Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 4 Q46-60
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Question 46:
A network engineer deploys OSPF in a multi-area network. Routers in area 1 do not see inter-area routes from area 2, although the ABR is advertising them. What is the most likely cause?
A) Area 1 is configured as a totally stub area.
B) Type-1 LSAs are not being advertised by the ABR.
C) The backbone link has too high an IGP cost.
D) OSPF process IDs are mismatched on routers.
Answer: A)
Explanation:
OSPF area types dictate which LSAs are allowed into an area and which routes are visible to routers. A totally stub area is a specialized OSPF area designed to minimize the routing table size and reduce processing overhead. In a totally stub area, external Type-5 LSAs are suppressed, inter-area Type-3 summary LSAs are replaced with a default route, and only intra-area Type-1 LSAs and a default route are allowed. This design is particularly useful for branch routers with limited resources or for areas where detailed visibility of external networks is unnecessary.
In the scenario described, routers in area 1 do not see routes from area 2 despite the ABR advertising them. This behavior is consistent with the characteristics of a totally stub area. While the ABR generates Type-3 inter-area summary LSAs for other standard areas, in a totally stub area, it will not forward these inter-area summaries to reduce LSDB size and routing table entries. Instead, the routers rely on a default route provided by the ABR to reach networks outside the area. This is by design and is expected behavior.
Type-1 LSAs represent the router’s links within the area. If Type-1 LSAs were not advertised by the ABR, routers in the area would fail to form adjacency and would not receive any routes at all. Since intra-area and default routes are present, Type-1 LSAs are functioning correctly, so this is not the cause of missing inter-area routes.
High IGP metrics on the backbone links can influence path selection but do not prevent inter-area LSA propagation. Even if the cost is high, summary LSAs are still installed in the LSDB, and routers can reach networks using less preferred paths. Therefore, high backbone metrics cannot explain the missing inter-area routes.
OSPF process IDs are locally significant identifiers that allow multiple OSPF processes to run independently on a router. Mismatched process IDs between routers do not impact LSDB synchronization within the same process. If adjacency is formed and intra-area routes are visible, process ID mismatches are not the cause.
The root cause of missing inter-area routes in this scenario is the totally stub area configuration. By intentionally suppressing inter-area LSAs and using a default route, OSPF reduces routing table size and processing overhead in the stub area. Network engineers must account for these limitations when designing OSPF networks. While totally stub areas improve efficiency, they limit route visibility, so proper planning is required to ensure connectivity and traffic reachability. Understanding the implications of stub and totally stub areas is critical for troubleshooting scenarios where expected inter-area routes are missing. The correct configuration of stub areas ensures network stability, scalability, and predictable routing behavior.
Question 47:
A network engineer deploys BGP on a multi-homed edge router. Multiple paths are visible in the BGP table, but traffic uses 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 running iBGP instead of eBGP.
C) The next-hop of one path is unreachable.
D) BGP update suppression is enabled.
Answer: A)
Explanation:
BGP multipath allows routers to forward traffic across multiple equal-cost paths, increasing bandwidth utilization and providing redundancy. For multipath to be used, BGP requires paths to be equal according to its best-path selection rules, which consider several attributes, including AS path length, origin type, MED, local preference, weight, and next-hop reachability. If any of these attributes differ between candidate paths, BGP selects a single best path for forwarding, even if multiple paths exist in the BGP table.
In the scenario described, multiple paths exist in the table but only one is used. This indicates that BGP does not recognize the paths as equal. Differences in AS path length, origin type, or MED values are the most common reasons. Even slight differences, such as AS path prepending on one path, prevent BGP from considering the paths equal. BGP enforces strict equality to ensure deterministic routing behavior, which is critical for network stability and predictable traffic distribution.
Running BGP as iBGP or eBGP does not inherently prevent multipath usage. Both session types support multipath as long as the attributes meet the equality requirements. The presence of multiple paths in the table indicates that BGP is functioning correctly, and the limitation is attribute inequality rather than session type.
