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Question 121
Which BGP command prevents a router from advertising specific routes to a neighbor?
A) route-map
B) distribute-list
C) neighbor filter-list
D) prefix-list
Answer: D
Explanation:
In BGP, the prefix-list command is widely used to filter routes that are advertised or received from neighbors. This feature allows engineers to control which networks are shared or accepted in BGP peering sessions, providing both security and route optimization. Prefix-lists are applied either inbound or outbound using the neighbor command, giving precise control over prefixes based on network and subnet masks.
Option D) is correct because prefix-lists can specify individual routes or ranges of routes that should be permitted or denied, preventing unnecessary route advertisement and protecting sensitive or internal network information. Option A), route-map, is used for policy-based manipulation of routes but requires a prefix-list or other criteria as part of its match condition. Option B), distribute-list, is supported in some Cisco platforms but is less granular than prefix-lists. Option C), neighbor filter-list, is primarily used for AS-path filtering in BGP rather than exact prefix control.
For ENARSI candidates, mastering prefix-lists is critical because BGP is inherently a policy-driven protocol that requires fine-grained control over route propagation. Enterprises often deploy multiple Internet links or MPLS connections, and unfiltered route advertisement could result in route leakage, suboptimal path selection, or exposure of internal network topology. Prefix-lists, when combined with route-maps, allow engineers to implement sophisticated traffic engineering, controlling not only which routes are advertised but also their attributes like weight, local preference, or MED, influencing inbound and outbound traffic paths.
In addition to security, prefix-lists also improve network scalability and stability. By filtering unnecessary prefixes, engineers reduce the size of the BGP routing table, minimizing CPU utilization and memory consumption on routers, which is crucial in enterprise-grade deployments. They also facilitate routing convergence and predictable network behavior, essential for maintaining high availability. ENARSI-certified professionals are expected to understand how to configure, verify, and troubleshoot prefix-lists using commands like show ip bgp, show ip bgp neighbors, and show route-map, ensuring that policy objectives are met consistently across large enterprise networks.
Prefix-lists also work in tandem with route-maps, community tags, and BGP attributes to implement complex enterprise routing policies, including multi-homed Internet connections, backup links, or inter-AS traffic engineering. Proper use of prefix-lists prevents routing anomalies and ensures that traffic follows intended paths, supporting service-level agreements, high performance, and robust enterprise connectivity. Mastery of prefix-list usage is a core expectation for ENARSI candidates, reflecting both theoretical knowledge and practical configuration skills required for enterprise-scale networks.
Question 122
Which OSPF LSA type carries external route information into a normal area?
A) Type 2
B) Type 3
C) Type 5
D) Type 7
Answer: C
Explanation:
In OSPF, Type 5 LSAs are generated by ASBRs to advertise external routes into the OSPF domain. These LSAs carry reachability information for networks learned from external protocols like EIGRP, BGP, or static routes. Type 5 LSAs are flooded throughout all non-stubby areas, allowing internal routers to incorporate external networks into their routing tables.
Option C) is correct because Type 5 LSAs explicitly represent external routes, distinguishing them from inter-area Type 3 LSAs or network Type 2 LSAs. Option A), Type 2, is used for network LSAs generated by Designated Routers for multi-access networks, not external routes. Option B), Type 3, summarizes inter-area routes at ABRs. Option D), Type 7, is specific to NSSA areas, where external routes are first advertised as Type 7 and then translated to Type 5 by the ABR before entering the backbone.
ENARSI candidates must understand the role of Type 5 LSAs because external route injection is critical for enterprise network integration with WAN links, Internet, or other routing domains. Misconfiguring ASBRs or LSA types can lead to routing loops, blackholing, or LSA flooding, which can destabilize large networks. Proper management involves route summarization, filtering, and redistribution policies, which must be carefully applied to maintain network efficiency and predictability.
Type 5 LSAs also interact with OSPF cost calculations and SPF algorithm operations. When external routes are injected, they receive an external metric, often based on E1 or E2 types, which influences path selection within the OSPF topology. Candidates must understand the difference between E1 and E2 metrics, how Type 5 LSAs propagate, and how ABRs and backbone routers handle these routes. Additionally, in enterprise designs, Type 5 LSAs affect network convergence, scalability, and traffic engineering, making them critical for maintaining enterprise-grade OSPF performance and reliability.
By mastering Type 5 LSA behavior, ENARSI professionals can design hierarchical OSPF networks, integrate external connectivity effectively, and troubleshoot issues related to redistribution, route summarization, or LSA flooding, ensuring predictable, high-performance routing for large-scale enterprise deployments. This knowledge is a foundational skill for any advanced routing engineer pursuing Cisco ENARSI certification.
Question 123
Which command verifies the feasible successor paths in EIGRP?
