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Question 41
Which BGP attribute determines the preferred path within the same autonomous system?
A) AS path
B) Local preference
C) MED
D) Weight
Answer: B
Explanation:
In BGP, the decision process for choosing the best path to a destination involves multiple attributes evaluated in a specific order. Among these attributes, local preference is crucial for determining the preferred path within the same autonomous system (AS). Local preference is a non-transitive attribute, meaning it is not propagated to external BGP peers; it only affects routing decisions among routers in the same AS. Higher local preference values are preferred.
Option B) is correct because administrators can assign local preference values to favor certain exit points for outbound traffic, optimizing bandwidth usage, redundancy, and policy enforcement. For example, a network with multiple exit links to the Internet or other ASes can manipulate local preference to ensure critical traffic exits through the most reliable or cost-effective link.
Option A) AS path primarily influences inbound traffic and route selection from external peers. A shorter AS path is preferred by default, but it does not affect internal path preferences.
Option C) MED (Multi-Exit Discriminator) is used to suggest preferred entry points to neighboring ASes but does not enforce path selection within the local AS.
Option D) Weight is Cisco-specific and affects local routers only, making it useful for local traffic engineering but secondary to local preference in large-scale enterprise networks.
Local preference is often manipulated using route maps, prefix lists, or policy-based configurations. Enterprises leverage it to enforce internal routing policies, prioritize business-critical traffic, and balance loads across multiple exits. Misconfigured local preference can lead to suboptimal routing, congestion, or inefficient use of available links. Using local preference strategically allows network engineers to maintain predictable path selection, optimize redundancy, and implement traffic engineering policies without altering physical topology. It is particularly useful in large-scale WANs and multi-homed enterprises where internal consistency, SLAs, and cost management are crucial. Understanding the interplay between BGP attributes like local preference, AS path, MED, and weight enables precise control over traffic flows, ensuring a resilient, high-performance enterprise network.
Question 42
In OSPF, which area type blocks external route advertisements?
A) Backbone area
B) Stub area
C) NSSA
D) Totally stubby area
Answer: D
Explanation:
OSPF supports various area types to optimize routing efficiency and minimize unnecessary LSA flooding. A totally stubby area is a special configuration that blocks all external route advertisements (Type 5 LSAs) from entering the area. This helps reduce routing table size, CPU utilization, and unnecessary SPF calculations for routers in the area. Totally stubby areas only allow a default route to reach external destinations.
Option D) is correct because totally stubby areas are designed specifically to eliminate external route flooding while still allowing inter-area routing through a default route. This is ideal for branch offices or remote networks with limited resources, where only connectivity to the backbone and internal networks is required.
Option A) the backbone area (Area 0) is not restrictive; it carries all LSA types.
Option B) a standard stub area blocks only external Type 5 LSAs but allows summary LSAs (Type 3) from other areas.
Option C) NSSA (Not So Stubby Area) allows limited external route injection via Type 7 LSAs while still providing summarization benefits.
Implementing totally stubby areas reduces routing overhead, improves convergence, and enhances network predictability. For example, remote branches connected to the enterprise backbone via WAN links can avoid flooding of external BGP or OSPF external routes, which reduces memory and CPU usage on lower-end routers. Network engineers often combine totally stubby areas with route summarization at ABRs to minimize LSA propagation further, improving scalability and operational stability. This configuration also simplifies troubleshooting because the limited routing table ensures predictable path selection. It is especially beneficial in large-scale enterprise networks with thousands of endpoints, where efficiency, stability, and rapid convergence are critical for maintaining business continuity and application performance. Totally stubby areas, when used properly, balance operational simplicity and functional connectivity.
Question 43
Which EIGRP feature ensures backup paths are immediately available if the primary fails?
A) Feasible successor
B) Variance
C) Stub routing
D) Load balancing
Answer: A
Explanation:
EIGRP maintains a topology table that contains all learned routes, not just the best route. A key feature of EIGRP is the feasible successor concept. Feasible successors are backup paths that satisfy the feasibility condition: the reported distance must be less than the feasible distance of the primary path. These routes are stored in the topology table and can be immediately installed into the routing table if the primary path fails, enabling rapid convergence without recalculating the SPF or querying neighbors.
Option A) is correct because feasible successors ensure instantaneous failover, which is critical for enterprise networks with strict uptime requirements. They reduce convergence time significantly compared to other routing protocols, which may need to recalculate or query the network to find an alternate path.
