LSAs describe a router’s own links within an area. Blocking these LSAs would prevent adjacency formation and intra-area routing. Since routers in area 13 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 connected areas. Since only area 13 is affected in this scenario, the backbone is operational, making it an unlikely cause of the issue.

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 13 is configured as a totally stub 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 to external networks. Network engineers must ensure that the ABR injects a default route using the “default-information originate” command. Proper configuration balances the benefits of reduced router resource utilization with the need for external connectivity. Misconfigured stub areas or omitted default route injection can isolate routers from external networks, creating operational and troubleshooting challenges. Documentation, verification, and testing of area types and default route injection are essential for predictable OSPF behavior, stable network operation, and scalable designs. Network engineers must carefully plan area types, validate default route injection, and ensure consistent network behavior to maintain optimal OSPF performance and reliable connectivity. Correct configuration of totally stub areas reduces memory usage, simplifies the LSDB, and ensures external reachability through the injected default route while preventing unexpected connectivity failures.

Question 92:

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 bandwidth utilization. For multipath to operate, the 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 result in BGP selecting a single best path, even if multiple paths exist in the BGP table. This strict equality ensures predictable routing, prevents forwarding loops, and guarantees consistent traffic distribution across the network.

In the scenario described, 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 include AS path prepending, or the origin type may differ, causing BGP to select a single best path and ignore other paths for forwarding. Understanding these strict equality requirements is essential when designing multi-homed BGP networks. Failure to align attributes prevents full utilization of all available paths, reduces redundancy, and may cause congestion on a single link, limiting overall network performance.

BGP session type, whether iBGP or eBGP, does not prevent multipath if equality conditions are met. Both session types support multipath, and the presence of multiple paths in the BGP table confirms correct BGP operation.

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. Network 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 fully 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, enhances resilience, and ensures efficient utilization of available paths. Planning, attribute management, and operational verification are crucial for successful deployment in multi-homed BGP networks.

Question 93:

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 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, 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 and alerts but do not stop CSPF from attempting to compute a feasible path.

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 incorrect TE colors or mismatched 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 intended path have compatible attributes, including TE colors, administrative groups, and bandwidth availability. 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 94:

A network engineer deploys OSPF in a multi-area network. Routers in area 14 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 14 is configured as a totally stub 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 managing routing table size, controlling LSA propagation, and optimizing router resource utilization. A totally stub area is a configuration that prevents the installation of both external Type-5 LSAs and inter-area Type-3 summary LSAs while allowing a default route injected by the ABR. This configuration is often used in branch offices or remote sites where routers have limited CPU or memory resources. By filtering summary LSAs, the LSDB size and routing table are reduced, making it easier for routers to manage routing information efficiently while still providing access to external networks through a default route. The ABR plays a critical role in injecting the default route to provide connectivity to destinations outside the area, ensuring that even with filtered LSAs, routers can reach external networks reliably.

In this scenario, routers in area 14 are not receiving inter-area routes from area 0 despite the ABR advertising Type-3 summary LSAs. This behavior aligns with the operational characteristics of a totally stub area. By design, Type-3 summary LSAs are filtered, leaving only the default route to external destinations. If the default route is missing or misconfigured, routers in area 14 will lose connectivity to networks outside the area even though intra-area routing is operational and the ABR is functioning correctly. Troubleshooting requires verification of the area type and ensuring proper default route injection on the ABR. Understanding the behavior of totally stub areas is critical when designing OSPF multi-area networks because failure to inject a default route can result in missing external routes despite correct ABR operation. Proper area type planning allows network engineers to reduce router memory usage and LSDB size while maintaining essential connectivity.

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 14 maintain connectivity within the area, 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 14 is affected, the backbone is operational, making it an unlikely cause.

The root cause is that area 14 is configured as a totally stub 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 to external networks. Network engineers must ensure the ABR injects a default route using the “default-information originate” command. Proper configuration balances reduced resource utilization 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 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 ensure consistent network behavior to maintain optimal OSPF performance and reliable connectivity. Correct configuration of totally stub areas reduces memory usage, simplifies the LSDB, and ensures external reachability through the injected default route while preventing unexpected connectivity failures.