A) show ip eigrp neighbors
B) show ip eigrp topology
C) show ip route eigrp
D) debug eigrp packets
Answer: B
Explanation:
In EIGRP, the show ip eigrp topology command displays all feasible successors and their associated metrics. Feasible successors are backup paths that satisfy the feasibility condition, which ensures loop-free routing. The command lists the successor, feasible distance (FD), reported distance (RD), and interface, allowing engineers to validate route selection and potential load-balancing paths.
Option B) is correct because the EIGRP topology table is the primary source of metric-based decision-making, showing both primary and backup routes. Option A), show ip eigrp neighbors, shows adjacency status and reliability but not feasible successors. Option C), show ip route eigrp, shows only the installed routes in the routing table, excluding feasible successors. Option D), debug eigrp packets, is for troubleshooting packet-level communication and does not provide a persistent view of the topology table.
Understanding feasible successors is critical for ENARSI candidates because unequal-cost load balancing, fast convergence, and redundancy depend on these paths. EIGRP maintains loop-free routes using successor and feasible successor logic, ensuring that backup paths can be quickly promoted to the routing table if a primary path fails. By examining the topology table, engineers can also analyze metric calculations, delay, bandwidth, and reliability, confirming that EIGRP routes are optimal and compliant with network design.
In enterprise networks, the ability to predict EIGRP behavior using the topology table is essential for designing resilient and efficient routing solutions. It allows engineers to plan variance configurations, adjust metrics, and implement traffic engineering to maximize link utilization while maintaining reliability. ENARSI-certified professionals must demonstrate the ability to interpret the topology table, understand feasible distance versus reported distance, and apply this knowledge to multi-area, multi-link enterprise topologies, ensuring predictable and stable routing outcomes.
The show ip eigrp topology command is also valuable for troubleshooting network convergence issues, identifying potential loops, and validating load-balancing configurations. Proper interpretation enables engineers to ensure enterprise-grade EIGRP stability, optimal traffic distribution, and compliance with high availability requirements, which are core competencies tested in the ENARSI exam.
Question 124
Which HSRP state is active after preemption is enabled and priority is highest?
A) Listen
B) Speak
C) Active
D) Standby
Answer: C
Explanation:
In HSRP, the router with the highest priority becomes Active if preemption is enabled. HSRP is a First Hop Redundancy Protocol that ensures continuous gateway availability in enterprise networks. Preemption allows a higher-priority router that comes online to take over the Active role, overriding an existing Active router with a lower priority.
Option C) is correct because the Active router forwards packets to the virtual IP address, providing uninterrupted gateway functionality. Option A), Listen, is a transient state where routers monitor the network but do not forward traffic. Option B), Speak, occurs when a router actively participates in election messaging but is not forwarding traffic. Option D), Standby, is the backup router ready to take over if the Active fails.
ENARSI candidates must understand HSRP states to ensure redundancy and high availability in enterprise LANs. HSRP design requires priority planning, preemption configuration, and interface reliability assessment to prevent network outages. By enabling preemption, network engineers ensure that the most capable router assumes responsibility, improving performance, reliability, and predictability in failover scenarios.
HSRP also integrates with other features like tracking interfaces or objects to adjust priority dynamically. This allows the Active router to maintain high availability while responding to network failures, making HSRP a critical component of enterprise LAN resilience and operational continuity. Understanding these states, preemption behavior, and priority manipulation enables ENARSI professionals to design redundant, high-performance gateway solutions that meet strict SLA requirements.
Question 125
Which OSPF area type blocks external Type 5 LSAs but allows inter-area routes?
A) Normal Area
B) Stub Area
C) NSSA
D) Totally Stubby Area
Answer: B
Explanation:
In OSPF, a Stub Area is configured to block external Type 5 LSAs, preventing external routes from being advertised into the area while still allowing inter-area Type 3 routes. Stub areas reduce the routing table size, LSA processing, and SPF calculation overhead, which is particularly useful for branch or remote sites with limited router resources.
Option B) is correct because stub areas rely on the ABR to provide a default route to external destinations, ensuring connectivity without the overhead of processing external LSAs. Option A), Normal Area, carries all LSA types including Type 5 and Type 3. Option C), NSSA, allows external routes via Type 7 LSAs that are translated to Type 5 at the ABR. Option D), Totally Stubby Area, blocks Type 5 and Type 3 inter-area LSAs, only allowing a single default route.
ENARSI candidates must understand stub area design because it significantly impacts OSPF scalability, network convergence, and routing efficiency. Configuring stub areas correctly ensures low-resource routers can participate in OSPF without maintaining a full external route database. Misconfiguration may lead to routing blackholes, unreachable destinations, or unnecessary SPF calculations, which can destabilize large enterprise networks.
