Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 13 Q181-195
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Question 181:
A network engineer deploys OSPF in a multi-area network. Some routers in a remote branch cannot 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 access list
C) OSPF process IDs are mismatched
D) The backbone area is configured as NSSA
Answer: A)
Explanation:
OSPF area types play a critical role in determining route propagation and the size of the routing table. A totally stubbed area is designed to minimize the size of the routing table in branch or remote areas. In a totally stub area, Type-5 LSAs, which carry external routes redistributed from other protocols like BGP, are blocked from entering the area. Instead, the ABR injects a default route to allow connectivity to external destinations. If the default route is missing or misconfigured, routers in the totally stub area cannot reach networks external to OSPF. Verification involves checking the area type using “show ip ospf” and examining the routing table using “show ip route” to ensure a default route is present. Proper configuration of the ABR ensures that external connectivity is maintained while keeping the LSDB small, improving efficiency and performance in resource-constrained branch routers.
In this scenario, the inability of branch routers to reach external networks indicates that the area is configured as a totally stubbed area. Engineers must ensure that the ABR advertises a default route into the area to allow connectivity. Proper configuration provides predictable routing, efficient network operation, and reduced LSDB complexity. Monitoring ABR behavior, verifying default route injection, and confirming area types are critical steps in maintaining OSPF stability and external reachability.
Filtering Type-3 LSAs with an access list could prevent inter-area routing, but this would only block inter-area routes and not necessarily block external routes if a default route exists. The specific issue of external route inaccessibility points to the area type rather than LSA filtering.
OSPF process IDs are locally significant, and mismatched process IDs do not prevent LSA propagation across correctly configured areas. Adjacency formation and LSA exchange rely on area types, network statements, and interface configurations rather than process ID alignment.
If the backbone area were configured as NSSA, it would affect route redistribution and Type-7 LSA behavior, but in this scenario, the ABRs and backbone are operational. The problem is isolated to the remote branch area, confirming that the area type is the root cause.
The root cause is the branch area being a totally stubbed area. Ensuring the ABR advertises a default route allows routers in the area to reach external destinations. Engineers should verify area type configuration, ABR behavior, and default route propagation. Proper configuration supports minimal LSDB size, efficient routing, and stable network operation. Totally stubbed areas are essential for reducing routing complexity in remote branches while maintaining connectivity to external networks. Correct configuration ensures predictable convergence, reduces CPU and memory usage, and allows branch routers to access external resources efficiently. Understanding area types and ABR functionality is critical in multi-area OSPF deployments. Proper planning, verification, and monitoring of area types, default routes, and ABR configuration ensure reliable external connectivity and optimized OSPF performance.
Question 182:
A network engineer configures BGP multipath in a multi-homed enterprise network. Multiple paths are present in the BGP table, but only a single path is actively used for 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 paths to be used for load balancing and redundancy. To forward traffic along multiple paths, BGP requires candidate paths to be equal in all relevant attributes, including AS path, origin type, MED, local preference, and next-hop reachability. The BGP path selection process is deterministic to avoid loops, so any attribute mismatch prevents multipath forwarding even if multiple paths appear in the BGP table. Verification can be done using “show bgp ipv4 unicast” to compare attributes of candidate paths. Ensuring attribute equality enables traffic to be distributed across multiple paths, optimizing bandwidth utilization and improving redundancy. Proper configuration of multipath forwarding provides predictable traffic distribution and prevents congestion on a single path.
All neighbors being in iBGP does not prevent multipath. Multipath functionality is supported with iBGP-learned routes as long as attributes align. iBGP or eBGP deployment affects route propagation but does not override the attribute equality requirement for multipath forwarding.
An unreachable next-hop prevents a specific path from being installed in the routing table. Since multiple paths are present, reachability is not the issue. Engineers can verify next-hop reachability using “show ip route” and ping tests.
BGP route dampening temporarily suppresses unstable routes. It does not prevent stable paths from being used for traffic forwarding. Dampening only affects routes considered unstable due to frequent withdrawals and re-advertisements.
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 for traffic. Correct attribute alignment enhances redundancy, load balancing, and network performance. Misalignment may lead to underutilization of available paths and congestion on the active path. Understanding BGP multipath requirements is essential for multi-homed network design. Verification and continuous monitoring of attributes and traffic flow ensure proper multipath operation. Proper configuration allows predictable traffic distribution, high availability, and optimized bandwidth usage. Correctly configured multipath BGP enhances resilience, prevents bottlenecks, and ensures stable enterprise network operation.
