Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 5 Q61-75
Visit here for our full Cisco 300-410 exam dumps and practice test questions.
Question 61:
A network engineer configures OSPF on a multi-area network. Routers in area 3 are not receiving routes to other areas, although the ABR is advertising Type-3 summary LSAs. What is the most likely cause?
A) Area 3 is configured as a totally stubbed area.
B) Type-1 LSAs are not being advertised by the ABR.
C) The backbone area 0 is experiencing high SPF computation times.
D) OSPF process IDs are different across routers.
Answer: A)
Explanation:
OSPF uses different area types to optimize routing efficiency and minimize resource usage on routers. A totally stub area is configured to prevent the installation of external Type-5 LSAs and inter-area Type-3 summary LSAs into the area, except for a default route injected by the ABR. This design is intended for branch routers or areas with limited memory and processing capacity, ensuring the routing table contains only the most essential information. The ABR still advertises Type-3 summary LSAs to other non-stub areas, but these LSAs are blocked from entering the totally stubbed area. Instead, a default route is provided, allowing routers in the totally stubbed area to reach networks outside their own area.
In the scenario described, routers in area 3 do not receive inter-area routes despite the ABR advertising them. This is consistent with a totally stub area configuration, where Type-3 LSAs are intentionally filtered. The routers rely solely on a default route to reach external destinations. If the default route is missing or improperly injected, routers cannot reach destinations outside the area. This behavior is expected in a totally stub area, and network engineers must account for the limited route visibility when designing OSPF deployments.
Type-1 LSAs describe the router’s own links within an area. If these were not advertised, adjacency formation within the area would fail, and no routes would be exchanged. Since routers maintain intra-area connectivity, Type-1 LSAs are functioning correctly and are not the cause of missing inter-area routes.
High SPF computation times in the backbone can delay convergence, but do not prevent LSAs from being advertised to other areas. Routers will eventually receive the Type-3 LSAs once SPF completes. Therefore, high SPF computation time is unlikely to be the root cause.
OSPF process IDs are locally significant identifiers. Differences in process IDs on routers do not prevent adjacency formation or LSA exchange as long as routers are correctly configured to participate in the same OSPF instance. Mismatched process IDs are not responsible for the behavior described.
The root cause is that area 3 is configured as a totally stubbed area. This configuration filters all inter-area Type-3 summary LSAs except for a default route. Network engineers must ensure that the ABR injects a default route to maintain connectivity for routers in the totally stubbed area. A proper understanding of OSPF area types is essential when troubleshooting missing routes. While totally stub areas reduce routing table size and improve router efficiency, they limit the visibility of external and inter-area routes. The ABR must be configured to provide a default route, and network designers must consider traffic patterns to avoid connectivity issues. Misconfiguring area types can lead to apparent route loss, even though the OSPF domain is functioning correctly. Engineers must plan area types carefully to balance resource efficiency with reachability requirements. In production networks, failure to inject default routes in totally stub areas can prevent users from accessing critical resources outside the area. Ensuring that area types are documented and default route injection is correctly implemented is crucial for predictable OSPF behavior, efficient resource utilization, and reliable network operation. The behavior observed in area 3 aligns with the design of a totally stub area and highlights the importance of understanding area type characteristics in OSPF network design and troubleshooting.
Question 62:
A network engineer configures BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic uses only a single path. What is the most likely cause?
A) The paths are not equal in AS path, origin, or MED attributes.
B) BGP is configured only with iBGP neighbors.
C) One path has an unreachable next-hop.
D) BGP update suppression is enabled.
Answer: A)
Explanation:
BGP multipath allows traffic to utilize multiple equal-cost paths for redundancy and better bandwidth utilization. However, BGP applies strict equality rules before multiple paths can be installed in the forwarding table. Attributes that must be identical include AS path length, origin type, MED, local preference, and next-hop reachability. Any difference in these attributes causes BGP to select a single best path, even though other paths are present in the table. This ensures deterministic routing and prevents forwarding loops.
In the scenario, multiple paths exist in the BGP table, but only one is used for forwarding. This indicates that the paths are not considered equal due to differences in AS path length, origin type, or MED. For instance, an extra AS path prepending or different origin codes can result in BGP choosing a single best path. This strict behavior is necessary to maintain stability and avoid inconsistent traffic distribution across unequal paths.
BGP session type, whether iBGP or eBGP, does not prevent multipath if paths meet the equality criteria. Both session types can support multipath, and the presence of multiple paths in the table indicates the protocol is functioning correctly.
Next-hop reachability is essential for installation in the forwarding table. If a next-hop is unreachable, the path will not be selected. Since multiple paths are visible in the table, reachability is not the limiting factor.
BGP update suppression reduces the rate at which updates are sent to neighbors to conserve control-plane resources, but does not influence local path selection. Therefore, update suppression cannot explain why only one path is used.
