Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 12 Q166-180
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Question 166:
A network engineer configures 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 requires all routers within the same routing domain to share the same autonomous system (AS) number to form neighbor relationships and exchange routing information. The AS number is included in EIGRP hello packets and identifies the routing domain. If routers on the same subnet have mismatched AS numbers, they will not recognize each other as valid neighbors, preventing adjacency formation and route exchange. In a multi-area network, this can cause partial segmentation, inconsistent routing tables, and delayed convergence. Network engineers can verify neighbor relationships using commands like “show ip eigrp neighbors” and inspect AS numbers with “show running-config” to ensure consistency. Proper alignment of AS numbers ensures that all routers form neighbor relationships and participate in route propagation across the network.
In this scenario, routers fail to form neighbor relationships on the same subnet, strongly indicating AS number mismatches. Correcting the AS number ensures routers recognize each other and can exchange routes, maintaining network stability and proper convergence. Engineers must also verify network statements, interface IP addresses, and passive interface configurations to ensure that all interfaces participating in EIGRP are actively engaged. Documenting AS numbers and maintaining consistency across all routers is critical in large networks with multiple EIGRP domains to avoid inadvertent mismatches that disrupt routing.
Passive interfaces prevent hello packets from being sent on specific interfaces, which can inhibit neighbor formation. However, if multiple routers on the same subnet fail to form neighbors, a passive interface misconfiguration is less likely the cause than an AS mismatch, which affects the entire routing domain.
Different K values determine how EIGRP calculates metrics for route selection, but do not prevent neighbor formation. Routers with mismatched K values can still exchange routes, although route selection may differ due to metric calculations.
Split-horizon prevents routers from advertising routes back out the interface on which they were learned. This may affect routing in hub-and-spoke topology, but does not prevent neighbor formation or route exchange across routers on the same subnet.
The root cause is mismatched AS numbers. Ensuring all routers share the same AS number is crucial for establishing neighbor relationships, exchanging routes, and maintaining network stability. Proper verification of AS numbers, interface configurations, and adjacency status allows predictable route propagation and efficient network convergence. Understanding the importance of AS numbers in EIGRP prevents neighbor formation failures and partial network segmentation. Network engineers must confirm AS number alignment to achieve consistent routing, maintain robust connectivity, and optimize performance. Correct AS configuration supports reliable connectivity, reduces the risk of route loss, and ensures operational stability across the network. By ensuring AS consistency, enterprise networks maintain stable routing tables, efficient convergence, and predictable EIGRP behavior, which is essential for large-scale multi-area deployments. Proper AS number configuration supports robust EIGRP operation, predictable route propagation, and overall network reliability.
Question 167:
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 on all participating interfaces to form neighbor adjacencies. OSPFv3 uses link-local addresses as the source and destination of hello packets. Without link-local addresses, routers cannot identify neighbors, preventing adjacency formation, LSA exchange, and proper routing. Link-local addresses enable routers to communicate directly on the same link, independent of global IPv6 addresses. Proper link-local addressing is essential for stable neighbor formation, consistent route propagation, and predictable network convergence.
In this scenario, routers fail to form adjacencies, strongly suggesting that link-local addresses are missing or misconfigured. Verification involves using “show ipv6 interface brief” and “show ipv6 ospf neighbor” to inspect interface configurations and neighbor status. Correct configuration requires assigning link-local addresses and enabling OSPFv3 on all participating interfaces. Additionally, unique router IDs must be configured, serving as unique identifiers in the OSPFv3 LSDB. Proper router ID configuration prevents adjacency conflicts and ensures database consistency.
Duplicate router IDs can cause adjacency conflicts and LSDB inconsistencies, but hello packets can still be exchanged if link-local addresses are configured. Duplicate IDs generate warning messages and logs indicating conflicts that must be resolved.
Area authentication mismatches affect the acceptance of LSAs but do not prevent adjacency formation. Hello packets are still exchanged, and initial neighbor relationships can form, though authenticated LSAs may be rejected.
IPv6 unicast routing being disabled globally prevents forwarding of IPv6 traffic, but does not stop hello packets sent using link-local addresses. Full OSPFv3 operation, including route propagation and LSA flooding, requires IPv6 routing to be enabled.
