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Question 141
Which OSPF type of LSA carries external route information into the OSPF domain?
A) Type 1
B) Type 2
C) Type 3
D) Type 5
Answer: D
Explanation:
In OSPF, Link-State Advertisements (LSAs) are used to communicate routing information throughout the network. Among these, Type 5 LSAs are specifically responsible for carrying external route information injected into the OSPF domain from other routing protocols or autonomous systems. These LSAs are generated by Autonomous System Boundary Routers (ASBRs) and allow OSPF to integrate routes learned from protocols like BGP, EIGRP, or RIP into its link-state database.
Option D) is correct because Type 5 LSAs represent external networks, providing OSPF routers with necessary information to calculate routes to destinations outside their OSPF autonomous system. Option A), Type 1 LSAs, describe router links within the area. Option B), Type 2 LSAs, describe network links for multi-access networks like Ethernet segments. Option C), Type 3 LSAs, carry summary route information between OSPF areas but not external routes.
Understanding Type 5 LSAs is critical for ENARSI candidates because large enterprise networks often connect to external networks or other autonomous systems. Integrating external routes correctly ensures reachability, loop-free routing, and network stability. Misconfiguring Type 5 LSA propagation can result in routing loops, blackholing traffic, or suboptimal paths, which can severely degrade enterprise network performance.
Network engineers must also be familiar with OSPF area types and their impact on Type 5 LSAs. For instance, stub areas block Type 5 LSAs to reduce routing table size and SPF computation overhead, while NSSA areas convert Type 5 LSAs to Type 7 for controlled propagation. Commands like show ip ospf database external and debug ip ospf events allow verification of Type 5 LSA generation and distribution.
Furthermore, understanding LSA flooding, aging, and SPF recalculation is vital. Each LSA contains sequence numbers, age timers, and checksums to ensure consistency across the OSPF domain. This knowledge is essential for troubleshooting OSPF convergence issues, route flapping, or network instability. ENARSI candidates must demonstrate the ability to design and optimize OSPF networks, ensuring external routes are efficiently incorporated while maintaining fast convergence and minimal network overhead.
Proper management of Type 5 LSAs also enhances interoperability with other routing protocols, enabling seamless enterprise-to-enterprise connectivity, multi-AS environments, and dynamic integration of external network resources. Mastery of Type 5 LSAs prepares engineers to tackle real-world enterprise challenges, optimize OSPF performance, and maintain reliable, scalable routing topologies.
Question 142
Which EIGRP packet type is used to discover neighbors and maintain adjacency?
A) Update
B) Hello
C) Query
D) Reply
Answer: B
Explanation:
EIGRP relies on several packet types to perform its operations, including Hello, Update, Query, Reply, and Acknowledgment packets. Among these, the Hello packet plays a foundational role in neighbor discovery and adjacency maintenance.
Option B) is correct because Hello packets are periodically sent out on all EIGRP-enabled interfaces to discover neighbors, establish adjacencies, and monitor the status of active connections. The Hello interval defines how frequently these packets are sent, and the Hold timer specifies how long a router waits for a Hello from a neighbor before declaring it down. Option A), Update, carries routing information to neighbors but does not initiate adjacency. Option C), Query, requests routing information when a route becomes unavailable. Option D), Reply, responds to Query packets to provide route information.
Understanding Hello packets is critical for ENARSI candidates because neighbor relationships form the backbone of EIGRP routing. Without successful neighbor discovery, EIGRP cannot exchange routing information, calculate feasible distances, or establish feasible successors. The Hello protocol also supports optional features like authentication, split-horizon, and bandwidth optimization, all of which are key considerations in enterprise network design.
Hello packets are sent using multicast addresses (224.0.0.10 for IPv4), reducing unnecessary traffic while ensuring routers within the same network segment can detect each other. Routers maintain a neighbor table that records interface, hold time, uptime, and state information, which is critical for troubleshooting connectivity or route flapping issues. Commands like show ip eigrp neighbors provide visibility into neighbor status, while debug eigrp packets offers real-time insight into Hello message exchanges.
Hello intervals and Hold timers must be carefully tuned for high-latency or WAN links. Misconfigured timers can lead to false neighbor loss, route recalculations, and network instability. ENARSI candidates must understand the interaction between Hello packets, topology table updates, and route propagation mechanisms to ensure loop-free, high-availability EIGRP networks.
Proper comprehension of Hello packet operations also enables troubleshooting complex enterprise topologies, including dual-homed branches, redundant links, and dynamically changing WAN environments. Mastery of this concept equips engineers to design resilient networks that maintain continuous connectivity and efficient routing performance across large-scale enterprise deployments.
