Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 14 Q196-210

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Question 196:

A network engineer deploys OSPF on a multi-area network. Remote branch routers cannot reach external networks redistributed from BGP. What is the most likely cause?

A) The remote area is configured as a totally stubbed area
B) Type-3 LSAs are filtered by an ACL
C) OSPF process IDs are mismatched
D) Area 0 is down

Answer: A)

Explanation:

OSPF area types are crucial in controlling how routing information is propagated and the size of the LSDB. A totally stubbed area is specifically designed to minimize LSDB size and routing table complexity, which is useful for branch offices with limited resources. In a totally stub area, Type-5 LSAs, which carry external routes redistributed from protocols such as BGP, are blocked from entering the area. This prevents routers within the totally stub area from directly learning external routes. Instead, the ABR injects a default route into the area so that all routers can reach external destinations. If this default route is missing or misconfigured, routers in the totally stub area cannot reach external networks. Verification includes using “show ip ospf” to confirm area type and “show ip route” to ensure a default route is present. Proper ABR configuration ensures that branch routers have connectivity to external destinations while minimizing LSDB size and resource consumption.

In this scenario, remote branch routers cannot reach external networks despite the backbone and ABRs functioning properly. This strongly suggests that the area is configured as a totally stubbed area. Engineers must verify the ABR configuration and ensure that a default route is being properly advertised. Proper configuration guarantees predictable routing, efficient operation, and optimized resource usage. Monitoring ABR behavior, confirming default route propagation, and verifying area types are crucial for stable OSPF operation and external reachability.

Filtering Type-3 LSAs via an ACL would only affect inter-area routes, not external routes, if a default route exists. Therefore, the inability to reach external network points points to the area type rather than LSA filtering.

If Area 0 were down, inter-area routing would be disrupted network-wide, not only in the branch. Because only the remote branch is affected, the backbone is operational, confirming that the area type is the root cause.

OSPF process IDs are locally significant and do not affect inter-area LSA propagation. Mismatched process IDs may generate configuration warnings, but will not prevent default routes from being received if area types are correctly configured.

The root cause is the branch area being a totally stubbed area. Ensuring that the ABR injects a default route allows routers in the area to reach external networks. Engineers should verify area types, ABR behavior, and default route propagation. Totally stub areas reduce LSDB size, conserve router resources, and maintain network stability while still allowing connectivity to external networks. Proper planning, monitoring, and verification of stub areas ensure stable OSPF operation, predictable convergence, and reliable external reachability. Correct deployment prevents routing issues, maintains efficiency, and supports enterprise or service provider requirements. Understanding totally stub areas and ABR functionality is essential for designing scalable OSPF networks. Proper configuration supports stable routing, minimal overhead, and efficient propagation of external routes through default route injection.

Question 197:

A network engineer configures BGP multipath in a multi-homed enterprise network. Multiple paths appear in the BGP table, but only one path is actively used. What is the most likely cause?

A) Candidate paths differ in AS path, origin, or MED attributes
B) All BGP neighbors are configured as iBGP
C) One path has an unreachable next-hop
D) BGP route dampening is enabled

Answer: A)

Explanation:

BGP multipath allows multiple equal-cost paths to be simultaneously used for traffic forwarding, providing redundancy and load balancing. Multipath operation requires that candidate paths be identical in attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP uses a deterministic path selection process to prevent loops, so any discrepancy in these attributes will prevent multipath forwarding even if multiple paths are visible in the BGP table. Verification can be performed using “show bgp ipv4 unicast” to inspect the attributes of candidate paths. Ensuring attribute equality allows traffic to be distributed across multiple paths, optimizing bandwidth usage and enhancing redundancy. Proper configuration provides predictable traffic distribution, prevents congestion on a single path, and improves overall network performance.

All neighbors being in iBGP does not prevent multipath functionality. Multipath is supported with both iBGP and eBGP neighbors as long as the attribute equality requirements are met. Neighbor type does not override the need for identical attributes.

An unreachable next-hop prevents a specific path from being installed in the routing table. Since multiple paths appear in the BGP table, next-hop reachability is not the problem. Verification can be done using “show ip route” and ping tests.

BGP route dampening temporarily suppresses unstable routes but does not prevent stable paths from being used. Multipath operation works as long as paths meet the equality requirements.

