Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 10 Q136-150
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Question 136:
A network engineer configures EIGRP for a branch office. Some routers do not exchange routes with neighbors despite being on the same subnet. What is the most likely cause?
A) Passive interface is enabled.
B) Mismatched EIGRP AS numbers.
C) K values are different.
D) Split-horizon is enabled on the interface.
Answer: B)
Explanation:
EIGRP is a distance vector routing protocol that relies on neighbor relationships to exchange routing updates. One of the primary requirements for forming a neighbor adjacency is that all routers must belong to the same autonomous system (AS). The AS number identifies the EIGRP routing domain and is included in hello packets sent by routers. If routers have different AS numbers, they will reject each other’s hello packets, preventing neighbor establishment and route exchange. This can result in partial network connectivity, routing table inconsistencies, and delayed convergence.
In this scenario, some routers on the same subnet fail to exchange routes. This strongly suggests a mismatch in the configured AS numbers. Verification involves checking the running configuration and using commands like “show ip eigrp neighbors” to confirm that the routers are attempting to form neighbor relationships and identifying any mismatched AS numbers. Correct configuration of the AS number across all routers in the EIGRP domain is critical to ensuring proper neighbor formation and the reliable exchange of routing information.
Enabling a passive interface prevents EIGRP hello packets from being sent out on a specific interface. While this can prevent neighbor formation on that interface, it would not explain a broader issue affecting multiple routers on the same subnet unless all interfaces were misconfigured as passive.
Different K values influence the calculation of EIGRP metrics, determining route selection and feasibility. However, mismatched K values do not prevent neighbor formation. Routers can still establish adjacencies and exchange routing information, though some routes may not be considered feasible successors if metrics are inconsistent.
Split-horizon, which prevents a router from advertising a route back out the interface from which it was learned, affects route propagation but does not prevent the initial neighbor formation process. Disabling or enabling split-horizon can influence route visibility in specific topologies like hub-and-spoke, but it does not explain a complete failure to exchange routes among neighbors.
The root cause is the mismatch of EIGRP AS numbers. Ensuring all routers share the same AS number is fundamental to forming neighbor relationships, exchanging routes, and achieving network convergence. Network engineers must verify AS numbers, interface configurations, and adjacency status using diagnostic commands. Properly matching AS numbers ensures a consistent and stable routing environment, avoids incomplete routing tables, and prevents unreachable networks. Documentation, validation, and periodic audits are essential for maintaining operational EIGRP networks, particularly in large enterprise deployments with multiple branches. Correct configuration facilitates predictable route propagation, efficient use of network resources, and seamless connectivity across all routers in the EIGRP domain. Understanding the significance of AS numbers in EIGRP is crucial for avoiding neighbor formation failures and ensuring optimal routing performance across the enterprise network.
Question 137:
A network engineer deploys OSPFv3 in an IPv6 network. Routers fail to establish neighbor relationships after configuration. 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 IPv6 link-local addresses for neighbor discovery and adjacency formation. Every interface participating in OSPFv3 must have a valid link-local address because OSPFv3 uses these addresses as the source and destination of hello packets. Without a link-local address, routers cannot identify neighbors or establish adjacencies, preventing the exchange of LSAs and proper route propagation. This reliance on link-local addresses is a key difference between OSPFv3 and OSPFv2, which primarily uses global addresses. Link-local addresses ensure that hello packets are delivered reliably within the local link, allowing routers to form adjacencies and exchange routing information consistently.
In this scenario, routers fail to establish neighbor relationships. This indicates that link-local addresses are missing or incorrectly configured. Verification commands such as “show ipv6 interface brief” and “show ipv6 ospf neighbor” can confirm the presence of link-local addresses and reveal adjacency failures. Proper configuration of link-local addresses, along with enabling OSPFv3 on all relevant interfaces, ensures that neighbors form successfully and LSAs are exchanged. Assigning unique router IDs for OSPFv3 operation complements link-local addressing and provides consistent route identification across the network.
Duplicate router IDs can disrupt OSPFv3 operation and cause adjacency issues, but do not prevent initial hello packet transmission if link-local addresses are properly configured. Conflicts generate specific log messages that help troubleshoot the problem.
Area authentication mismatch affects the acceptance of LSAs from neighbors but does not block initial adjacency formation. Hello packet exchange and neighbor negotiation can still occur even if authentication fails.
Disabling IPv6 unicast routing globally prevents the router from forwarding IPv6 traffic but does not stop OSPFv3 from attempting hello packet exchange using link-local addresses. However, full OSPFv3 operation and LSA propagation may be impacted.