Question 143
Which BGP attribute influences path selection across different autonomous systems?
A) Local preference
B) AS-path
C) Weight
D) Community
Answer: B
Explanation:
BGP path selection across autonomous systems relies heavily on the AS-path attribute, which lists all the AS numbers that a route has traversed. The AS-path ensures loop-free inter-AS routing and is a primary factor when BGP routers compare multiple paths to the same prefix.
Option B) is correct because BGP selects the path with the shortest AS-path when multiple routes exist to the same destination, assuming all other attributes are equal. Option A), local preference, is used for outbound path selection within the same AS. Option C), weight, is a Cisco-specific attribute influencing local router preference, not inter-AS decisions. Option D), community, is a tagging mechanism used for policy routing and traffic engineering but does not directly determine the best path.
For ENARSI candidates, understanding AS-path is essential for designing multi-homed enterprise BGP networks connected to multiple ISPs. The AS-path attribute allows engineers to enforce routing policies, prevent loops, and implement traffic engineering strategies that balance inbound and outbound flows. Manipulation of AS-path through prepending enables control over how external networks perceive the best route to reach the enterprise, which is particularly important for redundancy and failover scenarios.
Verification commands like show ip bgp, show ip bgp neighbors, and debug ip bgp events allow engineers to monitor AS-path propagation, detect routing anomalies, and validate path selection. Mismanagement of AS-path can result in suboptimal routing, congestion, or even routing loops, highlighting its significance in enterprise-level BGP deployments.
AS-path also interacts with other attributes like NEXT_HOP, MED, local preference, and route reflectors to ensure robust scalable network design. For enterprises, proper AS-path configuration ensures efficient use of multi-homed connections, reduces latency, and maintains high availability. ENARSI candidates must be proficient in AS-path understanding, BGP policy configuration, and troubleshooting to optimize inter-AS routing efficiency, prevent routing conflicts, and maintain overall network stability.
Question 144
Which OSPF feature prevents Type 5 LSAs from entering a specific area?
A) Stub area
B) NSSA
C) Backbone area
D) Totally stubby area
Answer: A
Explanation:
In OSPF, stub areas are designed to limit external LSA flooding, improving network scalability and reducing router resource usage. A stub area blocks Type 5 LSAs, meaning routes learned from external sources such as other autonomous systems are replaced with a default route (0.0.0.0).
Option A) is correct because stub areas prevent Type 5 LSAs from entering, ensuring routers within the area do not store extensive external routing information, which reduces memory usage, SPF calculation complexity, and convergence times. Option B), NSSA, allows limited external route injection through Type 7 LSAs. Option C), backbone area, carries all LSA types including Type 5. Option D), totally stubby area, blocks Type 3 and Type 5 LSAs except for the default route, but the standard stub area is the fundamental implementation that stops Type 5 propagation.
For ENARSI candidates, understanding stub areas is crucial when designing enterprise networks with remote branch offices, limited router resources, or low-speed WAN links. By restricting external LSAs, stub areas optimize SPF calculations, reduce routing table size, and accelerate convergence, which is essential for maintaining network stability and performance. Commands such as area X stub, show ip ospf database, and show ip ospf neighbor allow engineers to configure and verify stub behavior.
Designing stub areas requires careful consideration of inter-area route dependencies, ABR responsibilities, and default route injection. Misconfiguration can lead to routing blackholes, unreachable external networks, or suboptimal routing, which can significantly impact enterprise operations. ENARSI candidates must demonstrate the ability to implement stub areas effectively while maintaining connectivity, redundancy, and optimal performance across multi-area OSPF networks.
Stub areas also interact with other OSPF features such as virtual links, NSSAs, and totally stubby areas, providing a toolkit for scalable hierarchical network design. Mastery of this concept allows network engineers to efficiently distribute routes, maintain loop-free environments, and ensure predictable network behavior in complex enterprise deployments.
Question 145
Which EIGRP attribute ensures loop-free backup routes to a destination?
A) Feasible distance
B) Feasible successor
C) Reported distance
D) Variance
Answer: B
Explanation:
EIGRP ensures loop-free routing using the concept of feasible successors. A feasible successor is a backup route to a destination whose reported distance is less than the feasible distance of the primary path. This guarantee prevents routing loops by ensuring that any backup path is downstream of the current route.