The root cause is that the paths differ in AS path, origin, or MED attributes. Network engineers must align these attributes to enable multipath forwarding. Correct configuration ensures effective load balancing, redundancy, and optimal utilization of available links. Understanding BGP multipath requirements is critical for designing multi-homed environments and ensuring that traffic leverages all available paths. Misalignment of these attributes limits traffic distribution and can lead to congestion on a single link, even when multiple paths are available. Properly configuring AS path, origin, and MED ensures that multipath functionality operates as intended, maximizing network performance and redundancy. This knowledge is essential for engineers deploying BGP in multi-homed networks to fully utilize bandwidth and maintain predictable traffic flow. Monitoring and adjusting BGP attributes is a key part of operational best practices to enable multipath deployment while avoiding unintended single-path traffic behavior.
Question 63:
A network engineer deploys RSVP-TE tunnels in an MPLS network. Tunnels fail to establish, even though all links have sufficient bandwidth. What is the most likely cause?
A) Link attribute constraints, such as TE colors, prevent CSPF from finding a feasible path.
B) RSVP authentication is mismatched.
C) RSVP soft-state refresh timers are too long.
D) The IGP metric of the path is misconfigured.
Answer: A)
Explanation:
MPLS Traffic Engineering (TE) allows network operators to create explicit paths based on bandwidth availability, link attributes, and administrative constraints. RSVP-TE is the protocol used to reserve bandwidth and establish these tunnels. Constrained Shortest Path First (CSPF) evaluates all candidate paths to ensure they satisfy TE constraints, including available bandwidth, TE colors, and administrative groups. Even when all links report sufficient bandwidth, CSPF may fail to compute a feasible path if one or more attributes do not match the requested constraints. Mismatched TE colors, administrative groups, or link configurations are the most common cause of RSVP-TE tunnel failures under these conditions.
RSVP authentication ensures that only authorized routers can reserve bandwidth. Mismatched authentication prevents tunnel establishment but does not stop CSPF from attempting to compute a path. Authentication failures generate log messages but do not directly affect path feasibility.
Soft-state refresh timers maintain RSVP reservations for established tunnels. If timers are too long, tunnels may expire or fail to maintain state, but initial path computation is unaffected.
IGP metrics affect unconstrained SPF computations, but CSPF considers both bandwidth and link attributes. Even if a path has the lowest IGP metric, CSPF will reject it if it does not meet TE constraints, such as unavailable TE colors or restricted administrative groups.
The root cause is mismatched link attributes preventing CSPF from finding a feasible path. Engineers must verify that all links along the intended path have attributes compatible with the requested TE constraints, including TE colors, administrative groups, and sufficient bandwidth. Proper alignment enables CSPF to compute a valid path, allowing RSVP-TE tunnels to be established. Understanding the relationship between link attributes, constraints, and CSPF computation is essential for predictable MPLS TE behavior, traffic engineering efficiency, and optimal utilization of network resources. Accurate configuration ensures reliable tunnel establishment, optimal network performance, and predictable behavior, avoiding failures caused by attribute mismatches. Proper planning and verification of TE constraints prevent unexpected tunnel failures and ensure that MPLS TE tunnels function as designed.
Question 64:
A network engineer deploys OSPF across multiple areas. Routers in area 4 are not receiving inter-area routes from area 0, although the ABR is advertising Type-3 summary LSAs. What is the most likely cause?
A) Area 4 is configured as a totally stubbed area.
B) Type-1 LSAs are not being advertised by the ABR.
C) Backbone area 0 is down.
D) OSPF process IDs are mismatched.
Answer: A)
Explanation:
OSPF area types allow network engineers to optimize routing behavior and reduce resource utilization on routers. A totally stub area is a specialized OSPF area designed to limit the number of LSAs that routers must process. In a totally stub area, external Type-5 LSAs and inter-area Type-3 summary LSAs are blocked, except for a single default route injected by the ABR. This type of configuration is particularly useful for branch offices or remote sites with limited router resources, as it reduces the size of the LSDB and the routing table, ensuring efficient use of memory and CPU resources.
In the scenario described, routers in area 4 are not receiving inter-area routes from area 0, despite the ABR advertising Type-3 summary LSAs. This behavior is consistent with a totally stubbed area configuration, where the ABR intentionally filters Type-3 LSAs to the area. Routers in the totally stub area rely solely on the default route provided by the ABR to reach destinations outside the area. If the default route is missing or improperly configured, connectivity to external networks and other areas is lost, even though the ABR is functioning correctly and Type-3 LSAs are being generated for other areas. This is a common source of confusion during troubleshooting because the LSAs exist in the ABR but do not propagate to the totally stubbed area by design.