The root cause is missing link-local addresses. Ensuring that each interface participating in OSPFv3 has a valid link-local address is crucial for adjacency formation, hello packet exchange, and LSA propagation. Proper configuration guarantees stable OSPFv3 operation, predictable convergence, and accurate routing table updates. Network engineers must verify link-local addresses, enable OSPFv3, assign unique router IDs, and monitor neighbor formation to maintain operational network performance. Understanding the role of link-local addresses in IPv6 networks is essential for successful OSPFv3 deployment. Proper planning, verification, and documentation help prevent connectivity issues and ensure that routing information flows efficiently across the network. By ensuring correct link-local addressing, engineers can achieve reliable OSPFv3 neighbor formation, maintain consistent route propagation, and provide predictable network convergence across all areas.
Question 138:
A network engineer configures 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 Label Distribution Protocol (LDP) relies on underlying IGP connectivity for neighbor discovery and session establishment. LDP uses TCP port 646 to exchange hello and label mapping messages between routers. Routers must have IP reachability provided by the IGP to establish LDP sessions. If there is no IGP adjacency or proper routing between routers, LDP messages cannot reach the neighbor, and the session fails to establish. IGP adjacency is therefore foundational for LDP operation because it ensures that routers can communicate directly and exchange label information to establish label-switched paths. Without proper IGP connectivity, LDP cannot function, preventing MPLS from forwarding traffic efficiently.
In this scenario, routers fail to establish LDP sessions. This commonly occurs due to missing IGP adjacency or routing inconsistencies that prevent neighbor reachability. Verification steps include checking the IGP routing table, examining neighbor status using “show ip ospf neighbor” or “show ip eigrp neighbors,” and pinging neighbors to confirm reachability. Correct IGP configuration and adjacency formation are critical for enabling LDP, exchanging label information, and establishing MPLS forwarding.
Mismatched LDP transport addresses may prevent session establishment in specific configurations where manually assigned addresses differ. However, in most default deployments, LDP uses the loopback interface or primary interface addresses, making this a less likely cause.
MPLS not being globally enabled prevents label switching but does not inherently prevent LDP from attempting neighbor discovery on interfaces where MPLS is configured. Labels will not be distributed until MPLS is enabled, but LDP discovery still occurs.
LDP hello timers determine how frequently hello messages are sent. While longer timers may delay session establishment, they do not prevent eventual session formation if underlying connectivity exists.
The root cause is missing IGP adjacency. Ensuring proper IGP reachability allows LDP messages to be exchanged, neighbors to be discovered, and label-switched paths to be established. Network engineers must verify IGP operation, adjacency status, and interface configuration to maintain stable MPLS networks. Correct IGP configuration enables LDP sessions to form, allows label distribution, and ensures MPLS forwarding operates efficiently. Planning, verification, and monitoring are essential for preventing session failures, maintaining predictable traffic engineering, and providing redundancy in service provider networks. Understanding the relationship between IGP and LDP is critical for deploying MPLS successfully, ensuring optimal path selection, and achieving network resilience.
Question 139:
A network engineer configures OSPF in a multi-area network. Routers in area 50 cannot reach external networks redistributed from BGP. What is the most likely cause?
A) Area 50 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 play a significant role in how routing information is propagated throughout an OSPF domain. A totally stub area is designed to simplify routing within a remote area by restricting routers from installing external Type-5 LSAs and inter-area Type-3 summary LSAs, except for a default route. This reduces the size of the link-state database and conserves router resources such as memory and CPU. Totally stub areas are often deployed in branch offices or remote sites with limited resources, where full external routing information is unnecessary. The routers inside the totally stub area rely on a default route advertised by the ABR to reach destinations outside the area. This configuration ensures efficient routing while maintaining a simplified routing table.
In this scenario, routers in area 50 cannot reach external networks redistributed from BGP. This behavior is consistent with a totally stub area configuration, where Type-5 LSAs and external Type-3 LSAs are filtered, preventing routers from learning specific external routes. Verification involves checking the area type using “show ip ospf” and ensuring that the ABR advertises a default route using the “default-information originate” command. Correct configuration guarantees that routers can access external destinations while still benefiting from reduced link-state database size and optimized router performance. Network engineers must plan ABR configurations carefully to ensure default route advertisement to totally stub areas.
Blocking Type-3 LSAs via an ACL could also prevent inter-area routes from being installed, but the symptom of complete external route loss in a single area strongly points toward a totally stubbed area configuration rather than an access-list issue.
If area 0, the backbone area, were down, inter-area routing would fail throughout the entire OSPF network, affecting multiple areas. Since only area 50 is impacted, the backbone area is functioning correctly.
OSPF process IDs are locally significant and do not affect the propagation of external LSAs or area reachability. Mismatched process IDs would not prevent routers from receiving redistributed external routes if adjacency and area types are correctly configured.