Stub area configuration is critical in enterprise campus, branch offices, and hierarchical OSPF topologies, where network engineers aim to balance routing efficiency with redundancy and availability. Candidates should be proficient in commands like show ip ospf, show ip ospf database, and show ip route to verify stub area behavior and ensure that default routes propagate correctly from the ABR, supporting both internal and external connectivity seamlessly. Mastery of stub areas, NSSAs, and totally stubby areas is essential for designing scalable, efficient, and reliable OSPF networks as required by Cisco ENARSI standards.
Question 126
Which EIGRP metric component considers interface reliability for path calculation?
A) Bandwidth
B) Delay
C) Load
D) Reliability
Answer: D
Explanation:
In EIGRP, the reliability metric reflects the historical stability of an interface, influencing how routes are selected. EIGRP uses a composite metric based on bandwidth, delay, reliability, load, and MTU. By default, reliability and load are not used in metric calculation unless explicitly configured. The reliability value ranges from 1 to 255, with 255 representing 100% reliable interfaces.
Option D) is correct because the reliability metric is specifically used to evaluate interface stability and can impact feasible distance calculation if configured. Option A), Bandwidth, measures the slowest link in the path and significantly affects the default EIGRP metric. Option B), Delay, reflects propagation and processing delays and also contributes to the metric calculation. Option C), Load, indicates current interface utilization but is not part of default metric calculation unless K-values are adjusted.
Understanding reliability is critical for ENARSI candidates because enterprise networks often encounter fluctuating link conditions on WAN interfaces, MPLS connections, or multi-access networks. Properly configured reliability ensures that unstable or error-prone links are less likely to be selected as primary paths, enhancing network resilience and predictable routing behavior.
EIGRP’s feasible distance (FD) and reported distance (RD) calculations also consider reliability if K-values are modified. A route with higher reliability may become a feasible successor, providing backup paths that are robust under varying network conditions. Engineers must understand how to configure reliability thresholds, monitor interface performance using commands like show interfaces, show ip eigrp topology, and show ip route, and adjust EIGRP parameters to optimize enterprise routing behavior.
In enterprise deployments, reliability monitoring allows operators to implement automatic traffic rerouting, load balancing, and backup path promotion, minimizing packet loss, jitter, and downtime. This is particularly critical in WAN environments with satellite links, DSL, or wireless connectivity, where interface quality can vary frequently. ENARSI professionals are expected to know how to manipulate EIGRP K-values to prioritize reliability when needed, ensuring that high-availability paths are chosen over paths with higher bandwidth but lower stability.
Furthermore, integrating reliability with network monitoring tools and SNMP traps allows proactive identification of potential network degradation. By aligning EIGRP routing decisions with interface health, enterprises maintain SLAs and service continuity, reducing troubleshooting overhead and improving operational efficiency. Mastery of this metric and its impact on route selection, feasible successors, and network stability is essential for achieving ENARSI certification.
Question 127
Which command displays OSPF routers within the same area and their priorities?
A) show ip ospf neighbor
B) show ip ospf database
C) show ip route ospf
D) debug ospf events
Answer: A
Explanation:
The show ip ospf neighbor command provides a detailed overview of OSPF adjacency states, router IDs, priorities, interface types, and neighbor relationships within a particular area. OSPF relies on priority values to elect the Designated Router (DR) and Backup Designated Router (BDR) on broadcast and non-broadcast multi-access networks.
Option A) is correct because it explicitly shows neighbor priorities, states (e.g., Full, Loading), and interface information, allowing engineers to verify OSPF DR/BDR elections and adjacency formation. Option B), show ip ospf database, lists LSAs but does not directly reveal neighbor priorities. Option C), show ip route ospf, shows the OSPF routing table without detailed neighbor or election information. Option D), debug ospf events, provides real-time OSPF activity but is not a persistent verification tool and can overwhelm routers with large networks.
ENARSI candidates must understand OSPF neighbor relationships because misconfigured priorities, dead intervals, or interface types can prevent DR/BDR elections, causing routing inconsistencies, incomplete LSAs, or slower convergence. Monitoring neighbor priorities ensures that the correct router assumes the DR role, providing optimal LSA flooding and minimal SPF recalculations.
In enterprise environments, OSPF adjacency management impacts network stability, SPF calculation efficiency, and convergence times. High-priority routers are typically more capable devices with better processing power or redundant links, ensuring that DR responsibilities are handled by reliable routers. Low-priority routers can act as backups, preventing unnecessary LSA processing and improving network efficiency.
Proper use of the show ip ospf neighbor command allows network engineers to validate configuration consistency, identify neighbor mismatches, troubleshoot adjacency issues, and plan upgrades without affecting operational networks. ENARSI candidates must also interpret neighbor states (e.g., Init, 2-Way, Full) to understand the OSPF adjacency formation process, including hello packets, dead intervals, and DR/BDR election dynamics.
Furthermore, combining this command with show ip ospf interface provides insights into OSPF network types, timers, and interface reliability, helping engineers optimize enterprise routing hierarchies, summarize routes effectively, and maintain high availability. Mastery of OSPF neighbor monitoring is essential for large-scale deployments where fast convergence, hierarchical design, and loop-free routing are critical performance and reliability factors.