Option B) variance allows unequal-cost load balancing but does not guarantee backup availability for immediate failover.
Option C) stub routing restricts query propagation but does not provide backup paths for instant failover.
Option D) load balancing distributes traffic but does not automatically provide feasible successors for rapid convergence.
The feasibility condition is a fundamental mechanism that prevents routing loops while allowing backup paths to be precomputed. In practical enterprise deployments, feasible successors ensure resilient WAN connections, maintain application performance during link failures, and support redundant network design. By storing multiple backup paths, EIGRP enables intelligent traffic engineering and fault tolerance. Network engineers must ensure that feasible successors are available on critical routes to prevent downtime, optimize bandwidth usage, and maintain high availability for voice, video, and mission-critical applications. This mechanism demonstrates EIGRP’s efficiency in rapid convergence, deterministic routing, and reliable backup path selection, which are essential for modern enterprise networks.
Question 44
Which BGP mechanism allows grouping routes for policy application across multiple prefixes?
A) Route maps
B) Prefix lists
C) Route aggregation
D) BGP communities
Answer: D
Explanation:
BGP supports advanced policy control using communities, which are optional transitive attributes that can be applied to groups of prefixes. Communities allow operators to tag routes and influence routing decisions, filtering, or redistribution policies without modifying each route individually. For example, an enterprise network can assign a community to all prefixes from a specific customer or region, enabling consistent policy application across multiple prefixes.
Option D) is correct because BGP communities are widely used in traffic engineering, route filtering, and multi-homed environments. Communities can signal preferred paths, blackhole traffic, or indicate customer-specific routing rules, providing a powerful and flexible mechanism to manage complex BGP deployments.
Option A) route maps are used to implement policy but act on individual prefixes, while communities allow bulk management.
Option B) prefix lists define which prefixes are permitted or denied but do not provide the grouping and tagging flexibility of communities.
Option C) route aggregation combines multiple routes into a summarized route but is not primarily a policy grouping mechanism.
Using BGP communities in enterprise and service provider networks enhances scalability, maintainability, and operational efficiency. Network engineers can define policies that automatically apply to entire sets of prefixes based on business requirements or SLAs, reducing the risk of misconfiguration. Communities are also useful for multi-AS deployments, influencing inbound or outbound traffic patterns predictably. Enterprises benefit from streamlined management, reduced administrative complexity, and precise traffic engineering, especially in large-scale WANs or when interacting with multiple ISPs. When combined with route maps and prefix lists, communities provide granular yet flexible control over routing behavior, ensuring predictable performance and high availability.
Question 45
Which feature allows OSPF routers to prevent routing loops in a hub-and-spoke topology?
A) Stub area
B) NSSA
C) Totally stubby area
D) Opaque LSAs
Answer: A
Explanation:
In hub-and-spoke topologies, remote sites (spokes) often require a simplified routing design to reduce overhead and prevent routing loops. OSPF stub areas restrict the propagation of external Type 5 LSAs into the area. By allowing only intra-area routes and a default route to exit the area, stub areas prevent spokes from learning external routes directly from other areas, effectively eliminating the possibility of routing loops between branches.
Option A) is correct because stub areas are designed for remote or resource-constrained locations, ensuring limited routing information is distributed, which maintains predictable path selection and network stability.
Option B) NSSA allows external route injection using Type 7 LSAs, providing more flexibility than standard stub areas but not necessarily preventing all potential routing loops.
Option C) totally stubby areas further restrict routing but are typically applied to branch sites; while they reduce routing overhead, their primary purpose is not loop prevention but efficiency.
Option D) opaque LSAs carry application-specific or traffic engineering information, not for preventing loops.
Using stub areas in hub-and-spoke networks simplifies topology management, reduces SPF calculations, and ensures that all spokes rely on the hub for external connectivity. By centralizing external route knowledge in the hub, network engineers can control traffic patterns, enforce security policies, and maintain redundancy without overloading spoke routers. This practice is widely recommended in large-scale enterprise deployments with multiple branches, providing efficiency, stability, and loop-free operations. Proper configuration ensures that all remote sites receive connectivity to backbone resources while preventing unnecessary LSA propagation, which could lead to instability or routing loops. Stub areas also facilitate faster convergence and resource-efficient routing, essential in WAN architectures with limited bandwidth and CPU-constrained devices.
Question 46
Which EIGRP feature prevents query propagation to specific routers in a network?
