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Question 81
Which OSPF type prevents external LSAs from flooding into an area?
A) Stub
B) Totally Stubby
C) NSSA
D) Backbone
Answer: B
Explanation:
The Totally Stubby Area (TSA) in OSPF is a specialized configuration that blocks external LSAs (Type 5) from entering an area. This is particularly useful for reducing the size of the link-state database and minimizing CPU and memory usage on routers in the area. By preventing external LSAs, the area receives only interior routes (Type 1 and Type 2 LSAs) and a default route, simplifying routing and enhancing convergence times.
Option B) is correct because Totally Stubby Areas combine the properties of stub areas with the additional restriction of blocking Type 3 summary LSAs from other areas except the default route advertised by the ABR. A normal stub area, A), blocks only Type 5 LSAs but still allows inter-area Type 3 LSAs. NSSA, C), allows Type 7 LSAs to carry external routes into the area, which are later converted to Type 5 LSAs by the ABR. The backbone, D), carries all LSAs and cannot be stub or totally stubby.
Totally Stubby Areas are widely used in enterprise branch networks, where routers often have limited processing power and only require default routing to reach external destinations. By advertising only a default route and internal prefixes, OSPF significantly reduces memory and CPU consumption while maintaining connectivity to all other areas. This is particularly critical in multi-area OSPF networks with hundreds of routers, where uncontrolled LSA flooding could cause instability or convergence delays.
Designing an OSPF topology with Totally Stubby Areas also supports simplified troubleshooting and predictable traffic patterns. With fewer LSAs, the link-state database becomes easier to manage, and route calculation becomes faster, leading to quicker adaptation during link or router failures. The ABR plays a pivotal role in TSA, ensuring that only the default route and necessary summary routes propagate into the area while preventing unnecessary flooding of external routes.
In addition, implementing TSA reduces the likelihood of routing loops and misconfigurations within branch areas. This configuration is particularly effective in enterprises with multiple remote offices or branch sites where external routes are not required locally, but connectivity to the backbone is essential. By mastering the principles of Totally Stubby Areas, network engineers can optimize OSPF deployment for efficiency, scalability, and network stability, which is crucial for passing the ENARSI exam and designing robust enterprise networks.
Question 82
Which BGP attribute influences incoming traffic from neighboring ASes?
A) Weight
B) Local preference
C) MED
D) AS-PATH
Answer: C
Explanation:
The Multi-Exit Discriminator (MED) attribute in BGP is designed to influence incoming traffic from external Autonomous Systems. When an enterprise network is multi-homed to two or more ISPs, MED signals the preferred path for inbound traffic. A lower MED value indicates a more preferred route, and neighboring ASes typically select the path with the lowest MED to reach the advertised prefixes.
Option C) is correct because MED affects inter-AS path selection, whereas Local Preference, B), influences internal outbound traffic within the AS. Weight, A), is a Cisco-specific attribute that affects outbound route selection locally but is not propagated to other ASes. AS-PATH, D), is used primarily for loop prevention and path length calculation.
MED is particularly useful in multi-homed enterprise networks to achieve optimized traffic engineering. For example, if two links exist to separate ISPs, assigning a lower MED to the preferred link ensures that most inbound traffic from the neighboring AS will traverse that link, reducing congestion on the other connection. Proper MED configuration can prevent asymmetric routing issues, maintain predictable latency, and improve overall network performance.
It is important to understand that MED is non-transitive by default, meaning it is only evaluated by the neighboring AS directly connected to the announcing router. If MED values need to influence downstream ASes, they must be explicitly propagated. In addition, MED can be combined with BGP communities, route filtering, and local policies to fine-tune traffic flow between external ASes. Misconfigurations can lead to suboptimal routing, uneven link utilization, or unnecessary load on backup connections.
Enterprises that rely on redundant Internet connections benefit significantly from MED-based traffic engineering. It allows network engineers to control ingress traffic patterns without impacting internal routing or requiring complex manipulations of other BGP attributes. Understanding the practical application and limitations of MED is essential for designing highly available, scalable, and predictable WAN topologies and is a critical knowledge area for the ENARSI certification exam.
Question 83
Which EIGRP feature allows unequal-cost load balancing?
A) Feasible successor
B) Variance
C) Maximum-paths
D) Successor
Answer: B
Explanation:
The variance feature in EIGRP enables unequal-cost load balancing, allowing traffic to utilize backup paths with higher metric values than the primary route. By default, EIGRP only balances traffic across equal-cost paths (successors). Variance multiplies the feasible distance of the primary route by a configured value, allowing feasible successors whose metrics fall within this range to participate in load balancing.
Option B) is correct because variance directly controls unequal-cost path utilization, while feasible successors, A), only provide immediate failover but do not influence load balancing. Maximum-paths, C), limits the number of equal-cost paths installed in the routing table. Successor, D), represents the primary route selected by EIGRP.