Option B) is correct because feasible successors are stored in the topology table and can immediately be promoted to the routing table if the primary path fails, providing fast convergence and network stability. Option A), feasible distance, represents the total metric to a destination but does not by itself guarantee loop-free backup paths. Option C), reported distance, is the metric advertised by a neighbor but requires comparison to the feasible distance to ensure safety. Option D), variance, allows unequal-cost load balancing but does not inherently prevent loops.
Understanding feasible successors is critical for ENARSI candidates because enterprise networks rely on redundancy, high availability, and rapid recovery from failures. Feasible successors enable EIGRP to switch to backup paths without recalculating the entire topology, minimizing downtime and avoiding traffic loss. Commands like show ip eigrp topology allow engineers to identify feasible successors, monitor backup path readiness, and verify metric calculations.
Feasible successors also interact with K-values, variance, and load balancing to provide flexible, optimized routing. Proper configuration ensures efficient utilization of secondary links, rapid failover, and reliable traffic distribution. ENARSI candidates must master this concept to design resilient EIGRP deployments capable of handling WAN, enterprise campus, and dual-homed branch scenarios with minimal disruption.
By mastering feasible successors, engineers can ensure that redundant paths are immediately available, network loops are prevented, and enterprise networks maintain high performance and reliability even under changing network conditions. This capability is a core aspect of EIGRP’s design and a critical skill for any candidate preparing for the Cisco 300-410 ENARSI exam.
Question 146
Which EIGRP metric component represents the lowest bandwidth along the path?
A) Delay
B) Bandwidth
C) Reliability
D) Load
Answer: B
Explanation:
EIGRP calculates its routing metric using a composite metric formula that incorporates multiple factors such as bandwidth, delay, reliability, load, and MTU. Among these, the bandwidth component represents the lowest bandwidth along a path, which significantly impacts the feasible distance (FD) calculation used to determine the best route.
Option B) is correct because EIGRP uses the minimum bandwidth on the path to influence the metric, meaning that a single slow link in a path can heavily affect route selection. Option A), delay, is another metric component reflecting cumulative propagation and queuing delays but does not directly indicate bandwidth limitations. Option C), reliability, measures the historical error rate of a link. Option D), load, reflects current traffic utilization.
For ENARSI candidates, understanding the bandwidth metric is critical for designing enterprise WAN and campus networks where links of varying speeds exist. Bandwidth awareness ensures that EIGRP selects paths that maximize throughput, minimize congestion, and optimize network efficiency. Misinterpreting this metric can result in suboptimal routing, underutilized links, or bottleneck congestion, which can impact critical enterprise applications.
EIGRP uses K-values to weight metric components, giving network engineers the flexibility to prioritize bandwidth, delay, or other parameters depending on network requirements. The default K-values give emphasis to bandwidth and delay, making these the dominant factors in metric calculation. Commands like show ip eigrp topology and show ip eigrp interfaces help verify metric contributions and ensure that bandwidth considerations are accurately reflected in route selection.
Furthermore, bandwidth metrics interact with feasible successors, unequal-cost load balancing (variance), and route summarization. Proper comprehension allows engineers to fine-tune EIGRP for redundant WAN links, multi-path load balancing, and scalable hierarchical topologies, which is essential for large enterprise environments.
Understanding how bandwidth impacts EIGRP also improves troubleshooting capabilities, allowing engineers to identify why certain routes are preferred over others, even when multiple viable paths exist. In practice, this knowledge enables the design of networks that deliver predictable performance, efficient failover, and optimal utilization of available links, which is crucial for enterprises relying on mission-critical applications and high-throughput connectivity.
Question 147
Which BGP attribute determines outbound traffic preference for a router?
A) Local preference
B) MED
C) Weight
D) AS-path
Answer: A
Explanation:
In BGP, the local preference (LocalPref) attribute is used to determine which outbound path a router prefers when multiple paths to the same destination exist. Local preference is propagated within an autonomous system (iBGP) and allows enterprise engineers to control traffic routing based on administrative policies.
Option A) is correct because LocalPref directly influences path selection for outbound traffic, giving higher preference to specific exit points from the AS. Option B), MED (multi-exit discriminator), influences incoming traffic selection for external peers but is not used internally. Option C), weight, is a Cisco-specific attribute that only affects the local router and is not propagated. Option D), AS-path, influences path selection across autonomous systems rather than within the local AS.
ENARSI candidates must understand LocalPref because it is fundamental for traffic engineering, failover, and multi-homed ISP environments. For instance, if an enterprise connects to multiple ISPs, LocalPref can be configured to prefer one ISP for outbound traffic while retaining redundancy. Commands such as show ip bgp or show bgp summary allow engineers to verify LocalPref values and troubleshoot path selection issues.