The root cause is missing link-local addresses. Ensuring every interface participating in OSPFv3 has a valid link-local address is essential for adjacency formation, LSA exchange, and network convergence. Proper configuration guarantees stable OSPFv3 operation, predictable route propagation, and reliable connectivity. Network engineers must verify link-local addresses, enable OSPFv3, and assign unique router IDs to maintain operational stability. Understanding the role of link-local addresses in OSPFv3 is critical for deploying IPv6 networks successfully. Correct configuration allows seamless adjacency formation, uninterrupted LSA flooding, and predictable convergence, which are essential for efficient and reliable IPv6 network operation.
Question 168:
A network engineer deploys MPLS LDP in a service provider network. Some routers fail to form 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 for neighbor discovery and label distribution. LDP communicates over TCP port 646, using hello messages and label mapping exchanges. Without IGP adjacency, routers cannot discover neighbors or exchange LDP messages, preventing session formation and blocking label-switched path creation essential for MPLS forwarding. LDP cannot operate independently of the IGP; connectivity between routers is critical. Verification involves inspecting the IGP routing table, 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 establishment. Engineers must confirm interface IP addressing and full IGP convergence to maintain reliable MPLS operations. Proper IGP configuration provides a foundation for LDP operation, enabling predictable MPLS forwarding and label distribution.
Mismatched LDP transport addresses can prevent session formation if manually configured incorrectly, but default LDP deployments typically rely on loopback or primary interface addresses, making this less likely.
MPLS not being globally enabled prevents label forwarding but does not stop LDP from attempting neighbor discovery. LDP discovery still occurs even if MPLS is inactive, though labels cannot be used until MPLS is enabled.
LDP hello timers determine how frequently hello messages are sent, but do not prevent neighbor formation. Longer timers may delay session establishment, but will not block LDP operation if 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 adjacencies, interface configurations, and LDP session status to maintain stable MPLS networks. Understanding the dependency of LDP on IGP is critical for network design and operations. Proper planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Stable IGP adjacencies provide a foundation for LDP operation, enabling reliable traffic forwarding, optimal performance, and high availability in service provider networks.
Question 169:
A network engineer configures RSVP-TE tunnels in an MPLS network. Despite sufficient bandwidth being available, the tunnels fail to establish. What is the most likely cause?
A) CSPF cannot find a feasible path due to link attribute constraints
B) RSVP authentication is mismatched
C) RSVP soft-state refresh timers are too long
D) IGP metrics are misconfigured
Answer: A)
Explanation:
RSVP-TE enables traffic-engineered label-switched paths (LSPs) across MPLS networks, allowing explicit path selection and bandwidth reservation for specific traffic flows. The Constrained Shortest Path First (CSPF) algorithm calculates feasible paths based on multiple constraints, including available bandwidth, TE colors, administrative groups, and link attributes. Even when links indicate sufficient bandwidth, RSVP-TE tunnel setup may fail if CSPF cannot compute a feasible path that satisfies all configured constraints. Link attribute constraints may include mismatched administrative groups, conflicting TE colors, or improper bandwidth reservations along candidate paths. When CSPF fails to find a valid path, the RSVP-TE tunnel cannot be established, and traffic will not traverse the intended route. Verification involves inspecting CSPF calculations using commands such as “show mpls traffic-eng tunnels” or “show mpls traffic-eng topology” to ensure that all constraints along the path are satisfied. Proper configuration of TE link attributes, administrative groups, and bandwidth reservations is crucial to ensure tunnel establishment and reliable MPLS traffic engineering.
RSVP authentication mismatches affect signaling between routers. Authentication must be configured consistently to ensure that signaling messages are accepted. While authentication errors can prevent tunnel establishment, in scenarios where CSPF cannot compute a path, the issue is not authentication but path feasibility.
RSVP soft-state refresh timers maintain tunnel state by periodically refreshing resource reservations. Timers that are too long may delay the detection of failed tunnels or slow recovery, but they do not affect the initial path computation. Proper timer configuration ensures state consistency but does not prevent initial tunnel creation if CSPF can compute a valid path.
IGP metrics influence the shortest path calculation for basic routing, but CSPF evaluates TE constraints in addition to IGP metrics. Even with correctly configured IGP metrics, CSPF will reject paths that do not meet TE requirements. Therefore, misconfigured IGP metrics may impact path selection but will not prevent CSPF from evaluating feasibility if other constraints are satisfied.