Next-hop reachability is essential for path installation. If a next-hop is unreachable, the path will not be installed. However, in this scenario, the paths appear in the table, indicating that next-hop connectivity is intact. Therefore, next-hop issues are not the cause.
BGP update suppression reduces control-plane overhead by limiting the frequency of advertisements to neighbors. While it affects propagation of updates, it does not prevent local path selection for multipath forwarding. The presence of multiple paths in the table indicates updates are received, so suppression is not relevant here.
The root cause is that BGP considers the paths unequal due to differences in AS path, origin, or MED attributes. Correcting these discrepancies, for example by adjusting prepending or MED values, allows BGP to recognize multiple equal-cost paths and install them in the forwarding table for multipath use. Understanding the strict equality requirements for BGP multipath ensures predictable traffic distribution and effective utilization of available paths, which is crucial for multi-homed network deployments. Proper planning and attribute alignment are essential for maximizing redundancy and bandwidth utilization.
Question 48:
A network engineer deploys RSVP-TE tunnels in an MPLS network. The 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 between routers is mismatched.
C) RSVP soft-state refresh intervals are too long.
D) The IGP metric of the desired path is misconfigured.
Answer: A)
Explanation:
MPLS Traffic Engineering allows explicit path selection based on available bandwidth, administrative constraints, and link attributes. RSVP-TE is used to reserve bandwidth and establish explicit tunnels. Constrained Shortest Path First (CSPF) computes paths that meet all specified constraints. Even if all links report sufficient bandwidth, tunnels may fail to establish if CSPF cannot find a path that satisfies the required link attributes, such as TE colors, administrative groups, or explicit exclusions. This is the most common cause of RSVP-TE tunnel failure when bandwidth is sufficient.
RSVP authentication ensures only authorized routers can establish reservations. While a mismatch could prevent tunnel establishment, CSPF still computes the path independently. Authentication mismatches typically result in log messages rather than preventing path computation.
Soft-state refresh intervals maintain RSVP state for established tunnels. Incorrect refresh intervals may lead to premature expiration or tunnel teardown but do not affect initial feasibility calculations.
IGP metrics influence unconstrained SPF calculations, but TE tunnels use CSPF, which evaluates bandwidth and link attributes. Even if the path has the lowest IGP cost, CSPF will reject it if any constraints are violated. Misconfigured metrics alone do not explain tunnel failure if TE constraints are not satisfied.
The root cause is therefore the link attribute constraints preventing CSPF from computing a feasible path. Network engineers must ensure that all links along the intended path have attributes compatible with the requested TE constraints, including TE colors, administrative groups, and bandwidth availability. Once these attributes are correctly configured, CSPF can compute a valid path, and the RSVP-TE tunnel will successfully establish. Understanding the interaction of link attributes, constraints, and CSPF computation is critical for predictable MPLS TE network operation, optimal traffic engineering, and efficient utilization of network resources. Proper attribute alignment ensures reliable tunnel establishment and performance predictability.
Question 49:
A network engineer configures OSPF on a multi-area network. Some routers in area 2 cannot reach external networks, even though the ABR advertises a default route. What is the most likely cause?
A) Area 2 is configured as a stub area.
B) Type-2 LSAs are not being advertised by the ABR.
C) The backbone area is down.
D) OSPF process IDs are mismatched.
Answer: A)
Explanation:
OSPF area types determine which LSAs are allowed into a particular area and which routes are visible to the routers within it. A stub area is configured to suppress external Type-5 LSAs from entering the area to reduce routing overhead on routers within that area. Instead of receiving all external routes, routers in a stub area rely on a default route injected by the ABR to reach networks outside of OSPF. This helps reduce the size of the routing table and minimizes resource consumption on less capable routers, such as branch routers.
In the scenario described, routers in area 2 cannot reach external networks despite the ABR advertising a default route. This situation typically occurs if the routers are configured as stub areas but the ABR is not injecting a proper default route or the routers are configured as totally stub areas and rely entirely on the default route. Stub areas allow only intra-area and inter-area routes to be installed in the routing table. The external routes from Type-5 LSAs are not imported, so reachability to external networks depends entirely on the injected default route. If the default route is missing or improperly configured, routers will not have a path to external destinations.