Question 183:
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 reachability between routers for neighbor discovery and label distribution. LDP uses TCP port 646 to exchange hello messages and label mappings. Without IGP adjacency, routers cannot communicate, preventing LDP session establishment and the creation of label-switched paths required for MPLS forwarding. LDP does not operate independently of the IGP; IP connectivity is essential. Verification includes checking IGP routing tables, adjacency status using “show ip ospf neighbor” or “show ip eigrp neighbors,” and connectivity tests using ping or traceroute. Proper IGP configuration ensures that LDP messages reach neighbors and sessions are established. Engineers must ensure interface IP addressing, full IGP convergence, and proper MPLS configuration.
Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly. However, in typical deployments, LDP uses 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, though LSPs cannot forward traffic until MPLS is active.
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 operation allows LDP messages to 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 the dependency of LDP on 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 form the foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.
Question 184:
A network engineer deploys EIGRP in a multi-area network. Some routers fail to establish neighbor relationships on the same subnet. 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 relies on the concept of an autonomous system (AS) number to identify which routers belong to the same routing domain. The AS number ensures that routers only establish neighbor relationships with devices within the same domain. If routers on the same subnet have mismatched AS numbers, they will not recognize each other as valid neighbors, preventing adjacency formation. This leads to incomplete route propagation, inconsistent routing tables, and delayed convergence. Engineers can verify neighbor relationships using “show ip eigrp neighbors” and check the AS numbers using “show running-config.” Ensuring AS numbers are consistent across all routers is critical for EIGRP stability and reliable routing.
In this scenario, routers on the same subnet fail to establish neighbor relationships, strongly indicating an AS number mismatch. Correcting the AS numbers allows routers to recognize each other, form adjacencies, and exchange routing information. Network engineers must also ensure that interface IP addresses are correctly configured and that no passive interfaces inadvertently block hello packets. Proper AS alignment is especially crucial in large enterprise networks with multiple EIGRP domains, as mismatches can cause network segmentation and operational instability.
Enabling a passive interface prevents hello packets from being sent on that interface, which can block neighbor formation. While this could affect a single interface, widespread neighbor failures across multiple routers indicate a more systemic cause, such as an AS number mismatch.
Different K values influence the EIGRP metric calculation but do not prevent neighbor formation. Mismatched K values may affect route selection, but will not stop routers from establishing EIGRP adjacencies.
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 block neighbor formation on the same subnet.
The root cause is mismatched AS numbers. Ensuring that all routers share the same AS number is essential for neighbor formation, 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, partial network segmentation, and routing inconsistencies. Correct AS configuration supports predictable route propagation, stable convergence, and robust connectivity. Proper deployment across multi-area networks requires careful planning, verification, and monitoring of AS numbers to prevent disruptions. Ensuring AS number consistency allows routers to form reliable adjacencies, maintain complete routing tables, and ensure predictable network behavior.
Question 185:
A network engineer deploys OSPFv3 in an IPv6 network. Some routers fail to form neighbor adjacencies. 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 uses link-local addresses as the source and destination for hello packets on each interface. Link-local addresses are mandatory for adjacency formation and neighbor discovery in IPv6 networks. Without valid link-local addresses, routers cannot communicate directly on the link, preventing the formation of neighbor adjacencies, LSDB synchronization, and proper LSA exchange. Verification involves using commands such as “show ipv6 interface brief” to check link-local address configuration and “show ipv6 ospf neighbor” to inspect adjacency status. Correctly configured link-local addresses ensure that routers can establish adjacencies, exchange LSAs, and converge appropriately. Network engineers must also ensure unique router IDs are configured to prevent LSDB conflicts.
In this scenario, routers fail to form adjacencies, strongly indicating missing or misconfigured link-local addresses. Proper configuration allows hello packets to be exchanged, adjacencies to form, and LSDB synchronization to occur. Ensuring all participating interfaces have valid link-local addresses is critical for stable OSPFv3 operation, predictable convergence, and reliable route propagation.
Duplicate router IDs can cause LSDB conflicts, potentially impacting database synchronization, but hello packets can still be exchanged if link-local addresses exist. Router ID conflicts generate warning messages but are less likely to prevent adjacency formation entirely.
Area authentication mismatches affect LSA acceptance but do not prevent adjacency formation. Hello packets are still sent and received, though authenticated LSAs may be rejected if authentication is inconsistent.