Question 128
Which BGP attribute influences outbound traffic selection from a local AS?
A) Local Preference
B) MED
C) AS-Path
D) Weight
Answer: D
Explanation:
In BGP, the weight attribute is a Cisco-specific attribute that influences outbound traffic selection for routers within the local Autonomous System (AS). Weight is a local, non-transitive attribute, meaning it is not advertised to other BGP peers and only affects the local router’s path selection process. The highest weight value is preferred when multiple routes to the same prefix exist.
Option D) is correct because manipulating weight directly changes which exit point the local router uses for outbound traffic without affecting upstream or downstream BGP peers. Option A), Local Preference, influences path selection within the entire AS, impacting all routers in the AS rather than just the local router. Option B), MED (Multi-Exit Discriminator), is used to influence inbound traffic from external ASes, not local outbound selection. Option C), AS-Path, helps in avoiding loops and influencing path selection but is less granular than weight for a local router.
ENARSI candidates must understand weight because it provides a simple yet powerful mechanism for traffic engineering. By adjusting weight values, network engineers can influence which ISP or peering link carries outbound traffic, optimizing bandwidth utilization, reducing latency, and avoiding congestion. Weight is particularly useful in dual-homed Internet connections or multi-ISP enterprise environments where granular control over path selection is necessary without modifying policies network-wide.
Proper configuration of weight requires careful planning. Over-reliance on weight may inadvertently create imbalanced traffic or underutilized links, so it is often used alongside route-maps, prefix-lists, and BGP attributes to implement more sophisticated traffic engineering strategies. ENARSI professionals are expected to verify weight assignment using show ip bgp, validate BGP best path selection, and troubleshoot unexpected routing behavior.
Weight also interacts with other BGP attributes, including Local Preference, AS-Path, and MED. Understanding these interactions is crucial for predictable route selection and outbound traffic control. For example, even if a route has the highest Local Preference, a higher weight on another path will cause the local router to prefer that path, making weight a decisive factor in enterprise outbound routing policies. Proper mastery ensures predictable enterprise-grade routing, redundancy, and load balancing.
Question 129
Which routing protocol supports unequal-cost load balancing by default?
A) OSPF
B) EIGRP
C) RIP
D) BGP
Answer: B
Explanation:
EIGRP is unique among common routing protocols because it supports unequal-cost load balancing using the variance command. This feature allows traffic to be distributed over multiple paths that do not have identical metrics but meet the feasibility condition, ensuring loop-free routing. The variance value multiplies the best feasible distance, and any route with a metric less than or equal to this product becomes a feasible successor eligible for load balancing.
Option B) is correct because EIGRP calculates feasible successors and uses variance to load balance traffic across multiple unequal-cost paths. Option A), OSPF, only supports equal-cost multi-path (ECMP) by default. Option C), RIP, does allow equal-cost load balancing but not unequal-cost. Option D), BGP, does not perform traditional load balancing but may use multipath features with careful attribute manipulation.
ENARSI candidates must understand EIGRP’s unequal-cost load balancing because it allows network engineers to maximize link utilization, reduce congestion, and provide redundancy in complex enterprise topologies. Without this feature, traffic would be limited to equal-cost paths, potentially leaving high-capacity links underutilized.
Proper configuration involves calculating feasible successors, understanding the impact of K-values on metric calculations, and verifying load balancing using show ip route, show ip eigrp topology, and debug eigrp packets. Engineers must ensure that all paths chosen meet the feasibility condition, which prevents routing loops by confirming that the reported distance of a successor is less than the feasible distance of the current path.
EIGRP’s unequal-cost load balancing also interacts with QoS, WAN optimization, and MPLS designs, making it a critical tool for enterprise WAN routing, traffic engineering, and high availability. ENARSI certification emphasizes deep understanding of path selection, variance configuration, and metric manipulation to ensure predictable and efficient routing across multi-link enterprise networks.
Question 130
Which command summarizes EIGRP routes on an interface?
A) ip summary-address eigrp
B) aggregate-address
C) network summary
D) redistribute summary
Answer: A
Explanation:
In EIGRP, the ip summary-address eigrp command is used on an interface to summarize multiple networks into a single route, reducing routing table size and improving convergence. Summarization is particularly useful in hub-and-spoke topologies or multi-area networks to limit route propagation.
Option A) is correct because it applies summary routes directly on an interface, controlling what routes are advertised to neighbors. Option B), aggregate-address, is used in BGP and not EIGRP. Option C), network summary, is not a valid Cisco command. Option D), redistribute summary, applies only when redistributing routes between protocols.