Type-2 LSAs are used in OSPF to describe transit networks within a broadcast or non-broadcast multi-access area. If Type-2 LSAs were missing, the routers would not be able to form adjacencies on transit networks, and OSPF would fail more broadly. Since the problem is limited to external network reachability, this is unlikely the cause.
The backbone area is essential for inter-area communication. If the backbone were down, inter-area routing would fail completely, and routers would not receive summary routes from other areas. However, the scenario suggests that internal and inter-area routing is functional, making backbone failure unlikely.
OSPF process IDs are locally significant identifiers and do not affect adjacency formation or LSA propagation across areas. Mismatched process IDs between routers affect only whether routers belong to the same OSPF instance. Since the routers form adjacencies and receive intra-area routes, mismatched process IDs are not the cause.
The root cause of missing external network reachability in area 2 is the stub area configuration. Stub areas intentionally block external Type-5 LSAs, requiring routers to rely on the default route injected by the ABR. Network engineers must ensure that stub areas are correctly configured and that the ABR injects the appropriate default route to enable connectivity to external networks. Proper understanding of stub areas, default route injection, and area types is crucial for designing OSPF networks that balance resource efficiency and reachability. Misconfiguration or omission of the default route will result in routers being unable to reach external destinations, even though the internal OSPF topology is functioning correctly. The solution involves verifying the stub area configuration and ensuring the ABR is correctly injecting a default route to maintain connectivity.
Question 50:
A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths are visible 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 running iBGP instead of eBGP.
C) The next-hop of one path is unreachable.
D) BGP update suppression is enabled.
Answer: A)
Explanation:
BGP multipath allows traffic to utilize multiple equal-cost paths simultaneously, providing better bandwidth utilization and redundancy. However, BGP enforces strict equality rules for multipath installation. Paths must match in several key attributes: AS path length, origin type, MED, local preference, and next-hop reachability. If any of these attributes differ between candidate paths, BGP will select a single best path for forwarding, even if multiple paths exist in the table. This strict enforcement ensures deterministic routing and prevents loops or inconsistent traffic distribution.
In this scenario, multiple paths exist in the BGP table, but only one is used for forwarding. This is indicative of unequal attributes between the paths. Differences such as AS path prepending on one path, varying origin types (IGP, EGP, incomplete), or different MED values can cause BGP to disregard additional paths for forwarding. Even small attribute differences prevent multipath from being applied, resulting in a single path being selected.
BGP session type, whether iBGP or eBGP, does not inherently prevent multipath usage. Both iBGP and eBGP can support multipath as long as the paths meet the equality criteria. The presence of multiple paths in the table indicates that BGP is operating correctly, and the limitation is due to attribute inequality rather than session type.
Next-hop reachability is necessary for a path to be installed. If a next-hop were unreachable, BGP would not install the path, and it would not appear in the table. Since multiple paths are visible, next-hop reachability is not an issue in this case.
BGP update suppression affects the frequency of update propagation to neighbors but does not influence the local selection of multiple paths for forwarding. Because multiple paths are already in the table, update suppression is not relevant to the observed behavior.
The root cause is that BGP considers the paths unequal due to differences in AS path, origin, or MED attributes. Correcting these differences allows BGP to recognize multiple equal-cost paths and install them in the forwarding table for multipath forwarding. Understanding the equality requirements of BGP multipath is essential for network engineers designing multi-homed environments to optimize traffic distribution, ensure redundancy, and maximize bandwidth utilization. Proper configuration of AS paths, origin types, and MED attributes ensures that multipath functionality operates as intended.
Question 51:
A network engineer deploys RSVP-TE tunnels in an MPLS network. Despite sufficient bandwidth on all links, tunnels fail to establish. What is the most likely cause?
A) Link attribute constraints, such as TE colors, prevent CSPF from finding a feasible path.
B) RSVP authentication between routers is mismatched.
C) RSVP soft-state refresh intervals are too long.
