Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 9 Q121-135
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Question 121:
A network engineer is configuring EIGRP for a large enterprise network. After enabling EIGRP, some routers fail to form neighbor relationships with others on the same subnet. What is the most likely cause?
A) Mismatched K values between routers.
B) EIGRP AS numbers are different.
C) Passive interface is configured on one router.
D) Split-horizon is disabled on a hub router.
Answer: B)
Explanation:
EIGRP is a distance vector routing protocol that relies on neighbor relationships to exchange routing information. Routers can only establish neighbor relationships if they share the same autonomous system (AS) number, among other parameters. The AS number is a locally significant identifier that defines the EIGRP domain. Routers with mismatched AS numbers will not form neighbors, regardless of physical connectivity or correct IP addressing. The neighbor relationship is foundational for EIGRP operation because routes are only advertised to routers that have successfully formed a neighbor relationship. Without forming neighbors, the routing table will remain incomplete, and network segments will experience reachability issues.
In this scenario, some routers on the same subnet fail to form neighbor relationships. This is indicative of an AS number mismatch. Even though the routers are physically connected and configured on the correct subnet, the EIGRP process will ignore potential neighbors with different AS numbers. Correct configuration of the AS number across all routers in the EIGRP domain is crucial for consistent and stable routing operation. Verification commands like “show ip eigrp neighbors” and “show running-config” are essential tools for identifying misconfigurations.
Mismatched K values, which define the metric calculation parameters, can cause routes not to be advertised or considered feasible successors, but do not prevent neighbor establishment. Routers with different K values can still form neighbor relationships, but routing decisions may differ if metric calculations are inconsistent.
A passive interface configuration prevents the router from sending EIGRP hello packets out the interface. While this can prevent neighbor formation on that interface, it is typically intentional and only affects the specific interface. In this case, multiple routers on the same subnet fail to form neighbors, making the passive interface scenario less likely.
Split-horizon, which prevents a router from advertising a route back out of the interface it was learned, can impact route propagation but does not prevent neighbor formation. Disabling split-horizon on a hub router is relevant for specific topologies, such as hub-and-spoke, but does not affect initial EIGRP neighbor negotiation.
The root cause is the mismatched EIGRP AS number. Ensuring all routers within the same EIGRP domain use the same AS number is critical for neighbor establishment, route exchange, and network convergence. Proper configuration avoids routing table inconsistencies, unreachable networks, and convergence delays. Network engineers should maintain consistent documentation and automated configuration templates to prevent misconfigurations. Neighbor verification and monitoring are essential to detect issues promptly. In large networks, small AS number mismatches can lead to significant connectivity problems, making proactive configuration checks vital. Ensuring all routers share the same AS number guarantees the successful formation of neighbor relationships, which is the foundation for all subsequent route exchange and convergence processes in EIGRP networks.
Question 122:
A network engineer deploys OSPFv3 in an IPv6 network. After configuration, the routers are not exchanging LSAs. What is the most likely cause?
A) Link-local addresses are missing on interfaces.
B) OSPFv3 process ID is mismatched.
C) Area type is configured as a stub.
D) IPv6 routing is disabled globally.
Answer: A)
Explanation:
OSPFv3, the version of OSPF used for IPv6 networks, relies on link-local addresses for neighbor formation and LSA exchange. Every interface participating in OSPFv3 must have a valid IPv6 link-local address because OSPFv3 routers use these addresses to identify neighbors and establish adjacencies. Link-local addresses are required for sending and receiving hello packets, which initiate the neighbor negotiation process. Without a link-local address, routers cannot form neighbors, failing to exchange LSAs. Since OSPFv3 operates independently of global IPv6 addresses, the absence of link-local addresses directly prevents adjacency formation, causing the network to appear as if OSPFv3 is not functioning.
In this scenario, routers are not exchanging LSAs after OSPFv3 configuration. This suggests that link-local addresses are either missing or improperly configured on the interfaces participating in OSPFv3. Verification commands such as “show ipv6 interface brief” and “show ipv6 ospf neighbor” can help identify missing or misconfigured link-local addresses. Correct configuration involves ensuring each interface has a link-local address and enabling OSPFv3 with the appropriate router ID and area assignment.
Mismatched OSPFv3 process IDs do not prevent neighbor formation. OSPFv3 process IDs are locally significant identifiers used to distinguish different OSPF instances on a router. Neighbor formation and LSA exchange are not affected by differences in process ID across routers.
Stub area configuration affects the propagation of external and summary LSAs but does not prevent the formation of neighbor relationships. Routers can still exchange intra-area LSAs and form adjacencies even in stub areas.