ENARSI candidates must understand route summarization for EIGRP because it reduces unnecessary LSA or routing table entries, improves convergence times, and supports scalable enterprise designs. Proper summarization prevents routing table bloat in large networks, which can degrade performance, increase CPU usage, and cause delayed convergence during network changes.
Summarization is often applied at ABRs, hub routers, or border routers, ensuring that only relevant summary routes are advertised to spokes or downstream routers. Commands like show ip route, show ip eigrp topology, and debug eigrp packets help validate summarization and confirm that backup or primary paths are functioning as expected. Mastery of ip summary-address eigrp and its implications is essential for implementing efficient, resilient, and scalable enterprise routing topologies, which is a core ENARSI skill.
Question 131
Which OSPF LSA type describes inter-area routes between ABRs?
A) Type 1
B) Type 2
C) Type 3
D) Type 4
Answer: C
Explanation:
In OSPF, Type 3 LSAs, also known as Summary LSAs, are generated by Area Border Routers (ABRs) to describe networks located in other areas. These LSAs are flooded from the ABR into connected areas, allowing routers to learn about destinations outside their local area without maintaining detailed intra-area topology.
Option C) is correct because Type 3 LSAs summarize inter-area routes. Option A), Type 1 LSAs, are Router LSAs describing router links within an area. Option B), Type 2 LSAs, are Network LSAs, describing multi-access networks with a DR. Option D), Type 4 LSAs, describe ASBRs for external route reachability and are used by ABRs to advertise paths to external routers.
Understanding Type 3 LSAs is essential for ENARSI candidates because large enterprise networks rely on multi-area OSPF designs to scale efficiently. Instead of flooding every router in the autonomous system with complete topology information, ABRs condense multiple intra-area LSAs into a Type 3 LSA, optimizing routing table size, SPF calculation, and network convergence times.
The generation of Type 3 LSAs involves summarization and route advertisement. ABRs examine their routing tables and, based on configuration and area boundaries, create Type 3 LSAs advertising networks reachable through themselves. These LSAs do not include link-level details but rather provide destination prefixes and metrics, ensuring routers in other areas can compute shortest paths without excessive memory or CPU load.
Proper configuration and understanding of Type 3 LSAs are vital for enterprise-scale deployments where multiple areas exist. Misconfigured summarization can lead to routing black holes, suboptimal paths, or routing loops. ENARSI professionals must know commands such as show ip ospf database, show ip route, and debug ospf events to verify correct LSA propagation, validate SPF calculation, and troubleshoot convergence issues.
Additionally, knowledge of how Type 3 LSAs interact with external LSAs (Type 5), area types (e.g., stub or NSSA), and route summarization allows engineers to implement scalable OSPF topologies while ensuring redundancy, loop prevention, and optimal path selection. By mastering Type 3 LSAs, ENARSI candidates can design enterprise networks that are resilient, efficient, and capable of handling high-density multi-area OSPF deployments.
Question 132
Which protocol automatically summarizes routes at classful boundaries by default?
A) OSPF
B) EIGRP
C) RIP
D) BGP
Answer: C
Explanation:
RIP (Routing Information Protocol), particularly RIP version 1, automatically summarizes routes at classful boundaries. This behavior means that subnets of a classful network are advertised as a single network prefix to neighboring routers, which can reduce routing table entries but potentially cause routing inconsistencies if discontiguous networks exist.
Option C) is correct because RIP performs automatic classful summarization unless disabled with the no auto-summary command in RIP version 2. Option A), OSPF, never performs automatic summarization; it uses explicit manual summarization. Option B), EIGRP, also performs automatic summarization only at network boundaries unless disabled. Option D), BGP, does not automatically summarize; route aggregation is configured manually using aggregate-address or route-maps.
Understanding automatic summarization is critical for ENARSI candidates because it affects route advertisement behavior, convergence, and network design. In enterprise networks with discontiguous subnets, automatic summarization may inadvertently advertise incorrect prefixes, leading to routing black holes or unreachable destinations. For example, if two subnets of 10.0.0.0/8 exist in different locations, RIP version 1 may advertise only 10.0.0.0/8 to all neighbors, ignoring the actual subnetting details.
In contrast, RIP version 2 supports classless routing but still retains the option for automatic summarization at major network boundaries. Configuring no auto-summary allows for precise subnet advertisement, essential for modern enterprise topologies using variable-length subnet masks (VLSM) or complex hierarchical designs. ENARSI professionals must understand how summarization affects convergence, especially in WAN environments, where route updates over slow links can consume bandwidth unnecessarily.
Additionally, network engineers must be adept at using debug ip rip and show ip route commands to confirm the advertised prefixes, detect summarization errors, and troubleshoot connectivity issues. In enterprise networks with multiple routing protocols, careful consideration of automatic summarization prevents routing loops, misrouting, and suboptimal path selection, ensuring that critical services maintain high availability and redundancy. Mastery of RIP summarization behavior is foundational for exam scenarios and practical network implementations covered under ENARSI objectives.