LocalPref interacts with other attributes such as AS-path, MED, and communities, which together allow fine-grained control over BGP route selection. Misconfiguration can lead to suboptimal routing, asymmetric traffic patterns, or underutilized links, which can degrade performance in enterprise networks. ENARSI candidates should be proficient in configuring LocalPref using route-maps, policy statements, and prefix lists to achieve desired traffic patterns.
Understanding LocalPref also enables network engineers to implement scalable, policy-driven routing architectures. For example, during maintenance or traffic spikes, LocalPref adjustments can redirect traffic dynamically without impacting network stability. Mastery of LocalPref ensures enterprise networks remain predictable, resilient, and optimized, particularly when managing complex, multi-homed BGP topologies with high-availability requirements.
Question 148
Which OSPF area type allows limited external routes using Type 7 LSAs?
A) Stub area
B) NSSA
C) Totally stubby area
D) Backbone area
Answer: B
Explanation:
An OSPF Not-So-Stubby Area (NSSA) allows limited external routes to enter an area while blocking standard Type 5 LSAs. Instead, Type 7 LSAs carry these external routes, which are then converted to Type 5 at the ABR for inter-area propagation.
Option B) is correct because NSSAs combine the benefits of stub areas (reduced SPF computation and smaller routing tables) with controlled injection of external routes, which is essential for enterprise branch offices that need limited external connectivity. Option A), stub area, blocks all Type 5 LSAs without exceptions. Option C), totally stubby area, blocks Type 3 and Type 5 LSAs except for a default route. Option D), backbone area, allows all LSA types.
Understanding NSSA is vital for ENARSI candidates because large enterprises often deploy branch offices or remote sites with limited computing resources. NSSAs allow external route reachability while maintaining OSPF scalability, fast convergence, and reduced memory usage. Configuration involves commands like area X nssa, optionally combined with no-summary to create a totally NSSA.
Type 7 LSAs carry important attributes including external metric, forwarding address, and route tag, enabling seamless redistribution from other protocols such as EIGRP or BGP. At the ABR, Type 7 LSAs are translated to Type 5, ensuring the backbone and other areas can reach these external routes. Misconfiguration may result in routing blackholes, unreachable external destinations, or inefficient paths, which can severely impact branch operations.
ENARSI candidates should also understand the interaction between NSSAs, stub areas, totally stubby areas, and the backbone to design hierarchical OSPF topologies that balance performance, scalability, and simplicity. Proper implementation ensures reduced SPF calculations, optimized memory usage on branch routers, and loop-free external route propagation, which are critical for enterprise networks with constrained devices or WAN links.
Question 149
Which EIGRP feature enables unequal-cost load balancing?
A) Feasible distance
B) Variance
C) Feasible successor
D) K-values
Answer: B
Explanation:
EIGRP supports unequal-cost load balancing through the variance feature, allowing traffic to be distributed across multiple paths with metrics within a configurable ratio of the best path. This enables better utilization of network links that would otherwise remain idle.
Option B) is correct because variance multiplies the best path metric by a specified factor, and any route with a metric less than or equal to this value becomes eligible for load sharing. Option A), feasible distance, defines the primary path metric but does not allow unequal-cost load balancing. Option C), feasible successor, ensures loop-free backup paths but is not used for load balancing. Option D), K-values, weight metric components but do not directly enable unequal-cost distribution.
Understanding variance is crucial for ENARSI candidates designing enterprise WAN networks with redundant links of varying speeds. By adjusting variance, traffic can be distributed efficiently across high-speed and lower-speed links without compromising network stability. Commands such as show ip eigrp topology allow engineers to verify which routes are eligible for load balancing.
Variance interacts with feasible successors, route summarization, and K-values to ensure loop-free, predictable traffic distribution. Misconfiguration may result in asymmetric traffic, routing loops, or overutilization of slower links, affecting performance. Engineers must carefully calculate feasible distances and variance values to maximize bandwidth utilization while maintaining high availability.
Using variance strategically enhances network resilience, particularly in multi-homed WAN or campus networks, by ensuring that backup links contribute to traffic forwarding rather than remaining idle. ENARSI candidates must master variance to optimize traffic flow, improve throughput, and maintain predictable network performance across complex enterprise environments with diverse link capacities.
Question 150
Which BGP attribute controls routing preference on a single router only?