IPv6 unicast routing being disabled globally prevents forwarding of IPv6 packets, but does not block the exchange of hello packets via link-local addresses.
The root cause is missing link-local addresses. Ensuring proper link-local address configuration on all participating interfaces is essential for adjacency formation, LSA exchange, and stable OSPFv3 operation. Network engineers must verify link-local addresses, enable OSPFv3 on all interfaces, and ensure unique router IDs are configured. Understanding the role of link-local addresses is critical for deploying IPv6 networks. Correct configuration ensures seamless adjacency formation, uninterrupted LSA flooding, and predictable network convergence. Proper link-local addressing maintains stable routing, reliable connectivity, and optimal IPv6 network performance. Continuous monitoring and verification are required to maintain OSPFv3 stability and efficient operation.
Question 186:
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 reachability between routers to establish neighbor relationships and exchange label mapping information. LDP uses TCP port 646 to communicate hello messages and label information. Without IGP adjacency, routers cannot communicate with neighbors, preventing LDP session formation and the creation of label-switched paths required for MPLS forwarding. LDP cannot operate independently of the IGP, making the connectivity provided by the IGP critical. 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. Ensuring proper IGP operation allows LDP messages to reach neighbors and enables session establishment. Engineers must confirm IP addressing and full IGP convergence to maintain reliable MPLS forwarding. Proper IGP configuration forms the foundation for LDP operation, ensuring predictable label distribution and traffic forwarding.
Mismatched LDP transport addresses may prevent session establishment if manually configured incorrectly, but default LDP implementations typically use loopback or interface addresses, making this scenario less likely.
MPLS not being enabled globally prevents label forwarding but does not stop LDP from attempting neighbor discovery. Discovery will still occur, though LSPs cannot forward traffic until MPLS is active.
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. Ensuring proper IGP configuration allows LDP messages to 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 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 187:
A network engineer deploys OSPF in a multi-area network. Some branch routers cannot 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 access list
C) Area 0 is down
D) OSPF process IDs are mismatched
Answer: A)
Explanation:
OSPF uses area types to control the flow of LSAs and determine which routes are propagated within each area. A totally stubbed area is designed to minimize the size of the LSDB and routing table for branch or remote areas. In a totally stub area, Type-5 LSAs, which carry external routes such as those redistributed from BGP, are blocked from entering the area. This prevents routers within the area from directly learning external routes. Instead, the ABR injects a default route into the area, allowing routers to reach external networks without maintaining detailed external routing information. If the default route is missing or misconfigured, routers in the totally stub area cannot reach external networks. Verification involves using “show ip ospf” to confirm area type and “show ip route” to check for the presence of a default route. Proper ABR configuration ensures that the default route is advertised into the stub area, maintaining external connectivity while reducing the LSDB size and conserving router resources.
In this scenario, branch routers are unable to reach external networks despite the backbone and ABRs functioning correctly. This strongly indicates that the area is configured as a totally stubbed area. Engineers must verify ABR configuration to ensure default routes are properly injected and that stub area behavior aligns with design goals. Correct configuration of totally stubbed areas ensures predictable routing behavior, efficient LSDB usage, and stable convergence. It also helps minimize memory and CPU usage on resource-constrained branch routers.
Filtering Type-3 LSAs with an access list affects inter-area route propagation but does not prevent external routes if a default route is present. The specific issue of inability to reach external networks points to an area-type configuration rather than LSA filtering.
If Area 0 were down, it would affect all inter-area routing, not just branch routers. Since only the branch area is affected, the backbone is operational, confirming the issue lies with the area configuration.
OSPF process IDs are locally significant and do not affect LSA propagation across areas. Mismatched process IDs may cause configuration inconsistencies locally, but will not prevent routers from receiving default routes if area types are correctly configured.
The root cause is the branch area being configured as a totally stubbed area. Ensuring the ABR injects a default route allows routers in the area to reach external networks redistributed from BGP. Engineers must verify area types, ABR behavior, and default route propagation. Proper configuration optimizes LSDB size, ensures stable convergence, and maintains external connectivity. Totally stub areas are particularly useful for large networks with many remote branches, reducing routing table complexity while still providing necessary reachability to external networks. Correct planning, configuration, and monitoring of stub areas ensures stable and efficient OSPF operation. Proper deployment prevents connectivity issues, maintains predictable routing behavior, and ensures efficient use of network resources, supporting both enterprise and service provider requirements.