D) The IGP metric of the desired 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 and establish these tunnels. Constrained Shortest Path First (CSPF) evaluates candidate paths against specified constraints, including link bandwidth, TE colors, and administrative group attributes. Even if all links report sufficient bandwidth, CSPF may fail to find a feasible path if any constraints are not satisfied. Link attribute mismatches, such as unavailable TE colors or administrative exclusions, are the most common cause of RSVP-TE tunnel failures when bandwidth is adequate.
RSVP authentication ensures that only authorized routers can establish reservations. A mismatch prevents the establishment of a tunnel but does not prevent CSPF from attempting to compute a path. Authentication errors typically generate log messages and do not directly cause CSPF computation to fail.
Soft-state refresh intervals maintain RSVP state for existing tunnels. Incorrect refresh intervals may cause premature expiration or tunnel teardown but do not affect initial tunnel computation or feasibility determination.
IGP metrics influence unconstrained SPF calculations but do not directly affect CSPF computation, which evaluates both link attributes and available bandwidth. Even if a link has the lowest IGP cost, CSPF will reject it if it violates TE constraints.
The root cause is therefore the mismatch of link attributes preventing CSPF from computing a feasible path. Engineers must ensure that all links along the intended 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 and enables RSVP-TE tunnels to establish successfully. Understanding the interplay of link attributes, constraints, and CSPF computation is critical for predictable MPLS TE operation, optimal traffic engineering, and efficient utilization of network resources. Accurate configuration ensures reliable tunnel establishment, predictable performance, and proper network behavior.
Question 52:
A network engineer configures OSPF on a multi-area network. Some routers in area 3 are not receiving default routes for external destinations even though the ABR is advertising them. What is the most likely cause?
A) Area 3 is configured as a totally stub area without a default route injection.
B) Type-1 LSAs are blocked by access control lists.
C) The backbone area is misconfigured with a high cost link.
D) OSPF process IDs are different across routers.
Answer: A)
Explanation:
OSPF supports different area types to optimize routing behavior and reduce resource utilization. A totally stub area is designed to prevent all external Type-5 LSAs and inter-area Type-3 summaries from being installed in the area’s routers, except for a single default route injected by the ABR. The default route acts as a catch-all to external networks and ensures reachability without the need to maintain a complete set of external routes in the LSDB.
In the scenario described, routers in area 3 are not receiving external routes despite the ABR advertising them. This is typically caused by the area being configured as a totally stub area but the ABR not injecting a default route, or the configuration omitting the “default-information originate” command. Totally stub areas rely entirely on this default route to reach external destinations. Without it, routers in the area cannot reach networks outside the OSPF domain.
Type-1 LSAs describe a router’s own links within an area. If Type-1 LSAs were blocked, adjacency formation would fail, and the routers would not exchange any LSAs at all. Since routers can exchange intra-area routes, LSAs are functioning correctly, and this is not the cause.
Backbone area misconfiguration, such as high-cost links, can influence path selection but does not prevent default route injection into a stub area. Routers still receive default routes from the ABR if configured correctly, so high IGP cost alone cannot explain missing default routes.
OSPF process IDs are locally significant and only matter on the local router. Mismatched process IDs do not prevent adjacency formation across different routers within the same area if other settings allow neighbor formation. Therefore, process ID differences are not a likely explanation for the missing default route.
The root cause is that the totally stub area configuration suppresses external LSAs, and the ABR has not injected a default route. Network engineers must ensure that stub areas are correctly configured and that the ABR provides the default route. Proper configuration involves using “area X stub no-summary” for totally stub areas and ensuring that default-information is propagated. Understanding area types and the behavior of totally stub areas is crucial for ensuring network reachability and predictable OSPF behavior, particularly in large networks where minimizing routing table size and LSDB processing is important. Failure to inject a default route into a totally stub area prevents routers from reaching external networks, even though the ABR is functional and internal OSPF routes are present.
Question 53:
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) The next-hop of one of the paths is unreachable.
D) BGP update suppression is enabled.
