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Question 61
Which OSPF network type uses DR and BDR elections?
A) Point-to-point
B) Broadcast
C) Non-broadcast
D) Point-to-multipoint
Answer: B
Explanation:
In OSPF, the network type determines how routers discover neighbors and exchange link-state advertisements (LSAs). Broadcast networks, such as Ethernet LANs, use Designated Router (DR) and Backup Designated Router (BDR) elections to reduce LSA flooding. DR and BDR are elected among all OSPF routers on the same broadcast segment to centralize the distribution of routing information.
Option B) is correct because in broadcast networks, DR manages communication by receiving LSAs from all routers and distributing them to all other routers on the segment, while BDR serves as a standby in case the DR fails. This reduces unnecessary flooding and minimizes processing overhead on routers.
Option A) point-to-point networks do not require DR or BDR elections because there are only two routers communicating directly, eliminating the need for centralized LSA management.
Option C) non-broadcast networks also may have DR/BDR elections, but they require manual neighbor configuration, making them different from true broadcast networks.
Option D) point-to-multipoint networks may not require DR/BDR elections, depending on configuration, because routers may communicate directly without central coordination.
DR and BDR elections are crucial in large enterprise networks with multiple routers on the same Ethernet segment. DR election is based on OSPF router priority, and the router with the highest priority becomes DR. BDR is elected in a similar manner. If priorities are equal, the router with the highest Router ID wins. This election ensures optimal use of network bandwidth and faster convergence. If a DR fails, the BDR immediately takes over, maintaining network stability. Proper configuration of OSPF priorities and Router IDs is essential to prevent unexpected DR/BDR changes during network upgrades or router reboots. Misconfigured elections can lead to suboptimal routing, LSA flooding, and increased CPU usage. Network engineers often implement manual priority adjustments to control DR selection in critical segments, ensuring predictable behavior and reducing operational risk.
Question 62
Which routing protocol uses EIGRP metric based on bandwidth and delay by default?
A) OSPF
B) EIGRP
C) RIP
D) BGP
Answer: B
Explanation:
EIGRP uses a composite metric that by default considers bandwidth and delay to determine the best path to a destination. This combination allows EIGRP to select optimal routes based on performance characteristics rather than just hop count, providing better network efficiency and stability. The metric formula includes bandwidth, delay, load, and reliability, but by default, K1 and K3 are set to give weight to bandwidth and delay while ignoring load and reliability.
Option B) is correct because bandwidth represents the slowest link along a path, while delay is cumulative along the route. Together, they provide a practical metric for modern enterprise networks where high-speed links and low-latency paths are preferred.
Option A) OSPF uses cost based on bandwidth only, not delay.
Option C) RIP uses hop count as its sole metric, which is simplistic and can lead to suboptimal path selection.
Option D) BGP uses path attributes rather than metrics based on link characteristics to influence routing decisions.
EIGRP’s metric calculation provides granular control over path selection, allowing network engineers to fine-tune routing behavior for performance-sensitive applications. Proper EIGRP configuration ensures fast convergence, minimal routing loops, and efficient use of high-speed links. By adjusting K-values, engineers can influence how much weight is given to each component of the metric. For instance, increasing the weight of delay may prioritize low-latency paths for voice or video traffic, while bandwidth prioritization ensures data-intensive applications use high-speed links. Understanding the EIGRP metric formula is critical for network design, traffic engineering, and troubleshooting. Without this understanding, routes may not behave as intended, leading to bottlenecks or uneven traffic distribution. EIGRP’s ability to use multiple metrics also supports redundancy and load balancing, as feasible successors with slightly higher metrics can act as backup paths for rapid failover. This makes EIGRP highly suitable for complex enterprise networks with varying link speeds and performance requirements. Proper documentation and monitoring of metrics ensure predictable routing behavior and maintain high availability in multi-site deployments.
Question 63
Which OSPF LSA type describes external routes redistributed into OSPF?
A) Type 1
B) Type 2
C) Type 3
D) Type 5
Answer: D
Explanation:
Type 5 LSAs are used in OSPF to describe routes redistributed from other routing protocols, such as RIP, EIGRP, or BGP, into OSPF. These LSAs are known as external LSAs and are flooded throughout the OSPF autonomous system, except stub or totally stubby areas. They carry the external route prefix, cost, and type (E1 or E2), allowing OSPF routers to incorporate external networks into their routing tables.
Option D) is correct because Type 5 LSAs are generated by Autonomous System Boundary Routers (ASBRs) to propagate information about external networks. E1 external routes include internal OSPF cost plus external cost, while E2 routes consider only external cost.
Option A) Type 1 LSAs describe router links within an area.
Option B) Type 2 LSAs describe network segments for multi-access networks.
Option C) Type 3 LSAs summarize inter-area routes from ABRs.
External LSAs are crucial for interconnecting OSPF with other protocols and providing reachability to external destinations. ASBRs manage the generation and propagation of Type 5 LSAs, ensuring that routers throughout the network can route to redistributed networks effectively. Proper configuration of OSPF area types is essential; for example, in stub areas, Type 5 LSAs are blocked to reduce routing table size and improve convergence, requiring alternative methods like default routes to reach external networks. Type 5 LSAs also allow administrators to perform traffic engineering, controlling metrics to influence path selection for external routes. Misconfiguration can result in unreachable external networks, suboptimal routing, or excessive LSA flooding, stressing router CPU and memory. Understanding the distinction between E1 and E2 is vital because it affects how external route cost interacts with OSPF internal topology. E1 is more suitable for environments where internal path cost should influence external route selection, while E2 is common when external cost should dominate. Monitoring and optimizing Type 5 LSA propagation ensures scalable, stable, and efficient enterprise OSPF deployments.