The root cause in this scenario is CSPF being unable to compute a feasible path due to link attribute constraints. Network engineers must ensure consistency of TE colors, administrative groups, and bandwidth reservations across all links along the intended path. Properly configured and verified TE attributes allow CSPF to compute valid paths, enabling RSVP-TE tunnels to be established successfully. Engineers should monitor tunnel status, inspect CSPF logs, and validate link attributes using management tools to confirm path feasibility. Understanding the interaction between CSPF, RSVP-TE, and TE link attributes is critical for reliable MPLS traffic-engineered networks. Misconfigured constraints can prevent tunnel establishment, reduce redundancy, and degrade network performance. Correct planning, attribute alignment, and monitoring prevent failures and optimize MPLS utilization, ensuring predictable LSP behavior, efficient bandwidth usage, and high availability in enterprise and service provider networks.
Question 170:
A network engineer deploys BGP multipath in a multi-homed enterprise network. Multiple paths appear in the BGP table, but traffic is only forwarded through a single path. What is the most likely cause?
A) Paths are not equal 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 simultaneously for redundancy and load balancing. For multipath to function, candidate paths must be identical in key attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP uses a strict deterministic selection process to prevent routing loops and maintain stable network behavior. When multiple paths exist in the BGP table, but traffic flows only through a single path, it typically indicates that the paths differ in AS path, origin, or MED attributes. Verification involves inspecting the BGP table using commands such as “show bgp ipv4 unicast” to compare attributes among candidate paths. Ensuring equality across all attributes allows BGP multipath to forward traffic simultaneously along multiple paths, providing both redundancy and efficient bandwidth utilization.
Configuring all BGP neighbors as iBGP does not prevent multipath. Multipath can operate over iBGP-learned routes provided the equality requirements are met. The protocol treats iBGP and eBGP differently for path selection, but attribute equality remains the primary requirement for multipath.
An unreachable next-hop prevents a specific path from being installed in the routing table. Since multiple paths are visible in the BGP table, reachability is not the issue in this scenario. Verification of next-hop reachability using “show ip route” and ping confirms that all paths are valid.
BGP route dampening temporarily suppresses routes to improve stability. Dampening may prevent unstable paths from being used, but it does not affect stable, visible paths in the BGP table that meet equality criteria.
The root cause is unequal attributes among candidate paths. AS path, origin type, or MED differences prevent multipath forwarding, even though multiple paths exist in the table. Engineers must align these attributes to enable effective load balancing and redundancy. Proper monitoring and adjustment of BGP attributes ensure efficient traffic distribution, high availability, and optimized bandwidth usage. Misalignment of attributes can lead to underutilization of available paths, congestion on a single active path, and reduced redundancy. Understanding BGP’s strict equality requirements is essential for multi-homed network design. Correct multipath deployment allows predictable traffic distribution, efficient bandwidth use, and reliable redundancy. Engineers should continually verify BGP table attributes and adjust as needed to maintain optimal performance in enterprise networks. Proper configuration ensures multipath forwarding, predictable routing behavior, and effective network resilience in multi-homed environments.
Question 171:
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 for IP reachability between routers to establish neighbor relationships and exchange label information. LDP uses TCP port 646 to send hello messages and label mapping information. Without IGP adjacency, routers cannot communicate, preventing LDP sessions from forming. This stops the creation of label-switched paths, which are essential for MPLS traffic forwarding. LDP does not provide its own discovery mechanism independent of the IGP, so connectivity through the IGP is critical. Verification involves checking the IGP routing table, adjacency status with commands like “show ip ospf neighbor” or “show ip eigrp neighbors,” and connectivity tests using ping or traceroute. Ensuring that IGP adjacencies exist allows LDP messages to reach neighbors and sessions to be established. Engineers must confirm proper IP addressing on interfaces and full IGP convergence to support reliable MPLS operations. Proper IGP operation forms the foundation for LDP functionality, enabling predictable label distribution and traffic forwarding.
Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly, but 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. LDP discovery occurs even when MPLS is inactive, though label-switched paths 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 the ability to form LDP sessions if connectivity exists.