Question 95:

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, traffic distribution, and bandwidth utilization. For multipath to operate, 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 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 distribution across the network.

In the 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 may differ, causing BGP to choose a single best path while ignoring other viable paths for forwarding. Understanding these strict equality requirements is essential when designing multi-homed BGP networks. Failure to align attributes prevents full utilization of all available paths, reduces redundancy, and may cause congestion on a single link, negatively impacting 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 confirms correct BGP operation.

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.

The root cause is that the paths are not equal 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 and adhering to multipath equality requirements is essential for fully 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, enhances network resilience, and ensures efficient utilization of available paths. Planning, attribute management, and operational verification are crucial for successful deployment in multi-homed BGP networks.

Question 96:

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 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 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 to compute a feasible path.

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 97:

A network engineer configures OSPF on a multi-area network. Routers in area 15 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 15 is configured as a totally stub 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 the propagation of routing information and optimizing router resource usage. A totally stub area is a specific configuration that prevents both external Type-5 LSAs and inter-area Type-3 summary LSAs from being installed in the routing table, while still allowing a default route injected by the ABR. This configuration is often deployed in branch offices or remote sites where routers have limited memory or processing power. By filtering LSAs, the LSDB and routing table size are minimized, simplifying the routing process while still providing access to external networks. The ABR is responsible for injecting a default route to enable routers in the totally stub area to reach networks outside the area.

In this scenario, routers in area 15 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 stub area, where Type-3 LSAs are filtered by design. Routers in area 15 rely solely on the injected default route for external connectivity. If the default route is missing or misconfigured, routers will not reach destinations outside the area, despite intra-area routes being functional and the ABR operating correctly. Troubleshooting requires verification of the area type configuration and ensuring that the ABR is configured to inject the default route. Understanding the characteristics of totally stub areas is crucial when designing multi-area OSPF networks, as failure to inject a default route can result in missing external routes despite proper ABR operation.

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 15 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 15 is affected, the backbone area is operational, making this an unlikely cause.

The root cause is that area 15 is configured as a totally stub 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 as the path to external networks. Network engineers must ensure that the ABR injects a default route using “default-information originate.” Proper configuration balances reduced resource utilization with the need for external connectivity. Misconfigured stub areas or missing default route injection can isolate routers from external networks, creating operational and troubleshooting challenges. Documentation, verification, and testing of area types and default route injection are essential for predictable OSPF behavior, stable network operation, and efficient scaling. Correct configuration of totally stub areas reduces memory usage, simplifies the LSDB, and ensures external reachability through the injected default route while preventing unexpected connectivity failures.

Question 98:

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 bandwidth utilization. For multipath to work, 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 will cause BGP to select a single best path even if multiple paths are present in the BGP table. This deterministic behavior ensures predictable routing, prevents forwarding loops, and guarantees consistent traffic distribution across the network.

In the 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 may include AS path prepending, or the origin type may differ, causing BGP to select a single best path and ignore other viable paths. Understanding these strict equality requirements is crucial for deploying multipath in multi-homed networks. Failure to align attributes prevents full utilization of all available paths, reduces redundancy, and may cause congestion on a single link, 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. Network 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 fully 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, enhances network resilience, and ensures efficient utilization of available paths. Planning, attribute management, and operational verification are crucial for successful deployment in multi-homed BGP networks.

Question 99:

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 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 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.

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 100:

A network engineer configures OSPF on a multi-area network. Routers in area 16 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 16 is configured as a totally stub 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 control the distribution of routing information and optimize the use of router resources. A totally stub area is a specific configuration that prevents the installation of both external Type-5 LSAs and inter-area Type-3 summary LSAs in the routing table, while still allowing a default route injected by the ABR. This type of area is often used in branch offices or remote locations where routers have limited CPU or memory resources. Filtering Type-3 summary LSAs and external LSAs reduces the size of the LSDB and routing table, simplifying routing decisions and minimizing resource usage. The ABR is responsible for injecting a default route into the totally stub area to allow routers to reach external networks.