Disabling global IPv6 routing prevents traffic forwarding and IPv6 operation but does not inherently prevent OSPFv3 from functioning if the interfaces have link-local addresses. However, full routing functionality will not be available without enabling IPv6.
The root cause is the absence or misconfiguration of link-local addresses on interfaces. Ensuring that each interface participating in OSPFv3 has a valid link-local address is essential for adjacency formation, hello packet exchange, and LSA propagation. Correct configuration ensures reliable OSPFv3 operation, predictable network convergence, and proper route distribution. Network engineers must verify link-local addresses, enable OSPFv3, assign router IDs, and check neighbor relationships to maintain operational OSPFv3 networks. Understanding the role of link-local addresses is crucial in IPv6 networks because OSPFv3 uses these addresses as the foundation for all routing exchanges. Proper deployment guarantees stable OSPFv3 operation, seamless neighbor formation, and consistent LSA propagation across the network.
Question 123:
A network engineer configures MPLS LDP on a service provider network. Some routers do not establish LDP sessions with their neighbors. What is the most likely cause?
A) IGP adjacency is missing between the routers.
B) LDP transport addresses are mismatched.
C) MPLS is not globally enabled.
D) LDP discovery timers are too long.
Answer: A)
Explanation:
MPLS Label Distribution Protocol (LDP) relies on underlying IGP connectivity for session establishment between neighbors. LDP requires that routers have reachability to each other, typically verified by the IGP. If there is no IGP adjacency between two routers, they cannot communicate directly, preventing the establishment of LDP sessions. LDP uses TCP port 646 for communication between neighbors, and this communication depends on successful IP routing provided by the IGP. Without IGP reachability, LDP discovery messages cannot be exchanged, resulting in a failed session establishment.
In this scenario, some routers do not establish LDP sessions with neighbors. This is commonly caused by missing IGP adjacency or incorrect routing that prevents direct communication. Verification involves checking the IGP routing table, adjacency status, and pinging neighbor routers to ensure reachability. Establishing proper IGP adjacencies and ensuring correct routing solves LDP session establishment issues.
Mismatched LDP transport addresses, which define the source address used for LDP messages, can cause sessions to fail, but typically only in specific configurations where manually assigned addresses are used. In most default deployments, LDP uses the loopback interface or primary interface address as the transport address, and mismatches are less common.
MPLS not being globally enabled prevents label switching but does not directly prevent LDP from attempting session establishment on interfaces where MPLS is enabled. However, labels will not be distributed without MPLS.
LDP discovery timers affect how frequently LDP messages are sent, but even with longer timers, eventual session establishment should occur if the underlying connectivity is correct. Delays do not prevent session formation permanently.
The root cause is missing IGP adjacency between routers. Ensuring that all routers have proper IGP connectivity is critical for LDP session establishment, label distribution, and MPLS functionality. Network engineers must verify IGP reachability, adjacency, and routing correctness to maintain a stable MPLS network. Correct IGP operation ensures LDP sessions form correctly, enabling MPLS traffic engineering, label switching, and optimal data plane performance. Proper planning, verification, and monitoring of IGP connectivity are essential for robust MPLS operation across service provider networks.
Question 124:
A network engineer configures EIGRP in a network but notices that certain routes are not appearing in the routing table even though they exist on neighboring routers. What is the most likely cause?
A) The routes are passive on the neighbor router.
B) The feasible distance is higher than the reported distance.
C) The routes are filtered by a distribute-list.
D) The router has split-horizon enabled on the interface.
Answer: C)
Explanation:
EIGRP is a distance vector routing protocol that relies on the exchange of routing updates between neighbors. Route visibility in the routing table depends on the proper advertisement of routes from neighboring routers and the absence of filtering mechanisms. Distribute-lists in EIGRP allow administrators to filter routes that are either sent to neighbors or accepted into the routing table. When a distribute-list is applied incorrectly or too restrictively, certain routes may be blocked from being installed in the routing table despite being advertised by neighbors. This results in incomplete routing tables and potential connectivity issues within the network.
In this scenario, certain routes exist on neighboring routers but are missing in the local routing table. This strongly suggests that a distribute-list is filtering those routes, preventing their installation. Verification commands such as “show ip route” and “show running-config” are essential to identify the presence of distribute-lists and understand their impact. Correctly configuring distribute-lists ensures that only desired routes are filtered while maintaining full connectivity for essential networks.
Routes being passive on the neighbor router affects whether the neighbor sends updates, but passive interfaces do not prevent a local router from receiving existing routes if other neighbors are advertising them. Therefore, while relevant to EIGRP behavior, passive interfaces are unlikely to be the root cause in this scenario where neighbors are actively advertising routes.