Question 133
Which EIGRP feature prevents routing loops by ensuring a feasible successor?
A) Split horizon
B) Feasibility condition
C) Route poisoning
D) Hold-down timer
Answer: B
Explanation:
EIGRP uses the feasibility condition to prevent routing loops and ensure loop-free path selection. The feasibility condition requires that a feasible successor’s reported distance (RD) to a destination must be less than the current router’s feasible distance (FD). This condition guarantees that any alternate path is closer to the destination than the current router’s path, maintaining loop-free routing in all scenarios.
Option B) is correct because the feasibility condition directly governs the eligibility of feasible successors in EIGRP. Option A), Split horizon, prevents routing loops on multi-access networks by prohibiting the advertisement of a route back onto the interface from which it was learned. Option C), Route poisoning, marks failed routes as unreachable and helps accelerate convergence but does not inherently prevent loops. Option D), Hold-down timers, delay route reinstatement to stabilize routing, but they are not used in EIGRP.
For ENARSI candidates, understanding the feasibility condition is critical because it underpins EIGRP’s loop-free routing mechanism. Without it, network loops could propagate quickly, causing unstable routing tables, delayed convergence, and network downtime. Feasible successors also provide instant backup routes without recomputation, enhancing network resiliency and redundancy.
Configuring and monitoring feasible successors involves using show ip eigrp topology and show ip route commands to identify successors and feasible successors for each destination. Enterprise engineers must ensure K-values are correctly configured, as these influence metric calculations and feasible successor eligibility. Adjusting K-values incorrectly could lead to feasible successor loss, reducing redundancy and increasing recovery time during link failures.
Furthermore, understanding feasible successors allows ENARSI candidates to implement advanced EIGRP features like unequal-cost load balancing using variance. Only feasible successors are considered when evaluating variance paths, ensuring traffic is balanced across safe, loop-free routes. Effective application of the feasibility condition helps enterprise networks maintain high availability, optimized traffic distribution, and minimal disruption during topology changes.
Mastery of the feasibility condition also prepares candidates to troubleshoot complex topologies, including hub-and-spoke, dual-homed WAN, and multi-area networks, ensuring that backup paths are utilized correctly and routing loops are never introduced even in dynamic network environments.
Question 134
Which BGP feature limits route advertisement to specific neighbors?
A) Route-map
B) Prefix-list
C) Community attribute
D) All of the above
Answer: D
Explanation:
BGP allows selective route advertisement using route-maps, prefix-lists, and community attributes, giving network engineers granular control over path selection, traffic engineering, and policy enforcement.
Option D) is correct because all three methods can restrict route advertisement. Route-maps provide flexible conditional policies based on prefix, next-hop, AS-path, or community attributes. Prefix-lists define specific IP ranges to permit or deny advertisements, acting as a first-level filter. Community attributes tag routes for selective propagation, allowing routers to interpret policies consistently across multiple peers or ASes.
ENARSI candidates must understand these features because controlling BGP route advertisement is essential for enterprise interconnections, multi-homing, and traffic engineering. Without proper filtering, enterprises may advertise unwanted routes, receive suboptimal paths, or violate peering agreements. These features also protect against misconfigurations that could cause routing loops, route leaks, or prefix hijacking.
Route-maps often combine multiple conditions, enabling engineers to apply complex policy decisions, such as preferring a primary ISP for some prefixes while sending backup routes elsewhere. Prefix-lists simplify this by providing lightweight, high-performance filters, ensuring only authorized prefixes are advertised. Community attributes extend filtering capabilities across the network, particularly useful in large-scale multi-AS enterprise or provider networks, allowing consistent policy enforcement without per-peer manual configuration.
Proficiency with these techniques also involves verification using commands like show ip bgp neighbors, show ip bgp, and debug ip bgp events, enabling network engineers to confirm policy compliance, validate outbound advertisement, and troubleshoot anomalies. For enterprises, implementing these controls enhances network predictability, SLA adherence, and operational security, all critical for ENARSI-level certification objectives.
Question 135
Which OSPF area type prevents Type 5 LSA flooding from external routes?
A) Stub
B) NSSA
C) Totally stubby
D) Backbone
Answer: A
Explanation:
In OSPF, a stub area is designed to prevent external routes (Type 5 LSAs) from flooding into the area. This reduces routing table size, simplifies SPF calculations, and ensures that routers in stub areas do not store unnecessary information about external destinations.
Option A) is correct because stub areas block Type 5 LSAs and allow only a default route to reach external networks. Option B), NSSA (Not-So-Stubby Area), allows limited Type 5-like LSAs (Type 7) to be injected for ASBRs within the area. Option C), Totally stubby area, blocks Type 3 and Type 5 LSAs, allowing only a default route for inter-area and external traffic. Option D), Backbone (Area 0), must carry all types of LSAs and cannot be stubbed.