The root cause is missing IGP adjacency. Ensuring proper IGP configuration allows LDP messages to reach neighbors, enabling label-switched path creation and MPLS forwarding. Engineers should monitor IGP adjacency, interface configurations, and LDP session status to maintain stable MPLS operations. Understanding the dependency of LDP on IGP is critical for network design and troubleshooting. Proper planning, configuration, and verification prevent session failures, support predictable label distribution, and ensure efficient MPLS traffic forwarding. Stable IGP adjacencies provide a foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.
Question 172:
A network engineer configures OSPF in a multi-area network. Some routers in a remote area cannot reach external networks redistributed from BGP. What is the most likely cause?
A) The area is configured as a totally stubbed area
B) Type-3 LSAs are blocked by an ACL
C) Area 0 is down
D) OSPF process IDs are mismatched
Answer: A)
Explanation:
OSPF area types define the behavior of route propagation and LSA flooding. Totally stub areas are designed to reduce routing table size and LSDB complexity, particularly in remote branch areas with limited resources. In a totally stub area, Type-5 LSAs, which carry external routes such as those redistributed from BGP, are blocked. 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 destinations without maintaining full external routing information. Without the default route, routers cannot access external networks redistributed from BGP. Verification can be performed using commands like “show ip ospf” to check area types and “show ip route” to inspect the routing table for default routes. Proper ABR configuration ensures that default routes are injected, allowing branch routers to reach external networks while reducing routing overhead.
In this scenario, routers in the branch area cannot reach external networks, indicating that the area is configured as a totally stubbed area. Engineers must verify the ABR configuration to ensure the default route is correctly advertised. Proper configuration allows efficient routing, predictable convergence, and reduced LSDB complexity. Maintaining consistent area types and default route propagation is critical for network stability and external connectivity.
Blocking Type-3 LSAs via an ACL would restrict inter-area routes but would not account for the total inability to reach external routes when a default route is not present.
If Area 0, the backbone area, were down, all inter-area routing would be impacted, not just a single branch area. Since only the branch area is affected, the backbone is operational.
OSPF process IDs are locally significant, and mismatched process IDs would not prevent LSA flooding or external route reception if adjacency and area types are correct.
The root cause is the branch area being configured as a totally stubbed area. Ensuring the ABR injects a default route allows branch routers to reach external networks redistributed from BGP. Verification includes checking route tables, inspecting ABR configuration, and confirming area type. Totally stub areas optimize OSPF operation in large networks while maintaining connectivity to external networks. Correct configuration reduces routing overhead, simplifies the LSDB, and ensures stable operation. Network engineers must carefully plan ABR configuration, validate area types, and monitor default route propagation. Proper configuration prevents connectivity issues, maintains predictable OSPF behavior, and enables remote branches to access external destinations efficiently. Understanding totally stub areas is crucial for deploying OSPF in multi-area environments to balance routing efficiency with connectivity requirements.
Question 173:
A network engineer deploys BGP multipath in a multi-homed network. Multiple paths appear in the BGP table, but traffic only traverses a single path. What is the most likely cause?
A) Paths are not equal 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 to enhance load balancing and redundancy. Multipath forwarding requires all candidate paths to have identical attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP uses a deterministic path selection process to prevent loops and maintain network stability. When multiple paths appear in the BGP table but only one path is used, it usually indicates that candidate paths differ in AS path, origin type, or MED. Verification involves inspecting the BGP table using commands like “show bgp ipv4 unicast” and comparing the attributes among candidate paths. Aligning these attributes allows multipath forwarding to function, distributing traffic across multiple paths and optimizing bandwidth utilization.
All neighbors being in iBGP does not prevent multipath. Multipath can operate over iBGP-learned routes provided attribute equality conditions are met. iBGP and eBGP do not inherently limit multipath operation as long as attributes align.
An unreachable next-hop prevents a specific path from being installed in the routing table. Since multiple paths appear in the BGP table, reachability is not the issue in this scenario. Engineers can confirm next-hop reachability using “show ip route” and ping tests.
BGP route dampening temporarily suppresses routes to reduce instability. Dampening does not affect stable paths that meet equality requirements and appear in the BGP table.