Question 188:
A network engineer deploys BGP multipath in a multi-homed network. Multiple paths exist in the BGP table, but traffic is only forwarded through 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 traffic forwarding to enhance load balancing and redundancy. Multipath requires candidate paths to be equal in key attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP uses a strict deterministic path selection process to prevent loops and maintain stability. When multiple paths appear in the BGP table, but traffic only traverses a single path, it usually indicates that candidate paths differ in one or more critical attributes such as AS path, origin type, or MED. Verification involves using commands like “show bgp ipv4 unicast” to inspect the attributes of all candidate paths. Aligning these attributes enables multipath forwarding, distributing traffic across multiple paths, and optimizing bandwidth utilization. Proper multipath configuration ensures redundancy, prevents congestion on a single path, and enhances network performance.
All neighbors beginning with IBGP do not prevent multipath. Multipath is supported with both iBGP and eBGP neighbors, provided the attribute equality conditions are met. The type of BGP neighbor does not override the requirement for identical attributes among candidate paths.
An unreachable next-hop prevents a specific path from being installed in the routing table. Since multiple paths are present in the BGP table, reachability is not the issue. Engineers can verify next-hop reachability using commands such as “show ip route” and by performing ping tests.
BGP route dampening temporarily suppresses unstable routes to maintain routing stability. It does not prevent stable paths from being used for traffic forwarding. Multipath will work as long as paths are stable and meet equality requirements.
The root cause is unequal attributes among candidate paths. Differences in AS path, origin, or MED prevent multipath forwarding even if multiple paths exist in the table. Engineers must align attributes to enable simultaneous use of multiple paths. Correct attribute alignment ensures redundancy, balanced traffic distribution, and optimal utilization of available bandwidth. Misalignment can result in traffic congestion, underutilization of available paths, and reduced network resilience. Understanding BGP multipath requirements is essential for designing multi-homed networks. Proper verification and monitoring of attributes and traffic flow ensure multipath operation functions as intended. Correct configuration provides predictable traffic distribution, high availability, and efficient bandwidth usage. Proper multipath BGP deployment enhances network performance, prevents bottlenecks, and ensures stable operation in enterprise and service provider environments. Continuous monitoring and verification are essential for maintaining multipath functionality and optimizing network efficiency.
Question 189:
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 reachability between routers for establishing neighbor relationships and exchanging label mapping information. LDP uses TCP port 646 to send hello messages and label mappings. Without IGP adjacency, routers cannot communicate with neighbors, preventing LDP session establishment and blocking the creation of label-switched paths needed for MPLS forwarding. LDP cannot operate independently of the IGP; connectivity provided by the IGP is essential. Verification involves checking the IGP routing table, 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 ensures LDP messages can reach neighbors, enabling session formation and label distribution. Engineers must confirm IP addressing, interface configuration, and full IGP convergence to maintain reliable MPLS forwarding. Proper IGP configuration is foundational for LDP operation, ensuring predictable label distribution, stable traffic forwarding, and efficient network performance.
Mismatched LDP transport addresses may prevent session establishment if manually configured incorrectly. However, in most deployments, LDP uses 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 will still occur, though LSPs cannot forward traffic until MPLS is active.
LDP hello timers define the interval for sending hello messages, but do not prevent neighbor formation. Longer timers may delay session establishment, but will not block LDP if IGP connectivity exists.
The root cause is missing IGP adjacency. Ensuring proper IGP operation allows LDP messages to 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 traffic forwarding. Stable IGP adjacencies provide the foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.
Question 190:
A network engineer configures 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 branch area is configured as a totally stubbed area
B) Type-3 LSAs are filtered by an ACL
C) Area 0 is down
D) OSPF process IDs are mismatched
Answer: A)
Explanation:
OSPF area types control the propagation of LSAs and routing information within a network. A totally stubbed area is specifically designed to minimize the LSDB size and reduce routing table complexity for remote branches. In a totally stub area, Type-5 LSAs, which carry external routes such as those redistributed from BGP, are not allowed into the area. This prevents routers in the totally stub area from directly learning external routes. Instead, the ABR injects a default route into the area so that routers can reach external networks without maintaining detailed external routing information. If this default route is missing or misconfigured, branch routers will not be able to reach external destinations. Verification involves checking area types with “show ip ospf” and examining the routing table with “show ip route” to ensure a default route is present. Proper ABR configuration ensures the default route is injected into the totally stub area, maintaining external connectivity while keeping the LSDB size minimal, which is critical for branch routers with limited resources.