Unequal-cost load balancing is particularly valuable in enterprise WAN networks, where multiple links may have different bandwidths or latencies. By applying a variance, traffic can be distributed across both high-speed and slightly slower backup links, optimizing bandwidth utilization and reducing congestion. For example, a primary link with a metric of 10 and a backup link with a metric of 15 could both carry traffic if a variance of 2 is applied (10 × 2 = 20), ensuring the backup link is efficiently used.
Using variance requires careful planning to avoid suboptimal routing or packet reordering. Network engineers must analyze feasible successors, link metrics, and traffic characteristics to configure appropriate variance values. When combined with offset lists, policy-based routing, and QoS, variance allows enterprise networks to maximize redundancy and performance while maintaining loop-free, reliable routing.
EIGRP’s unequal-cost load balancing is one of its most powerful capabilities, enabling resilient, high-throughput WAN designs without requiring complex overlay technologies. Understanding variance, its interaction with feasible successors, and its implications for network convergence and stability is crucial for ENARSI exam candidates and network professionals designing robust, high-performance enterprise networks.
Question 84
Which BGP attribute prevents routing loops across multiple ASes?
A) MED
B) AS-PATH
C) Local preference
D) Next-hop
Answer: B
Explanation:
The AS-PATH attribute in BGP is a critical component for preventing routing loops across Autonomous Systems. AS-PATH records the sequence of AS numbers a route has traversed. When a BGP router receives a route advertisement containing its own AS number in the AS-PATH, it rejects the route, thereby preventing a loop.
Option B) is correct because AS-PATH provides loop detection across AS boundaries. MED, A), influences ingress traffic selection but does not prevent loops. Local Preference, C), affects internal outbound path selection, while Next-hop, D), simply specifies the IP address of the next router for packet forwarding.
AS-PATH also serves as a path selection metric in BGP. When multiple routes exist to the same prefix, the route with the shortest AS-PATH is typically preferred, promoting optimal path selection while inherently discouraging routing loops. AS-PATH is essential for multi-homed environments, especially when an enterprise connects to multiple ISPs or peers with other autonomous systems.
Network engineers can manipulate AS-PATH through AS-PATH prepending, which artificially increases the path length to make certain routes less preferred. This is a common strategy in BGP traffic engineering, enabling control over both inbound and outbound traffic. Improper AS-PATH configurations can lead to unintended route preferences, asymmetric routing, or traffic congestion, making careful understanding critical for enterprise network design.
In addition, AS-PATH plays a pivotal role in BGP policy enforcement, security, and loop prevention. Combined with communities, MED, and local preference, AS-PATH allows for granular control over route selection, ensuring scalable, stable, and predictable inter-AS routing. Mastering AS-PATH functionality is fundamental for ENARSI exam candidates and network architects implementing redundant, high-performance BGP WAN topologies.
Question 85
Which OSPF LSA type advertises ASBR location to other areas?
A) Type 3
B) Type 4
C) Type 5
D) Type 7
Answer: B
Explanation:
Type 4 LSAs in OSPF are generated by Area Border Routers (ABRs) to advertise the location of Autonomous System Boundary Routers (ASBRs) to other areas. This allows internal OSPF routers to determine the next hop for reaching external routes learned via redistribution into OSPF. Type 4 LSAs ensure that routers outside the ASBR’s area can correctly route packets to external destinations.
Option B) is correct because Type 4 LSAs specifically indicate ASBR reachability. Type 3 LSAs, A), summarize networks between areas. Type 5 LSAs, C), carry external routes, and Type 7 LSAs, D), are used in NSSAs to carry external routes within a not-so-stubby area.
Type 4 LSAs are essential for maintaining accurate inter-area routing in multi-area OSPF networks. When an ASBR redistributes external routes, ABRs generate Type 4 LSAs to inform all routers in other areas how to reach the ASBR. This mechanism ensures loop-free, efficient routing to external destinations while preserving OSPF’s hierarchical design.
In enterprise networks, misconfiguration or omission of Type 4 LSAs can lead to blackholing of external traffic, asymmetric routing, or routing loops. Network designers often combine Type 4 LSAs with route summarization and area design strategies to reduce link-state database size and optimize OSPF convergence times. Effective understanding of Type 4 LSAs is vital for ENARSI exam preparation, as it underpins the principles of OSPF external route reachability, hierarchical routing, and enterprise WAN scalability.
Question 86
Which BGP feature controls outbound traffic within an AS?