ENARSI candidates must understand stub areas because they reduce resource consumption, improve convergence, and simplify large-scale OSPF topologies. Stub areas are commonly deployed in branch office networks where detailed external routing information is unnecessary, and routers only need to reach internal destinations or exit points via ABRs.
Proper configuration of stub areas involves router ospf area X stub commands and verification with show ip ospf database and show ip route. Engineers must also plan carefully to avoid misconfiguration that could prevent external traffic from reaching the branch or cause routing black holes. Understanding the interaction between stub areas, NSSA, and totally stubby areas allows enterprises to optimize OSPF hierarchy, reduce SPF calculations, and maintain high network efficiency.
Stub area deployment ensures that branch routers maintain minimal routing state while still reaching the broader network efficiently, making it an essential OSPF design strategy for large-scale enterprise networks. Mastery of this concept aligns with ENARSI objectives related to scalable OSPF design, route summarization, and controlled LSA propagation.
Question 136
Which EIGRP metric component determines bandwidth influence on path selection?
A) Delay
B) Bandwidth
C) Load
D) Reliability
Answer: B
Explanation:
In EIGRP, the composite metric used for path selection is influenced by several parameters: bandwidth, delay, load, and reliability. Among these, bandwidth plays a crucial role as it represents the minimum bandwidth along a path to the destination, impacting the metric calculation and determining which path is most favorable.
Option B) is correct because EIGRP calculates the feasible distance by considering the lowest bandwidth along the route, combined with cumulative delay. Option A), delay, also influences the metric but primarily measures transit time across links rather than the path’s capacity. Option C), load, accounts for current interface utilization but is optional and less frequently weighted in calculations. Option D), reliability, measures link stability over time and contributes minimally to EIGRP’s default metric.
Understanding the bandwidth component is critical for ENARSI candidates because it directly affects route selection in enterprise networks, especially where multiple paths exist with varying capacities. Bandwidth allows EIGRP to prefer higher-capacity links, ensuring efficient traffic distribution and optimal utilization of available resources. Without proper consideration of bandwidth, EIGRP may select a slower or less efficient path, causing latency, congestion, and suboptimal network performance.
Network engineers can adjust EIGRP metric calculation using K-values, which weight bandwidth, delay, reliability, and load differently to meet network design objectives. For example, in high-performance enterprise networks, giving more weight to bandwidth ensures that traffic favors high-capacity links while secondary links are reserved for backup or low-priority traffic. Commands like show ip eigrp topology, show ip route, and debug eigrp packets are essential for monitoring and verifying path selection.
Bandwidth also interacts with unequal-cost load balancing via the variance command, enabling multiple paths to be used if their metrics fall within a defined range of the feasible distance. This capability allows enterprises to maximize throughput while maintaining loop-free routing, a central tenet of EIGRP’s design. Mastery of how bandwidth affects metrics prepares ENARSI candidates for designing robust, scalable, and efficient WAN topologies, particularly in scenarios involving dual-homed branches, redundant high-speed links, and variable traffic patterns.
Question 137
Which OSPF area type allows external route injection while blocking Type 5 LSAs externally?
A) Stub
B) NSSA
C) Totally stubby
D) Backbone
Answer: B
Explanation:
A Not-So-Stubby Area (NSSA) is a unique OSPF area type that allows an Autonomous System Boundary Router (ASBR) to inject external routes into the area while preventing external Type 5 LSAs from flooding into the rest of the network. NSSAs achieve this by generating Type 7 LSAs, which are later translated to Type 5 LSAs by ABRs when propagating outside the NSSA.
Option B) is correct because NSSA combines the benefits of stub areas—limited external flooding—while still supporting internal ASBRs. Option A), stub, blocks all external LSAs and does not allow ASBRs to inject external routes. Option C), totally stubby area, blocks both inter-area and external LSAs except for a default route, limiting flexibility. Option D), backbone, is unrestricted and carries all LSA types.
Understanding NSSA is essential for ENARSI candidates because large enterprise networks often have branch offices that need to advertise external routes without exposing the entire OSPF domain to unnecessary external LSA flooding. NSSAs minimize SPF calculations within the area, optimize memory usage on branch routers, and maintain high convergence speed.
Implementing NSSAs involves careful configuration using area X nssa commands, along with redistribution of external routes via ASBRs. Candidates must also understand NSSA to normal area translation, LSA type conversion, and the role of ABRs in propagating Type 7 LSAs. Proper verification using show ip ospf database, show ip route, and debug ip ospf events ensures correct external route propagation.
NSSA deployment enhances scalability, operational efficiency, and routing hierarchy in enterprise networks. By mastering NSSAs, ENARSI candidates can design branch office OSPF implementations that optimize memory usage, improve convergence, and ensure controlled external routing dissemination while maintaining overall network performance and stability.
Question 138
Which routing protocol uses autonomous system paths to prevent loops between ASes?