Answer: A)
Explanation:
BGP multipath functionality allows multiple equal-cost paths to be installed in the forwarding table to optimize bandwidth usage and provide redundancy. However, BGP strictly requires that candidate paths meet equality criteria before they are considered for multipath installation. The key attributes that must match include AS path length, origin type, MED, local preference, and next-hop reachability. Any differences among these attributes will cause BGP to select only a single best path, leaving other paths unused in the forwarding plane even though they appear in the BGP table.
In this scenario, multiple paths are visible in the BGP table but only one is being used. This indicates that BGP does not consider the paths equal due to differences in AS path, origin type, or MED attributes. For example, if one path has AS path prepending or a different MED, BGP treats it as inferior, and multipath forwarding is not applied. This behavior ensures deterministic routing and prevents potential routing loops or inconsistent traffic distribution.
BGP session type, whether iBGP or eBGP, does not inherently prevent multipath usage. Both iBGP and eBGP support multipath as long as the equality conditions are satisfied. Since multiple paths are present in the table, the session type is functioning correctly and is not the limiting factor.
Next-hop reachability is required for installation of a path. If the next-hop were unreachable, the path would not be installed and would not appear in the table. Since the paths are present, reachability is not an issue.
BGP update suppression reduces the frequency of updates sent to neighbors to conserve control-plane resources but does not affect local path selection for forwarding. Therefore, update suppression does not explain why only one path is used.
The root cause is that the paths are not equal in AS path, origin, or MED attributes. Network engineers must align these attributes to allow BGP to recognize multiple equal-cost paths and enable multipath forwarding. Correctly configuring AS path, origin, and MED ensures that multiple paths are installed in the forwarding table, providing better bandwidth utilization, redundancy, and predictable traffic distribution. Understanding BGP multipath requirements and attribute equality is crucial for optimizing multi-homed network environments. Proper alignment of these attributes allows traffic to leverage all available paths, enhancing network performance and resilience.
Question 54:
A network engineer deploys RSVP-TE tunnels in an MPLS network. Tunnels fail to establish even though bandwidth is sufficient on all links. 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 between routers.
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 operators to establish explicit paths based on available bandwidth, administrative constraints, and link attributes. RSVP-TE is used to reserve bandwidth along a computed path. Constrained Shortest Path First (CSPF) is responsible for calculating paths that satisfy all constraints, including link bandwidth, TE colors, and administrative group attributes. Even when sufficient bandwidth exists, tunnels may fail if CSPF cannot find a path that meets all constraints. Link attribute mismatches, such as incorrect TE colors or missing administrative groups, are the most common cause of RSVP-TE tunnel failure in scenarios where bandwidth is not a limiting factor.
RSVP authentication ensures only authorized routers can establish reservations. While a mismatch prevents reservation installation, CSPF will still compute a path independently. Authentication failures generate logs but do not affect the feasibility calculation itself.
Soft-state refresh intervals maintain RSVP state for already established tunnels. Incorrect refresh intervals can cause premature tunnel teardown but do not impact initial path computation.
IGP metrics affect unconstrained SPF calculations, but TE tunnels rely on CSPF, which evaluates bandwidth and link attributes. Even if a path has the lowest IGP metric, CSPF will reject it if it does not satisfy TE constraints.
The root cause is therefore mismatched link attributes preventing CSPF from finding a feasible path. Network engineers must verify that all links along the intended tunnel path have attributes compatible with requested TE constraints, including TE colors, administrative groups, and available bandwidth. Proper alignment allows CSPF to compute a valid path, and RSVP-TE tunnels can successfully establish. Understanding the interaction between link attributes, constraints, and CSPF computation is critical for predictable MPLS TE operation, effective traffic engineering, and efficient utilization of network resources. Correct attribute configuration ensures reliable tunnel establishment, optimized network performance, and predictable behavior.
Question 55:
A network engineer deploys OSPF across multiple areas. Some routers in area 1 are not receiving inter-area routes from area 0, even though the ABR is advertising them. What is the most likely cause?
A) Area 1 is configured as a stub area.
B) Type-1 LSAs are not being advertised by the ABR.
C) Backbone area 0 is overloaded.
D) OSPF process IDs are mismatched.