A) Local Preference
B) MED
C) Weight
D) AS-PATH
Answer: A
Explanation:
Local Preference is a fundamental BGP attribute that influences outbound traffic selection within a single Autonomous System (AS). It allows network engineers to dictate which egress path routers should prefer when multiple exit points exist. Unlike MED, which influences inbound traffic from external ASes, Local Preference affects internal BGP (iBGP) route selection and is propagated to all iBGP peers within the AS.
Option A) is correct because Local Preference provides control over internal route selection. MED, B), only affects how external ASes choose paths to your network. Weight, C), is Cisco-specific and locally significant but does not propagate. AS-PATH, D), is used primarily for loop prevention and path length evaluation between ASes.
Local Preference is configured on a per-route basis using route maps, prefix lists, or policy statements, enabling enterprises to implement sophisticated traffic engineering strategies. For instance, in a multi-homed enterprise with two upstream ISPs, administrators may assign a higher Local Preference to routes through the preferred ISP to ensure outbound traffic follows the optimal path. This guarantees efficient utilization of bandwidth, predictable latency, and improved redundancy.
One of the key advantages of Local Preference is its simplicity and consistency across the AS. By propagating this value to all iBGP routers, the AS ensures that traffic consistently exits via the preferred path, eliminating unexpected route variations that can occur if routers select paths independently. Additionally, Local Preference works well with BGP communities and policy filtering, allowing granular control over outbound traffic based on prefix, neighbor, or geographic location.
Implementing Local Preference correctly also reduces the likelihood of asymmetric routing, which can cause performance issues in applications sensitive to packet order or latency, such as VoIP, video conferencing, or financial transaction networks. Enterprises frequently leverage Local Preference to balance load, optimize redundancy, and improve failover response, which is critical for maintaining high availability in WAN topologies.
Understanding Local Preference is essential for ENARSI candidates, as it forms the basis of BGP traffic engineering strategies, inter-AS path optimization, and enterprise-scale network stability. Mastery of this attribute allows network architects to design predictable, efficient, and scalable networks, minimizing operational complexity and ensuring compliance with business SLA requirements.
Question 87
Which EIGRP metric component reflects link delay between routers?
A) Bandwidth
B) Delay
C) Reliability
D) Load
Answer: B
Explanation:
The Delay component in EIGRP metrics measures the cumulative transmission delay along a path between two routers. It is expressed in tens of microseconds and represents the time required to send a packet across each hop. Delay, together with Bandwidth, Reliability, Load, and MTU, forms the composite EIGRP metric used to determine the best path to a destination.
Option B) is correct because Delay directly influences the metric calculation, giving preference to paths with lower propagation or queuing delays. Bandwidth, A), measures the lowest link bandwidth along the path, reflecting the maximum data-carrying capacity. Reliability, C), represents the probability of successful transmission over time, and Load, D), indicates the current utilization of the link.
EIGRP combines Delay and Bandwidth using a formula to calculate the composite metric. By default, Reliability and Load are not used in metric calculation but can be optionally enabled for fine-tuned traffic engineering. Delay is additive across each hop, meaning longer paths with slower links accumulate higher delay values, reducing their preference in path selection.
In enterprise WAN designs, Delay becomes crucial in high-latency environments, such as satellite or multi-hop MPLS networks. Administrators can manipulate Delay values artificially through interface configuration to influence EIGRP routing decisions, allowing traffic to favor specific paths even if they are longer physically but have higher available bandwidth. This is particularly useful for traffic engineering, load balancing, and failover planning.
Furthermore, understanding Delay is critical when configuring variance for unequal-cost load balancing. Feasible successors must have a metric within the variance threshold, which is affected by the Delay component. Misconfigurations in Delay settings can lead to suboptimal routing, congestion, and increased latency, impacting mission-critical applications. EIGRP’s delay metric also interacts with features like stub routing, summarization, and route filtering, emphasizing its importance for designing efficient, loop-free, and resilient enterprise networks.
Mastery of Delay in EIGRP is vital for ENARSI exam candidates because it forms the foundation of metric calculation, path selection, and advanced traffic engineering strategies. Correctly leveraging Delay ensures high-performance, reliable, and scalable routing across complex enterprise topologies.
Question 88
Which OSPF area type allows external routes but blocks type 3 LSAs?
A) Standard
B) Stub
C) NSSA
D) Totally Stubby
Answer: C
Explanation:
The Not-So-Stubby Area (NSSA) is a unique OSPF area type that permits the introduction of external routes via Type 7 LSAs while restricting Type 5 LSAs from entering the area. This allows an NSSA to have internal redistribution of external routes without flooding the entire OSPF network, preserving the hierarchical design and reducing the size of the link-state database.
Option C) is correct because NSSA specifically allows external route redistribution while maintaining controlled LSA propagation. Standard areas, A), allow all LSAs. Stub areas, B), block Type 5 LSAs entirely but allow inter-area Type 3 LSAs. Totally Stubby areas, D), block both Type 5 and Type 3 LSAs, only allowing a default route from the ABR.