The root cause is that area 50 is configured as a totally stubbed area. While this setup optimizes routing table size and conserves resources, it requires proper ABR default route advertisement to maintain connectivity to external networks. Network engineers must validate that ABRs are correctly configured, verify route propagation with “show ip route” and “show ip ospf database,” and confirm that routers in the stub area can reach external destinations. Proper understanding and planning of totally stubbed areas are essential in large enterprise networks to balance resource optimization with network reachability. Misconfigurations can result in isolation from external networks, impacting business-critical applications. Ensuring proper ABR default route injection guarantees that the benefits of totally stub areas are achieved without sacrificing connectivity, providing predictable and efficient routing behavior across the network.
Question 140:
A network engineer deploys BGP multipath on a multi-homed network. Multiple paths are listed in the BGP table, but traffic only flows 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 traffic to be forwarded across multiple equal-cost paths, increasing redundancy, improving bandwidth utilization, and providing load balancing. For multipath to function correctly, the candidate paths must match exactly in key attributes, including AS path, origin type, MED, local preference, and next-hop reachability. Any difference in these attributes causes BGP to select a single best path according to its deterministic decision process. This ensures stable routing, prevents loops, and maintains predictable behavior, but it also prevents alternate paths from being utilized unless the attributes are identical.
In this scenario, multiple paths appear in the BGP table, but traffic only flows through one path. This indicates that the candidate paths differ in AS path, origin, or MED attributes. For instance, one path may have AS path prepending or a different origin type, causing BGP to consider it less preferable. Network engineers should verify the attributes of candidate paths using commands such as “show bgp ipv4 unicast” and “show ip route” to identify discrepancies. Aligning attributes ensures that multiple paths are treated as equal and can be used for multipath forwarding. Proper configuration improves traffic distribution, redundancy, and overall network efficiency.
All neighbors being in iBGP does not prevent multipath as long as the equality criteria are met. iBGP supports multipath for equal-cost paths, provided that AS path, origin, MED, and next-hop attributes are identical.
An unreachable next-hop prevents a path from being installed in the routing table. However, the scenario indicates that multiple paths exist in the BGP table, so reachability is not the issue.
BGP route dampening stabilizes routing by suppressing flapping routes, but does not affect the selection of multipath routes if they meet equality requirements. Multipath forwarding still occurs once the suppressed routes recover.
The root cause is that the candidate paths are not equal in AS path, origin, or MED attributes. Network engineers must ensure that these attributes are consistent across all candidate paths to enable multipath forwarding. Proper configuration facilitates load balancing, increases redundancy, and prevents traffic congestion on a single path. Monitoring BGP tables, adjusting path attributes as needed, and validating multipath operation are crucial for maintaining predictable routing behavior in multi-homed networks. Understanding BGP’s strict equality requirements for multipath is essential for achieving optimal network performance and resilience. Correct multipath deployment ensures efficient bandwidth utilization, enhances redundancy, and provides predictable traffic flow, contributing to stable and reliable network operations.
Question 141:
A network engineer configures RSVP-TE tunnels in an MPLS network. Tunnels fail to establish, even though links report sufficient bandwidth. 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:
MPLS Traffic Engineering allows operators to establish explicit paths through a network based on bandwidth availability, administrative constraints, and link attributes. RSVP-TE is used to signal and reserve resources along these paths. The Constrained Shortest Path First (CSPF) algorithm calculates feasible paths by evaluating all potential routes against traffic engineering constraints, including available bandwidth, TE colors, and administrative group settings. Even if links report sufficient bandwidth, tunnels may fail if CSPF cannot find a path that satisfies all constraints. For example, incorrect TE color configurations, mismatched administrative groups, or conflicting constraints can prevent CSPF from selecting a valid path, resulting in RSVP-TE tunnel setup failures.
In this scenario, tunnels fail to establish despite adequate bandwidth. This indicates that CSPF is unable to compute a feasible path due to link attribute constraints. Verification involves checking TE link attributes, examining CSPF calculations, and reviewing RSVP-TE logs. Correct configuration of TE attributes ensures that CSPF identifies a valid path, and RSVP-TE tunnels can be successfully established. Network engineers must ensure that administrative groups, bandwidth constraints, and TE color assignments are consistent across all links in the intended path. Proper planning includes validating TE attributes, simulating CSPF calculations, and testing tunnel setups before deployment.
RSVP authentication mismatches affect the security of tunnel signaling but do not influence CSPF’s path computation. Authentication failures prevent tunnels from being established only if signaling is rejected, but path feasibility is determined independently.
Soft-state refresh timers maintain RSVP-TE tunnel state but do not affect initial path computation. Longer timers may delay state refreshes, but do not block tunnel establishment if a feasible path exists.