A) Feasible successor
B) Stub routing
C) Variance
D) Summarization
Answer: B
Explanation:
In EIGRP, stub routing is a feature that is used to optimize routing in hub-and-spoke topologies. Stub routers are typically branch routers that do not need to propagate routes or be queried by other routers for network information. Enabling stub routing on a router ensures that query messages for routes to external networks or other areas do not propagate to that router, thus preventing unnecessary CPU and memory utilization, and reducing convergence time in case of network failures.
Option B) is correct because stub routing ensures that spokes rely on the hub router for knowledge of the rest of the network, which reduces overhead and increases efficiency. EIGRP calculates feasible successors for failover, but stub routers are not burdened with this information, making them lightweight.
Option A) feasible successor refers to backup routes that satisfy the feasibility condition and are ready to be installed in the routing table if the primary path fails. While crucial for fast convergence, feasible successors do not prevent query propagation.
Option C) variance allows unequal-cost load balancing, spreading traffic across multiple paths but does not prevent query propagation or control topology information dissemination.
Option D) summarization reduces the number of routes advertised to neighbors by combining multiple routes into a single summarized route, but it does not inherently stop query propagation.
Stub routing is essential in large enterprise environments, particularly in WAN topologies where multiple branches are connected to a central hub. Without stub routing, all routers would need to process queries for all destinations, leading to excessive network traffic and potential delays. By marking certain routers as stub routers, network engineers can ensure predictable and controlled traffic patterns, reduce memory and CPU usage on branch devices, and improve overall network performance. Properly configured stub routing also contributes to loop-free, stable, and highly available network operations, as branches only forward necessary information while the central hub maintains a comprehensive view of the network topology.
Question 47
Which OSPF LSA type is used to advertise external routes into the OSPF domain?
A) Type 1
B) Type 2
C) Type 5
D) Type 3
Answer: C
Explanation:
In OSPF, external routes learned from other routing protocols (like BGP or static routes) are advertised into the OSPF domain using Type 5 LSAs, also called AS-external LSAs. These LSAs allow OSPF routers to know about networks outside the OSPF autonomous system, facilitating communication between internal OSPF routers and external destinations.
Option C) is correct because Type 5 LSAs are generated by Autonomous System Boundary Routers (ASBRs) and flooded throughout the OSPF domain, except into stub or totally stubby areas where they are intentionally blocked to reduce overhead. Type 5 LSAs carry information about the network prefix, external cost, and metric type, which can be either Type 1 (E1) for additive cost or Type 2 (E2) for fixed cost.
Option A) Type 1 LSAs are router LSAs generated by every router and describe the router itself and its interfaces within an area. They are not used for external routes.
Option B) Type 2 LSAs are network LSAs generated by Designated Routers (DRs) for multi-access networks and describe connected routers.
Option D) Type 3 LSAs are summary LSAs generated by Area Border Routers (ABRs) to advertise inter-area routes.
Understanding the role of Type 5 LSAs is essential in enterprise networks where BGP, static routes, or other routing protocols are redistributed into OSPF. These LSAs allow internal routers to reach external networks without having native knowledge of the external routing protocol. When designing OSPF areas, network engineers must carefully plan where Type 5 LSAs are propagated to avoid flooding unnecessary information into resource-constrained areas, such as stub areas, where only a default route is sufficient. Misconfiguring external LSAs can result in suboptimal routing, loops, or increased convergence time, impacting business-critical applications. Proper management of Type 5 LSAs, combined with summarization and area design, ensures efficient, stable, and scalable OSPF operation, especially in large enterprise environments with multiple ASBRs and external connections.
Question 48
Which BGP attribute is Cisco-specific and used to influence local path selection?
A) Weight
B) Local preference
C) AS path
D) MED
Answer: A
Explanation:
In BGP, the weight attribute is a Cisco-proprietary parameter that affects local path selection on a router. It is the first criterion evaluated in the BGP best path selection process, making it a highly effective tool for influencing which path the local router will prefer for routing traffic. Unlike other attributes, weight is not propagated to other routers; it is purely local, providing network engineers with a way to control outbound traffic at a particular router without affecting the broader network.
Option A) is correct because weight allows administrators to assign preference to specific routes, ensuring that critical traffic exits through a desired link or interface. For example, if a router has multiple BGP peers advertising the same prefix, assigning a higher weight to one peer guarantees that traffic prefers that path locally.