The root cause is attribute differences among candidate paths. Differences in AS path, origin type, or MED prevent multipath forwarding. Engineers must align attributes to enable multiple paths for traffic distribution. Correct attribute alignment ensures redundancy, balanced traffic, and optimized bandwidth utilization. Misalignment can lead to underutilization of available paths, congestion on the active path, and reduced network resilience. Understanding BGP multipath requirements is critical for designing multi-homed networks. Continuous verification of attributes and monitoring of traffic flow ensure proper multipath operation. Proper configuration allows predictable traffic distribution, high availability, and efficient bandwidth usage, supporting stable enterprise network performance and reliability. Proper multipath BGP deployment enhances network redundancy, prevents bottlenecks, and ensures consistent traffic distribution. Continuous monitoring and attribute verification are essential for multipath functionality and network optimization.

Question 198:

A network engineer deploys MPLS LDP in a service provider network. Some routers fail to establish LDP sessions with neighbors. What is the most likely cause?

A) Missing IGP adjacency between routers
B) LDP transport addresses are mismatched
C) MPLS is not enabled globally
D) LDP hello timers are too long

Answer: A)

Explanation:

MPLS LDP depends on the underlying IGP to provide IP connectivity between routers in order to establish neighbor relationships and exchange label mapping information. LDP uses TCP port 646 to send hello messages and label updates. Without IGP adjacency, routers cannot communicate with neighbors, preventing LDP session formation and blocking the creation of label-switched paths required for MPLS traffic forwarding. LDP cannot operate independently of the IGP; connectivity provided by the IGP is essential. Verification involves checking the IGP routing table, adjacency status using “show ip ospf neighbor” or “show ip eigrp neighbors,” and performing connectivity tests using ping or traceroute. Proper IGP operation allows LDP messages to reach neighbors, enabling session formation and label distribution. Engineers must ensure interface IP addressing, full IGP convergence, and proper MPLS configuration to maintain reliable MPLS forwarding. Proper IGP configuration forms the foundation for LDP operation, ensuring predictable label distribution, stable traffic forwarding, and efficient network performance.

Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly. However, most LDP deployments use loopback or interface addresses by default, making this scenario less likely.

MPLS not being globally enabled prevents label forwarding but does not stop LDP from attempting neighbor discovery. Discovery will still occur, though LSPs cannot forward traffic until MPLS is enabled.

LDP hello timers define the frequency of hello messages, but do not prevent neighbor formation. Longer timers may delay session establishment, but do not block LDP if IGP connectivity exists.

The root cause is missing IGP adjacency. Proper IGP configuration ensures that LDP messages can reach neighbors, enabling label-switched path formation and MPLS forwarding. Engineers should monitor IGP adjacency, interface configuration, and LDP session status to maintain stable MPLS networks. Understanding the dependency of LDP on IGP is critical for network design and troubleshooting. Proper planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Stable IGP adjacencies provide the foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.

Question 199:

A network engineer deploys OSPF in a multi-area network. Remote branch routers are unable to reach networks external to OSPF, even though ABRs and the backbone are operational. What is the most likely cause?

A) The remote area is configured as a totally stubbed area
B) Type-3 LSAs are filtered by an ACL
C) OSPF process IDs are mismatched
D) Area 0 is down

Answer: A)

Explanation:

OSPF area types determine the routing information that is allowed into each area. A totally stubbed area is specifically designed to reduce the size of the LSDB and minimize routing table entries in branch areas. In such areas, Type-5 LSAs, which carry external routes redistributed from protocols like BGP, are blocked from entering. Instead, the ABR injects a default route into the totally stubbed area to provide external connectivity. If the default route is missing or misconfigured, routers in the totally stub area cannot reach external networks. Verification involves using commands like “show ip ospf” to confirm area type and “show ip route” to check for the default route. Ensuring the ABR properly injects the default route allows remote routers to reach external networks while keeping the LSDB minimal, which conserves memory and CPU resources on branch routers.

In this scenario, branch routers cannot access external networks even though the backbone and ABRs are operational. This strongly suggests that the area is configured as a totally stubbed area. Engineers must confirm ABR behavior and verify that the default route is being advertised. Proper configuration ensures predictable routing, efficient network operation, and optimized LSDB usage. Monitoring ABRs and verifying default route propagation are critical for maintaining stable OSPF operation and external connectivity.