A) Weight
B) Local preference
C) AS-path
D) MED
Answer: A
Explanation:
The Weight attribute in BGP is Cisco-proprietary and controls routing preference locally on a router. Unlike Local Preference, which is propagated within the AS, weight only affects the router that assigns it. Routes with the highest weight are preferred for outbound traffic from that router.
Option A) is correct because Weight allows granular, router-specific path selection without influencing other routers in the autonomous system. Option B), Local Preference, influences all routers within the AS. Option C), AS-path, affects inter-AS path selection. Option D), MED, influences how external neighbors select routes into the AS.
ENARSI candidates must understand Weight for fine-tuning traffic flows, implementing backup links, and controlling route selection on critical edge routers. Weight is commonly configured using route-maps and prefix lists to influence path selection for specific prefixes. Verification commands like show ip bgp allow engineers to see assigned weight values and troubleshoot path selection anomalies.
Weight is particularly useful in dual-homed enterprise networks, where administrators want to prefer one ISP or link locally without impacting global path selection policies. Mismanagement can result in unintended traffic patterns, asymmetric routing, or underutilized links.
Mastery of the Weight attribute also provides insight into BGP decision-making order, which considers Weight before Local Preference, AS-path, and MED. Understanding this order is crucial for ENARSI candidates designing highly available, multi-homed enterprise networks, ensuring predictable traffic routing and fast failover capabilities. Weight configuration, combined with other BGP attributes, allows engineers to implement sophisticated traffic engineering policies and maintain enterprise network stability and performance.
Question 151
Which OSPF router type generates Type 3 LSAs for inter-area routing?
A) ABR
B) ASBR
C) Internal Router
D) Backbone Router
Answer: A
Explanation:
In OSPF, the Area Border Router (ABR) plays a pivotal role in interconnecting different areas within an autonomous system. ABRs maintain separate link-state databases for each area and are responsible for generating Type 3 LSAs, which represent inter-area routes. These LSAs allow routers in one area to learn about destinations in another area without having full knowledge of the other area’s topology.
Option A) is correct because ABRs summarize and distribute routes across areas, optimizing OSPF scalability and reducing SPF calculation overhead. Option B), ASBR (Autonomous System Boundary Router), redistributes external routes from other protocols into OSPF using Type 5 LSAs. Option C), an internal router, only generates Type 1 LSAs describing its own links. Option D), backbone routers, reside in Area 0 but only act as regular OSPF routers unless they are also ABRs.
ENARSI candidates must understand the ABR function for large-scale enterprise networks with multiple OSPF areas. ABRs are crucial in implementing hierarchical OSPF topologies, which improve convergence, minimize routing loops, and optimize network performance. Misconfigurations can lead to lost inter-area connectivity, excessive SPF recalculations, or routing blackholes.
ABRs also enable route summarization at the area boundary, reducing the size of routing tables in non-backbone areas, which is critical for branch offices with limited resources. Commands like show ip ospf border-routers and show ip ospf database help verify Type 3 LSAs and ABR functionality.
By mastering ABR operations, ENARSI candidates can design OSPF networks that scale effectively, maintain predictable performance, and facilitate rapid fault isolation. ABRs, when properly deployed, ensure that enterprise networks remain hierarchical, modular, and resilient, even in scenarios with multiple branch sites, WAN links, and inter-area traffic demands. Understanding the ABR’s role is indispensable for anyone preparing for the ENARSI exam.
Question 152
Which EIGRP table stores backup routes that are loop-free?
A) Routing table
B) Topology table
C) Neighbor table
D) ARP table
Answer: B
Explanation:
EIGRP maintains multiple tables to facilitate efficient routing and fast convergence, and the topology table is specifically responsible for storing all feasible successors, which are backup routes that are loop-free. Feasible successors are paths whose reported distance (RD) is less than the feasible distance (FD) of the primary route, ensuring loop-free alternate paths.
Option B) is correct because the topology table contains all learned routes, including primary successors and feasible successors, which enable rapid failover if the primary route fails. Option A), the routing table, contains only the active routes used for forwarding traffic. Option C), the neighbor table, stores information about directly connected EIGRP neighbors, including their hold timers and state information. Option D), the ARP table, maps IP addresses to MAC addresses and is unrelated to routing decisions.
ENARSI candidates need to grasp the topology table because it directly impacts EIGRP’s fast convergence and network resilience. When a primary route fails, EIGRP can immediately promote a feasible successor from the topology table to the routing table without requiring a full recomputation of routes, minimizing packet loss and downtime. Commands like show ip eigrp topology allow engineers to visualize feasible successors and understand the network’s redundancy.