Option B) local preference is used to influence path selection across all routers in the same AS, affecting internal route choice rather than purely local selection.
Option C) AS path determines route preference based on path length through autonomous systems and primarily affects inbound traffic.
Option D) MED is used to suggest a preferred entry point into an AS for external neighbors but does not influence local decision-making directly.
The weight attribute is vital for traffic engineering in multi-homed enterprises or service provider networks, allowing administrators to control routing behavior without altering physical connectivity. It is particularly useful for failover scenarios, load distribution, or prioritizing certain WAN links over others. By assigning weight strategically, network engineers can enforce routing policies, maintain predictable traffic patterns, and enhance overall network performance. Mismanagement of weight can result in suboptimal routing or traffic congestion, so careful planning and consistent application are critical. Weight, combined with other BGP attributes like local preference, AS path, and MED, allows for a granular, flexible, and robust routing policy framework, supporting the high availability and reliability requirements of modern enterprise networks.
Question 49
Which OSPF feature reduces SPF calculations in multi-access networks?
A) NSSA
B) DR/BDR election
C) Stub area
D) Totally stubby area
Answer: B
Explanation:
In multi-access networks, such as Ethernet LANs, multiple OSPF routers may exist on the same segment. Without optimization, each router would generate numerous LSAs and perform SPF calculations for every topology change, resulting in high CPU utilization and potential instability. OSPF solves this problem through the Designated Router (DR) and Backup Designated Router (BDR) election process.
Option B) is correct because the DR centralizes LSA dissemination on the segment, reducing the number of LSAs exchanged and the number of SPF calculations each router must perform. The BDR serves as a standby to maintain redundancy in case the DR fails. All other routers form adjacencies only with the DR and BDR rather than with every other router, significantly reducing routing overhead.
Option A) NSSA allows limited external route injection and area flexibility but does not specifically reduce SPF calculations on multi-access networks.
Option C) Stub areas minimize external route flooding but are not designed to optimize multi-access segment efficiency directly.
Option D) Totally stubby areas restrict external and summary LSAs but also do not address multi-access SPF optimization.
Using DR/BDR elections improves OSPF scalability, particularly in enterprise campus environments with dozens of routers per broadcast domain. By centralizing LSA handling, convergence times are faster, CPU load is lower, and the network can support more routers without performance degradation. Proper DR/BDR election is critical; routers with higher priority are preferred as DRs, and tie-breaking rules consider router ID. Misconfigured DR/BDR priorities or conflicts can lead to suboptimal routing or temporary LSA inconsistencies. Engineers must carefully plan network design, broadcast segmentation, and election parameters to ensure resilient, efficient OSPF operation, particularly in large-scale enterprise networks with complex topologies. DR/BDR functionality is fundamental for maintaining predictable, scalable, and loop-free routing.
Question 50
Which EIGRP setting allows unequal-cost load balancing across multiple paths?
A) Feasible successor
B) Variance
C) Stub routing
D) Metric weights
Answer: B
Explanation:
EIGRP provides a robust mechanism for load balancing across multiple paths using the variance command. By default, EIGRP supports equal-cost load balancing, but variance allows traffic to traverse paths with metrics up to a certain multiple of the best path metric. This enables unequal-cost load balancing, efficiently utilizing multiple links and improving bandwidth usage without sacrificing stability.
Option B) is correct because variance multiplies the feasible distance of the best path, including additional feasible successors that meet the condition. These routes are then installed in the routing table, enabling traffic distribution across multiple paths, even when their metrics differ.
Option A) feasible successors are backup paths stored in the topology table for rapid failover but do not control traffic distribution.
Option C) stub routing prevents query propagation in hub-and-spoke topologies, enhancing efficiency but not load balancing.
Option D) metric weights influence route selection but are not used for unequal-cost load balancing.
Proper use of variance enables enterprises to maximize link utilization, reduce congestion, and improve redundancy. When combined with EIGRP metrics like bandwidth, delay, reliability, and load, variance allows fine-grained traffic engineering tailored to network requirements. Network engineers must ensure feasible successors exist and that variance does not introduce routing loops. By balancing performance, redundancy, and operational simplicity, variance ensures optimal utilization of available paths while maintaining rapid convergence and fault tolerance, critical for high-performance enterprise networks supporting voice, video, and data traffic simultaneously.
Question 51
Which BGP attribute determines the preferred path when multiple routes exist from the same AS?