Answer: A)
Explanation:
OSPF supports several area types, including standard areas, stub areas, totally stub areas, and not-so-stubby areas (NSS A). The purpose of these area types is to control which LSAs are allowed into an area to optimize router resource usage and limit the size of the LSDB. A stub area is configured to block external Type-5 LSAs from entering the area. Instead, the area relies on a default route injected by the ABR to reach external destinations. Stub areas can still receive Type-3 inter-area summary LSAs from other OSPF areas, but if incorrectly configured or combined with other settings, certain routes may not propagate.
In this scenario, routers in area 1 are not seeing inter-area routes from the backbone, area 0, despite ABR advertisements. This behavior is consistent with a stub area configuration that intentionally suppresses external LSAs to reduce routing table size. If area 1 is configured as a totally stub area or incorrectly configured, Type-3 summary LSAs representing inter-area routes may be blocked. Instead, routers rely on a default route injected by the ABR. If the default route is missing or misconfigured, routers will fail to reach networks outside their own area.
Type-1 LSAs describe router links within an area. If they were not advertised by the ABR, routers would not form OSPF adjacencies, and no LSAs would be received at all. Since intra-area routes are functional, Type-1 LSAs are correctly advertised and are not the cause.
An overloaded backbone area may cause delayed SPF computations or increased convergence times, but it does not prevent the propagation of LSAs to stub areas. Even in a busy backbone, LSAs are still advertised to other areas; only convergence performance may degrade.
OSPF process IDs are locally significant identifiers. Mismatched process IDs between routers do not affect the ability of the ABR to advertise inter-area routes. Adjacencies form based on neighbor IP configurations and authentication, not process ID.
The root cause is that area 1 is configured as a stub area. Stub areas are designed to reduce routing table size and resource usage by blocking certain LSAs and relying on default routes. Network engineers must carefully verify stub area configuration and ensure default route injection by the ABR. Proper planning ensures reachability to inter-area destinations without overwhelming routers with excessive LSAs. Understanding stub area behavior is critical to troubleshooting scenarios where expected inter-area routes do not appear in the LSDB, even though ABRs are correctly advertising routes in other areas. Misconfiguration or omission of default route injection can prevent connectivity to external networks or other areas.
Question 56:
A network engineer deploys BGP multipath on a multi-homed edge router. Multiple paths appear in the BGP table, but traffic is using 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 multiple paths to be used simultaneously in the forwarding plane to improve bandwidth utilization and redundancy. However, BGP strictly enforces equality conditions for multipath installation. To be considered equal, paths must have the same AS path length, origin type, MED value, local preference, and reachable next-hop. If any of these attributes differ, BGP will select a single best path and ignore the others in terms of forwarding, even though they appear in the BGP table.
In the scenario described, multiple paths are visible in the table, but only one is used for traffic. This indicates that the paths do not meet the equality requirements. Common differences include variations in AS path length due to prepending, different origin types (IGP, EGP, incomplete), or MED discrepancies. BGP uses strict criteria to ensure deterministic routing and avoid potential loops or inconsistent traffic forwarding.
BGP session type, whether iBGP or eBGP, does not inherently prevent multipath. Both session types support multipath, provided the equality conditions are satisfied. The presence of multiple paths in the table confirms that BGP is operating correctly and receiving routes from neighbors.
Next-hop reachability is necessary for a path to be installed in the forwarding table. In this case, all paths are present in the BGP table, indicating that next-hop reachability is not an issue.
BGP update suppression affects how often updates are sent to neighbors but does not influence local path selection for forwarding. Since multiple paths exist in the table, update suppression is not relevant to the observed behavior.
The root cause is that the paths are not equal in AS path, origin, or MED attributes. Network engineers must align these attributes to allow BGP to recognize multiple equal-cost paths and enable multipath forwarding. Correct configuration ensures optimal bandwidth utilization, redundancy, and predictable traffic distribution. Understanding BGP multipath equality requirements is essential for multi-homed network deployments to fully leverage available paths, maximize network performance, and achieve high availability. Misalignment of attributes prevents multipath forwarding and can limit redundancy and effective traffic distribution.