Type-1 LSAs represent a router’s links within an area. If Type-1 LSAs were not being advertised, routers in area 4 would fail to form adjacencies, and no routes would be present in the LSDB. Since intra-area routing is operational, Type-1 LSAs are being correctly advertised, and this is not the cause of missing inter-area routes.
If the backbone area were down, inter-area routing would fail, and all areas connected to the backbone would be affected. However, only area 4 is experiencing missing routes, indicating that the backbone is operational and not the root cause.
OSPF process IDs are locally significant identifiers and do not affect the propagation of LSAs between routers. Differences in process IDs do not prevent adjacency formation or inter-area LSA exchange within the same OSPF instance. Therefore, mismatched process IDs are not relevant in this scenario.
The root cause of missing inter-area routes in area 4 is the totally stubbed area configuration. The ABR is functioning correctly, but the area type prevents Type-3 LSAs from being installed, relying exclusively on a default route. Network engineers must ensure that default route injection is properly configured on the ABR to maintain connectivity for routers in the totally stubbed area. Understanding the behavior of totally stubbed areas is crucial for troubleshooting scenarios where routes are unexpectedly missing. Proper configuration balances the benefits of reduced resource usage with the need for network reachability. Failure to inject a default route can lead to loss of external connectivity despite correct ABR operation. Network engineers should carefully plan area types and verify that all required default routes are distributed to ensure reliable inter-area communication. Proper documentation and configuration checks prevent connectivity issues and ensure that totally stubbed areas function as intended without causing unexpected route loss. The behavior in area 4 illustrates the importance of understanding OSPF area types and their effects on route propagation.
Question 65:
A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic continues to use only one path. What is the most likely cause?
A) The paths are not equal in AS path, origin, or MED attributes.
B) BGP is configured only with iBGP neighbors.
C) One path has an unreachable next-hop.
D) BGP update suppression is enabled.
Answer: A)
Explanation:
BGP multipath functionality is designed to leverage multiple equal-cost paths for redundancy and optimal bandwidth utilization. However, BGP enforces strict equality rules before multiple paths can be installed in the forwarding table. Attributes that must match for multipath to operate include AS path length, origin type, MED, local preference, and next-hop reachability. Any difference in these attributes results in a single best path selection, even if multiple paths appear in the BGP table. This strict behavior ensures deterministic routing and prevents inconsistencies or routing loops.
In this scenario, multiple paths are visible in the BGP table, but only one path is used for forwarding. This indicates that the paths do not satisfy equality conditions due to differences in AS path, origin type, or MED. For example, an extra AS path prepending or a different origin code can cause BGP to select a single path, despite the presence of additional viable routes. This ensures predictable traffic flow and avoids potential misrouting or uneven load distribution.
BGP session type, whether iBGP or eBGP, does not prevent multipath functionality if equality conditions are met. Both iBGP and eBGP support multipath, and the presence of multiple paths in the table confirms that BGP is receiving updates correctly.
Next-hop reachability is critical for installing a path in the forwarding table. Since all paths are visible, next-hop connectivity is not the limiting factor.
BGP update suppression reduces the rate of updates sent to neighbors but does not influence local path selection. Therefore, update suppression cannot explain why only one path is being used for forwarding.
The root cause is that the paths are unequal in AS path, origin, or MED attributes. Network engineers must align these attributes to enable BGP multipath functionality. Correctly configuring AS path, origin type, and MED ensures that multiple paths are installed in the forwarding table, improving redundancy, load balancing, and bandwidth utilization. Understanding the requirements for multipath in BGP is essential for multi-homed network designs, as misaligned attributes can prevent traffic from utilizing all available paths, limiting network performance. Network engineers must monitor and adjust BGP attributes carefully to ensure effective traffic distribution and predictable routing behavior. Proper configuration maximizes the benefits of multipath forwarding, enhances redundancy, and prevents traffic congestion on a single link, even when multiple paths exist. Effective use of multipath forwarding requires careful planning, consistent attribute management, and thorough verification to fully utilize available network resources while maintaining stable routing behavior.
Question 66:
A network engineer deploys 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) Link attribute constraints, such as TE colors, prevent CSPF from finding a feasible path.
B) RSVP authentication is mismatched.
C) RSVP soft-state refresh timers are too long.
D) The IGP metric of the path is misconfigured.
Answer: A)
Explanation:
MPLS Traffic Engineering allows network operators to create explicit paths based on available bandwidth, administrative constraints, and link attributes. RSVP-TE is used to reserve bandwidth along these paths. Constrained Shortest Path First (CSPF) evaluates candidate paths to ensure all TE constraints, including link bandwidth, TE colors, and administrative groups, are met. Even when sufficient bandwidth exists, tunnels can fail if CSPF cannot identify a feasible path that satisfies all specified attributes. Link attribute mismatches, such as incorrect TE colors or missing administrative group markings, are the most common reason for RSVP-TE tunnel failure when bandwidth is adequate.