IGP metrics influence shortest-path calculations but cannot override TE constraints. Even if the lowest-metric path exists, CSPF will reject it if it does not satisfy bandwidth, administrative, or other TE constraints.
The root cause is that CSPF cannot find a feasible path due to link attribute constraints. Proper configuration of TE attributes, bandwidth availability, and administrative group settings is critical for successful RSVP-TE deployment. Network engineers must monitor CSPF computations, verify TE configurations, and perform ongoing tunnel validation to maintain operational MPLS networks. Ensuring consistent TE attributes guarantees predictable tunnel establishment, reliable traffic engineering, and optimal network performance. Understanding the interaction between CSPF, TE attributes, and RSVP-TE signaling is essential for deploying MPLS networks effectively and maintaining network resilience and redundancy.
Question 142:
A network engineer configures OSPF in a multi-area network. Some routers in area 100 cannot reach external networks redistributed from BGP. What is the most likely cause?
A) Area 100 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 control the propagation of routing information across the network and are used to optimize routing table size, reduce CPU load, and manage LSDB complexity. A totally stub area is designed to limit the type of LSAs that routers in the area can receive. Specifically, totally stub areas block Type-5 LSAs, which carry external routes such as those redistributed from BGP, and Type-3 LSAs from other areas, except for a default route. This configuration reduces routing overhead and is often used in branch or remote sites with limited router resources. Routers in totally stub areas rely on a default route injected by the ABR to reach external destinations, instead of maintaining a full set of external and inter-area routes.
In this scenario, routers in area 100 cannot reach external networks redistributed from BGP. This behavior strongly indicates that the area is configured as a totally stubbed area. Verification involves checking the OSPF area type using commands like “show ip ospf” and ensuring that the ABR advertises a default route using “default-information originate.” Proper configuration guarantees that routers in the stub area can reach external destinations while maintaining a simplified LSDB. Network engineers must ensure ABRs are correctly configured to advertise default routes and that area type documentation is consistent across the network.
Blocking Type-3 LSAs using an ACL could also prevent inter-area route propagation. However, ACL blocking is usually a deliberate action for security or traffic engineering purposes. In this scenario, the symptom of complete external route loss, particularly in a single area, aligns more closely with the behavior of a totally stubbed area.
If area 0, the backbone area, were down, all inter-area routing would fail, impacting multiple areas. Since only area 100 is affected, the backbone area is functioning normally.
OSPF process IDs are locally significant and do not influence LSA propagation across areas. Mismatched process IDs would not prevent external route reception if adjacency and area types are correctly configured.
The root cause is that area 100 is configured as a totally stubbed area. While this configuration optimizes routing table size and reduces resource usage, it requires correct ABR configuration to ensure routers in the area receive a default route for external reachability. Network engineers must verify ABR advertisements, monitor OSPF database consistency, and confirm that external connectivity is maintained. Understanding totally stubbed areas is critical for large enterprise networks to balance resource optimization with connectivity. Properly configured, totally stubbed areas simplify network management, reduce routing overhead, and maintain predictable routing behavior. Misconfigurations can lead to isolation from external networks, affecting critical services and user connectivity. Network engineers must validate configurations using “show ip route,” “show ip ospf database,” and ABR inspection to ensure the benefits of totally stub areas are realized without sacrificing reachability. By planning area types carefully, engineers can optimize performance while maintaining reliable access to external networks and achieving consistent, predictable OSPF operation.
Question 143:
A network engineer deploys BGP multipath on a multi-homed network. Multiple paths are listed 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 functionality allows traffic to be forwarded across multiple equal-cost paths for load balancing and redundancy. For multipath to operate correctly, candidate paths must be identical in certain attributes: AS path, origin type, MED, local preference, and next-hop reachability. Any differences in these attributes cause BGP to select a single best path according to its deterministic decision process. This decision ensures routing stability, prevents loops, and maintains predictable behavior, but it also prevents the simultaneous utilization of other candidate paths that do not meet the equality criteria.
In this scenario, multiple paths appear in the BGP table, but traffic is forwarded only through a single path. This suggests that the candidate paths differ in AS path, origin, or MED attributes. For example, a path may have an additional AS path prepending or a different origin type, which makes BGP prefer one path over others. Verification involves using commands such as “show bgp ipv4 unicast” and “show ip route” to compare path attributes. Aligning these attributes allows multiple paths to be used for traffic forwarding, optimizing bandwidth utilization and redundancy.
All BGP neighbors being iBGP does not prevent multipath usage. iBGP supports multipath as long as the equality conditions are satisfied. Multipath forwarding can operate over multiple iBGP-learned routes if they match the necessary attributes.
An unreachable next-hop would prevent a path from being installed in the routing table. However, multiple paths are present in the BGP table in this scenario, indicating that next-hop reachability is not an issue.