NSSA is particularly valuable in branch office or multi-area enterprise deployments where routers at the edge need to advertise external routes, such as default routes or routes learned via redistribution from EIGRP or RIP, without overwhelming the backbone with unnecessary LSAs. The ABR converts Type 7 LSAs to Type 5 LSAs before injecting them into other OSPF areas, ensuring consistent reachability and loop-free routing.
Enterprise network engineers frequently leverage NSSA for incremental expansion, allowing new areas to advertise external routes safely. NSSA also interacts with summarization, route filtering, and redistribution policies, giving administrators precise control over LSA propagation. This prevents link-state database growth, excessive CPU utilization, and OSPF instability in large networks.
Proper understanding of NSSA is critical for ENARSI exam candidates because it demonstrates knowledge of OSPF hierarchical design, area types, and external route management. By mastering NSSA, engineers can optimize branch office connectivity, WAN efficiency, and enterprise network scalability, ensuring that external routes are accessible without compromising OSPF performance or convergence.
Question 89
Which BGP mechanism ensures predictable routing when multiple exit links exist?
A) Local Preference
B) AS-PATH Prepending
C) Weight
D) Route Reflector
Answer: B
Explanation:
AS-PATH Prepending is a BGP technique used to influence external AS routing decisions by artificially lengthening the AS-PATH of a route. By adding additional copies of the AS number to the AS-PATH attribute, an AS can make certain exit points appear less preferable to neighboring networks, thereby controlling the distribution of inbound traffic across multiple links.
Option B) is correct because AS-PATH Prepending directly affects how external ASes perceive route desirability. Local Preference, A), influences internal route selection. Weight, C), is locally significant on a Cisco router but does not propagate. Route Reflector, D), is used for iBGP scalability and does not directly control exit point selection.
AS-PATH Prepending is widely used in multi-homed enterprise environments with redundant ISP connections. For example, if two exit links exist, prepending the AS number on one link makes it appear less attractive to upstream providers, steering most inbound traffic toward the preferred exit. This technique balances load, reduces congestion, and improves network predictability without modifying external routing policies.
While AS-PATH Prepending is effective, it is important to understand that its influence is heuristic rather than deterministic. Neighboring ASes may have other policies or attributes, such as MED, local preference, or route filtering, that could override the prepending effect. Therefore, network engineers often combine AS-PATH Prepending with other BGP attributes and communities to achieve precise traffic engineering.
From a strategic perspective, AS-PATH Prepending also supports resilience planning. In case of failure on the preferred exit, prepended paths become more attractive automatically, allowing traffic to reroute without additional configuration. This ensures high availability, predictable traffic distribution, and efficient utilization of redundant WAN links, which is critical in enterprise networks relying on multi-ISP connectivity.
Mastering AS-PATH Prepending is essential for ENARSI exam candidates because it demonstrates an understanding of BGP inbound traffic manipulation, multi-homing strategies, and enterprise WAN optimization, forming a core component of advanced routing and network design principles.
Question 90
Which EIGRP feature prevents routing updates from entering stub areas?
A) Feasible Successor
B) Stub Router
C) Variance
D) Maximum-paths
Answer: B
Explanation:
The Stub Router feature in EIGRP allows administrators to configure a router to limit the type of routing updates it receives, effectively preventing unnecessary routes from entering branch or stub areas. By declaring a router as a stub, it will advertise only directly connected routes, summary routes, and optionally a default route, while ignoring certain external routes.
Option B) is correct because the stub configuration restricts route propagation into low-resource areas, ensuring efficient routing and reduced CPU/memory usage. Feasible Successor, A), provides backup routes but does not restrict updates. Variance, C), allows unequal-cost load balancing. Maximum-paths, D), limits the number of equal-cost paths installed in the routing table.
Stub areas are particularly valuable in branch office networks, satellite sites, and remote locations where routers have limited processing capabilities. By using stub routers, network engineers prevent unnecessary routing updates, reduce link-state chatter, and enhance convergence speed in WAN topologies. This configuration also minimizes the risk of routing loops in distributed enterprise networks.
The stub feature can be combined with other EIGRP mechanisms like route summarization, variance, and feasible successors to optimize traffic engineering and ensure reliable network performance. Proper planning is essential because misconfiguring stub routers may lead to loss of reachability for certain external routes, impacting business-critical applications.
Understanding the Stub Router concept is crucial for ENARSI candidates, as it demonstrates the ability to design scalable, efficient, and stable enterprise WANs. By implementing stubs effectively, engineers can maintain predictable routing behavior, reduce overhead, and optimize resource utilization in branch and remote office deployments.
Question 91
Which OSPF feature prevents routing loops between two areas?