The topology table also plays a central role in unequal-cost load balancing, where feasible successors can be included in forwarding based on the variance command. A deep understanding of how EIGRP manages the topology table ensures that network engineers can design scalable, highly available enterprise networks that leverage redundancy efficiently.
By knowing which routes are safe for immediate use, ENARSI candidates can troubleshoot routing loops, misconfigurations, and convergence issues more effectively. Mastery of the EIGRP topology table supports optimized WAN and campus routing, rapid failover, and policy-based routing strategies, which are crucial for enterprise environments with high availability requirements.
Question 153
Which OSPF LSA type describes external routes redistributed into OSPF?
A) Type 2
B) Type 5
C) Type 3
D) Type 1
Answer: B
Explanation:
In OSPF, external routes redistributed from other routing protocols are represented by Type 5 LSAs. These LSAs allow OSPF routers to advertise external network reachability throughout the OSPF autonomous system, ensuring that routers can forward packets to destinations not originated within OSPF.
Option B) is correct because Type 5 LSAs carry important attributes such as external metric, forwarding address, and route tags, which help in routing decisions and traffic engineering. Option A), Type 2 LSAs, describe network routers on broadcast networks. Option C), Type 3 LSAs, describe inter-area routes generated by ABRs. Option D), Type 1 LSAs, describe router links within an area.
ENARSI candidates must understand Type 5 LSAs for enterprises that redistribute EIGRP, BGP, or static routes into OSPF. Type 5 LSAs ensure that external destinations are reachable while maintaining loop-free topology within OSPF. Commands like show ip ospf database external provide visibility into Type 5 LSAs and help engineers verify correct redistribution and metric assignment.
Misconfigurations related to Type 5 LSAs can result in routing loops, unreachable external networks, or excessive SPF recalculations. Proper metric selection and route tagging during redistribution allow for fine-grained control over external path selection. This is crucial for multi-protocol enterprise networks, where maintaining consistent routing policies and predictable traffic patterns across internal and external networks is necessary.
By mastering Type 5 LSAs, ENARSI candidates can design hybrid networks with multiple protocols, ensuring efficient redistribution, proper route advertisement, and loop-free connectivity. Type 5 knowledge also enables effective traffic engineering, policy enforcement, and scalability, which are core requirements for enterprise-grade OSPF deployments.
Question 154
Which BGP attribute is used to prefer shorter AS paths?
A) AS-path
B) Local preference
C) Weight
D) MED
Answer: A
Explanation:
The AS-path attribute in BGP records the sequence of autonomous systems a route has traversed. BGP uses the shortest AS-path as a primary criterion to prefer paths with fewer AS hops, promoting more direct and efficient routing across the Internet or between enterprise ASes.
Option A) is correct because BGP evaluates the AS-path length to determine path preference. Shorter paths are preferred because they usually indicate fewer intermediary networks, potentially reducing latency and complexity. Option B), Local Preference, affects internal path selection within an AS. Option C), Weight, is Cisco-specific and only influences local routers. Option D), MED, suggests preferred entry points to neighboring ASes but does not evaluate path length.
For ENARSI candidates, understanding AS-path is crucial for designing multi-homed BGP networks where multiple providers or peers exist. AS-path manipulation allows network engineers to influence inbound traffic, avoid routing loops, and implement policy-based routing decisions. Commands like show ip bgp and show bgp summary help visualize AS-paths and analyze how BGP selects routes based on path length.
Incorrectly interpreting or configuring AS-path can result in suboptimal routing, longer paths, asymmetric traffic flows, or underutilized links. By mastering AS-path, ENARSI candidates can ensure predictable BGP behavior, implement traffic engineering policies, and achieve optimal performance across enterprise WANs. AS-path awareness is also critical when redistributing routes into BGP or coordinating between multiple autonomous systems, ensuring that routing policies are consistent, loop-free, and resilient.
Question 155
Which OSPF area type blocks Type 3 and Type 5 LSAs except a default route?
A) Stub area
B) Totally stubby area
C) NSSA
D) Backbone area
Answer: B
Explanation:
A Totally Stubby Area in OSPF blocks all Type 3 and Type 5 LSAs from entering the area, except for a default route (0.0.0.0) injected by the ABR. This design reduces routing table size, SPF calculation overhead, and memory requirements, making it ideal for branch routers or low-resource devices.