BGP route dampening stabilizes routing by suppressing flapping routes. While this may temporarily prevent a route from being used, it does not affect the selection of multipath routes when attributes are equal, and the routes are stable.
The root cause is that the candidate paths are not equal in AS path, origin, or MED attributes. Ensuring equality among these attributes is essential for multipath to function. Correct configuration enables load balancing, improves redundancy, and prevents congestion on a single path. Network engineers must monitor BGP attributes, adjust configurations, and validate multipath behavior to achieve predictable and efficient traffic distribution. Understanding BGP’s strict equality requirements is crucial for multi-homed environments to ensure optimal network performance, reliable routing, and redundancy. Proper multipath deployment improves bandwidth utilization, enhances resilience, and maintains stable traffic flow. Verification and monitoring are necessary to confirm that multipath traffic is operating as intended, preventing bottlenecks and ensuring optimal network resource utilization across the enterprise.
Question 144:
A network engineer configures RSVP-TE tunnels in an MPLS network. Tunnels fail to establish, even though sufficient bandwidth is available. 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:
MPLS Traffic Engineering enables explicit path selection through a network based on available resources, administrative constraints, and link attributes. RSVP-TE is used to signal and reserve resources along these paths. The Constrained Shortest Path First (CSPF) algorithm calculates feasible paths by evaluating all candidate routes against TE constraints, including available bandwidth, TE colors, and administrative groups. Even when links report sufficient bandwidth, tunnels can fail if CSPF cannot find a path that satisfies all configured constraints. Incorrect link attributes, such as misconfigured TE colors, missing administrative group markings, or conflicting path constraints, can prevent CSPF from computing a valid path, resulting in tunnel setup failure.
In this scenario, tunnels fail to establish despite adequate bandwidth. This strongly suggests that CSPF cannot compute a feasible path due to link attribute constraints. Verification includes examining TE link attributes, reviewing CSPF computations, and inspecting RSVP-TE logs for errors. Correct configuration of TE attributes, administrative groups, and bandwidth availability ensures that CSPF can compute a valid path, allowing RSVP-TE tunnels to be established successfully. Network engineers should perform TE simulations, validate path feasibility, and monitor tunnel setup to ensure predictable and reliable operation. Proper configuration also involves verifying that all links along the intended path have consistent TE attributes and sufficient bandwidth to satisfy the tunnel’s requirements.
RSVP authentication mismatches affect signaling security but do not impact CSPF path computation. Authentication failures will prevent tunnel establishment only if the signaling message is rejected, independent of path feasibility.
RSVP soft-state refresh timers maintain tunnel state and prevent premature expiration, but do not influence CSPF path calculation. Longer timers may delay state updates, but do not prevent tunnel setup if a feasible path exists.
IGP metrics affect unconstrained SPF computation but cannot override TE constraints. CSPF will reject paths that fail to meet bandwidth, administrative group, or TE color requirements regardless of IGP metric values.
The root cause is CSPF being unable to find a feasible path due to link attribute constraints. Proper configuration of TE attributes, administrative groups, and bandwidth availability is essential for successful RSVP-TE tunnel deployment. Ensuring correct configuration guarantees predictable tunnel establishment, efficient traffic engineering, and optimal MPLS network performance. Network engineers must verify link attributes, monitor CSPF computations, and validate tunnel setup to maintain operational MPLS networks. Understanding CSPF, TE attributes, and RSVP-TE signaling is critical for effective MPLS deployment, ensuring reliable tunnel establishment, redundancy, and predictable traffic flow. Proper planning, configuration, and monitoring prevent tunnel failures and support resilient, high-performance MPLS networks.
Question 145:
A network engineer configures EIGRP on a large enterprise network. Some routers fail to form 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 on a hub router.
Answer: A)
Explanation:
EIGRP requires neighbor relationships to exchange routing information effectively. One of the most critical requirements for forming an adjacency is that all routers must be in the same autonomous system (AS). The AS number is included in EIGRP hello packets and identifies the routing domain to which a router belongs. If routers are configured with different AS numbers, they will not recognize each other as neighbors, preventing the exchange of routes and potentially causing partial connectivity issues or network segmentation. Verification involves checking the running configuration of all routers using commands such as “show running-config” and examining neighbor status using “show ip eigrp neighbors.” Matching AS numbers across all routers ensures proper adjacency formation and full route exchange.
In this scenario, routers fail to form neighbor relationships even though they share the same subnet. This strongly indicates that AS numbers are mismatched. Correcting the AS numbers ensures that routers recognize each other and can exchange routing updates. Proper EIGRP configuration also includes verifying interface IP addresses, network statements, and ensuring that the interfaces participating in EIGRP are correctly configured. Network engineers should document AS numbers and maintain consistency throughout the deployment, especially in large-scale enterprise networks where multiple EIGRP domains may exist.