Filtering Type-3 LSAs via ACLs affects inter-area routes but does not prevent external routes if a default route is present. Since the problem is external connectivity, the area type configuration is the primary cause.

If Area 0 were down, inter-area routing would fail across the network, not just the branch. Since only the branch is affected, the backbone is functional, confirming that the area type is the root cause.

OSPF process IDs are locally significant and do not affect inter-area LSA propagation. Mismatched process IDs may trigger local warnings, but will not prevent external routes if area types are properly configured.

The root cause is the branch area being a totally stubbed area. Ensuring the ABR injects a default route allows routers to reach external networks. Engineers must verify area type, ABR configuration, and default route propagation. Totally stub areas reduce LSDB size, conserve resources, and maintain network stability while providing essential connectivity to external networks. Proper planning, monitoring, and verification of stub areas ensure stable OSPF operation, predictable convergence, and reliable external reachability. Correct deployment prevents routing issues, maintains efficiency, and supports enterprise and service provider requirements. Understanding totally stub areas and ABR behavior is essential for scalable OSPF network design. Proper configuration ensures stable routing, minimal overhead, and efficient propagation of external routes through default route injection.

Question 200:

A network engineer configures BGP multipath in a multi-homed network. Multiple paths appear in the BGP table, but only one path is used for forwarding traffic. What is the most likely cause?

A) Paths differ in AS path, origin, or MED attributes
B) All BGP neighbors are configured as iBGP
C) One path has an unreachable next-hop
D) BGP route dampening is enabled

Answer: A)

Explanation:

BGP multipath allows multiple equal-cost paths to be used simultaneously for forwarding traffic, improving redundancy and load balancing. Multipath operation requires candidate paths to be identical in all key attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP follows a deterministic path selection process to prevent routing loops, so any discrepancy in attributes prevents multiple paths from being used even if they appear in the BGP table. Verification involves using “show bgp ipv4 unicast” to compare AS path, origin type, MED, and local preference values of candidate paths. Proper alignment of attributes enables traffic to be distributed across multiple paths, optimizing bandwidth utilization and improving redundancy. Proper configuration ensures predictable traffic distribution, prevents congestion on a single path, and enhances overall network performance.

All neighbors being in iBGP does not prevent multipath. Multipath is supported with both iBGP and eBGP neighbors as long as attribute equality requirements are met. Neighbor type does not override the need for identical attributes.

BGP route dampening temporarily suppresses unstable routes but does not prevent stable paths from being used for traffic forwarding. Multipath operation works as long as paths meet attribute equality requirements.

The root cause is differences in critical attributes among candidate paths. Differences in AS path, origin type, or MED prevent multipath forwarding. Engineers must align these attributes to enable multiple paths to carry traffic. Correct attribute alignment enhances redundancy, ensures balanced traffic distribution, and optimizes bandwidth utilization. Misalignment can result in underutilization of available paths, congestion on the active path, and reduced network resilience. Understanding BGP multipath requirements is essential for multi-homed network design. Continuous verification and monitoring of attributes and traffic flow ensure proper multipath operation. Proper configuration allows predictable traffic distribution, high availability, and efficient bandwidth usage, supporting stable enterprise network performance. Correct multipath BGP deployment improves redundancy, prevents bottlenecks, and ensures consistent traffic distribution across the network.

Question 201:

A network engineer deploys MPLS LDP in a service provider network. Some routers fail to establish LDP sessions with neighbors. What is the most likely cause?

A) Missing IGP adjacency between routers
B) LDP transport addresses are mismatched
C) MPLS is not enabled globally
D) LDP hello timers are too long

Answer: A)

Explanation:

MPLS LDP depends on the underlying IGP to provide IP connectivity between routers for establishing neighbor relationships and exchanging label mapping information. LDP uses TCP port 646 to communicate hello messages and label updates. Without IGP adjacency, routers cannot communicate, preventing LDP session formation and blocking the creation of label-switched paths required for MPLS traffic forwarding. LDP cannot operate independently of the IGP; connectivity through the IGP is essential. Verification involves checking the IGP routing table, neighbor adjacency status with commands like “show ip ospf neighbor” or “show ip eigrp neighbors,” and performing connectivity tests using ping or traceroute. Proper IGP operation allows LDP messages to reach neighbors, enabling session formation and label distribution. Engineers must ensure interface IP addressing, full IGP convergence, and proper MPLS configuration. Proper IGP configuration provides the foundation for LDP operation, ensuring predictable label distribution, stable traffic forwarding, and efficient network performance.

Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly. However, default LDP deployments typically use loopback or interface addresses, making this scenario less common.

MPLS not being globally enabled prevents label forwarding but does not stop LDP from attempting neighbor discovery. Discovery still occurs even if MPLS is inactive, though LSPs cannot forward traffic until MPLS is enabled.

LDP hello timers define the frequency of hello messages, but do not prevent neighbor formation. Longer timers may delay session establishment, but do not block LDP if IGP connectivity exists.

The root cause is missing IGP adjacency. Proper IGP configuration ensures that LDP messages can reach neighbors, enabling label-switched path formation and MPLS forwarding. Engineers should monitor IGP adjacency, interface configurations, and LDP session status to maintain stable MPLS networks. Understanding LDP’s dependency on the IGP is critical for network design and troubleshooting. Correct planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Stable IGP adjacencies provide the foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.

Question 202:

A network engineer configures EIGRP across multiple routers in a large enterprise network. Some routers on the same subnet are not forming EIGRP neighbor relationships. What is the most likely cause?

A) Mismatched EIGRP AS numbers
B) Passive interface is enabled
C) K values are different
D) Split-horizon is enabled

Answer: A)

Explanation:

EIGRP uses the autonomous system (AS) number to identify routers that belong to the same routing domain. All routers participating in EIGRP must share the same AS number to establish neighbor relationships and exchange routing information. When routers on the same subnet have mismatched AS numbers, they will fail to recognize each other as valid neighbors, preventing adjacency formation. This failure results in incomplete route propagation, inconsistent routing tables, and delayed convergence, potentially causing network segmentation and routing loops. Verification involves using “show ip eigrp neighbors” to inspect neighbor relationships and “show running-config” to confirm AS numbers. Ensuring consistent AS numbers across all routers is essential for EIGRP stability and proper operation.

In this scenario, routers on the same subnet are unable to form neighbors, strongly indicating an AS number mismatch. Correcting the AS numbers allows routers to recognize each other, establish adjacencies, and propagate routing information throughout the network. Engineers should also verify interface IP addressing, ensure that passive interfaces are not inadvertently configured, and confirm that routers can reach each other at Layer 3. Proper AS number alignment is critical in large networks with multiple EIGRP domains because mismatches can segment the network, disrupt routing, and degrade performance.

A passive interface prevents hello packets from being sent on that interface, blocking neighbor formation. While this could affect a single interface, widespread neighbor failures across multiple routers suggest a systemic issue, such as an AS number mismatch rather than a passive interface configuration.

Differences in K values affect metric calculation but do not prevent neighbor formation. Mismatched K values influence route selection, potentially resulting in suboptimal routing, but do not block adjacency establishment.

Split-horizon prevents a router from advertising a route back out the interface on which it was learned. While it affects route propagation in certain topologies, it does not prevent neighbor formation on the same subnet.

The root cause is mismatched AS numbers. Ensuring all routers share the same AS number is crucial for neighbor formation, proper route propagation, and network stability. Network engineers should verify AS numbers, interface configurations, and adjacency status to maintain proper EIGRP operation. Understanding the importance of AS number alignment prevents neighbor formation failures, routing inconsistencies, and network segmentation. Proper AS configuration supports predictable route propagation, stable convergence, and reliable connectivity. Correct deployment across multi-area networks requires careful planning, verification, and monitoring of AS numbers. Ensuring consistent AS numbers allows routers to form reliable adjacencies, maintain complete routing tables, and guarantee predictable network behavior. Proper AS number configuration ensures efficient EIGRP operation, robust connectivity, and optimal network performance. Continuous verification and monitoring of AS numbers prevent operational issues and support stable multi-area EIGRP networks.

Question 203:

A network engineer deploys OSPFv3 in an IPv6 network. Some routers fail to form neighbor adjacencies on certain links. What is the most likely cause?