The feasible distance versus reported distance relationship determines route selection within EIGRP. A feasible successor must have a reported distance less than the feasible distance to qualify as a backup path. However, this does not block valid routes from being installed in the routing table if they are feasible successors or primary successors. Metric differences alone cannot explain missing routes that are otherwise present on the neighbor.
Split-horizon is a mechanism that prevents a router from advertising a route back out the interface from which it was learned. While this can impact route propagation in certain topologies, it does not typically prevent a router from receiving and installing routes from other interfaces. In most point-to-point or multi-access networks, split-horizon alone would not cause the complete absence of routes in the routing table.
The root cause is the presence of a distribute-list filtering routes. Ensuring correct distribution and filtering of routes is critical for network stability, reachability, and performance. Administrators must carefully plan and document distribution lists to prevent unintended route omissions. Verification of routing tables, careful design of filtering policies, and ongoing monitoring are essential to maintain consistent and complete network connectivity. Proper configuration ensures that important routes are received and installed in the routing table, supporting efficient network operation and preventing reachability issues caused by inadvertent route filtering. Understanding the role of distribute-lists in EIGRP enables engineers to balance route control with network reliability, ensuring that routing information flows as intended while limiting unnecessary propagation of routes that may not be relevant.
Question 125:
A network engineer is deploying OSPFv3 in an IPv6 network. After configuring the protocol, routers do not form neighbor relationships. What is the most likely cause?
A) Link-local addresses are missing on the interfaces.
B) Router IDs are duplicated across the network.
C) Area authentication is mismatched.
D) IPv6 unicast routing is disabled globally.
Answer: A)
Explanation:
OSPFv3, designed for IPv6 networks, relies heavily on link-local addresses for establishing neighbor relationships and exchanging LSAs. Every interface participating in OSPFv3 must have a valid link-local address because these addresses are used as the source and destination for OSPFv3 hello packets. Neighbor relationships cannot form without link-local addresses, as OSPFv3 cannot identify and communicate with adjacent routers. The importance of link-local addresses is a fundamental difference from OSPFv2, which relies primarily on IPv4 addresses. In OSPFv3, global IPv6 addresses are not sufficient for adjacency formation.
In this scenario, routers fail to form neighbor relationships after OSPFv3 configuration. This indicates that link-local addresses are either missing or misconfigured on the participating interfaces. Verification commands such as “show ipv6 interface brief” and “show ipv6 ospf neighbor” can confirm whether link-local addresses exist and whether hello packets are being sent and received. Correctly configuring link-local addresses and enabling OSPFv3 on the appropriate interfaces ensures successful neighbor establishment and LSA exchange.
Router ID duplication can prevent proper OSPFv3 operation and cause adjacency issues. However, OSPFv3 routers will still attempt to form neighbor relationships using link-local addresses, and adjacency failures due to router ID conflicts typically generate specific log messages.
Area authentication, while important for securing OSPF exchanges, does not prevent adjacency formation if link-local addresses are missing. Authentication mismatches cause the router to reject LSAs from neighbors, but not prevent hello packet exchange and neighbor negotiation initially.
Disabling IPv6 unicast routing globally prevents the router from forwarding IPv6 packets, but does not inherently prevent OSPFv3 from sending hello packets from a link-local address if the interface has one configured. However, the lack of IPv6 routing may affect full operation and LSA flooding.
The root cause is the absence or misconfiguration of link-local addresses. Ensuring that each interface participating in OSPFv3 has a valid link-local address is critical for adjacency formation, hello packet exchange, and LSA propagation. Proper configuration ensures stable OSPFv3 operation, predictable network convergence, and correct route distribution. Engineers should verify link-local addresses, enable OSPFv3, assign unique router IDs, and check neighbor relationships to maintain operational networks. Understanding link-local addresses is essential in IPv6 networks because OSPFv3 uses them as the foundation for routing protocol communication. Correct deployment guarantees stable OSPFv3 operation, seamless neighbor formation, and consistent LSA propagation across the network. Proper planning and verification prevent connectivity issues and maintain reliable routing information flow across the network.
Question 126:
A network engineer is deploying MPLS LDP in a service provider network. Some routers fail to establish LDP sessions with their 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 the existence of underlying IGP connectivity between routers for session establishment. LDP uses TCP port 646 to communicate with neighbors, and routers must have IP reachability provided by the IGP to exchange hello and label mapping messages. If there is no IGP adjacency or proper routing between two routers, LDP messages cannot be exchanged, and the session will fail to establish. IGP adjacency is foundational because LDP cannot function independently of the underlying routing infrastructure. Without connectivity, LDP cannot discover neighbors, exchange label information, or maintain label-switched paths.