Option B) is correct because Totally Stubby Areas combine stub area principles with complete suppression of inter-area and external LSA types, simplifying the network for routers with minimal computational resources. Option A), stub area, blocks Type 5 LSAs but allows inter-area routes (Type 3 LSAs). Option C), NSSA, allows controlled external route injection using Type 7 LSAs. Option D), backbone area, allows all LSA types for full OSPF functionality.
ENARSI candidates need to understand Totally Stubby Areas to design scalable enterprise OSPF topologies where certain areas only require connectivity to a default route. Commands such as area X stub no-summary enable this configuration. Misconfiguration may result in lost connectivity, unreachable destinations, or routing loops.
Totally Stubby Areas are commonly used for branch offices with limited link bandwidth, where simplified routing ensures faster convergence and minimal SPF recalculations. By reducing LSA propagation, these areas improve network efficiency, minimize router CPU usage, and simplify troubleshooting. ENARSI candidates must be able to differentiate between stub, totally stubby, and NSSA areas to implement hierarchical OSPF networks that are both efficient and resilient.
Question 156
Which EIGRP metric determines the preferred path when multiple successors exist?
A) Feasible Distance
B) Reported Distance
C) Bandwidth
D) Delay
Answer: A
Explanation:
In EIGRP, the Feasible Distance (FD) plays a critical role in selecting the primary path when multiple successors are available. The FD represents the lowest calculated metric to reach a destination, considering cumulative bandwidth, delay, load, and reliability. This ensures that traffic always takes the most optimal and efficient path through the network. EIGRP uses the Diffusing Update Algorithm (DUAL) to compute feasible distances for all learned routes and determine loop-free paths.
Option A) is correct because the feasible distance directly influences successor selection. Option B), Reported Distance (RD), is the distance advertised by a neighbor to reach a destination and is used to identify feasible successors but does not determine the primary route alone. Option C), Bandwidth, and Option D), Delay, are components of the EIGRP composite metric used to calculate FD but individually do not dictate path selection.
For ENARSI candidates, understanding FD is essential for designing resilient, high-performance enterprise networks using EIGRP. Proper calculation of FD ensures fast convergence, loop-free routing, and optimal utilization of network resources. Commands like show ip eigrp topology allow visualization of feasible distances and help troubleshoot issues when traffic is not following expected paths.
Feasible distance also interacts with feasible successors, which are backup routes that meet the feasibility condition (RD < FD). This enables EIGRP to provide rapid failover without recalculating the entire topology, reducing downtime in enterprise networks. Misconfiguration or misunderstanding of FD can lead to suboptimal routing, traffic blackholes, or slow convergence, especially in complex topologies with multiple links and unequal-cost paths.
By mastering feasible distance, network engineers can tune EIGRP metrics for WAN and campus designs, implement policy-based routing, and optimize redundant paths for maximum reliability. It is foundational for the ENARSI exam, as candidates must demonstrate precise knowledge of routing decisions, metric calculations, and path selection in enterprise-grade EIGRP deployments.
Question 157
Which BGP attribute determines path selection preference within an AS?
A) Weight
B) Local Preference
C) AS-path
D) MED
Answer: B
Explanation:
Within a single autonomous system, Local Preference is the primary BGP attribute used to influence route selection and determine the preferred exit point for outbound traffic. The higher the local preference value, the more likely a path is chosen over others. This attribute is critical for traffic engineering, load balancing, and policy enforcement within enterprise networks.
Option B) is correct because Local Preference is propagated throughout the AS and ensures consistent routing decisions across all internal routers. Option A), Weight, is Cisco-specific and only affects the local router; it is not propagated. Option C), AS-path, affects inter-AS routing decisions by preferring shorter paths. Option D), MED, suggests preferred entry points to neighboring ASes but is ignored for internal path selection unless explicitly configured.
ENARSI candidates need to understand Local Preference because it allows enterprises with multiple upstream providers to control outbound traffic efficiently. Commands like show ip bgp and show bgp summary provide visibility into local preference values, enabling network engineers to verify policy application and predict traffic flow.
Mismanagement of Local Preference can result in suboptimal routing, asymmetric traffic patterns, or overutilization of certain WAN links. Proper use ensures consistent path selection, optimized bandwidth utilization, and adherence to organizational routing policies. It also facilitates scalable, predictable BGP designs, especially in multi-homed or redundant topologies where control over path preference is critical.
By mastering Local Preference, ENARSI candidates can design enterprise BGP networks that prioritize preferred paths, enforce routing policies, and maintain high availability, making it a key concept for the 300-410 exam.
Question 158
Which OSPF feature reduces routing table size in branch networks?