A) ABR
B) Backbone Area
C) LSA Filtering
D) Route Summarization
Answer: A
Explanation:
The Area Border Router (ABR) is a fundamental OSPF feature that maintains loop-free routing between different OSPF areas. ABRs are routers that connect one or more OSPF areas to the backbone area (Area 0). They act as intermediaries, receiving, processing, and filtering LSAs to ensure that routes from one area are correctly advertised to another without creating loops.
Option A) is correct because ABRs maintain distinct area link-state databases and handle inter-area route calculations. Backbone Area, B), refers to Area 0, which all OSPF areas must connect to, but it does not prevent loops on its own. LSA Filtering, C), selectively blocks LSAs but is a manual process rather than an inherent loop-prevention mechanism. Route Summarization, D), aggregates multiple routes to reduce table size but is not sufficient alone to prevent loops.
The ABR plays a critical role in hierarchical OSPF design, ensuring that intra-area routes remain separate from inter-area routes. It receives Type 1 and Type 2 LSAs from internal routers, calculates shortest paths for internal routes, and then advertises summarized Type 3 LSAs into neighboring areas. By doing this, ABRs control the distribution of link-state information, which inherently prevents routing loops and promotes scalable enterprise network design.
ABRs also allow flexibility in network growth. By connecting multiple areas to the backbone, administrators can segment traffic for better stability, convergence, and fault isolation. This is crucial for large enterprise networks with multiple WAN links or geographically dispersed sites, where uncontrolled LSA propagation could cause excessive CPU usage, memory consumption, and instability.
Additionally, ABRs work in conjunction with area types like stub, NSSA, and totally stubby areas to further control LSA flooding. This ensures that branch offices or low-resource routers are not overwhelmed by unnecessary routing information while still maintaining connectivity to the backbone and external destinations. Proper understanding of ABRs is vital for ENARSI exam candidates, as it demonstrates the ability to implement loop-free, scalable, and highly resilient OSPF designs.
By leveraging ABRs, enterprise network engineers can achieve predictable routing behavior, optimized resource utilization, and efficient network convergence, which are key objectives for high-performance OSPF implementations.
Question 92
Which BGP attribute is Cisco-specific and influences path selection locally?
A) Weight
B) Local Preference
C) MED
D) Community
Answer: A
Explanation:
The Weight attribute is a Cisco-proprietary BGP feature that influences path selection locally on a router without being propagated to other BGP peers. Weight is the highest priority attribute in Cisco routers for determining the best path among multiple BGP routes. By default, a higher weight value makes the route more preferred for outbound traffic.
Option A) is correct because Weight is router-specific, does not propagate to neighboring routers, and allows administrators to control which BGP path is chosen without affecting external ASes. Local Preference, B), influences internal BGP path selection and is propagated to all iBGP peers. MED, C), communicates preference to external ASes. Community, D), is used for tagging and policy purposes but does not directly select a path locally.
Weight is frequently used in multi-homed enterprise environments to control traffic exiting the AS. For example, if a company has two links to the same ISP, administrators can assign a higher weight to the preferred exit path, ensuring that all outbound traffic uses the optimal link while leaving the other as a backup. This guarantees predictable routing behavior and better utilization of available bandwidth.
Unlike Local Preference, which affects all routers in an AS, Weight provides fine-grained control on individual routers. This can be particularly beneficial in edge routers or critical WAN devices where deterministic traffic routing is required. Weight also simplifies traffic engineering, as it requires no complex route-maps or community tagging when the goal is purely local path selection.
Proper use of Weight is essential for ENARSI candidates because it demonstrates knowledge of BGP traffic engineering, path manipulation, and enterprise WAN optimization. Understanding Weight also helps in scenarios involving failover planning, as it can automatically direct traffic to secondary paths without manual intervention. This ensures high availability, predictable routing, and optimized network performance in multi-homed deployments.
Weight is particularly critical in large-scale BGP deployments, where multiple exit points, redundant connections, and varying ISP policies require precise control of traffic flows. By mastering this attribute, engineers can design robust, scalable, and resilient BGP networks in line with enterprise performance requirements.
Question 93
Which OSPF LSA type is used to advertise external routes into an area?
A) Type 3
B) Type 4
C) Type 5
D) Type 7
Answer: C
Explanation:
Type 5 LSAs in OSPF are designed to advertise external routes into an OSPF domain. These routes originate outside the OSPF autonomous system, such as from EIGRP, RIP, or static routes redistributed into OSPF, and are flooded throughout all non-stub areas of the network. Type 5 LSAs enable routers to learn about external destinations, ensuring interoperability with other routing protocols and seamless WAN connectivity.