In this scenario, some routers fail to establish LDP sessions. This is commonly due to missing IGP adjacency or routing inconsistencies that prevent neighbor reachability. Verification steps include checking the IGP routing table, confirming adjacency with commands like “show ip route” or “show ip ospf neighbor,” and pinging neighbor routers. Ensuring correct IGP operation and adjacency is critical for establishing LDP sessions successfully.
Mismatched LDP transport addresses, which specify the source address used for LDP messages, can prevent sessions from forming in specific configurations but are less common in default deployments where loopback addresses are used.
MPLS not being enabled globally affects label switching but does not necessarily prevent LDP from attempting neighbor establishment on interfaces where MPLS is configured. However, without global MPLS, label distribution cannot be completed.
LDP hello timers determine how frequently hello messages are sent to neighbors. Longer timers may delay session establishment, but do not prevent eventual session formation if connectivity exists.
The root cause is missing IGP adjacency. Ensuring proper IGP connectivity guarantees that LDP messages can be exchanged, neighbors discovered, and label-switched paths established. Proper planning, configuration, and monitoring of IGP adjacencies are essential for stable MPLS networks. Correct IGP operation allows LDP to establish sessions, distribute labels, and enable MPLS forwarding. Network engineers must verify IGP reachability, adjacency status, and interface configuration to maintain a reliable MPLS environment. Proper IGP and LDP coordination ensures predictable traffic engineering, optimal path selection, and network resilience. Verification, documentation, and monitoring are critical for preventing unexpected session failures and maintaining efficient MPLS operations across the service provider network.
Question 127:
A network engineer configures OSPF on a large enterprise network. After deployment, certain routers in a remote area fail to receive external routes redistributed from BGP. What is the most likely cause?
A) The remote area is configured as a stub area.
B) Type-1 LSAs are filtered.
C) The backbone area is down.
D) OSPF process IDs are mismatched.
Answer: A)
Explanation:
OSPF area types influence how routing information is propagated throughout an OSPF domain. A stub area, commonly used in remote sites or branch offices, is designed to reduce LSDB size and limit the propagation of external Type-5 LSAs. When a stub area is configured, routers inside that area do not install external routes from outside the OSPF domain. Instead, they rely on a default route provided by the ABR to reach external destinations. This mechanism simplifies the routing table, reduces memory consumption, and optimizes CPU utilization on routers with limited resources.
In this scenario, remote routers fail to receive external routes redistributed from BGP. This indicates that the area is likely configured as a stub area. Stub areas prevent the installation of Type-5 LSAs, which are used to advertise external routes. Consequently, routers in the stub area can only use a default route to reach external networks. Verification involves checking area configurations with commands like “show ip ospf” and reviewing the ABR configuration for default route injection. Correctly configuring ABRs to advertise a default route ensures connectivity to external networks while maintaining the benefits of stub area optimization.
Type-1 LSAs describe a router’s own links within an area. Filtering Type-1 LSAs would prevent intra-area routing and neighbor formation, which is not the issue in this case since routers can communicate internally.
A down backbone area would prevent inter-area routing for all connected areas. Since only the remote area is affected and other areas can receive external routes, the backbone area is functioning correctly.
OSPF process IDs are locally significant identifiers and do not affect LSA propagation across areas. Differences in process IDs would not prevent external routes from being advertised or received by routers.
The root cause is that the remote area is configured as a stub area. While stub areas provide performance and scalability benefits, they require careful planning to ensure that routers can reach external networks via a default route. Network engineers must verify ABR configurations, confirm default route advertisement, and ensure that stub area settings align with network reachability requirements. Misconfigured stub areas or missing default routes can isolate routers from external destinations. Proper configuration ensures optimized LSDB size, efficient memory usage, and predictable routing behavior, while maintaining connectivity to external resources. Understanding the impact of OSPF area types is crucial for large-scale enterprise deployments to prevent unexpected reachability issues while benefiting from simplified routing tables and reduced processing overhead.
Question 128:
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 update dampening is enabled.
Answer: A)
Explanation:
BGP multipath allows multiple equal-cost paths to be installed in the routing table, improving redundancy, load balancing, and utilization of network bandwidth. For multipath to function correctly, candidate paths must be identical in certain attributes, including AS path length, origin type, MED, local preference, and next-hop reachability. If any of these attributes differ, BGP selects a single best path according to its deterministic decision process. This ensures predictable routing and prevents routing loops while maintaining stability in the network.