In this scenario, routers in area 16 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. By design, Type-3 LSAs are filtered in totally stub areas, leaving only the default route as the path to external networks. If the default route is not injected or misconfigured, routers in area 16 will not reach destinations outside the area, even though intra-area routes function properly and the ABR is operating correctly. Troubleshooting requires verification of the area type configuration and ensuring the ABR is configured to inject the default route. Understanding totally stub areas is crucial when designing multi-area OSPF networks, as failure to inject a default route can result in routers losing external connectivity despite proper ABR operation. Proper planning allows reduced LSDB size and memory usage while maintaining essential 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 16 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 16 is affected, the backbone is operational, making it 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 16 is configured as a totally stub 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 to external networks. Engineers must ensure the ABR injects a default route using “default-information originate.” Proper configuration balances optimized resource usage with the need for external connectivity. Misconfigured stub areas or missing default route injection can isolate routers from external networks, creating operational and troubleshooting challenges. Documentation, verification, 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 101:

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 bandwidth utilization. For multipath to work, 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 will cause BGP to select a single best path even if multiple paths exist in the BGP table. This behavior ensures predictable routing, prevents forwarding loops, and guarantees 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 may differ, causing BGP to select one path as the best 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 all available paths, reduces redundancy, and may lead to congestion on a single link, affecting overall network performance.

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.

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 fully 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 102:

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 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 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 computing a feasible path.

Soft-state refresh timers maintain the 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.

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 establishment failures and ensure RSVP-TE tunnels function as intended, supporting efficient and predictable MPLS network operation.

Question 103:

A network engineer deploys OSPF in a multi-area network. Routers in area 17 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 17 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 critical role in controlling the propagation of routing information and optimizing the use of router resources. A totally stub area is a configuration that prevents both external Type-5 LSAs and inter-area Type-3 summary LSAs from being installed in the routing table, while still allowing a default route injected by the ABR. This configuration is often deployed in branch offices or remote locations where routers have limited processing power and memory capacity. By filtering Type-3 summary LSAs and external LSAs, the LSDB size is reduced, and the routing table is simplified, allowing routers to operate efficiently with minimal resources. The ABR is responsible for injecting a default route to provide connectivity to networks outside the area, ensuring that routers in the totally stub area maintain access to external destinations despite the LSA filtering.

In this scenario, routers in area 17 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. By design, Type-3 LSAs are filtered, and routers rely solely on the injected default route for connectivity to external networks. If the default route is missing or misconfigured, routers will lose access to destinations outside the area, despite intra-area routes functioning correctly and the ABR operating properly. Troubleshooting requires verification of the area type configuration and ensuring that the ABR injects the default route using “default-information originate.” Understanding totally stub areas is essential when designing multi-area OSPF networks, as failure to inject a default route can result in missing external routes even when the ABR is operational. Proper planning and configuration allow network engineers to reduce LSDB size and resource consumption while maintaining essential external 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 17 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 17 is affected, the backbone area is operational, making it an unlikely cause.

The root cause is that area 17 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 to external networks. Engineers must ensure that the ABR injects a default route using “default-information originate.” Proper configuration balances optimized resource utilization 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 104:

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. For multipath to operate correctly, the candidate paths must match in multiple 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 present in the BGP table. This behavior ensures predictable routing, avoids forwarding loops, and provides 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 example, one path may contain AS path prepending, or the origin type may 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 networks. Failure to align attributes prevents full utilization of all available paths, reduces redundancy, and may lead to congestion on a single link, negatively 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 critical 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 use of all available paths. Planning, attribute management, and operational verification are essential for successful deployment in multi-homed BGP networks.

Question 105:

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 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 meets 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.

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.

IGP metrics influence unconstrained SPF computation, but CSPF evaluates 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.

Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 7 Q91-105

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