RSVP authentication ensures that only authorized routers can establish reservations. Mismatched authentication prevents tunnel establishment but does not affect CSPF path computation. Authentication errors generate log messages but do not stop the feasibility calculation.
Soft-state refresh timers maintain RSVP reservations for active tunnels. Incorrect refresh timers can lead to premature state expiration, but they do not prevent initial path computation.
IGP metrics affect unconstrained SPF calculations but CSPF considers both link attributes and bandwidth. Even a path with the lowest IGP metric will be rejected by CSPF if TE constraints are not satisfied.
The root cause is mismatched link attributes preventing CSPF from finding a feasible path. Network engineers must ensure that all links along the desired tunnel path have attributes compatible with requested TE constraints, including TE colors, administrative groups, and bandwidth availability. Proper alignment allows CSPF to compute a valid path, enabling RSVP-TE tunnels to establish successfully. Understanding the interaction between link attributes, constraints, and CSPF computation is essential for predictable MPLS TE operation, efficient traffic engineering, and optimal resource utilization. Accurate configuration ensures reliable tunnel setup, predictable network performance, and avoidance of failures caused by attribute mismatches. Proper verification and planning of TE attributes prevent unexpected tunnel establishment failures and ensure that MPLS TE tunnels function as intended, supporting efficient and predictable network behavior.
Question 67:
A network engineer configures OSPF on a multi-area network. Routers in area 5 are not receiving external routes even though the ABR is advertising default routes. What is the most likely cause?
A) Area 5 is configured as a stub area without default route injection.
B) Type-1 LSAs are blocked by access control lists.
C) Backbone area 0 is overloaded.
D) OSPF process IDs are mismatched.
Answer: A)
Explanation:
OSPF area types are designed to optimize routing efficiency, reduce LSDB size, and minimize resource usage on routers. A stub area prevents external Type-5 LSAs from being installed in the routers’ LSDB to reduce routing table complexity. Instead of receiving all external routes, routers rely on a default route injected by the ABR. Totally stub areas go further by also blocking inter-area Type-3 summary LSAs. In either case, the ABR must inject a default route to allow connectivity to external networks.
In the scenario described, routers in area 5 cannot reach external destinations despite ABR advertisements. This is consistent with a stub area configuration without proper default route injection. Stub areas intentionally suppress Type-5 LSAs, so if the ABR does not inject a default route, routers have no knowledge of external destinations. This behavior is a common source of confusion in OSPF troubleshooting because the ABR is functioning correctly, but the area’s configuration limits route visibility.
Type-1 LSAs describe a router’s links within an area. Blocking Type-1 LSAs would prevent adjacency formation, leading to loss of intra-area routing. Since routers in area 5 maintain connectivity within the area, Type-1 LSAs are not blocked and are functioning properly.
Backbone area overload can lead to delayed SPF computation and slower convergence, but it does not prevent ABRs from advertising default routes to stub areas. The routers’ inability to reach external networks is localized to the stub area, indicating that backbone overload is not the primary cause.
OSPF process IDs are locally significant identifiers. Differences in process IDs do not affect inter-area route propagation within the same OSPF instance. Mismatched process IDs are not responsible for missing external routes.
The root cause is that area 5 is a stub area without a properly injected default route. Network engineers must configure the ABR to advertise a default route using the “default-information originate” command, ensuring that routers in the stub area can reach external networks. Understanding stub area behavior is critical to ensure network reachability while maintaining routing efficiency. Correct configuration balances the benefits of reduced LSDB and routing table size with the requirement for connectivity to external networks. Failure to inject default routes in stub areas results in loss of external connectivity, despite a fully operational ABR and functioning intra-area routing. Engineers must carefully design stub areas, verify default route injection, and document configurations to prevent unexpected network isolation while leveraging the efficiency benefits of stub area configurations. Proper awareness of stub area behavior ensures predictable OSPF performance and reliable network operation.
Question 68:
A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic only uses a single path. What is the most likely cause?
A) The paths are not equal in AS path, origin, or MED attributes.
B) BGP is configured only with iBGP neighbors.
C) One path has an unreachable next-hop.
D) BGP update suppression is enabled.
Answer: A)
Explanation:
BGP multipath allows multiple equal-cost paths to be installed in the forwarding table to improve redundancy, load balancing, and bandwidth utilization. BGP enforces strict equality conditions before multiple paths can be installed in the forwarding table. Attributes that must match include AS path length, origin type, MED, local preference, and next-hop reachability. Any discrepancy among these attributes results in the selection of a single best path, even if multiple paths are available in the BGP table. This ensures deterministic traffic flow and avoids potential loops or inconsistent forwarding.