A) Local preference
B) Weight
C) MED
D) AS path
Answer: A
Explanation:
In BGP, the local preference attribute is used to determine the preferred path when multiple routes to the same destination exist within an Autonomous System (AS). Local preference is propagated to all routers within the same AS and indicates which path outbound traffic should use. By default, the highest local preference value is preferred. This makes local preference a critical tool for traffic engineering and controlling routing policies in enterprise or service provider networks.
Option A) is correct because local preference influences path selection internally across all routers in the AS, providing administrators with centralized control over route preference. For example, if a company has multiple connections to the Internet or different branches, the local preference attribute can ensure that traffic uses a primary route while providing backup paths for failover scenarios.
Option B) weight is Cisco-specific and only affects the local router, making it a local-only tool rather than an AS-wide solution.
Option C) MED (multi-exit discriminator) is used to suggest preferred entry points into an AS from external neighbors, not to control internal routing within the AS.
Option D) AS path length is evaluated when comparing external BGP routes and affects path selection for inbound traffic, but local preference takes precedence for internal preference decisions.
Understanding local preference is crucial for controlling routing patterns in large enterprise networks. It is particularly useful when multiple BGP peers exist and traffic needs to be steered through specific links for load balancing, redundancy, or cost optimization. Misconfiguring local preference can cause suboptimal routing, congestion, or unintended failover, so careful planning is essential. Local preference can also be combined with route maps to dynamically adjust preferences based on criteria such as prefix lists, next-hop IP addresses, or routing policies. By effectively managing local preference, network engineers can achieve predictable, stable, and highly available network behavior, while optimizing bandwidth usage and maintaining network performance even in complex, multi-homed environments.
Question 52
Which OSPF area type blocks external Type 5 LSAs but allows default routes?
A) Stub area
B) Totally stubby area
C) NSSA
D) Backbone area
Answer: B
Explanation:
In OSPF, a totally stubby area is a special configuration designed to reduce routing table size and minimize LSA flooding. This area type blocks external Type 5 LSAs while allowing only a default route to be advertised into the area. This reduces overhead for routers in branch locations or constrained environments, improving convergence time and CPU efficiency.
Option B) is correct because totally stubby areas are commonly used in hub-and-spoke topologies. Routers inside the area do not receive detailed external or inter-area routes; instead, they rely on a single default route pointing to the ABR, which handles all external routing. This setup simplifies the routing table on branch routers and limits SPF recalculation, especially during network topology changes.
Option A) stub areas block Type 5 LSAs but still allow summary Type 3 LSAs. They reduce external routing information but are less restrictive than totally stubby areas.
Option C) NSSA (Not-So-Stubby Area) allows external routes to be injected via Type 7 LSAs and converted to Type 5 at the ABR, primarily for scenarios where external redistribution is required.
Option D) backbone area (Area 0) is the core of OSPF and carries all inter-area and external LSAs; it does not restrict Type 5 LSAs.
Totally stubby areas are essential in large-scale enterprise deployments where branch routers have limited resources and do not need detailed routing information. By reducing the number of LSAs, network engineers can lower CPU utilization, conserve memory, and achieve faster network convergence. Proper implementation requires configuring the ABR with the no-summary command to prevent Type 3 LSAs from entering the area, ensuring that only a default route exists. Misconfigurations can lead to routing loops or unreachable networks, so careful planning is necessary. Totally stubby areas are particularly effective in WAN environments, ensuring predictable routing behavior, minimal LSA propagation, and efficient resource usage while maintaining connectivity to external networks via the ABR.
Question 53
Which EIGRP metric component reflects the reliability of a path?
A) Bandwidth
B) Delay
C) Load
D) Reliability
Answer: D
Explanation:
EIGRP uses a composite metric to select the best path to a destination. This metric combines bandwidth, delay, load, reliability, and MTU, though MTU is not used for path selection. Among these, the reliability component reflects the stability and dependability of the path, measured on a scale from 1 (unreliable) to 255 (highly reliable). Reliability is calculated based on interface error rates and link health over time, allowing EIGRP to prefer paths that are less prone to packet loss or failures.
Option D) is correct because reliability directly affects route selection in EIGRP. Paths with higher reliability are preferred when other metrics are equal, ensuring that critical enterprise traffic, such as voice, video, or sensitive data, traverses stable links. Administrators can also influence routing decisions using manual adjustments to interface parameters, enhancing network resilience.