Option C) is correct because Type 5 LSAs carry external network prefixes and their associated metrics. Type 3, A), represents summary LSAs for inter-area routes. Type 4, B), is used to advertise the location of ASBRs. Type 7, D), is used in NSSA areas to carry external routes before conversion to Type 5.
Type 5 LSAs include the external metric, route type (E1 or E2), and forwarding information, allowing routers to calculate cost to reach external networks. E1 routes include the internal OSPF cost plus external cost, while E2 routes only consider the external cost. This distinction affects route selection, traffic engineering, and path preference within the OSPF domain.
In enterprise network design, Type 5 LSAs are critical for multi-protocol integration, such as redistributing legacy RIP or EIGRP networks into OSPF, enabling a hybrid routing strategy. Type 5 LSAs are not allowed in stub or totally stubby areas, which forces administrators to rely on default routes or NSSA Type 7 LSAs for external reachability, preventing unnecessary LSA flooding in resource-constrained routers.
Understanding Type 5 LSAs is vital for ENARSI candidates, as it demonstrates knowledge of OSPF external route propagation, ASBR functions, and inter-protocol redistribution. Proper configuration ensures efficient, scalable, and loop-free routing for external destinations in enterprise networks, which is essential for predictable WAN connectivity and network stability.
Type 5 LSAs also support advanced enterprise features such as route summarization, traffic engineering, and load balancing. Misconfigurations can lead to route loops, suboptimal paths, and excessive LSA flooding, highlighting the importance of mastering this concept for complex OSPF deployments.
Question 94
Which EIGRP mechanism allows unequal-cost load balancing?
A) Feasible Successor
B) Variance
C) Stub Router
D) Maximum-paths
Answer: B
Explanation:
The Variance command in EIGRP allows for unequal-cost load balancing, enabling traffic to utilize multiple paths that are not of identical metric cost. By default, EIGRP only installs equal-cost paths in the routing table, but with variance, engineers can expand this behavior to include feasible successors whose metric falls within a configured multiple of the best path metric.
Option B) is correct because Variance directly allows traffic engineering across unequal-cost paths, improving bandwidth utilization and redundancy. Feasible Successor, A), provides backup paths but does not enable unequal-cost load balancing. Stub Router, C), restricts routes entering stub areas. Maximum-paths, D), only sets the maximum number of equal-cost paths.
The variance value is a multiplier applied to the lowest feasible metric to determine which backup routes qualify for load balancing. For example, setting a variance of 2 allows all feasible successors with metrics less than twice the lowest metric to participate in routing, providing flexible path utilization without compromising loop-free guarantees.
Unequal-cost load balancing is especially useful in WAN environments, MPLS networks, and multi-homed enterprise links, where links have varying bandwidth or latency characteristics. By utilizing variance, network engineers can achieve higher network efficiency, fault tolerance, and better utilization of expensive WAN circuits.
Additionally, variance works seamlessly with feasible successor computation, ensuring that only loop-free backup routes are considered for load balancing. Misconfiguration can lead to suboptimal routing, increased latency, or congestion on certain links, highlighting the need for careful metric calculation and planning.
For ENARSI candidates, mastering variance demonstrates the ability to implement sophisticated traffic engineering, optimize resource utilization, and design scalable EIGRP networks. Correct use of variance ensures that traffic flows efficiently across the network, leveraging both primary and backup paths while maintaining loop-free, resilient routing topologies.
Question 95
Which BGP feature reduces route propagation within the same AS?
A) Route Reflector
B) Confederation
C) Weight
D) Local Preference
Answer: A
Explanation:
Route Reflectors (RRs) are a BGP mechanism designed to reduce iBGP route propagation within a single AS. In traditional iBGP, every router must form a full mesh with all other iBGP peers, which becomes unsustainable in large networks. Route reflectors allow iBGP routers to advertise routes to other iBGP peers without requiring a full mesh, simplifying network design and reducing configuration complexity.
Option A) is correct because Route Reflectors centralize route advertisement, eliminating the need for full iBGP peerings. Confederations, B), split a single AS into sub-ASes to reduce complexity but do not directly solve route reflection. Weight, C), is a local path selection mechanism. Local Preference, D), affects internal path preference but does not reduce route propagation.
Route Reflectors introduce client and non-client relationships, where the RR advertises learned routes from clients to non-clients and vice versa, maintaining loop-free path propagation using the ORIGINATOR_ID and CLUSTER_LIST attributes. This design preserves BGP scalability while enabling efficient route dissemination across large enterprise or service provider networks.
RRs also enable policy enforcement, traffic engineering, and network hierarchy design by controlling which routes are reflected and to whom. In combination with confederations, communities, and local preference, RRs allow network engineers to implement complex routing policies while maintaining simplicity and scalability.