In the scenario described, multiple paths are present in the BGP table, but only one path is used for traffic. This indicates that BGP considers the paths unequal due to differences in AS path, origin type, or MED. For example, one path may have additional AS path prepending or a different origin code, causing BGP to select a single best path. This strict behavior ensures predictable routing and avoids uneven traffic distribution or suboptimal forwarding decisions.
BGP session type, whether iBGP or eBGP, does not prevent multipath if paths meet equality conditions. Both session types support multipath, and the presence of multiple paths in the table indicates that the routing protocol is functioning correctly.
Next-hop reachability is required for paths to be installed in the forwarding table. Since multiple paths are visible in the table, next-hop connectivity is not the limiting factor.
BGP update suppression reduces the rate of updates sent to neighbors to conserve control-plane resources, but it does not influence local path selection for forwarding. Therefore, update suppression is not responsible for traffic using a single path.
The root cause is that the paths are not equal in AS path, origin, or MED attributes. Network engineers must align these attributes to enable BGP multipath functionality. Correct configuration ensures effective traffic distribution, redundancy, and optimal bandwidth utilization. Understanding BGP multipath equality requirements is critical in multi-homed network environments to fully utilize available paths and prevent congestion on a single link. Misalignment of AS path, origin, or MED prevents traffic from leveraging multiple paths, limiting network performance and redundancy. Engineers should monitor BGP attributes, adjust configurations where necessary, and verify multipath operation to ensure predictable and balanced traffic forwarding. Proper configuration of multipath forwarding allows traffic to utilize all available links effectively, enhancing network resilience and efficiency.
Question 69:
A network engineer deploys 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) Link attribute constraints, such as TE colors, prevent CSPF from finding a feasible path.
B) RSVP authentication is mismatched.
C) RSVP soft-state refresh timers are too long.
D) The IGP metric of the path is misconfigured.
Answer: A)
Explanation:
MPLS Traffic Engineering allows explicit paths to be created based on bandwidth availability, administrative constraints, and link attributes. RSVP-TE is the protocol responsible for reserving bandwidth and establishing these tunnels. Constrained Shortest Path First (CSPF) evaluates candidate paths against all specified TE constraints, including bandwidth, TE colors, and administrative groups. Even when all links report sufficient bandwidth, tunnels may fail if CSPF cannot identify a feasible path that satisfies all constraints. Link attribute mismatches, such as incorrect TE colors or missing administrative group markings, are the most common cause of RSVP-TE tunnel failures under these conditions.
RSVP authentication ensures that only authorized routers can establish reservations. A mismatch prevents tunnel establishment but does not affect CSPF path computation. Authentication failures generate logs but do not directly impact feasibility calculations.
Soft-state refresh timers maintain RSVP state for active tunnels. Incorrect timers may cause premature tunnel teardown, but they do not prevent initial path computation.
IGP metrics influence unconstrained SPF computations, but CSPF considers both bandwidth and link attributes. Even if a path has the lowest IGP metric, CSPF will reject it if TE constraints are not satisfied, such as unavailable TE colors or administrative group restrictions.
The root cause is mismatched link attributes preventing CSPF from finding a feasible path. Engineers must ensure that all links along the desired tunnel path have attributes compatible with the requested TE constraints, including TE colors, administrative groups, and bandwidth availability. Proper alignment allows CSPF to compute a valid path, enabling RSVP-TE tunnels to establish successfully. Understanding the interaction between link attributes, constraints, and CSPF is essential for predictable MPLS TE operation, efficient traffic engineering, and optimal utilization of network resources. Accurate configuration ensures reliable tunnel establishment, optimal network performance, and predictable network behavior, avoiding failures caused by attribute mismatches. Verification of TE attributes and proper planning prevent unexpected tunnel establishment failures and ensure that MPLS TE tunnels function as intended, supporting efficient and predictable network operation.
Question 70:
A network engineer deploys OSPF in a multi-area network. Routers in area 6 are not receiving inter-area routes from area 0, although the ABR is advertising Type-3 summary LSAs. What is the most likely cause?
A) Area 6 is configured as a totally stubbed area.
B) Type-1 LSAs are blocked by ACLs.
C) Backbone area 0 is overloaded.
D) OSPF process IDs are mismatched.
Answer: A)
Explanation:
OSPF uses different area types to optimize routing efficiency and minimize the size of routing tables and the link-state database. A totally stubbed area is a specialized area that blocks the propagation of external Type-5 LSAs and inter-area Type-3 summary LSAs. The only route that routers in a totally stub area receive for destinations outside the area is a default route injected by the ABR. This configuration is particularly useful for branch offices or remote sites with limited router memory and processing capacity because it reduces the number of routes the routers must maintain while still providing external network connectivity through the default route.