Option A) bandwidth represents the minimum bandwidth along the path and is often the dominant factor in route selection. While important, bandwidth does not reflect path stability or error rates.
Option B) delay reflects the cumulative latency across a path and impacts traffic performance but does not measure link reliability.
Option C) load indicates current utilization on an interface but is dynamic and does not measure historical stability or fault rates.
Reliability is particularly significant in enterprise environments where high availability and fault tolerance are critical. EIGRP’s ability to consider reliability ensures that traffic avoids unstable or degraded links, maintaining consistent application performance. When combined with other metrics, network engineers can design intelligent routing policies that balance efficiency, redundancy, and performance, supporting business-critical applications and improving overall network resilience. Proper monitoring and tuning of reliability values allow administrators to proactively respond to failing links before they impact operations, ensuring predictable and stable enterprise network behavior even in complex topologies with multiple redundant paths.
Question 54
Which BGP feature allows selective route advertisement based on defined policies?
A) Route maps
B) Weight
C) Local preference
D) AS path prepending
Answer: A
Explanation:
BGP route maps are a powerful tool for implementing selective route advertisement, filtering, and modification of BGP attributes based on defined policies. Route maps allow network engineers to control which routes are advertised or accepted, modify attributes such as weight, local preference, or MED, and implement complex routing policies that meet enterprise or service provider requirements.
Option A) is correct because route maps provide granular control over routing decisions. They can be applied to both inbound and outbound BGP updates, enabling selective redistribution and policy-based routing. Route maps use match conditions (e.g., prefix lists, AS numbers, community strings) and set actions (e.g., modify attributes, deny, permit) to enforce network policies consistently.
Option B) weight is local-only and influences path selection on the router itself but does not provide selective advertisement capabilities.
Option C) local preference affects path selection within an AS but cannot filter or selectively advertise routes.
Option D) AS path prepending manipulates AS path length to influence inbound traffic from external neighbors but is not used for fine-grained selective advertisement.
Route maps are crucial for enterprise networks that must implement security policies, traffic engineering, and multi-homing strategies. By controlling advertisement selectively, administrators can prevent routing loops, optimize bandwidth utilization, enforce business rules, and maintain predictable routing behavior. Misconfigured route maps can lead to route loss, suboptimal routing, or network instability, so careful testing and planning are essential. Route maps can also integrate with BGP communities to provide even more flexible and scalable routing policies, enabling large enterprises or service providers to manage complex interconnections while maintaining network stability, efficiency, and compliance with organizational objectives.
Question 55
Which OSPF feature allows external routes in a stub area?
A) NSSA
B) Totally stubby area
C) Backbone area
D) Stub area
Answer: A
Explanation:
In OSPF, a Not-So-Stubby Area (NSSA) is designed to allow limited external route injection into a stub-like area. NSSA supports Type 7 LSAs, which carry external routing information while still maintaining most stub characteristics, such as blocking Type 5 LSAs. This makes NSSA suitable for branch offices that need to redistribute external routes (like static or BGP-learned routes) while keeping the routing table size small.
Option A) is correct because NSSA provides a balance between stub area simplicity and the ability to inject external routes, which are converted to Type 5 LSAs by the ABR when leaving the NSSA. This ensures internal routers have access to external networks without flooding the area with unnecessary routing information.
Option B) totally stubby areas block external LSAs entirely and only allow a default route, making them unsuitable when external route injection is needed.
Option C) backbone area is the main OSPF area (Area 0) and carries all route types; it does not provide stub-like functionality.
Option D) stub area blocks Type 5 LSAs but does not support injecting external routes from within the area.
NSSAs are widely used in large enterprise or multi-site deployments where branch routers require access to redistributed external networks while minimizing routing complexity. Proper configuration ensures that external routes are available only where necessary, conserving router resources, reducing SPF recalculations, and maintaining network stability. Misconfiguration can lead to unreachable networks or excessive LSA propagation, so careful planning of NSSA boundaries and ABR settings is critical. NSSA provides scalable, efficient, and flexible OSPF design, allowing enterprises to optimize both performance and routing table size while accommodating external network integration.
Question 56
Which OSPF authentication type uses MD5 for secure routing updates?