A) EIGRP
B) OSPF
C) BGP
D) RIP
Answer: C
Explanation:
Border Gateway Protocol (BGP) prevents routing loops between autonomous systems (ASes) by using the AS-path attribute, which lists all ASes a route has traversed. If a BGP router sees its own AS in the path, it rejects the route, ensuring loop-free inter-AS routing.
Option C) is correct because AS-path is fundamental to BGP’s loop prevention mechanism. Option A), EIGRP, relies on the feasibility condition for intra-AS loops but does not handle AS-level loops. Option B), OSPF, prevents loops using SPF calculation within an AS but cannot manage loops across AS boundaries. Option D), RIP, also prevents loops using hop-count limits but does not support AS-level routing.
For ENARSI candidates, mastering AS-path behavior is critical for enterprise networks that use multi-homed BGP connections to ISPs or partners. AS-path ensures policy enforcement, loop prevention, and route selection across multiple ASes. BGP engineers can manipulate AS-path with tools like AS-path prepending to influence routing decisions and control inbound traffic flows without violating loop prevention.
In addition to AS-path, BGP includes other attributes—NEXT_HOP, MED, local preference, and community tags—to optimize routing decisions. Understanding the interplay of these attributes allows network engineers to design scalable enterprise-to-ISP connectivity, prevent routing loops, and maintain consistent routing policies.
Verification using commands such as show ip bgp, show ip bgp summary, and debug ip bgp events allows engineers to confirm path integrity, detect anomalies, and validate policy enforcement. Effective BGP design is essential for large-scale enterprise networks, particularly in redundant or multi-provider environments, where loop prevention, path selection, and traffic engineering are critical to operational success and ENARSI exam objectives.
Question 139
Which EIGRP feature allows unequal-cost load balancing across multiple paths?
A) Feasible successor
B) Variance
C) Split horizon
D) Route poisoning
Answer: B
Explanation:
EIGRP supports unequal-cost load balancing using the variance command. This feature enables traffic to be distributed across multiple routes as long as their metric falls within a defined multiple of the feasible distance to the destination.
Option B) is correct because variance allows network engineers to use backup paths or secondary links for load sharing, improving bandwidth utilization. Option A), feasible successor, ensures loop-free backup paths but does not handle traffic distribution. Option C), split horizon, prevents advertisement loops on multi-access networks but is unrelated to load balancing. Option D), route poisoning, helps with convergence by marking failed routes but does not facilitate load sharing.
Understanding variance is critical for ENARSI candidates because enterprises often operate networks with multiple redundant links of varying bandwidth. By configuring variance appropriately, engineers can direct traffic over lower-cost primary paths while still leveraging secondary links, enhancing resiliency and throughput.
Implementation involves examining feasible distances using show ip eigrp topology and determining the variance multiplier to include additional paths safely. The variance must be carefully calculated because including paths with metrics too high could lead to suboptimal routing, increased latency, and potential congestion.
Variance also interacts with other EIGRP features, including load, K-values, and feasible successor selection, to maintain loop-free routing while distributing traffic efficiently. This ensures that enterprise networks can handle dynamic traffic loads and recover quickly from link failures without disrupting critical applications. ENARSI professionals must understand both theoretical and practical aspects of variance, including impact on routing tables, convergence, and resource utilization.
Question 140
Which BGP attribute influences outbound traffic selection for preferred paths?
A) Local preference
B) AS-path
C) MED
D) NEXT_HOP
Answer: A
Explanation:
In BGP, local preference is an attribute that determines which outbound path is preferred when multiple paths exist to the same destination within the same AS. A higher local preference value indicates a more preferred route, directly influencing intra-AS traffic selection.
Option A) is correct because local preference allows enterprise engineers to control outbound traffic flows, ensuring that preferred paths are used for critical destinations. Option B), AS-path, primarily prevents loops and influences inbound traffic selection rather than outbound. Option C), MED (Multi-Exit Discriminator), suggests preferred paths to neighboring ASes but is interpreted by external ASes, not for internal outbound selection. Option D), NEXT_HOP, indicates the next router for a destination but does not influence path preference internally.
For ENARSI candidates, understanding local preference is essential for multi-homed enterprise networks where traffic optimization, redundancy, and policy enforcement are key. Engineers can manipulate local preference using route-maps, prefix-lists, and policy rules to ensure traffic exits via the preferred ISP or peering point while maintaining loop-free routing and policy compliance.
Verification commands like show ip bgp, show ip bgp neighbors, and show route-map allow engineers to monitor the impact of local preference, validate policy application, and troubleshoot traffic flow issues. Mastery of local preference enables ENARSI candidates to design optimized, resilient BGP topologies that meet enterprise requirements for performance, security, and operational consistency. Proper use of local preference also enhances traffic engineering strategies, ensuring efficient utilization of redundant links and minimizing congestion.