In this scenario, multiple paths are present in the BGP table, but only one path is used for forwarding traffic. This behavior indicates that the candidate paths differ in AS path, origin type, or MED attributes. For example, AS path prepending on one route or differences in the origin type can cause BGP to choose a single best path. Network engineers must ensure that these attributes are consistent across all candidate paths to enable effective multipath forwarding. Verification commands such as “show bgp ipv4 unicast” and “show ip route” help identify differences in attributes and confirm which paths are considered equal.
iBGP neighbors do support multipath if the equality conditions are satisfied. Therefore, having all neighbors as iBGP does not prevent multipath usage.
An unreachable next-hop would prevent the installation of a path in the routing table, but since multiple paths appear in the BGP table, reachability is not the issue in this case.
BGP update dampening reduces the frequency of updates sent to neighbors to prevent route flapping. While it affects route advertisement, it does not prevent multipath selection locally on the router if multiple equal-cost paths are available.
The root cause is that the paths are not equal in AS path, origin, or MED attributes. Ensuring equality among candidate paths is critical for leveraging multipath forwarding, which improves network redundancy and load distribution. Proper configuration allows multiple paths to be used simultaneously, enhancing bandwidth utilization and preventing congestion on a single path. Network engineers must monitor BGP attributes, adjust configurations to align AS path, origin, and MED values, and verify multipath operation. Correct deployment ensures effective traffic distribution, predictable routing behavior, and efficient utilization of network resources. Understanding the strict requirements for BGP multipath is essential for multi-homed environments to achieve optimal performance and resilience.
Question 129:
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) Link attribute constraints prevent CSPF from finding a feasible path.
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 network operators to establish explicit paths through a network based on available bandwidth, administrative constraints, and link attributes. RSVP-TE is the signaling protocol used to reserve bandwidth along these paths. The Constrained Shortest Path First (CSPF) algorithm computes a feasible path by evaluating candidate paths against requested TE constraints, including available bandwidth, TE colors, and administrative groups. Even if all links report sufficient bandwidth, tunnels may fail if CSPF cannot find a path that satisfies all constraints. Mismatched link attributes, such as incorrect TE colors or missing administrative group markings, prevent CSPF from establishing a feasible path and result in failed tunnel setup.
In this scenario, tunnels fail to establish despite sufficient bandwidth. This indicates that CSPF cannot find a path meeting all constraints due to link attribute mismatches. Verification involves examining TE link attributes, CSPF computation results, and RSVP-TE logs to identify inconsistencies. Correct configuration of TE attributes ensures that CSPF can compute valid paths and that RSVP-TE tunnels are established successfully. Proper planning involves verifying bandwidth, administrative groups, and TE color assignments across the intended path.
RSVP authentication, while important for security, does not impact CSPF computation. Mismatched authentication prevents tunnels from being established, but is independent of path feasibility calculation.
Soft-state refresh timers maintain tunnel state but do not prevent the initial CSPF path computation. Longer timers may delay state refresh, but do not block path establishment if constraints are met.
IGP metrics influence the shortest-path calculation but do not override TE constraints. Even a path with the lowest IGP metric will be rejected by CSPF if it does not meet TE constraints, such as bandwidth or administrative attributes.
The root cause is the link attribute constraints preventing CSPF from finding a feasible path. Ensuring consistent TE attributes, proper administrative group configuration, and sufficient bandwidth along all links in the intended path is critical for successful RSVP-TE tunnel deployment. Correct configuration guarantees predictable MPLS TE operation, optimized bandwidth utilization, and reliable traffic engineering. Network engineers must verify link attributes, perform CSPF simulations, and monitor tunnel establishment to maintain efficient MPLS networks. Proper alignment of constraints ensures that RSVP-TE tunnels are established as intended, supporting redundancy, predictable performance, and optimized resource utilization. Understanding the interplay between CSPF, TE attributes, and RSVP-TE signaling is essential for maintaining operational MPLS networks and preventing tunnel failures.
Question 130:
A network engineer deploys OSPF in a multi-area network. Certain routers in area 23 are unable to reach external networks redistributed from BGP. What is the most likely cause?
A) Area 23 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 significantly impact the propagation of routing information. A totally stub area is a specific configuration that restricts routers within that area from installing both external Type-5 LSAs and inter-area Type-3 summary LSAs. This restriction reduces the routing table size and conserves router memory and CPU resources. Totally stub areas are often used in remote branches or sites with limited resources. In such areas, routers rely on a default route injected by the ABR to reach destinations outside the area. The ABR generates a Type-3 summary LSA as a default route, allowing external connectivity without the overhead of maintaining full external route information.