In this scenario, routers in area 6 are not receiving inter-area routes from area 0 despite the ABR advertising Type-3 summary LSAs. This is consistent with the behavior of a totally stub area, which intentionally filters Type-3 LSAs to reduce the routing table size. The routers in area 6 rely solely on the default route to reach external networks. If the default route is missing or improperly configured on the ABR, routers in the totally stub area cannot reach destinations outside their own area. Troubleshooting this situation requires verifying the area type configuration and ensuring that the ABR is properly injecting the default route to maintain connectivity.
Type-1 LSAs represent a router’s own links within an area. If these LSAs were blocked, adjacency formation within area 6 would fail, and routers would not be able to exchange intra-area routes. Since intra-area routes appear to be functioning correctly, Type-1 LSAs are being advertised and received correctly, and they are not the cause of the missing inter-area routes.
Backbone area overload could delay SPF computation and network convergence, but it does not prevent LSAs from being advertised to other areas once SPF calculations are complete. Since only area 6 is affected and other areas are likely receiving Type-3 LSAs normally, backbone overload is unlikely to be the root cause.
OSPF process IDs are locally significant and do not affect LSA propagation across areas as long as all routers are configured correctly within the same OSPF instance. Differences in process IDs do not prevent Type-3 LSA advertisement or default route injection, making this an unlikely cause of the problem.
The root cause is that area 6 is configured as a totally stubbed area. The ABR correctly advertises Type-3 summary LSAs, but the area type filters them out, leaving only a default route as the path to external networks. Network engineers must ensure that the ABR injects the default route to maintain connectivity. Understanding totally stub area behavior is critical when designing multi-area OSPF networks, as misconfigurations or omitted default route injections can lead to apparent route loss despite ABR functionality. Proper configuration balances reduced router resource usage with required connectivity, ensuring predictable and efficient network operation. Failing to inject a default route in a totally stub area prevents external network access, which can cause troubleshooting challenges if area types are not fully understood. Documenting area types and verifying default route injection is essential to prevent connectivity issues and ensure that network traffic can reach external destinations reliably while benefiting from the efficiency advantages of a totally stub area configuration.
Question 71:
A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic continues to use only a single path. What is the most likely cause?
A) The paths are not equal in AS path, origin, or MED attributes.
B) BGP is configured only with iBGP neighbors.
C) One path has an unreachable next-hop.
D) BGP update suppression is enabled.
Answer: A)
Explanation:
BGP multipath functionality allows traffic to leverage multiple equal-cost paths for redundancy, load balancing, and optimal bandwidth utilization. BGP enforces strict equality requirements before multiple paths are installed in the forwarding table. The attributes that must match include AS path length, origin type, MED, local preference, and next-hop reachability. Any difference in these attributes causes BGP to select a single best path, even if multiple paths are available in the BGP table. This ensures deterministic routing and avoids potential forwarding loops or inconsistent traffic distribution.
In the scenario described, multiple paths are present in the BGP table, but only one path is being used for traffic. This indicates that the paths are not considered equal by BGP due to differences in AS path, origin type, or MED. For example, if one path has AS path prepending or a different origin type, BGP will select the single best path based on its decision process, ignoring other viable paths for forwarding purposes. This behavior is necessary to maintain predictable routing and avoid suboptimal traffic distribution.
BGP session type, whether iBGP or eBGP, does not prevent multipath if paths meet equality conditions. Both iBGP and eBGP support multipath functionality, and the presence of multiple paths in the BGP table indicates that BGP is correctly receiving updates from neighbors.
Next-hop reachability is required for installation in the forwarding table. Since all paths appear in the table, next-hop connectivity is not a limiting factor in this case.
BGP update suppression affects the frequency of updates sent to neighbors to conserve control-plane resources but does not influence local path selection for forwarding. Therefore, update suppression cannot explain why only one path is used.
The root cause is that the paths are unequal in AS path, origin, or MED attributes. Network engineers must align these attributes to enable multipath forwarding. Proper configuration ensures effective traffic distribution, redundancy, and optimal use of bandwidth. Understanding BGP multipath equality requirements is critical in multi-homed network designs to fully utilize available paths and prevent congestion on a single link. Misalignment of AS path, origin, or MED attributes limits traffic utilization and redundancy, reducing network performance. Engineers must carefully monitor and adjust BGP attributes, verifying multipath behavior to ensure predictable traffic distribution. Properly configured multipath forwarding allows all available links to be leveraged effectively, improving network resilience, performance, and efficiency. This requires careful planning, attribute management, and operational verification to maximize the benefits of BGP multipath deployment.
Question 72:
A network engineer deploys 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) Link attribute constraints, such as TE colors, prevent CSPF from finding a feasible path.
B) RSVP authentication is mismatched.
C) RSVP soft-state refresh timers are too long.
D) The IGP metric of the path is misconfigured.