The root cause is unequal attributes among candidate paths. Differences in AS path, origin, or MED prevent multipath forwarding. Engineers must ensure attribute consistency to enable simultaneous use of multiple paths. Proper attribute alignment enhances redundancy, load balancing, and traffic optimization. Misalignment can lead to underutilization of available paths, congestion on a single path, and reduced network resilience. Understanding BGP’s strict equality requirements is crucial for multi-homed networks. Correct multipath configuration ensures predictable traffic distribution, efficient bandwidth utilization, and reliable redundancy. Continuous monitoring of BGP tables, attributes, and traffic flow is necessary to maintain optimized multipath operation. Proper configuration improves performance, prevents bottlenecks, and provides reliable network operation, supporting enterprise and service provider requirements for high availability and redundancy. Engineers must align BGP attributes, monitor multipath usage, and verify traffic distribution to achieve stable and predictable routing behavior.
Question 174:
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 mappings. LDP uses TCP port 646 to send hello messages and label mapping information. Without IGP adjacency, routers cannot communicate, preventing LDP session formation and the creation of label-switched paths required for MPLS forwarding. LDP does not provide its own neighbor discovery independent of the IGP; connectivity via the IGP is critical. Verification includes checking IGP routing tables, adjacency status using commands such as “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 sessions to be established. Engineers must confirm IP addressing and full IGP convergence to support reliable MPLS operations. Proper IGP configuration forms the foundation for LDP operation, enabling predictable label distribution and traffic forwarding.
Mismatched LDP transport addresses can prevent sessions if manually configured incorrectly, but default LDP implementations typically use loopback or primary 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 occurs even if MPLS is inactive, although label-switched paths 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 will not block LDP operation if 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 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 a foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.
Question 175:
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 requires all routers within the same autonomous system (AS) to have matching AS numbers to establish neighbor relationships and exchange routing information. The AS number identifies the routing domain, ensuring that routers only form adjacencies with routers in 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 results in routing inconsistencies, incomplete route propagation, and delayed network convergence. Engineers can verify neighbor relationships using commands such as “show ip eigrp neighbors” and confirm AS numbers using “show running-config.” Ensuring AS numbers match across all routers is crucial to maintaining EIGRP neighbor relationships and stable route propagation.
In this scenario, routers fail to form neighbor relationships on the same subnet, indicating an AS number mismatch. Correcting the AS number allows routers to recognize each other, establish adjacencies, and exchange routing information. Engineers must also ensure that the correct interfaces are enabled for EIGRP, that passive interfaces are not unintentionally configured, and that IP addressing is consistent. Proper AS number alignment is especially important in large networks with multiple EIGRP domains, as mismatches can result in partial network segmentation and operational instability.
A passive interface prevents hello packets from being sent on the configured interface, which can block neighbor formation. While it could cause issues on a single interface, widespread neighbor failures across multiple routers suggest a more systemic cause, such as an AS number mismatch.
Different K values influence metric calculation but do not prevent neighbor formation. Mismatched K values can lead to differences in route selection, but will not stop routers from forming EIGRP adjacencies.
Split-horizon prevents routers from advertising routes back out the interface on which they were learned, affecting route propagation in certain topologies but not adjacency formation.
The root cause is mismatched AS numbers. Ensuring all routers share the same AS number is critical for establishing neighbor relationships, exchanging routes, and maintaining network stability. Engineers must 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 potential routing loops. Correct AS configuration ensures predictable route propagation, robust connectivity, and efficient convergence. Proper EIGRP deployment across multi-area networks requires careful planning, verification, and monitoring of AS numbers to avoid disruptions. By confirming consistent AS numbers, network engineers maintain stable adjacencies, complete routing tables, and predictable network behavior. AS number alignment is essential for EIGRP to function efficiently, supporting network reliability, high availability, and optimal performance in enterprise networks. Proper configuration prevents routing inconsistencies, reduces troubleshooting complexity, and ensures seamless communication between all routers in the EIGRP domain.
Question 176:
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 relies on link-local addresses to establish neighbor adjacencies in IPv6 networks. Link-local addresses are required as the source and destination for OSPFv3 hello packets. Without a valid link-local address, routers cannot discover neighbors or exchange LSAs. Link-local addresses ensure direct communication between routers on the same link, independent of global IPv6 addresses. Proper configuration of link-local addresses is essential for adjacency formation, LSDB synchronization, and network convergence. Engineers can verify interface configurations using “show ipv6 interface brief” and neighbor status using “show ipv6 ospf neighbor.” Ensuring link-local addresses are correctly configured allows routers to form adjacencies, exchange LSAs, and maintain routing consistency. Unique router IDs must also be configured to prevent conflicts in the OSPFv3 LSDB.