In this scenario, routers in area 23 are unable to reach external networks redistributed from BGP. This behavior is consistent with a totally stub area configuration, where Type-5 LSAs and Type-3 summary LSAs for external routes are filtered, leaving only the default route. If the ABR fails to inject the default route, connectivity to external networks is lost. Troubleshooting involves checking the area type with commands such as “show ip ospf” and verifying that the ABR is configured to advertise the default route using “default-information originate.” Correct configuration ensures that routers in the stub area can reach external networks while still benefiting from reduced routing table size.
Blocking Type-3 LSAs via an ACL could prevent inter-area route propagation. However, this scenario specifically aligns with a totally stub area design, which inherently filters Type-3 LSAs except the default route.
A down backbone area would affect all connected areas, not just area 23. Since only this area experiences the issue, the backbone is functioning correctly.
OSPF process IDs are locally significant and do not affect LSA propagation across routers. Differences in process IDs would not prevent external route reception if adjacency and area configuration are correct.
The root cause is that area 23 is configured as a totally stubbed area. While this design optimizes resources and simplifies the routing table, it requires proper ABR configuration to ensure default routes are advertised. Misconfigurations can result in routers losing access to external networks. Network engineers must plan area types carefully, document configurations, and validate ABR default route advertisements. Proper verification using “show ip route” and “show ip ospf database” ensures that connectivity is maintained while optimizing performance. Understanding the behavior of totally stub areas is critical for network design, particularly in large-scale deployments where memory and CPU optimization are necessary. Correctly configuring stub areas allows a balance between efficiency and reachability, ensuring predictable network behavior and reliable access to external destinations.
Question 131:
A network engineer deploys BGP multipath on a multi-homed enterprise network. Multiple paths appear in the BGP table, but traffic is only forwarded through one path. What is the most likely cause?
A) Paths are not equal in AS path, origin, or MED attributes.
B) All 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 distributed across multiple equal-cost paths, improving redundancy and bandwidth utilization. For multipath to be effective, the candidate paths must match exactly in key attributes such as AS path length, origin type, MED, local preference, and next-hop reachability. If any of these attributes differ, BGP selects a single best path according to its decision algorithm. This deterministic selection ensures stable routing and prevents loops, but it also means that potential alternate paths are ignored if they are not equal in attributes.
In this scenario, multiple paths exist in the BGP table, yet only one path is used. This suggests that the candidate paths are not identical in AS path, origin, or MED attributes. For example, one route may have AS path prepending or a different origin type, leading BGP to choose a single best path for forwarding. Network engineers must ensure that multipath conditions are satisfied by aligning these attributes. Verification commands such as “show bgp ipv4 unicast” and “show ip route” help identify attribute differences and confirm which paths are considered equal.
All neighbors being in iBGP does not prevent multipath usage if equality requirements are met. iBGP supports multipath, provided the attributes of the candidate paths are identical.
An unreachable next-hop would prevent a path from being installed in the routing table. However, the presence of multiple paths in the BGP table indicates that reachability is not the issue in this case.
BGP route dampening reduces the frequency of updates to stabilize the routing environment. While it affects route advertisement, it does not prevent multipath selection locally if multiple paths meet the equality criteria.
The root cause is that candidate paths differ in AS path, origin, or MED attributes. Ensuring equality among these attributes is essential for BGP multipath to function correctly. Proper configuration allows multiple paths to be used for load balancing, improving redundancy and bandwidth utilization. Network engineers must monitor BGP attributes, adjust configurations as needed, and verify multipath operation to maintain balanced and predictable traffic forwarding. Understanding the strict equality requirements for BGP multipath is crucial in multi-homed environments to achieve optimal performance, reduce congestion, and maximize network efficiency. Correct deployment ensures traffic is distributed effectively, enhancing redundancy, performance, and resilience. Documentation, monitoring, and ongoing verification are key to maintaining proper multipath behavior and avoiding traffic concentration on a single path.
Question 132:
A network engineer configures RSVP-TE tunnels in an MPLS network. Tunnels fail to establish, even though all links report sufficient bandwidth. What is the most likely cause?
A) 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 setup based on bandwidth availability, administrative constraints, and link attributes. RSVP-TE signaling is used to reserve resources along these paths. The Constrained Shortest Path First (CSPF) algorithm computes feasible paths by evaluating all candidate routes against TE constraints such as available bandwidth, TE colors, and administrative groups. Even when sufficient bandwidth exists, tunnels may fail if CSPF cannot find a path that satisfies all configured constraints. Link attribute mismatches, such as incorrect TE colors or administrative group markings, prevent CSPF from establishing a valid path, resulting in tunnel setup failures.