For ENARSI candidates, understanding Route Reflectors is critical because they ensure iBGP scalability, redundancy, and loop prevention, which are essential in enterprise and service provider environments with hundreds or thousands of routers. Proper RR configuration enables predictable routing, high availability, and efficient network management.
Question 96
Which routing protocol supports fast convergence with feasible successor paths?
A) OSPF
B) EIGRP
C) BGP
D) RIP
Answer: B
Explanation:
EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco-developed routing protocol known for its rapid convergence and loop-free routing. One of its distinctive features is the feasible successor mechanism, which allows a backup path to be immediately available if the primary route fails. This is fundamental for maintaining network stability and ensuring minimal packet loss during topology changes.
Option B) is correct because EIGRP calculates a primary route using the best composite metric, which includes bandwidth, delay, reliability, load, and MTU. Feasible successors are identified as routes that satisfy the feasibility condition—their reported distance must be less than the feasible distance of the primary route, guaranteeing loop-free operation.
OSPF, A), uses the Dijkstra algorithm and converges rapidly within areas but does not inherently maintain precomputed backup paths like EIGRP. BGP, C), is an external routing protocol and relies on path attributes rather than precomputed feasible successors. RIP, D), is slow to converge due to its distance-vector mechanism, making it unsuitable for high-speed enterprise networks.
Feasible successors enable EIGRP to perform near-instant failover without recalculating the topology across the network. This is especially valuable in WAN environments with multiple redundant paths where delays in routing updates could disrupt applications, VoIP traffic, or critical enterprise services. By maintaining backup paths in the topology table, EIGRP avoids routing loops and packet loss, which enhances reliability and service quality.
Understanding feasible successors is critical for ENARSI candidates because it demonstrates mastery of advanced EIGRP concepts, traffic engineering, and high-availability design principles. By leveraging this mechanism, network engineers can ensure that enterprise networks maintain consistent connectivity even during partial link failures, optimize bandwidth utilization across multiple paths, and design resilient hierarchical topologies that minimize downtime.
Furthermore, feasible successors also allow for unequal-cost load balancing when combined with the variance command, improving network efficiency by distributing traffic over multiple viable paths. This makes EIGRP a preferred choice in complex enterprise networks requiring scalability, predictability, and robust failover capabilities.
Question 97
Which OSPF area type blocks external Type 5 LSAs?
A) Stub
B) Backbone
C) NSSA
D) Totally Stubby
Answer: A
Explanation:
Stub areas in OSPF are designed to reduce routing table size and LSA flooding, enhancing stability in routers with limited resources. One of the defining characteristics of a stub area is that it blocks external Type 5 LSAs, which advertise routes from outside the OSPF autonomous system. Instead of receiving these LSAs, routers in stub areas rely on a default route to reach external networks, simplifying routing and reducing CPU and memory usage.
Option A) is correct because stub areas are intended for branch or remote sites where full external route visibility is unnecessary. The Backbone, B) (Area 0), connects all OSPF areas but does not block Type 5 LSAs. NSSA (Not-So-Stubby Area), C), allows limited external LSAs in Type 7 format to be converted to Type 5. Totally Stubby Areas, D), extend stub behavior by blocking inter-area Type 3 LSAs in addition to Type 5 LSAs.
By blocking Type 5 LSAs, stub areas minimize LSA propagation overhead, conserve memory, and ensure that routers can focus on critical intra-area routing decisions. This is particularly important for branch offices, small routers, or remote locations where network resources are constrained and excessive LSA flooding could cause slow convergence or instability.
OSPF stub area design requires careful planning. Routers connecting stub areas must be ABRs, which inject a default route to allow traffic to reach external networks while maintaining loop-free topologies. This design ensures predictable routing, reduced flooding, and simplified administration, which are key objectives in large-scale enterprise deployments.
For ENARSI exam candidates, mastering stub area configuration demonstrates expertise in hierarchical OSPF design, efficient LSA management, and enterprise scalability. Understanding when and how to implement stub areas enables network engineers to balance performance, stability, and routing efficiency, ensuring that the network remains resilient, optimized, and capable of supporting complex enterprise applications without overloading smaller routers.
Question 98
Which BGP attribute controls outbound traffic preference from an AS?
A) Weight
B) Local Preference
C) MED
D) AS Path
Answer: B
Explanation:
The Local Preference attribute in BGP is used to control outbound traffic selection across all routers within the same autonomous system. Unlike Weight, which is router-specific, Local Preference is propagated to all iBGP peers, enabling uniform policy enforcement throughout the AS. A higher Local Preference value makes a route more preferred, guiding outbound traffic toward the most efficient exit point.
Option B) is correct because Local Preference directly influences internal path selection and allows administrators to engineer traffic flows for load balancing, failover, or preferred ISP routing. Weight, A), is local to a single router and does not propagate. MED, C), signals preference to external ASes. AS Path, D), affects route selection by evaluating the number of AS hops but does not enforce internal policy.