In this scenario, routers fail to form adjacencies, suggesting that link-local addresses are missing or misconfigured. Correcting this ensures hello packets can be exchanged, adjacencies are established, and the OSPFv3 LSDB synchronizes properly across the network. Engineers should verify link-local addresses and ensure OSPFv3 is enabled on all participating interfaces. Proper configuration maintains network stability, predictable convergence, and consistent route propagation.
Duplicate router IDs can cause LSDB conflicts and adjacency issues, but hello packets can still be exchanged if link-local addresses exist. Conflicts generate log messages and warnings, but missing link-local addresses directly prevent adjacency formation.
Area authentication mismatches affect the acceptance of LSAs but do not block initial adjacency formation. Hello packets are still sent and received, though authenticated LSAs may be rejected if the authentication does not match.
IPv6 unicast routing being disabled globally prevents forwarding of IPv6 traffic, but does not block the exchange of hello packets via link-local addresses.
The root cause is missing link-local addresses. Ensuring every interface participating in OSPFv3 has a valid link-local address is critical for adjacency formation, LSA exchange, and network convergence. Proper configuration ensures stable OSPFv3 operation, predictable route propagation, and reliable connectivity. Network engineers must verify link-local addresses, enable OSPFv3, and assign unique router IDs to prevent conflicts. Understanding the role of link-local addresses in OSPFv3 is essential for IPv6 deployment. Correct configuration enables seamless adjacency formation, uninterrupted LSA flooding, and predictable convergence, supporting efficient and reliable IPv6 network operation. Engineers should continuously monitor adjacency status and interface configurations to maintain stable OSPFv3 networks. Proper link-local addressing ensures connectivity, stable routing, and optimized performance in IPv6 networks.
Question 177:
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 underlying IGP connectivity to provide reachability between routers for neighbor discovery and label distribution. LDP uses TCP port 646 to exchange hello messages and label mapping information. Without IGP adjacency, routers cannot communicate, preventing LDP session formation and blocking label-switched path creation necessary for MPLS forwarding. LDP does not operate independently of the IGP; IP connectivity is essential. Verification involves checking IGP routing tables, adjacency status with commands like “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity using ping or traceroute. Ensuring IGP adjacency allows LDP messages to reach neighbors and enables session establishment. Engineers must confirm IP addressing and full IGP convergence to maintain reliable MPLS operation. Proper IGP configuration forms the foundation for LDP operation, ensuring predictable label distribution and traffic forwarding.
Mismatched LDP transport addresses can prevent sessions if manually configured incorrectly, but default LDP deployments typically 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. LDP discovery still occurs, though labels cannot be used 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 will not block LDP operation 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 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 provide a foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.
Question 178:
A network engineer configures OSPF in a multi-area network. Some branch routers cannot reach external networks redistributed from BGP. What is the most likely cause?
A) The area is configured as a totally stubbed area
B) Type-3 LSAs are blocked by an ACL
C) Area 0 is down
D) OSPF process IDs are mismatched
Answer: A)
Explanation:
OSPF area types define how LSAs are propagated and how routes are distributed across the network. A totally stubbed area is designed to reduce the size of the LSDB and routing table for routers in remote areas, such as branch offices. In a totally stub area, Type-5 LSAs, which carry external routes like those redistributed from BGP, are blocked from entering the area. This prevents routers inside 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 destinations. If the default route is not present or misconfigured, branch routers cannot reach external networks. Verification can be performed using commands such as “show ip route” to check if a default route is present and “show ip ospf” to confirm area types. Correct ABR configuration ensures that a default route is advertised into the totally stub area, allowing branch routers to reach external destinations while keeping the LSDB minimal and reducing CPU and memory usage.
In this scenario, branch routers are unable to reach external networks, indicating that the area is likely configured as a totally stubbed area. Ensuring the ABR advertises a default route resolves this issue. Proper configuration allows for optimized OSPF operation, efficient resource utilization, and predictable connectivity to external networks. Network engineers must carefully plan area types, verify ABR behavior, and monitor route propagation to maintain stable OSPF operation across multi-area networks.