In this scenario, tunnels do not establish despite adequate bandwidth. This indicates that CSPF fails due to link attribute mismatches or other TE constraints. Verification involves examining TE link attributes, CSPF computations, and RSVP-TE logs. Correctly configuring TE attributes ensures that CSPF identifies a valid path and that RSVP-TE tunnels can be established successfully. Engineers must ensure that administrative groups, bandwidth constraints, and TE colors are consistent across all links in the intended path. Proper planning includes reviewing all TE attributes, performing path simulations, and validating tunnel setups before deployment.
RSVP authentication, while important for security, does not affect CSPF path computation. Authentication mismatches prevent tunnel establishment but are separate from path feasibility issues.
RSVP soft-state refresh timers maintain tunnel state but do not prevent initial CSPF path computation. Longer timers may delay state refreshes, but will not block initial tunnel setup if CSPF finds a valid path.
IGP metrics influence unconstrained SPF computation but do not override TE constraints. Even the shortest IGP path will be rejected if it does not satisfy bandwidth or administrative requirements.
The root cause is CSPF being unable to find a feasible path due to link attribute constraints. Ensuring consistent TE attributes, sufficient bandwidth, and correct administrative group configuration is critical for successful RSVP-TE tunnel deployment. Correct configuration guarantees predictable MPLS TE operation, optimized resource usage, and reliable tunnel establishment. Engineers must monitor CSPF calculations, verify TE attributes, and perform ongoing tunnel validation to maintain operational MPLS networks. Proper understanding of CSPF, RSVP-TE, and TE constraints ensures tunnels are established as intended, supporting redundancy, traffic engineering, and network resilience. Planning, configuration verification, and monitoring are essential for maintaining predictable MPLS TE behavior and avoiding unexpected tunnel failures.
Question 133:
A network engineer configures EIGRP in 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 K values between routers.
B) EIGRP AS numbers are different.
C) Passive interface is configured on one router.
D) Split-horizon is disabled on a hub router.
Answer: B)
Explanation:
EIGRP is a distance vector routing protocol that establishes neighbor relationships to exchange routing information. One of the key requirements for neighbor establishment is that routers must belong to the same EIGRP autonomous system (AS). The AS number identifies the EIGRP routing domain and is locally significant on each router. Routers with mismatched AS numbers will not form neighbors even if they are physically connected and configured on the same subnet. This is because the AS number is included in EIGRP hello packets, and routers with different AS numbers reject these packets. Without forming neighbors, EIGRP cannot exchange routing updates, resulting in incomplete routing tables and potential network reachability issues.
In this scenario, some routers on the same subnet fail to form neighbor relationships, which strongly indicates an AS number mismatch. Troubleshooting involves checking the EIGRP configuration on all routers using commands like “show running-config” and “show ip eigrp neighbors” to confirm that the AS numbers match. Correct configuration of the AS number across all routers ensures proper neighbor establishment and enables the exchange of routing information. Network engineers should maintain consistent documentation of EIGRP domains and verify AS numbers during deployment or expansion to prevent neighbor formation issues.
Mismatched K values, which define the metric calculation parameters, can cause routes not to be advertised correctly, but do not prevent neighbor establishment. Neighbor relationships are formed based on AS number, hello intervals, hold timers, and interface configuration, independent of K values.
Passive interface configuration prevents EIGRP hello packets from being sent out on an interface. While this can prevent neighbor formation on that specific interface, it does not explain a widespread issue across multiple routers on the same subnet unless the interface is incorrectly configured on all routers.
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 neighbor formation. Disabling split-horizon is sometimes necessary in hub-and-spoke topologies, but it is unrelated to initial neighbor establishment.
The root cause is mismatched EIGRP AS numbers. Ensuring all routers in the same EIGRP domain use the same AS number is critical for establishing neighbor relationships, exchanging routes, and achieving network convergence. Proper configuration avoids incomplete routing tables, unreachable networks, and delayed convergence. Network engineers should validate AS numbers, interface configurations, and adjacency status using verification commands. Understanding the significance of AS numbers in EIGRP prevents neighbor formation failures and ensures stable, predictable routing behavior. Correct deployment facilitates seamless route propagation, efficient network operation, and optimized use of network resources while maintaining robust connectivity across the enterprise network.
Question 134:
A network engineer deploys OSPFv3 in an IPv6 network. Routers fail to exchange LSAs after configuration. What is the most likely cause?