A) None
B) Simple password
C) MD5 authentication
D) SHA-256
Answer: C
Explanation:
In OSPF, authentication ensures that routers only accept routing updates from trusted neighbors. MD5 authentication is widely used because it provides a secure mechanism to prevent unauthorized routers from injecting incorrect routes into the network. MD5 uses a cryptographic hash function, generating a checksum based on the password and the OSPF message content. This ensures data integrity and authenticity.
Option C) is correct because MD5 authentication is implemented by configuring an OSPF key chain or directly specifying an MD5 password on the interface connecting OSPF neighbors. This protects against common attacks, such as route injection or spoofing, by validating the source of the routing update. MD5 authentication is also resilient to plain-text password exposure, unlike simple password authentication, which can be easily captured from network traffic.
Option A) None indicates no authentication is applied, leaving OSPF updates vulnerable to interception or manipulation.
Option B) Simple password uses a clear-text password for authentication, which is susceptible to eavesdropping and is generally discouraged in enterprise environments.
Option D) SHA-256 is a stronger cryptographic hash but is not natively supported in standard OSPF implementations; some advanced versions or third-party vendors may support it.
Using MD5 authentication is critical in enterprise networks with multiple OSPF neighbors across WAN links or untrusted segments. It protects the routing infrastructure from man-in-the-middle attacks, route poisoning, and network misconfigurations. Proper implementation requires careful coordination of passwords and key IDs across all OSPF neighbors to avoid adjacency failures. Network engineers must also periodically rotate keys to maintain security compliance and reduce the risk of compromise. MD5 authentication, while not foolproof, provides a balance between security and performance, ensuring that enterprise routing remains both reliable and protected against malicious or accidental misconfigurations. This is particularly important in multi-area or multi-site deployments where unauthorized updates could severely disrupt network stability.
Question 57
Which EIGRP feature provides rapid convergence using feasible successors?
A) Split horizon
B) Diffusing Update Algorithm (DUAL)
C) Poison reverse
D) Passive interface
Answer: B
Explanation:
EIGRP uses the Diffusing Update Algorithm (DUAL) to calculate loop-free paths and ensure rapid convergence. DUAL relies on the concepts of successor and feasible successor to provide fast failover without recalculating the entire topology. A successor is the primary path to a destination, while a feasible successor is a backup path that meets the feasibility condition, ensuring loop-free operation.
Option B) is correct because DUAL enables EIGRP to switch immediately to a feasible successor if the primary path fails, minimizing downtime and maintaining high network availability. The feasibility condition ensures that the feasible successor’s reported distance is less than the feasible distance of the current successor, guaranteeing loop-free backup routes.
Option A) split horizon prevents routing information from being sent back out the interface it was learned on, helping avoid routing loops but not providing rapid convergence.
Option C) poison reverse is a method to explicitly mark routes as unreachable to prevent loops, which is a slower mechanism compared to feasible successors.
Option D) passive interface disables EIGRP updates on an interface, preventing neighbor formation without contributing to convergence.
DUAL is a key feature that differentiates EIGRP from other routing protocols. It combines the speed of distance-vector protocols with the loop-free guarantees of link-state protocols, making it highly efficient in enterprise environments. By maintaining feasible successors in the topology table, EIGRP ensures that a backup path is instantly available if a failure occurs, which is critical for networks supporting real-time applications such as voice and video. Additionally, DUAL allows EIGRP to optimize routing metrics dynamically, considering bandwidth, delay, load, and reliability to select the best paths. Proper configuration and monitoring of feasible successors improve redundancy and reduce the risk of traffic loss during link failures, ensuring robust and predictable routing behavior across complex enterprise topologies. Network engineers must also understand that the number of feasible successors and the design of metric calculations directly affect failover performance and network stability, making DUAL a cornerstone of resilient EIGRP deployments.
Question 58
Which BGP attribute influences path selection for incoming traffic from external peers?
A) Local preference
B) Weight
C) MED
D) AS path
Answer: C
Explanation:
The Multi-Exit Discriminator (MED) attribute in BGP is used to influence the preferred entry point for incoming traffic from external neighbors. It suggests which path should be preferred when multiple entry points exist between Autonomous Systems. A lower MED value is preferred, signaling external peers to choose a particular link. MED is particularly useful in multi-homed environments where traffic engineering for inbound flows is required.
Option C) is correct because MED is sent along with BGP updates to neighboring ASes and can be used to control traffic entering the AS without affecting internal routing decisions. It is not propagated beyond the neighboring AS unless specifically configured to do so.
Option A) local preference affects outbound path selection within the AS, not inbound traffic.