Question 57:
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) provides the capability to establish explicit paths based on available bandwidth, link attributes, and administrative constraints. RSVP-TE is the protocol responsible for reserving bandwidth and establishing these tunnels along computed paths. Constrained Shortest Path First (CSPF) calculates a feasible path that satisfies all required attributes, including link bandwidth, TE colors, and administrative group constraints. Even when links report sufficient bandwidth, tunnels may fail to establish if CSPF cannot find a path that meets the constraints. The most common reason is mismatched or missing link attributes, such as TE colors or administrative groups. If the path requested does not match the link attributes along any feasible route, CSPF cannot compute a valid path, and the RSVP-TE tunnel fails.
RSVP authentication ensures that only authorized routers can reserve bandwidth. Authentication mismatches can prevent a tunnel from being successfully established, but CSPF will still attempt to compute a path. Authentication errors generate log messages but do not prevent the feasibility calculation itself.
Soft-state refresh timers maintain RSVP state for existing tunnels. Incorrect refresh intervals may cause premature state expiration or tunnel teardown, but do not affect initial path computation or feasibility.
IGP metrics affect unconstrained SPF calculations, but TE tunnels rely on CSPF, which evaluates both bandwidth and link attributes. Even if a path has the lowest IGP metric, CSPF will reject it if it violates TE constraints, such as unavailable TE colors or administrative group mismatches.
The root cause is therefore the link attribute constraints that prevent CSPF from computing a feasible path. Network engineers must verify that all links along the intended path have attributes compatible with the requested TE constraints, including TE colors, administrative groups, and available bandwidth. Once aligned, CSPF can compute a valid path, allowing RSVP-TE tunnels to establish successfully. Understanding the relationship between link attributes, constraints, and CSPF is critical for predictable MPLS TE operation, traffic engineering efficiency, and optimal utilization of network resources. Proper configuration ensures reliable tunnel establishment, predictable performance, and high network efficiency, avoiding unexpected tunnel failures due to attribute mismatches.
Question 58:
A network engineer deploys OSPF in a multi-area environment. Routers in area 2 are unable to reach external networks even though the ABR advertises default routes. What is the most likely cause?
A) Area 2 is configured as a stub area without a default route injection.
B) Type-2 LSAs are blocked by access lists.
C) Backbone area 0 is down.
D) OSPF process IDs are mismatched.
Answer: A)
Explanation:
OSPF area types are designed to optimize routing efficiency and minimize resource utilization on routers. A stub area blocks external Type-5 LSAs from entering the area to reduce the size of the LSDB and the routing table. Instead of propagating all external routes, the ABR injects a default route, allowing routers in the stub area to reach destinations outside the OSPF domain without requiring detailed knowledge of every external network. A totally stub area goes even further, blocking inter-area Type-3 summaries and relying solely on the default route.
In this scenario, routers in area 2 cannot reach external networks, even though the ABR advertises default routes. This typically occurs when area 2 is configured as a stub area but the ABR is either misconfigured or not injecting the default route. Without the default route, routers in the stub area cannot reach external destinations, because Type-5 LSAs are suppressed. This behavior is consistent with the design of stub areas, where external routes are intentionally restricted, emphasizing the need for proper default route injection to maintain connectivity.
Type-2 LSAs describe transit networks in broadcast or NBMA areas. Blocking Type-2 LSAs would prevent adjacency formation on multi-access networks and would affect intra-area routing, not the visibility of external routes. Since intra-area routing is functional, Type-2 LSAs are not the issue.
A down backbone area would prevent inter-area routing and default route propagation to other areas. However, the ABR is operational and capable of advertising routes, so the backbone is not the limiting factor.
OSPF process IDs are locally significant identifiers that only affect the configuration on the local router. Mismatched process IDs do not prevent adjacency formation or route propagation within a correctly configured OSPF instance.