A) Missing link-local addresses on interfaces
B) Duplicate router IDs
C) Area authentication mismatch
D) IPv6 unicast routing is disabled globally

Answer: A)

Explanation:

OSPFv3 requires link-local addresses for neighbor discovery and adjacency formation in IPv6 networks. Link-local addresses serve as the source and destination for OSPFv3 hello packets and are essential for routers to communicate directly on the same link. Without valid link-local addresses, routers cannot discover neighbors or exchange LSAs, preventing adjacency formation and LSDB synchronization. Verification involves using “show ipv6 interface brief” to confirm link-local address configuration and “show ipv6 ospf neighbor” to inspect adjacency status. Proper configuration of link-local addresses ensures routers can establish adjacencies, exchange LSAs, and achieve network convergence. Engineers should also ensure unique router IDs are configured to prevent LSDB conflicts.

In this scenario, routers fail to form adjacencies, indicating missing or misconfigured link-local addresses. Correctly configured link-local addresses enable hello packets to be exchanged, adjacencies to form, and LSDBs to synchronize across the network. Ensuring all interfaces participating in OSPFv3 have link-local addresses is critical for predictable convergence, stable route propagation, and reliable network operation. Proper configuration supports efficient IPv6 deployment and high availability.

Duplicate router IDs can cause LSDB conflicts, but do not prevent hello packets from being sent. While conflicts may trigger warnings and impact LSA synchronization, they are unlikely to block adjacency formation entirely if link-local addresses exist.

Area authentication mismatches affect the acceptance of LSAs but do not prevent adjacency formation. Routers will still exchange hello packets, though LSAs may be rejected if authentication does not match.

Disabling IPv6 unicast routing globally prevents the forwarding of IPv6 packets but does not block hello packet exchange over link-local addresses.

The root cause is missing link-local addresses. Proper configuration ensures adjacency formation, LSA exchange, and stable OSPFv3 operation. Engineers must verify link-local addresses, enable OSPFv3 on all interfaces, and configure unique router IDs. Correct link-local addressing enables seamless adjacency formation, uninterrupted LSA flooding, and predictable convergence. Continuous verification and monitoring prevent operational issues and maintain stable OSPFv3 networks. Proper deployment ensures stable routing, reliable connectivity, and optimal IPv6 network performance, supporting efficient communication between all routers and robust LSDB synchronization. Proper configuration allows predictable route propagation, stable convergence, and reliable operation in IPv6 networks.

Question 204:

A network engineer deploys MPLS LDP in a service provider network. Some routers fail to establish LDP sessions with neighbors. What is the most likely cause?

A) Missing IGP adjacency between routers
B) LDP transport addresses are mismatched
C) MPLS is not enabled globally
D) LDP hello timers are too long

Answer: A)

Explanation:

MPLS LDP relies on the underlying IGP to provide IP connectivity between routers to establish neighbor relationships and exchange label mapping information. LDP uses TCP port 646 to communicate hello messages and label updates. Without IGP adjacency, routers cannot reach neighbors, preventing LDP session formation and blocking the creation of label-switched paths required for MPLS traffic forwarding. LDP cannot operate independently; IGP-provided connectivity is essential. Verification involves checking the IGP routing table, adjacency status using commands like “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity with ping or traceroute. Proper IGP operation ensures LDP messages reach neighbors, enabling session formation and label distribution. Engineers must ensure interface IP addressing, full IGP convergence, and MPLS configuration to maintain stable LDP operation. Proper IGP configuration provides a foundation for predictable label distribution, stable traffic forwarding, and efficient network performance.

Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly, but most deployments use loopback or interface addresses by default, making this scenario less common.

LDP hello timers define how often hello messages are sent, but do not prevent neighbor formation. Longer timers may delay session establishment, but do not block LDP if IGP connectivity exists.

The root cause is missing IGP adjacency. Proper IGP configuration ensures LDP messages reach neighbors, enabling label-switched path formation and MPLS forwarding. Engineers should monitor IGP adjacency, interface configuration, and LDP session status to maintain stable MPLS networks. Understanding LDP’s dependency on the IGP is critical for network design and troubleshooting. Correct planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Stable IGP adjacencies provide the foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.

Question 205:

A network engineer deploys OSPF in a multi-area network. Some remote branch routers cannot reach external networks redistributed from BGP. What is the most likely cause?