Answer: A)
Explanation:
MPLS Traffic Engineering allows operators to create explicit paths based on available bandwidth, administrative constraints, and link attributes. RSVP-TE is used to reserve bandwidth along these explicit paths. Constrained Shortest Path First (CSPF) evaluates candidate paths to ensure that all specified TE constraints are satisfied, including bandwidth availability, TE colors, and administrative groups. Even when all links report sufficient bandwidth, tunnels may fail to establish if CSPF cannot find a feasible path that satisfies all constraints. The most common cause of such tunnel failures is mismatched link attributes, such as incorrect TE colors, missing administrative groups, or incompatible configurations along the intended path.
RSVP authentication ensures that only authorized routers can establish reservations. Mismatched authentication prevents a tunnel from being established but does not affect CSPF path computation. Authentication failures generate logs but do not directly cause path feasibility issues.
Soft-state refresh timers maintain RSVP state for active tunnels. Incorrect refresh intervals may lead to premature tunnel teardown, but they do not prevent the initial computation of a feasible path by CSPF.
IGP metrics influence unconstrained SPF computations, but TE tunnel establishment relies on CSPF, which considers bandwidth and link attributes. Even if a path has the lowest IGP metric, CSPF will reject it if TE constraints are not met, such as when TE colors or administrative groups do not match the tunnel requirements.
The root cause is mismatched link attributes preventing CSPF from computing a feasible path. Engineers must ensure that all links along the tunnel path have attributes compatible with the requested TE constraints, including TE colors, administrative groups, and available bandwidth. Proper alignment allows CSPF to compute a valid path, enabling RSVP-TE tunnels to establish successfully. Understanding the relationship between link attributes, constraints, and CSPF computation is essential for predictable MPLS TE operation, traffic engineering efficiency, and optimal utilization of network resources. Accurate configuration ensures reliable tunnel setup, optimal performance, and predictable network behavior, avoiding failures caused by attribute mismatches. Proper verification and planning of TE attributes prevent unexpected tunnel establishment failures and ensure that MPLS TE tunnels function as intended, supporting efficient and predictable network operation.
Question 73:
A network engineer deploys OSPF across multiple areas. Routers in area 7 are not receiving inter-area routes from area 0, although the ABR is advertising Type-3 summary LSAs. What is the most likely cause?
A) Area 7 is configured as a totally stubbed area.
B) Type-1 LSAs are blocked by ACLs.
C) Backbone area 0 is down.
D) OSPF process IDs are mismatched.
Answer: A)
Explanation:
OSPF area types provide a method to optimize routing efficiency and control the number of routes maintained on routers. A totally stub area is a specialized configuration designed to reduce the number of LSAs that routers must process by filtering both external Type-5 LSAs and inter-area Type-3 summary LSAs. The only route available to routers in a totally stub area for destinations outside the area is a default route injected by the ABR. This area type is particularly useful for remote sites or branch offices with limited memory or CPU resources, as it reduces the routing table size while still providing connectivity to external networks.
In this scenario, routers in area 7 are not receiving inter-area routes from area 0, even though the ABR advertises Type-3 summary LSAs. This behavior is consistent with a totally stub area, where the area type filters Type-3 LSAs and relies solely on the default route for inter-area connectivity. If the default route is missing or misconfigured, routers cannot reach external networks, despite ABR functionality and Type-3 LSAs being advertised to other areas. Understanding the behavior of totally stub areas is crucial when designing OSPF networks, as failure to inject a default route can create the appearance of missing routes even though the network is operational. Properly configuring the ABR to inject a default route is essential for maintaining connectivity in totally stub areas.
Type-1 LSAs describe the router’s links within an area. If Type-1 LSAs were blocked, intra-area adjacency formation would fail, and routers would not exchange routes within the area. Since routers in area 7 maintain intra-area routing, Type-1 LSAs are functioning correctly and do not explain missing inter-area routes.
A down backbone area would prevent inter-area routing for all areas connected to it. Since only area 7 is affected and other areas appear to receive Type-3 LSAs normally, backbone failure is not the root cause.
OSPF process IDs are locally significant identifiers that do not affect LSA propagation across areas. Differences in process IDs do not prevent Type-3 LSA advertisement or default route injection within a correctly configured OSPF instance.
The root cause is that area 7 is configured as a totally stubbed area. The ABR is correctly advertising Type-3 summary LSAs, but the area type prevents them from being installed, leaving only a default route as the path to external networks. Network engineers must ensure that the ABR is configured to inject a default route using the “default-information originate” command. Properly designing area types is essential to balance reduced resource utilization with required network reachability. Failure to inject default routes can isolate routers in stub areas from external networks. Engineers must carefully plan and verify area types and default route injection to ensure predictable OSPF behavior, maintain network connectivity, and optimize routing efficiency. Proper documentation and configuration checks prevent unexpected connectivity issues and ensure that totally stubbed areas operate as intended, providing reduced routing table size without sacrificing access to external networks.