Blocking Type-3 LSAs with an ACL could restrict inter-area route propagation, but it would not prevent access to external routes if a default route is properly injected. This scenario specifically indicates external route inaccessibility due to the area type configuration, not LSA filtering.
If Area 0 were down, it would affect the entire OSPF backbone and all inter-area routing, not just branch routers. Since only branch routers are affected, the backbone is operational and not the root cause.
OSPF process IDs are locally significant and do not affect inter-area route propagation. Mismatched process IDs only impact local configuration and do not prevent the distribution of LSAs between areas if adjacency and area types are correct.
The root cause is the branch area being configured as a totally stubbed area. Ensuring the ABR advertises a default route allows branch routers to reach external networks redistributed from BGP. Engineers should verify area types, ABR configuration, and default route advertisement. Totally stub areas optimize OSPF operation in large networks, reduce LSDB size, and conserve resources while maintaining connectivity to external networks. Correct configuration ensures stable operation, predictable convergence, and efficient routing. Network engineers must monitor route tables and ABR behavior to prevent connectivity issues, maintain predictable routing behavior, and optimize OSPF efficiency. Proper configuration of stub areas supports reliable external connectivity, minimizes routing complexity, and improves network performance. Understanding totally stub areas is critical for designing scalable OSPF networks. Effective deployment ensures stable routing, reduced overhead, and efficient propagation of external routes through a default route mechanism.
Question 179:
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 are not equal 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 with equal attributes to be used simultaneously for redundancy and load balancing. Multipath forwarding requires candidate paths to be identical in critical 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 is only forwarded through a single path, it usually indicates that candidate paths differ in AS path, origin type, or MED attributes. Verification can be performed using “show bgp ipv4 unicast” to compare attributes among candidate paths. Ensuring attribute equality allows multipath forwarding to distribute traffic across multiple paths, optimizing bandwidth utilization and improving redundancy.
All neighbors being in BGP does not prevent multipath. Multipath works with both iBGP and eBGP neighbors, provided the equality requirements are satisfied. iBGP and eBGP differences do not affect multipath eligibility as long as attributes align.
An unreachable next-hop prevents a specific path from being installed in the routing table. Since multiple paths are visible in the BGP table, next-hop reachability is not the issue. Engineers can verify reachability using “show ip route” or ping commands.
BGP route dampening suppresses unstable paths to enhance stability. It does not prevent stable, visible paths from being used for traffic forwarding.
The root cause is unequal attributes among candidate paths. Differences in AS path, origin, or MED prevent multipath forwarding. Engineers must align these attributes to enable simultaneous use of multiple paths. Correct configuration ensures redundancy, efficient bandwidth usage, and optimal traffic distribution. Misalignment can result in congestion on a single active path and underutilization of available bandwidth. Understanding BGP multipath requirements is essential in multi-homed networks. Proper verification and attribute alignment maintain predictable traffic distribution, high availability, and network resilience. Engineers should continuously monitor BGP tables, attributes, and traffic flows to ensure multipath operation remains functional. Proper multipath configuration enhances performance, reduces bottlenecks, and supports stable enterprise network operation. Attribute consistency is critical for leveraging multiple paths for load balancing and redundancy.
Question 180:
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 form neighbor relationships and exchange label mapping information. LDP uses TCP port 646 to send hello messages and label mapping updates. Without IGP adjacency, routers cannot communicate with neighbors, preventing LDP session formation and the creation of label-switched paths essential for MPLS traffic forwarding. LDP cannot operate independently of the IGP; IP connectivity is required. Verification involves checking the IGP routing table, adjacency status with “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 sessions to be established. Engineers must confirm IP addressing and full IGP convergence to maintain reliable MPLS operation. Proper IGP configuration forms the foundation for LDP operation, ensuring predictable label distribution and efficient traffic forwarding.
Mismatched LDP transport addresses can prevent sessions if manually configured incorrectly, but default LDP implementations 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 occurs even when MPLS is inactive, although label-switched paths cannot forward traffic until MPLS is active.
LDP hello timers determine how often hello messages are sent, but do not prevent neighbor formation. Longer timers may delay session establishment, but do not block LDP operation 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 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 provide a foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.