A) The remote area is configured as a totally stubbed area
B) Type-3 LSAs are filtered by an ACL
C) OSPF process IDs are mismatched
D) Area 0 is down

Answer: A)

Explanation:

OSPF area types determine the flow of routing information and the size of the LSDB within each area. A totally stubbed area is designed to minimize the LSDB and reduce routing table complexity, particularly for remote branches with limited resources. In a totally stub area, Type-5 LSAs, which carry external routes such as those redistributed from BGP, are blocked from entering. This prevents routers within the area from directly learning external routes. Instead, the ABR injects a default route into the totally stub area, allowing routers to reach external destinations without maintaining detailed external route information. If the default route is missing or misconfigured, branch routers cannot reach external networks. Verification involves checking the area type with “show ip ospf” and confirming the presence of the default route in the routing table with “show ip route.” Proper ABR configuration ensures that branch routers have connectivity to external networks while keeping the LSDB size small, which conserves memory and CPU resources on branch routers.

In this scenario, remote branch routers cannot access external networks despite the ABRs and backbone functioning properly. This strongly indicates that the area is configured as a totally stubbed area. Engineers must ensure the ABR is correctly injecting a default route into the area. Proper configuration guarantees predictable routing, efficient operation, and optimized LSDB usage. Monitoring ABR behavior, verifying default route propagation, and confirming area types are crucial for stable OSPF operation and external connectivity.

Filtering Type-3 LSAs via an ACL affects inter-area route propagation but does not prevent external routes if a default route exists. Since the problem involves external connectivity, the area type configuration is the primary factor.

If Area 0 were down, inter-area routing would fail across the network, not just the branch. Because only the branch area is affected, the backbone is operational, confirming that the area type is the root cause.

OSPF process IDs are locally significant and do not affect inter-area LSA propagation. Mismatched process IDs may trigger local warnings, but do not prevent routers from receiving default routes if area types are correctly configured.

The root cause is the branch area being a totally stubbed area. Ensuring that the ABR injects a default route allows routers to reach external networks. Engineers must verify area type, ABR configuration, and default route propagation. Totally stub areas reduce LSDB size, conserve resources, and maintain network stability while providing essential connectivity to external networks. Proper planning, monitoring, and verification of stub areas ensure stable OSPF operation, predictable convergence, and reliable external reachability. Correct deployment prevents routing issues, maintains efficiency, and supports enterprise and service provider requirements. Understanding totally stub areas and ABR behavior is essential for designing scalable OSPF networks. Proper configuration ensures stable routing, minimal overhead, and efficient propagation of external routes through default route injection.

Question 206:

A network engineer configures BGP multipath in a multi-homed network. Multiple paths appear in the BGP table, but traffic only uses one path. What is the most likely cause?

A) Paths differ in AS path, origin, or MED attributes
B) All BGP neighbors are configured as iBGP
C) One path has an unreachable next-hop
D) BGP route dampening is enabled

Answer: A)

Explanation:

BGP multipath allows multiple equal-cost paths to be used simultaneously for forwarding traffic, enhancing redundancy and load balancing. Multipath operation requires that candidate paths have identical attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP follows a strict deterministic path selection process to prevent routing loops, so any differences in these attributes prevent multipath forwarding even if multiple paths are visible in the BGP table. Verification involves using “show bgp ipv4 unicast” to inspect AS path, origin type, MED, and local preference values of all candidate paths. Proper alignment of attributes allows traffic to be distributed across multiple paths, optimizing bandwidth utilization and improving redundancy. Correct configuration ensures predictable traffic distribution, prevents congestion on a single path, and enhances overall network performance.

All neighbors being in iBGP does not prevent multipath. Multipath works with both iBGP and eBGP neighbors as long as attribute equality requirements are satisfied. Neighbor type does not override the requirement for identical attributes.

An unreachable next-hop prevents a specific path from being installed in the routing table. Since multiple paths appear in the BGP table, next-hop reachability is not the issue. Verification can be performed using “show ip route” and ping tests.

BGP route dampening suppresses unstable routes temporarily but does not prevent stable paths from being used for forwarding. Multipath works as long as paths meet attribute equality requirements.

The root cause is differences in key attributes among candidate paths. Differences in AS path, origin type, or MED prevent multipath forwarding. Engineers must align these attributes to enable multiple paths for traffic distribution. Correct attribute alignment enhances redundancy, ensures balanced traffic, and optimizes bandwidth usage. Misalignment may result in underutilization of available paths, congestion on the active path, and reduced network resilience. Understanding BGP multipath requirements is critical in multi-homed network design. Continuous verification of attributes and monitoring traffic flow ensures proper multipath operation. Proper configuration allows predictable traffic distribution, high availability, and efficient bandwidth usage, supporting stable enterprise network performance. Correct multipath deployment improves redundancy, prevents bottlenecks, and ensures consistent traffic distribution across the network.