A) Link-local addresses are missing on interfaces.
B) Router IDs are duplicated.
C) Area authentication is mismatched.
D) IPv6 unicast routing is disabled globally.
Answer: A)
Explanation:
OSPFv3, which is designed for IPv6 networks, relies on 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 a link-local address, routers cannot form neighbor adjacencies, preventing the exchange of LSAs and stopping route propagation. This is a critical distinction from OSPFv2, which primarily uses global IPv4 addresses for adjacency formation. The use of link-local addresses ensures consistent communication between adjacent routers on the same link and enables reliable LSA flooding throughout the network.
In this scenario, routers fail to exchange LSAs after OSPFv3 configuration. This strongly suggests that link-local addresses are missing or improperly configured. Verification commands such as “show ipv6 interface brief” and “show ipv6 ospf neighbor” can confirm the presence of link-local addresses and identify adjacency failures. Correctly configuring link-local addresses and enabling OSPFv3 on all participating interfaces ensures proper neighbor formation, LSA exchange, and routing table convergence. Proper planning involves assigning link-local addresses either automatically or manually and ensuring unique router IDs for OSPFv3 operation.
Duplicated router IDs can cause OSPFv3 adjacency issues, but do not prevent initial hello packet exchange if link-local addresses are correctly configured. Router ID conflicts typically generate specific log messages, which can help in troubleshooting.
Mismatched area authentication affects the acceptance of LSAs from neighbors but does not prevent neighbor formation itself. Hello packet exchange can still occur, and adjacencies can form even if authentication fails.
Disabling global IPv6 unicast routing prevents the router from forwarding IPv6 traffic, but does not prevent OSPFv3 from sending hello packets via link-local addresses if they exist. However, full network operation may be compromised without IPv6 routing 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 route distribution. Network engineers must verify link-local addresses, enable OSPFv3, assign unique router IDs, and monitor neighbor formation. Understanding the importance of link-local addresses in IPv6 networks allows for reliable OSPFv3 deployment, uninterrupted LSA propagation, and consistent network operation. Proper planning, verification, and documentation are critical to avoid connectivity issues and maintain efficient routing protocol performance across the enterprise network.
Question 135:
A network engineer configures MPLS LDP in a service provider network. Some routers fail to establish LDP sessions with their 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) is used to distribute labels and establish label-switched paths across an MPLS network. LDP relies on underlying IGP connectivity for neighbor discovery and session establishment. Routers must have IP reachability to each other, provided by the IGP, for LDP to function correctly. LDP uses TCP port 646 to exchange hello and label mapping messages. If there is no IGP adjacency or proper routing between routers, LDP messages cannot be exchanged, and the session fails to establish. IGP adjacency is a foundational requirement because LDP cannot operate independently of the IP routing infrastructure. Without IGP reachability, neighbors cannot be discovered, label information cannot be exchanged, and MPLS forwarding cannot function properly.
In this scenario, routers fail to establish LDP sessions with their neighbors. This typically indicates missing IGP adjacency or routing inconsistencies that prevent neighbor reachability. Verification involves checking the IGP routing table, ensuring adjacency formation, and pinging neighbor routers. Commands such as “show ip route,” “show ip ospf neighbor,” or “show ip eigrp neighbors” can confirm connectivity and identify the root cause. Ensuring proper IGP adjacency is critical for the successful establishment of LDP sessions, enabling label distribution and MPLS functionality.
Mismatched LDP transport addresses can prevent session formation in specific configurations where manually assigned transport addresses differ. However, in most default deployments, LDP uses loopback addresses or primary interface addresses, making this a less common cause.
MPLS not being globally enabled prevents label switching and forwarding, but does not inherently block LDP from attempting neighbor discovery on configured interfaces. However, label distribution will not occur until MPLS is enabled.
LDP hello timers determine how frequently hello messages are sent. Longer timers may delay session formation, but do not prevent eventual session establishment if connectivity exists.
The root cause is missing IGP adjacency. Proper IGP connectivity ensures that LDP messages can be exchanged, neighbors are discovered, and label-switched paths are established. Network engineers must verify IGP reachability, adjacency status, and interface configuration to maintain a reliable MPLS network. Correct IGP operation enables LDP sessions to form, labels to be distributed, and MPLS forwarding to operate efficiently. Planning, configuration verification, and monitoring of IGP and LDP are essential to prevent session failures, maintain predictable traffic engineering, and ensure redundancy in service provider networks. Understanding the relationship between IGP connectivity and LDP operation is critical for successful MPLS deployment, optimal path selection, and network resilience.