Option B) weight is a Cisco-specific attribute used for local path selection and is not advertised to external peers.
Option D) AS path affects path selection and loop prevention for external routes, but it does not provide explicit control over inbound traffic preference.
Proper use of MED is critical for enterprise networks with multiple connections to the same service provider or multiple providers, ensuring traffic enters the AS via the most optimal link. It supports load balancing, redundancy, and failover scenarios, improving overall network performance and reliability. Misconfiguring MED can lead to suboptimal routing, congestion on preferred links, or inefficient bandwidth usage. Network engineers must carefully coordinate MED values across peers and understand its interaction with local preference, AS path, and other attributes to achieve desired routing outcomes. Additionally, MED can be combined with route maps and BGP communities to dynamically adjust inbound traffic patterns, enabling scalable and flexible routing policies for large-scale enterprise networks. MED’s role in shaping inbound traffic makes it a critical tool for controlling how external users and partners access an enterprise network, ensuring predictable and optimized traffic flows.
Question 59
Which OSPF LSA type describes routes between different areas?
A) Type 1
B) Type 2
C) Type 3
D) Type 5
Answer: C
Explanation:
In OSPF, Type 3 LSAs (Summary LSAs) are used by Area Border Routers (ABRs) to advertise routes from one area to another. These LSAs summarize networks from a non-backbone area and provide inter-area connectivity. Type 3 LSAs allow OSPF to scale efficiently by reducing the amount of detailed link-state information propagated across areas.
Option C) is correct because ABRs generate Type 3 LSAs to ensure that routers in other areas can reach destinations outside their own area. Type 3 LSAs include the network prefix, subnet mask, and metric to reach the destination. They do not include interface details, which helps reduce routing table size and LSA flooding.
Option A) Type 1 LSAs describe router link states within an area and are flooded only within that area.
Option B) Type 2 LSAs describe network links in broadcast or non-broadcast multi-access networks and are confined to a single area.
Option D) Type 5 LSAs are external LSAs used to advertise routes redistributed from other protocols into OSPF.
Type 3 LSAs are fundamental to OSPF’s hierarchical design, which divides a network into areas to enhance scalability and reduce routing overhead. Without Type 3 LSAs, routers in different areas would not have a mechanism to learn about remote networks efficiently. ABRs are responsible for summarizing and propagating inter-area routes while maintaining loop-free topologies. Misconfigurations in summarization can lead to routing black holes, suboptimal paths, or unnecessary LSA flooding. Properly designed Type 3 LSA summarization optimizes SPF calculation times, reduces CPU and memory usage on routers, and enhances network stability. Enterprises often leverage summarization to reduce the size of routing tables on branch routers, improve convergence times, and support large multi-area OSPF networks efficiently.
Question 60
Which BGP configuration modifies route advertisement order to influence outbound traffic?
A) Weight
B) Local preference
C) AS path prepending
D) MED
Answer: C
Explanation:
AS path prepending in BGP is used to influence the selection of outbound traffic paths by artificially increasing the length of the AS path. BGP prefers routes with shorter AS paths. By prepending the local AS multiple times, administrators can make certain paths less attractive, steering outbound traffic through preferred links. This method is critical for traffic engineering, load balancing, and redundancy in multi-homed networks.
Option C) is correct because AS path prepending directly affects external BGP peers’ route selection. By increasing the AS path length on one path, traffic is biased toward alternate, shorter paths. This technique is widely used in enterprise networks connected to multiple ISPs or when multiple exit points exist to ensure optimal utilization and control over outbound traffic flows.
Option A) weight is Cisco-specific and only affects the local router’s path selection, not advertisement to external peers.
Option B) local preference affects internal path selection within an AS but does not directly influence outbound traffic from the AS.
Option D) MED influences how external peers select inbound paths into an AS, not outbound paths.
Correct implementation of AS path prepending requires careful planning to avoid asymmetric routing, suboptimal paths, or traffic congestion. It is often combined with BGP communities, route maps, and policy-based routing to create sophisticated traffic engineering strategies. Misuse of AS path prepending can unintentionally divert traffic, create loops, or increase latency. Enterprises leverage this technique to optimize connectivity, achieve predictable performance, and manage costs associated with multi-homed Internet or WAN connectivity. AS path prepending is also useful for redundancy planning, allowing engineers to control failover scenarios while minimizing disruptions in outbound traffic routing patterns.