Question 207:

A network engineer deploys MPLS LDP in a service provider network. Some routers fail to establish LDP sessions with neighbors. What is the most likely cause?

A) Missing IGP adjacency between routers
B) LDP transport addresses are mismatched
C) MPLS is not enabled globally
D) LDP hello timers are too long

Answer: A)

Explanation:

MPLS LDP depends on the underlying IGP for IP connectivity between routers in order to establish neighbor relationships and exchange label mapping information. LDP uses TCP port 646 to send hello messages and label updates. Without IGP adjacency, routers cannot communicate with neighbors, preventing LDP session formation and blocking the creation of label-switched paths required for MPLS traffic forwarding. LDP cannot operate independently; connectivity through the IGP is essential. Verification involves checking the IGP routing table, neighbor adjacency status using “show ip ospf neighbor” or “show ip eigrp neighbors,” and performing connectivity tests with ping or traceroute. Proper IGP operation allows LDP messages to reach neighbors, enabling session formation and label distribution. Engineers must ensure interface IP addressing, full IGP convergence, and MPLS configuration to maintain stable LDP operation. Proper IGP configuration ensures predictable label distribution, stable traffic forwarding, and efficient network performance.

Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly, but most deployments use loopback or interface addresses, making this less common.

LDP hello timers define the frequency of hello messages, but do not prevent neighbor formation. Longer timers may delay session establishment, but do not block LDP if IGP connectivity exists.

The root cause is missing IGP adjacency. Proper IGP configuration ensures that LDP messages reach neighbors, enabling label-switched path formation and MPLS forwarding. Engineers should monitor IGP adjacency, interface configuration, and LDP session status to maintain stable MPLS networks. Understanding LDP’s dependency on the IGP is critical for network design and troubleshooting. Correct planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Stable IGP adjacencies provide the foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.

Question 208:

A network engineer deploys OSPF in a multi-area network. Some branch routers are unable to reach external networks redistributed from BGP. What is the most likely cause?

A) The branch area is configured as a totally stubbed area
B) Type-3 LSAs are filtered by an ACL
C) OSPF process IDs are mismatched
D) Area 0 is down

Answer: A)

Explanation:

OSPF area types define the propagation of routing information and directly impact how routers learn about networks outside the OSPF domain. A totally stubbed area is designed to reduce the size of the LSDB and minimize routing table entries, which is particularly useful for remote branch routers with limited processing resources. In a totally stub area, Type-5 LSAs, which carry external routes from protocols like BGP, are blocked. As a result, routers in a totally stub area cannot learn external routes directly. Instead, the ABR injects a default route into the area to allow routers to reach external networks. If the default route is missing or misconfigured, branch routers will fail to reach external destinations. Verification can be performed using “show ip ospf” to confirm area type and “show ip route” to ensure the presence of a default route. Proper ABR configuration guarantees that remote routers have external connectivity while maintaining a minimal LSDB, conserving memory and CPU resources on branch routers.

In this scenario, branch routers cannot reach external networks even though the ABRs and backbone are operational. This strongly indicates that the area is configured as a totally stubbed area. Engineers should ensure that the ABR correctly injects a default route into the area. Correct configuration ensures predictable routing, optimized network performance, and reduced LSDB size. Monitoring ABR behavior, verifying default route propagation, and confirming area types are essential for stable OSPF operation and external reachability.

Filtering Type-3 LSAs with an ACL only impacts inter-area route propagation, not external routes, if a default route exists. The inability to reach external network points to a real type configuration rather than LSA filtering.

If Area 0 were down, inter-area routing would fail throughout the network, not just in the branch area. Since only the branch is affected, the backbone remains functional, confirming that the area type is the cause.

OSPF process IDs are locally significant and do not affect inter-area LSA propagation. Mismatched process IDs may trigger warnings, but do not prevent routers from receiving the default route if area types are correct.