Question 74:
A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic continues to use only one path. What is the most likely cause?
A) The paths are not equal in AS path, origin, or MED attributes.
B) BGP is configured only with iBGP neighbors.
C) One path has an unreachable next-hop.
D) BGP update suppression is enabled.
Answer: A)
Explanation:
BGP multipath allows multiple equal-cost paths to be used simultaneously in the forwarding plane to increase redundancy and bandwidth utilization. However, BGP applies strict equality rules to determine which paths are eligible for multipath forwarding. Paths must match in AS path length, origin type, MED, local preference, and next-hop reachability. Any discrepancy in these attributes prevents multipath installation and causes BGP to select a single best path. This ensures deterministic traffic flow and avoids routing loops or inconsistent forwarding behavior.
In the scenario described, multiple paths appear in the BGP table, but traffic only uses a single path. This indicates that the paths are unequal in AS path, origin, or MED attributes. For instance, if one path has additional AS path prepending or a different origin code, BGP selects the single best path based on its decision process, ignoring other viable paths for forwarding. This strict equality requirement ensures predictable and stable routing, which is critical in multi-homed environments.
BGP session type, whether iBGP or eBGP, does not prevent multipath if paths meet equality conditions. Both session types support multipath, and the presence of multiple paths in the BGP table confirms that BGP is receiving updates correctly from neighbors.
Next-hop reachability is necessary for path installation in the forwarding table. Since multiple paths are visible, next-hop connectivity is not the limiting factor in this scenario.
BGP update suppression limits the frequency of updates sent to neighbors to reduce control-plane load but does not affect local path selection. Therefore, update suppression cannot explain why only one path is used for traffic.
The root cause is that the paths are not equal in AS path, origin, or MED attributes. Network engineers must ensure that these attributes are aligned to enable BGP multipath forwarding. Proper configuration allows effective traffic distribution, redundancy, and optimal bandwidth utilization. Understanding multipath equality requirements is crucial for multi-homed networks, as misaligned attributes can prevent traffic from utilizing all available paths, leading to congestion on a single link. Engineers must monitor and adjust BGP attributes as needed and verify multipath operation to maintain predictable and balanced traffic forwarding. Correctly configured multipath forwarding maximizes the benefits of available paths, enhances network resilience, and ensures efficient utilization of resources.
Question 75:
A network engineer deploys 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) Link attribute constraints, such as TE colors, prevent CSPF from finding a feasible path.
B) RSVP authentication is mismatched.
C) RSVP soft-state refresh timers are too long.
D) The IGP metric of the path is misconfigured.
Answer: A)
Explanation:
MPLS Traffic Engineering allows network operators to establish explicit paths based on bandwidth, administrative constraints, and link attributes. RSVP-TE is the signaling protocol that reserves bandwidth along these paths. Constrained Shortest Path First (CSPF) computes a feasible path by evaluating all candidate paths against the requested TE constraints, which include bandwidth, TE colors, and administrative groups. Even when all links have sufficient bandwidth, tunnels may fail to establish if CSPF cannot find a path that satisfies all constraints. The most common cause of such failures is mismatched link attributes, such as incorrect TE colors, missing administrative group markings, or inconsistent configurations along the path.
RSVP authentication ensures that only authorized routers can reserve bandwidth. Mismatched authentication prevents tunnel establishment but does not interfere with CSPF path computation. Authentication failures generate logs and alarms, but the path feasibility process continues independently.
Soft-state refresh timers maintain RSVP state for active tunnels. If the refresh intervals are too long, the tunnel state may expire prematurely, but this does not prevent CSPF from computing a feasible path initially.
IGP metrics influence unconstrained SPF calculations, but CSPF considers both bandwidth and link attributes. Even if a path has the lowest IGP metric, CSPF will reject it if the requested TE constraints are not satisfied, such as when TE colors or administrative groups do not match.
The root cause is mismatched link attributes preventing CSPF from computing a feasible path. Engineers must verify that all links along the intended tunnel path have compatible attributes, including TE colors, administrative groups, and bandwidth. Proper alignment enables CSPF to find a valid path, allowing RSVP-TE tunnels to establish successfully. Understanding the interaction between link attributes, constraints, and CSPF computation is essential for predictable MPLS TE behavior, efficient traffic engineering, and optimal utilization of network resources. Accurate configuration ensures reliable tunnel establishment, optimal performance, and predictable network operation, avoiding failures caused by attribute mismatches. Verification of TE attributes and proper planning prevent unexpected tunnel establishment failures and ensure that MPLS TE tunnels function as intended, supporting efficient and predictable network behavior.