In this scenario, the remote branch routers are unable to access external networks despite the ABRs and backbone functioning correctly. This indicates that the branch area is configured as a totally stubbed area. Engineers must ensure the ABR is correctly injecting a default route into the stub area. Proper configuration guarantees predictable routing, efficient network operation, and optimized LSDB utilization. Monitoring ABR behavior, verifying default route propagation, and confirming area types are essential for stable OSPF operation and external reachability.
Filtering Type-3 LSAs via an ACL affects inter-area route propagation but does not prevent external routes if a default route is present. Since the problem involves a lack of external connectivity, the area type configuration is the primary factor.
If Area 0 were down, inter-area routing for all areas would fail, not just the remote branch area. Because only the branch is affected, the backbone is operational, confirming the area type is the root cause.
OSPF process IDs are locally significant and do not affect LSA propagation between areas. Mismatched process IDs may cause local 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 configured as a totally stubbed area. Ensuring the ABR injects a default route enables routers to reach external networks. Engineers must verify area types, ABR configuration, and default route propagation. Totally stub areas optimize LSDB size, reduce CPU and memory usage, and maintain network stability while allowing connectivity to external networks. Proper planning, monitoring, and verification of stub areas ensure stable OSPF operation, predictable convergence, and reliable external connectivity. Correct deployment prevents routing issues, maintains network efficiency, and supports both enterprise and service provider requirements.
Question 191:
A network engineer deploys BGP multipath in a multi-homed enterprise network. Multiple paths appear in the BGP table, but traffic only uses a single 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 for load balancing and redundancy. For multipath forwarding, all candidate paths must have identical attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP follows a deterministic path selection process to avoid loops, so any mismatch in attributes prevents multiple paths from being used simultaneously. When multiple paths appear in the BGP table but only a single path carries traffic, it indicates attribute differences among candidate paths. Verification can be done using “show bgp ipv4 unicast” to compare AS path, origin, MED, and local preference values. Aligning attributes allows multipath forwarding, distributing traffic across all eligible paths, optimizing bandwidth utilization, and improving redundancy. Proper configuration ensures predictable traffic distribution and prevents congestion on a single path.
All neighbors being in iBGP does not prevent multipath. Multipath functionality works with iBGP-learned routes as long as the attribute equality requirements are met. The neighbor type does not override attribute equality rules.
An unreachable next-hop prevents a path from being installed in the routing table. Since multiple paths appear in the BGP table, reachability is not the issue. Engineers can verify next-hop reachability using commands like “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 the paths meet equality requirements.
The root cause is unequal attributes among candidate paths. Differences in AS path, origin type, or MED prevent multipath forwarding. Engineers must ensure attributes are aligned to enable the simultaneous use of multiple paths. Correct attribute alignment improves redundancy, optimizes bandwidth, and provides balanced traffic distribution. Misalignment can cause underutilization of available paths and congestion on the active path. Understanding multipath requirements is essential for multi-homed network design. Verification and continuous monitoring of attributes and traffic flow ensure that multipath operation functions correctly. Proper configuration ensures predictable traffic distribution, high availability, and efficient bandwidth usage, supporting enterprise network performance and reliability.
Question 192:
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 reachability between routers 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 does not operate independently; IP connectivity through the IGP is critical. Verification involves checking the IGP routing table, neighbor adjacency status with commands like “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity using ping or traceroute. Ensuring proper IGP operation allows LDP messages to reach neighbors and enables session formation. Engineers must confirm IP addressing, interface configuration, and full IGP convergence to maintain reliable MPLS forwarding. 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. However, most LDP deployments use loopback or interface addresses, making this scenario less likely.
MPLS not being globally enabled prevents label forwarding but does not stop LDP from attempting neighbor discovery. Discovery occurs even when MPLS is inactive, though LSPs cannot forward traffic until MPLS is active.
LDP hello timers define the interval for sending 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 allows LDP messages to 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 essential 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 193:
A network engineer deploys EIGRP in a multi-area network. Some routers fail to establish neighbor relationships despite being on the same subnet. 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 relies on the autonomous system (AS) number to define which routers belong to the same routing domain. Routers must have identical AS numbers to establish neighbor relationships and exchange routing updates. If the AS numbers differ, routers on the same subnet will not recognize each other as valid neighbors, preventing adjacency formation. This can lead to incomplete route propagation, inconsistent routing tables, and delayed network convergence. Verification involves using “show ip eigrp neighbors” to view the neighbor table and “show running-config” to confirm AS numbers. Ensuring consistent AS numbers across all routers is crucial for EIGRP stability and proper route exchange.