A passive interface prevents EIGRP hello packets from being sent on a particular interface. While this can stop neighbor formation, it is usually applied intentionally to interfaces connected to end devices. Widespread neighbor failures across multiple routers on the same subnet are less likely to be caused by passive interfaces unless all relevant interfaces are misconfigured.
K values determine how EIGRP calculates its metrics and affect route selection. Mismatched K values do not prevent neighbor establishment; routers will still exchange routing updates, but routes may not be considered feasible successors if metrics differ.
Split-horizon prevents a router from advertising routes back out the interface from which they were learned. While split-horizon can affect route propagation in hub-and-spoke topologies, it does not prevent neighbor formation. Incorrect split-horizon settings could cause routing table inconsistencies, but not adjacency failures.
The root cause in this scenario is a mismatched EIGRP AS numbers. Ensuring that all routers share the same AS number is essential for forming neighbor relationships, exchanging routes, and achieving network convergence. Proper verification and documentation are critical to maintain operational stability and prevent partial network segmentation. Engineers must validate AS numbers, interface configurations, and neighbor status using diagnostic commands. Correct configuration ensures predictable route propagation, efficient resource utilization, and seamless connectivity. Understanding the significance of AS numbers in EIGRP is crucial for avoiding neighbor formation failures and maintaining a robust and reliable routing environment. Consistent AS number configuration across routers supports optimized network performance, prevents route loss, and ensures stable convergence in complex enterprise topologies.
Question 146:
A network engineer deploys OSPFv3 in an IPv6 network. Routers fail to exchange LSAs after configuration. 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 IPv6 link-local addresses to establish neighbor relationships and exchange LSAs. Each interface participating in OSPFv3 must have a valid link-local address because OSPFv3 uses these addresses as the source and destination of hello packets. Without link-local addresses, routers cannot form neighbor adjacencies, preventing LSAs from being exchanged and halting proper routing. This is a critical distinction between OSPFv2, which relies on IPv4 addresses, and OSPFv3, which requires link-local addresses for communication. Link-local addressing ensures that hello packets are delivered reliably to neighbors on the same link, allowing adjacency formation and consistent route propagation.
In this scenario, routers fail to exchange LSAs, which strongly suggests that link-local addresses are missing or incorrectly configured. Verification commands such as “show ipv6 interface brief” and “show ipv6 ospf neighbor” can confirm the presence of link-local addresses and reveal adjacency failures. Correctly configuring link-local addresses, along with enabling OSPFv3 on relevant interfaces, ensures neighbor formation and LSA propagation. Unique router IDs complement link-local addressing by providing distinct identifiers within the OSPFv3 domain, which are critical for stable routing.
Duplicate router IDs can cause adjacency issues, but do not prevent initial hello packets if link-local addresses exist. Conflicts generate log messages indicating router ID collisions, which can be corrected.
Area authentication mismatches affect LSA acceptance but do not prevent neighbor formation. Hello packets can still be exchanged and adjacencies established even if authentication fails.
IPv6 unicast routing being disabled globally prevents the router from forwarding traffic, but does not stop hello packets from being exchanged using link-local addresses. However, full OSPFv3 operation is impacted if routing is not enabled.
The root cause is missing link-local addresses. Ensuring each interface participating in OSPFv3 has a valid link-local address is essential for adjacency formation, hello packet exchange, and LSA flooding. Proper configuration guarantees stable OSPFv3 operation, predictable convergence, and correct routing table updates. Network engineers must verify link-local addresses, enable OSPFv3 on all interfaces, assign unique router IDs, and monitor neighbor formation. Understanding link-local address requirements is crucial for deploying OSPFv3 successfully in IPv6 networks. Correct planning and verification prevent connectivity issues, maintain route propagation, and provide consistent network operation. Proper link-local address assignment supports reliable adjacency formation, uninterrupted LSA exchange, and predictable convergence, which are essential for efficient and stable IPv6 network operation across enterprise environments.
Question 147:
A network engineer configures 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 Label Distribution Protocol (LDP) relies on underlying IGP connectivity to discover neighbors and establish sessions. LDP uses TCP port 646 to exchange hello and label mapping messages between routers. Routers must have IP reachability, provided by the IGP, to establish LDP sessions. If there is no IGP adjacency or routing between routers, LDP messages cannot be exchanged, and sessions fail to establish. IGP adjacency is essential because LDP does not operate independently of the routing infrastructure; neighbors must be reachable for label information to be distributed and label-switched paths to be set up.