The root cause is the branch area being a totally stubbed area. Ensuring that the ABR injects a default route enables routers in the area to reach external networks. Engineers should verify area type, ABR configuration, and default route propagation. Totally stub areas reduce LSDB size, conserve router resources, and maintain stability while still providing connectivity to external networks. Proper planning, monitoring, and verification of stub areas ensure predictable convergence, stable OSPF operation, and reliable external reachability. Correct deployment prevents routing issues, optimizes resource usage, and supports enterprise or service provider requirements. Understanding the behavior of totally stubbed areas and ABRs is critical for designing scalable OSPF networks. Proper configuration maintains stable routing, minimizes overhead, and ensures efficient propagation of external routes through default route injection.

Question 209:

A network engineer configures BGP multipath in a multi-homed network. Multiple paths appear in the BGP table, but only one path is actively used for traffic forwarding. What is the most likely cause?

A) Paths differ in AS path, origin, or MED attributes
B) All BGP neighbors are configured as iBGP
C) One path has an unreachable next-hop
D) BGP route dampening is enabled

Answer: A)

Explanation:

BGP multipath allows multiple equal-cost paths to be used simultaneously for forwarding traffic, providing redundancy and load balancing. Multipath operation requires candidate paths to be identical in attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP uses a deterministic path selection process to prevent routing loops. If there are differences in any of these attributes, only a single path is selected for forwarding traffic, even if multiple paths are visible in the BGP table. Verification involves using “show bgp ipv4 unicast” to inspect AS path, origin type, MED, and local preference of candidate paths. Proper alignment of these attributes allows traffic to be distributed across multiple paths, optimizing bandwidth utilization, improving redundancy, and ensuring high availability. Correct configuration guarantees predictable traffic distribution, prevents congestion on a single path, and enhances overall network performance.

All neighbors being in IBGP does not prevent multipath. Multipath can work with both iBGP and eBGP neighbors, provided attribute equality requirements are met. Neighbor type does not override the requirement for identical attributes.

An unreachable next-hop prevents a path from being installed in the routing table. Since multiple paths appear in the BGP table, next-hop reachability is not the issue. Verification can be done using “show ip route” and ping tests.

BGP route dampening suppresses unstable routes temporarily but does not prevent stable paths from being used. Multipath operation continues as long as candidate paths meet equality requirements.

The root cause is differences in critical attributes among candidate paths. Differences in AS path, origin type, or MED prevent multipath forwarding. Engineers must align these attributes to enable multiple paths to carry traffic. Proper attribute alignment improves redundancy, ensures balanced traffic distribution, and optimizes bandwidth usage. Misalignment can result in underutilization of available paths, congestion on the active path, and reduced network resilience. Understanding BGP multipath requirements is essential for multi-homed network design. Continuous verification of attributes and monitoring traffic flow ensures proper multipath operation. A correct configuration provides predictable traffic distribution, high availability, and efficient bandwidth usage, supporting stable enterprise network performance. Proper multipath BGP deployment improves redundancy, prevents bottlenecks, and ensures consistent traffic distribution across the network.

Question 210:

A network engineer deploys MPLS LDP in a service provider network. Some routers fail to establish LDP sessions with neighbors. What is the most likely cause?

A) Missing IGP adjacency between routers
B) LDP transport addresses are mismatched
C) MPLS is not enabled globally
D) LDP hello timers are too long

Answer: A)

Explanation:

MPLS LDP depends on the underlying IGP to provide IP connectivity between routers in order to establish neighbor relationships and exchange label mapping information. LDP uses TCP port 646 to send hello messages and label updates. Without IGP adjacency, routers cannot communicate with neighbors, preventing LDP session formation and blocking the creation of label-switched paths required for MPLS traffic forwarding. LDP cannot operate independently; connectivity through the IGP is essential. Verification involves checking the IGP routing table, neighbor adjacency status with “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity with ping or traceroute. Proper IGP operation ensures that LDP messages can reach neighbors, enabling session formation and label distribution. Engineers must ensure interface IP addressing, full IGP convergence, and proper MPLS configuration. Proper IGP configuration provides a foundation for predictable label distribution, stable traffic forwarding, and efficient network performance.

Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly, but default LDP deployments use loopback or interface addresses, making this scenario less common.

LDP hello timers define how frequently hello messages are sent, but do not prevent neighbor formation. Longer timers may delay session establishment, but do not block LDP if IGP connectivity exists.

Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 14 Q196-210

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