In this scenario, routers on the same subnet fail to form neighbors, suggesting an AS number mismatch. Correcting the AS numbers allows routers to recognize each other, form adjacencies, and propagate routing information. Network engineers must also verify interface IP configurations, ensure no passive interfaces are unintentionally enabled, and confirm that neighbor relationships are correctly formed. Proper AS alignment is especially important in large networks with multiple EIGRP domains, as mismatches can segment the network and disrupt routing.
Passive interfaces prevent hello packets from being sent, which blocks neighbor formation on that interface. While this could affect a single interface, widespread neighbor failures across multiple routers indicate a systemic issue, such as an AS number mismatch rather than a passive interface configuration.
Different K values influence the metric calculation in EIGRP, but do not prevent neighbor formation. Mismatched K values affect route selection, potentially causing suboptimal routing, but they will not block adjacency establishment between routers.
Split-horizon prevents a router from advertising a route back out the interface on which it was learned. This affects route propagation in certain topologies but does not prevent routers on the same subnet from establishing neighbor relationships.
The root cause is mismatched AS numbers. Ensuring all routers use the same AS number is essential for neighbor formation, 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 partial network segmentation. Correct AS configuration supports predictable route propagation, stable convergence, and reliable connectivity. Proper deployment across multi-area networks requires careful planning, verification, and monitoring of AS numbers to avoid disruptions. 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 optimized network performance. Continuous verification and monitoring of AS numbers prevent operational issues and support stable multi-area EIGRP networks.
Question 194:
A network engineer deploys OSPFv3 in an IPv6 network. Some routers fail to form neighbor adjacencies. 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 to form neighbor adjacencies in IPv6 networks. Link-local addresses serve as the source and destination for OSPFv3 hello packets, enabling 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 the presence of link-local addresses and “show ipv6 ospf neighbor” to check adjacency status. Proper configuration of link-local addresses ensures that routers can establish adjacencies, exchange LSAs, and converge the network effectively. Network engineers must also ensure unique router IDs are configured to avoid LSDB conflicts.
In this scenario, routers fail to form adjacencies, indicating missing or misconfigured link-local addresses. Correctly configured link-local addresses allow hello packets to be exchanged, adjacencies to form, and LSDBs to synchronize. Ensuring all participating interfaces have valid link-local addresses is critical for OSPFv3 operation, predictable convergence, and stable route propagation. Proper configuration supports efficient IPv6 deployment and network reliability.
Duplicate router IDs can cause LSDB conflicts, but do not prevent hello packets from being sent. While conflicts generate warnings and may impact LSA synchronization, they are unlikely to block adjacency formation entirely if link-local addresses are correctly configured.
Area authentication mismatches affect LSA acceptance but do not prevent adjacency formation. Routers will still exchange hello packets, though LSAs may be rejected if authentication does not match.
IPv6 unicast routing being disabled globally prevents forwarding of IPv6 packets, but does not block hello packet exchange over link-local addresses.
The root cause is missing link-local addresses. Ensuring proper link-local address configuration is essential for adjacency formation, LSA exchange, and stable OSPFv3 operation. Engineers should verify link-local addresses, enable OSPFv3 on all relevant interfaces, and confirm unique router IDs. Proper link-local addressing ensures seamless adjacency formation, uninterrupted LSA flooding, and predictable network convergence. Network engineers must monitor adjacency status and interface configuration to maintain stable OSPFv3 networks. Correct configuration guarantees stable routing, reliable connectivity, and optimal IPv6 network performance. Continuous verification and monitoring prevent operational issues and ensure predictable OSPFv3 behavior. Proper deployment allows efficient communication between all routers, stable LSDB synchronization, and reliable route propagation in IPv6 networks.
Question 195:
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 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 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 IGP routing tables, adjacency status using “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity with ping or traceroute. Proper IGP operation ensures that LDP messages reach neighbors, enabling session formation and label distribution. Engineers must ensure interface IP addressing, full IGP convergence, and proper MPLS configuration.
Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly, but default deployments typically use loopback or interface addresses, making this less common.
MPLS not being enabled globally prevents label forwarding but does not stop LDP from attempting neighbor discovery. Discovery will still occur, though label-switched paths cannot forward traffic until MPLS is active.
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.
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.