In this scenario, routers fail to establish LDP sessions. This is typically caused by missing IGP adjacency or routing inconsistencies preventing neighbor reachability. Verification involves checking the IGP routing table, examining adjacency status with “show ip ospf neighbor” or “show ip eigrp neighbors,” and pinging neighbor routers to confirm reachability. Ensuring proper IGP adjacency allows LDP messages to be exchanged, neighbors to be discovered, and label-switched paths to be established successfully. Network engineers must confirm that all interfaces participating in LDP have correct IP reachability and that the IGP is fully converged.
Mismatched LDP transport addresses may prevent session establishment in manually configured environments, but are less common in default deployments where LDP uses loopback or primary interface addresses.
MPLS not being globally enabled prevents label switching but does not stop LDP discovery attempts on configured interfaces. LDP sessions can still attempt neighbor discovery even if MPLS forwarding is not active.
LDP hello timers determine the interval at which hello messages are sent. While longer timers may delay session establishment, they do not prevent eventual neighbor discovery if underlying connectivity exists.
The root cause is missing IGP adjacency. Proper IGP configuration ensures that LDP messages can reach neighbors, allowing label distribution and MPLS forwarding. Engineers must verify IGP connectivity, adjacency status, and interface configuration to maintain a reliable MPLS network. Correct IGP operation allows LDP sessions to form, enables efficient label distribution, and ensures predictable traffic engineering. Understanding the dependency between IGP connectivity and LDP operation is critical for successful MPLS deployment. Proper planning, monitoring, and verification prevent session failures and support resilient, high-performance service provider networks. Network engineers must ensure that all IGP adjacencies are established and stable, providing the foundation for predictable LDP operation, successful tunnel establishment, and reliable MPLS traffic forwarding.
Question 148:
A network engineer deploys OSPFv3 in a large IPv6 network. Some routers fail to form neighbor relationships after configuration. 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 heavily on IPv6 link-local addresses for neighbor discovery and adjacency formation. Every interface that participates in OSPFv3 must have a valid link-local address because OSPFv3 uses these addresses as the source and destination of hello packets. If link-local addresses are missing, routers cannot identify each other as neighbors, preventing adjacency formation and halting the exchange of LSAs. Without successful neighbor formation, routing tables remain incomplete, and network convergence is disrupted. Link-local addresses are crucial because OSPFv3 communicates directly on the link, independent of global IPv6 addresses. This ensures that routers can exchange hello packets and establish adjacencies even if global addresses are not yet configured or are changing.
In this scenario, routers fail to form neighbor relationships, which strongly suggests that link-local addresses are missing or improperly configured. Verification involves using commands like “show ipv6 interface brief” to inspect link-local address assignments and “show ipv6 ospf neighbor” to confirm adjacency status. Configuring a unique link-local address for each interface and enabling OSPFv3 ensures proper neighbor formation, LSA exchange, and route propagation. Network engineers must also assign unique router IDs because OSPFv3 uses them to identify routers in the LSDB, complementing link-local communication for stable routing.
Duplicate router IDs can cause adjacency issues because OSPFv3 requires each router to have a unique identifier in the LSDB. However, initial hello packets can still be sent and received if link-local addresses exist. Duplicate router IDs generate log messages indicating conflicts that must be resolved to prevent database inconsistencies.
Area authentication mismatches affect the acceptance of LSAs from neighbors but do not prevent neighbor formation. Hello packets can still be exchanged, and adjacencies formed, even if authentication settings differ.
IPv6 unicast routing being disabled globally prevents forwarding of IPv6 traffic, but does not inherently stop hello packets from being 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 that every interface participating in OSPFv3 has a link-local address is essential for establishing neighbor relationships, exchanging LSAs, and maintaining routing table consistency. Proper configuration guarantees stable OSPFv3 operation, predictable convergence, and reliable network connectivity. Network engineers must verify link-local addresses, enable OSPFv3 on all relevant interfaces, and monitor neighbor formation to maintain operational stability. Understanding the importance of link-local addresses in IPv6 networks allows for reliable OSPFv3 deployment, uninterrupted LSA propagation, and predictable network behavior. Proper planning, configuration, and verification prevent connectivity issues and ensure efficient routing in enterprise networks. Correct link-local addressing supports seamless adjacency formation, consistent route propagation, and optimal IPv6 network performance.
Question 149:
A network engineer configures MPLS LDP in a service provider network. LDP sessions fail to establish between certain routers. 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 Label Distribution Protocol (LDP) relies on underlying IGP connectivity to discover neighbors and establish label distribution sessions. LDP uses TCP port 646 to exchange hello and label mapping messages. For LDP to successfully form neighbor relationships, routers must have IP reachability, which is provided by the IGP. Without proper IGP adjacency or routing, LDP messages cannot reach neighboring routers, and sessions fail. This is a foundational dependency because LDP operates over an established routing infrastructure and does not provide its own discovery mechanism independent of IGP. Missing IGP adjacency prevents neighbor discovery, blocking label exchange, and hindering the establishment of label-switched paths essential for MPLS forwarding.