The root cause is that area 2 is a stub area without a properly injected default route. Network engineers must ensure that stub areas receive a default route from the ABR to reach external networks. Proper configuration includes using the “area X stub” command on area interfaces and configuring the ABR to advertise default routes using “default-information originate.” Understanding stub area behavior is critical to ensure that routers have external connectivity while maintaining reduced routing table sizes and LSDB complexity. Failure to configure default route injection results in routers being unable to reach external destinations, even though internal OSPF routing is functional. Correctly designing stub areas balances efficiency with connectivity requirements, which is essential for predictable OSPF behavior in large, multi-area networks.
Question 59:
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 with iBGP only.
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, increasing redundancy and optimizing bandwidth utilization. To qualify for multipath, candidate paths must match across several attributes, including AS path length, origin type, MED, local preference, and reachable next-hop. Even minor differences in these attributes prevent multipath installation, ensuring deterministic routing and avoiding inconsistent forwarding behavior.
In the scenario described, multiple paths are present in the BGP table, but only one path is used for forwarding. This indicates that BGP considers the paths unequal due to differences in AS path, origin type, or MED. For example, if one path has an extra AS path prepended or a different origin type, BGP selects the single best path based on its decision process and does not leverage multipath. This behavior ensures predictable traffic routing and prevents potential loops.
BGP session type, whether iBGP or eBGP, does not prevent multipath as long as the equality conditions are met. Both iBGP and eBGP support multipath, and the presence of multiple paths in the table confirms that the routing protocol is receiving updates correctly.
Next-hop reachability is required for installation in the forwarding table. If the next-hop is unreachable, the path will not be used for forwarding. Since all paths appear in the table, next-hop reachability is not the limiting factor.
BGP update suppression limits the rate of updates sent to neighbors but does not influence local path selection for forwarding. Therefore, update suppression does not explain why only one path is being utilized.
The root cause is that the paths are unequal in key attributes. Network engineers must align AS path, origin, and MED values to enable BGP multipath forwarding. This ensures effective traffic distribution, increased redundancy, and better utilization of available bandwidth. Understanding multipath equality requirements is essential in multi-homed network designs to avoid underutilization of available links. Proper configuration of AS paths, origin types, and MED attributes guarantees that traffic leverages all available paths, enhancing network performance, resilience, and efficiency. Misalignment of these attributes limits redundancy and can lead to traffic congestion on a single link, even when multiple paths are available. Network engineers must carefully evaluate and adjust attributes to fully enable BGP multipath forwarding in multi-homed deployments.
Question 60:
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 operators to create explicit paths based on available bandwidth, link attributes, and administrative constraints. RSVP-TE is the protocol used to reserve bandwidth and establish tunnels along these paths. Constrained Shortest Path First (CSPF) evaluates candidate paths to ensure all TE constraints, such as bandwidth, TE colors, and administrative groups, are met. Even if sufficient bandwidth exists, tunnels may fail if CSPF cannot compute a path that satisfies all constraints. Link attribute mismatches, including incorrect TE colors or missing administrative group markings, are the most common reason for RSVP-TE tunnel establishment failure in situations where bandwidth is not an issue.
RSVP authentication prevents unauthorized routers from reserving bandwidth. While mismatched authentication prevents the tunnel from being established, CSPF still attempts path computation. Authentication errors generate log entries but do not directly cause path feasibility failures.
Soft-state refresh timers maintain RSVP reservations for active tunnels. If timers are too long, tunnels may expire prematurely, but initial path computation is unaffected.
IGP metrics affect unconstrained SPF calculations but do not impact CSPF, which considers both bandwidth and attribute constraints. Even if the lowest IGP cost path exists, CSPF will reject it if it violates TE constraints.
The root cause is mismatched link attributes preventing CSPF from computing a feasible path. Engineers must ensure that all links along the desired path have compatible attributes, including TE colors, administrative groups, and sufficient bandwidth. Proper attribute alignment allows CSPF to successfully calculate a path, enabling RSVP-TE tunnels to establish. Understanding the interaction between link attributes, constraints, and CSPF computation is essential for predictable MPLS TE behavior, effective traffic engineering, and efficient resource utilization. Accurate configuration ensures reliable tunnel setup, optimal performance, and predictable network operation, avoiding failures caused by attribute mismatches.