Local Preference is crucial in multi-homed enterprise networks where multiple exit points exist. By setting Local Preference, administrators can prioritize certain links over others for outbound traffic while still allowing alternative paths for redundancy. For example, traffic destined for the internet can be routed preferentially through the higher-capacity WAN link, while the secondary link serves as a backup, ensuring reliability and performance optimization.
In addition, Local Preference supports advanced traffic engineering scenarios. It can be combined with route maps, prefix lists, or communities to enforce complex routing policies. Proper configuration ensures that enterprise networks maintain predictable routing, prevent suboptimal path selection, and achieve high availability.
For ENARSI candidates, understanding Local Preference is essential because it demonstrates the ability to implement sophisticated BGP traffic management strategies, ensuring that large-scale enterprise networks operate efficiently, consistently, and resiliently. Mismanagement of Local Preference can result in unexpected traffic patterns, congestion, or suboptimal utilization of WAN links, making mastery of this attribute vital for professional network design and operation.
Question 99
Which EIGRP feature prevents routing loops while allowing backup paths?
A) Feasible Successor
B) Split Horizon
C) Variance
D) Passive Interface
Answer: A
Explanation:
The Feasible Successor in EIGRP is a backup route that satisfies the feasibility condition, ensuring that it is loop-free relative to the primary route. This mechanism allows EIGRP to maintain redundant paths without risking routing loops, enabling fast failover in case of primary path failure. Feasible successors are precomputed and stored in the topology table, making them immediately available for route installation in the routing table if the active path fails.
Option A) is correct because it combines loop prevention with rapid convergence, which is one of the hallmark strengths of EIGRP. Split Horizon, B), prevents routing loops by not advertising routes back out the interface from which they were learned. Variance, C), allows unequal-cost load balancing but does not inherently prevent loops. Passive Interface, D), stops sending routing updates on an interface but does not provide backup path computation.
The feasibility condition is critical: the reported distance of a potential backup route must be less than the feasible distance of the current primary route. This guarantees that the backup path does not create a routing loop while still providing a viable alternative. Feasible successors can be leveraged with the variance command to distribute traffic across multiple unequal-cost paths without compromising loop-free guarantees.
Feasible successors are especially useful in enterprise WAN topologies, where redundancy is crucial. In networks with multiple links or partial failures, feasible successors ensure high availability and minimal downtime, which is essential for applications like voice, video, and critical business services. This feature highlights EIGRP’s scalability and reliability advantages over other distance-vector protocols, particularly in complex enterprise environments.
ENARSI candidates must understand feasible successors because they demonstrate advanced routing knowledge, traffic engineering capabilities, and high-availability design expertise. Proper use allows engineers to implement resilient, efficient, and loop-free network topologies, aligning with enterprise objectives for predictable, fast-converging, and stable routing.
Question 100
Which OSPF LSA type identifies ASBRs for external route reachability?
A) Type 3
B) Type 4
C) Type 5
D) Type 7
Answer: B
Explanation:
Type 4 LSAs in OSPF are generated by Area Border Routers (ABRs) to advertise the location of Autonomous System Boundary Routers (ASBRs) to other areas. These LSAs allow internal routers to know the best path to reach external routes redistributed into OSPF, ensuring proper external reachability and loop-free routing. Type 4 LSAs are especially critical in multi-area OSPF networks where external routes originate from another AS or are redistributed from other protocols.
Option B) is correct because Type 4 LSAs specifically locate ASBRs. Type 3, A), advertises summary inter-area routes. Type 5, C), advertises external routes directly. Type 7, D), is used in NSSA areas to carry external routes before conversion to Type 5.
Type 4 LSAs carry the router ID of the ASBR and the associated cost to reach it, enabling internal OSPF routers to compute the shortest path to external destinations via the appropriate ABR. Without Type 4 LSAs, routers in non-backbone areas would lack proper information to reach external networks, potentially causing routing failures, loops, or suboptimal paths.
In enterprise environments, Type 4 LSAs are essential for multi-area OSPF topologies with external route redistribution. They ensure that routers in remote areas have consistent knowledge of ASBR locations, enabling predictable routing, redundancy, and rapid convergence. Understanding Type 4 LSAs is crucial for ENARSI candidates, as it demonstrates expertise in inter-area route advertisement, external route reachability, and advanced OSPF design principles.
Type 4 LSAs also interact with Type 5 LSAs and NSSA Type 7 LSAs, creating a complete picture of OSPF external routing. Misconfigurations can lead to incomplete routing tables, routing loops, or unreachable external destinations, highlighting the importance of correctly understanding and implementing this LSA type in enterprise networks.