In this scenario, LDP sessions fail to establish between certain routers. This suggests that the routers do not have proper IGP adjacency or routing between them. Verification involves checking IGP tables and adjacency status using commands like “show ip ospf neighbor” or “show ip eigrp neighbors.” Engineers can also use ping or traceroute to validate IP reachability between LDP neighbors. Ensuring proper IGP adjacency and connectivity allows LDP messages to be exchanged, neighbors to be discovered, and label-switched paths to be established successfully. Network engineers must ensure that all interfaces participating in LDP have correct IP addressing and that the IGP is fully converged across the network. Proper configuration of IGP adjacencies forms the foundation for successful LDP deployment, enabling reliable MPLS traffic forwarding.
Mismatched LDP transport addresses can prevent session establishment if the manually configured transport addresses differ, but in typical deployments, LDP uses loopback or primary interface addresses, making this less common.
MPLS not being enabled globally prevents label switching and forwarding, but does not stop LDP from attempting neighbor discovery. LDP can still attempt to establish sessions, although label exchange will not function until MPLS is activated.
LDP hello timers determine the frequency of hello messages. While longer timers may delay session formation, they do not prevent neighbor discovery if IGP connectivity exists.
The root cause is missing IGP adjacency. Ensuring that all routers have proper IGP reachability is critical for successful LDP session establishment. Proper IGP configuration allows LDP messages to be exchanged, enables label distribution, and ensures efficient MPLS forwarding. Network engineers must monitor IGP adjacency, interface configuration, and LDP session status to maintain predictable and stable MPLS operations. Understanding the dependency between IGP connectivity and LDP functionality is crucial for successful MPLS deployment. Proper planning, monitoring, and verification prevent session failures and support resilient, high-performance service provider networks. Network engineers must ensure all IGP adjacencies are established and stable, providing a reliable foundation for LDP operation, label-switched path formation, and efficient MPLS traffic forwarding.
Question 150:
A network engineer configures RSVP-TE tunnels in an MPLS network. The tunnels fail to establish, sh even though links have sufficient bandwidth. 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:
MPLS Traffic Engineering enables explicit path selection based on available resources, administrative constraints, and link attributes. RSVP-TE is used to signal and reserve bandwidth along these paths. The Constrained Shortest Path First (CSPF) algorithm evaluates all possible paths against configured TE constraints, such as bandwidth, TE colors, and administrative groups, to find a feasible path. Even if links report sufficient bandwidth, tunnels can fail if CSPF cannot identify a path that satisfies all constraints. For example, misconfigured TE colors, inconsistent administrative group assignments, or conflicting path requirements can prevent CSPF from computing a valid path, resulting in failed RSVP-TE tunnel establishment.
In this scenario, tunnels fail despite sufficient bandwidth, which suggests that CSPF cannot find a feasible path due to link attribute constraints. Verification involves checking TE link attributes, examining CSPF calculations, and reviewing RSVP-TE logs. Correct configuration of TE attributes ensures CSPF can compute a valid path, allowing RSVP-TE tunnels to be established successfully. Engineers must confirm that all links along the desired path meet bandwidth requirements, have consistent TE color and administrative group settings, and are operational. Proper planning includes validating TE configurations, simulating CSPF path calculations, and performing end-to-end tunnel testing before deployment. This ensures predictable tunnel establishment, efficient traffic engineering, and reliable MPLS operation.
RSVP authentication mismatches affect signaling security but do not prevent CSPF from computing paths. Authentication failures prevent tunnel establishment only if signaling is rejected, not due to path feasibility.
RSVP soft-state refresh timers maintain tunnel state but do not impact CSPF computation. Longer timers may delay state updates, but do not prevent tunnel setup if a feasible path exists.
IGP metrics influence unconstrained SPF calculations but cannot override TE constraints. CSPF will reject paths that do not satisfy bandwidth, administrative, or color constraints regardless of metric values.
The root cause is CSPF failing to find a feasible path due to link attribute constraints. Proper configuration of TE attributes, bandwidth, and administrative groups is essential for successful RSVP-TE deployment. Ensuring consistent link attributes across the network allows predictable tunnel establishment, efficient traffic engineering, and optimal MPLS performance. Engineers must monitor CSPF computations, verify TE configurations, and validate tunnel setups to maintain operational MPLS networks. Understanding the interaction between CSPF, TE attributes, and RSVP-TE signaling is critical for deploying MPLS networks successfully. Proper planning, configuration, and verification prevent tunnel failures and support resilient, high-performance MPLS operations, ensuring predictable traffic flow and optimized network resource utilization.