A) Route summarization
B) Stub area
C) Totally stubby area
D) NSSA
Answer: A
Explanation:
OSPF route summarization is a fundamental feature designed to reduce routing table size and optimize network efficiency, particularly in enterprise branch networks with limited resources. Summarization consolidates multiple contiguous routes into a single network advertisement, minimizing the number of entries propagated and decreasing SPF computation overhead.
Option A) is correct because summarization enables routers to advertise aggregate routes rather than individual subnets. Option B), stub area, limits external LSAs but does not perform summarization. Option C), totally stubby area, blocks all inter-area and external LSAs but also relies on a default route. Option D), NSSA, allows selective external route injection but does not inherently reduce routing table size via summarization.
ENARSI candidates must understand route summarization because it enhances scalability, reduces memory usage, and accelerates convergence, which is vital in enterprise networks with multiple branch offices and WAN links. Commands like area X range enable summarization at ABRs, while careful planning ensures no routing blackholes or overlaps occur.
Route summarization also improves network predictability, as fewer routes make troubleshooting easier and control plane stability higher. In hierarchical OSPF deployments, summarization at area boundaries limits LSA propagation, reducing router CPU usage and optimizing link-state database size. Misconfigured summarization can result in inaccessible subnets, routing loops, or asymmetric traffic, highlighting the importance of precision in configuration.
By mastering OSPF summarization, ENARSI candidates can design efficient, scalable, and resilient networks, ensuring optimal performance for enterprise-grade WANs, branch connectivity, and hierarchical OSPF topologies. Proper application of summarization is a core skill for the 300-410 exam, ensuring candidates can manage complex networks while maintaining high reliability.
Question 159
Which EIGRP feature allows unequal-cost load balancing?
A) Feasible successor
B) Variance command
C) Successor route
D) Topology table
Answer: B
Explanation:
EIGRP’s variance command is the primary mechanism for enabling unequal-cost load balancing, allowing traffic to be distributed across multiple paths with different metrics while maintaining loop-free routing. This improves link utilization, redundancy, and network resilience.
Option B) is correct because the variance multiplies the lowest feasible distance to identify additional paths eligible for load balancing. Option A), feasible successor, provides backup loop-free routes but does not influence unequal-cost forwarding alone. Option C), successor route, is the primary active route for forwarding. Option D), topology table, stores feasible successors and metrics but requires variance to perform unequal-cost load balancing.
For ENARSI candidates, understanding the variance command is essential to optimize enterprise WANs with multiple links of varying bandwidth. Commands like show ip eigrp topology and show ip route demonstrate how unequal-cost paths are installed in the routing table. Proper use ensures efficient bandwidth utilization without compromising EIGRP’s loop-free guarantees.
Misuse or misunderstanding of variance can lead to routing inefficiencies, suboptimal path selection, or asymmetric traffic flows, particularly in large-scale enterprise deployments. By mastering this feature, network engineers can implement resilient, high-performance networks that balance load dynamically and enhance redundancy, which is a critical requirement for the ENARSI exam and real-world enterprise operations.
Question 160
Which OSPF area allows injection of external routes using Type 7 LSAs?
A) Stub area
B) Totally stubby area
C) NSSA
D) Backbone area
Answer: C
Explanation:
An OSPF Not-So-Stubby Area (NSSA) is a specialized area type that allows external route injection via Type 7 LSAs, which are then translated to Type 5 LSAs by the ABR if necessary. NSSAs combine the benefits of stub areas—reducing external LSA propagation—with controlled external redistribution, ideal for branch networks that require external connectivity without full LSA exposure.
Option C) is correct because NSSA supports Type 7 LSAs, allowing limited redistribution of external routes while preserving routing table efficiency. Option A), stub area, blocks external LSAs entirely. Option B), totally stubby area, blocks both inter-area and external LSAs except a default route. Option D), backbone area, carries all LSA types without restrictions.
ENARSI candidates must understand NSSAs to design hybrid OSPF topologies that balance routing efficiency with external reachability. Commands such as area X nssa and redistribute allow engineers to configure NSSAs and control external route propagation. Misconfiguration can lead to unreachable external destinations, routing loops, or excessive SPF computation, making careful NSSA planning essential.
NSSAs are particularly valuable in multi-site enterprises with limited branch resources, where minimizing routing table entries and CPU usage is critical while still allowing access to essential external networks. Mastery of NSSA concepts enables candidates to design scalable, resilient, and efficient OSPF networks, a key skill for the ENARSI exam and enterprise routing operations.