Question 64
Which BGP attribute controls outbound traffic preference within the local AS?
A) MED
B) Weight
C) AS path
D) Local preference
Answer: D
Explanation:
Local preference in BGP is used to influence outbound traffic routing decisions within an Autonomous System. The path with the highest local preference is preferred for sending traffic to external networks. This attribute is propagated to all internal BGP (iBGP) peers, ensuring consistent routing behavior across the AS.
Option D) is correct because local preference enables administrators to direct traffic through preferred exit points, providing traffic engineering, redundancy, and optimized network performance. It is a crucial tool in multi-homed environments to control how traffic leaves the AS.
Option A) MED influences incoming traffic from external peers, not outbound traffic.
Option B) weight is Cisco-specific and affects local path selection but is not shared with other routers.
Option C) AS path influences route selection for external routes but does not allow explicit control of outbound traffic.
Local preference is a simple yet powerful mechanism to ensure predictable routing behavior within the AS. By assigning higher local preference values to preferred links, network engineers can ensure that traffic exits through the most desirable paths, optimizing bandwidth utilization and meeting service-level agreements. Misconfigurations can cause traffic asymmetry, congestion, or suboptimal routing, impacting performance-sensitive applications. Local preference is often combined with route maps, policy routing, and BGP communities to implement complex routing policies, such as cost-based routing, load balancing, and failover strategies. Understanding local preference propagation rules and default values is critical for multi-site enterprise networks, where incorrect configuration could inadvertently send large volumes of traffic through secondary or backup links. Local preference provides granular control over outbound flows while maintaining loop-free and stable iBGP operations, ensuring enterprise networks remain reliable and predictable. Proper documentation and monitoring of local preference assignments also help network teams quickly troubleshoot routing anomalies and adjust traffic patterns proactively.
Question 65
Which EIGRP feature prevents routing loops on a multi-access network?
A) Split horizon
B) Passive interface
C) Poison reverse
D) DUAL
Answer: A
Explanation:
Split horizon is an EIGRP and distance-vector protocol feature designed to prevent routing loops on multi-access networks. It works by preventing a router from advertising a route back out of the interface from which it was learned. This simple rule ensures that updates do not create loops, which can otherwise destabilize the network.
Option A) is correct because split horizon inherently reduces the chance of routing loops, especially on Ethernet or broadcast networks where multiple routers can see the same updates. It is a fundamental technique in distance-vector protocols.
Option B) passive interface prevents EIGRP neighbor formation on an interface but does not prevent loops for existing neighbors.
Option C) poison reverse explicitly advertises a route as unreachable to break loops, but split horizon is more efficient as it stops the problem preemptively.
Option D) DUAL ensures loop-free routing and rapid convergence but relies on feasible successors rather than interface-based filtering.
Split horizon is critical in enterprise networks with multiple routers on the same segment. By avoiding advertisement of routes back on the incoming interface, it minimizes unnecessary traffic and prevents loops, which could lead to routing table corruption, CPU overload, and network instability. Network engineers may disable split horizon in certain scenarios, such as with frame relay hub-and-spoke topologies, where traffic must be advertised back to a central hub. However, understanding the implications of disabling split horizon is essential to prevent loops. Combined with poison reverse and DUAL, split horizon ensures highly reliable, scalable, and loop-free network topologies, allowing EIGRP to provide rapid convergence and redundancy in multi-site enterprise deployments. Proper planning and monitoring of interface roles and update propagation are essential to maintaining a stable EIGRP environment and ensuring predictable network behavior.
Question 66
Which OSPF area type blocks Type 5 LSAs?
A) Backbone area
B) Stub area
C) Totally stubby area
D) Not-so-stubby area
Answer: B
Explanation:
In OSPF, stub areas are designed to reduce the size of the routing table and limit the number of LSAs flooded within the area. A stub area blocks Type 5 LSAs, which are external routes redistributed into OSPF from other protocols, thereby forcing routers inside the stub area to use a default route to reach external destinations. This reduces complexity, conserves router resources, and improves convergence times, particularly in large enterprise networks.
Option B) is correct because stub areas prevent the propagation of Type 5 LSAs into the area, allowing routers to maintain a smaller, more efficient routing table. The backbone area always accepts Type 5 LSAs, so A) is incorrect. Totally stubby areas, C), not only block Type 5 LSAs but also block Type 3 summary LSAs, except the default route, which is different from a regular stub area. Not-so-stubby areas, D), are special configurations where some Type 5 LSAs are allowed based on filtering rules, providing more flexibility but not fully blocking Type 5 LSAs.
The primary benefit of stub areas lies in resource optimization. By limiting the propagation of external LSAs, routers in stub areas require less CPU processing and memory to maintain their link-state database. This is particularly useful in branch offices or smaller segments of an enterprise network where external routes are less critical. Proper configuration requires the area to be explicitly designated as stub on all routers, and a default route is injected by the Area Border Router (ABR). Misconfigurations can cause routing failures if routers in the stub area cannot reach external networks or if ABRs do not correctly advertise the default route.
Administrators often choose stub areas in networks where stability, minimal overhead, and faster convergence are critical. This strategy is especially effective in multi-site enterprise deployments where branch offices connect to a central backbone. By strategically designing OSPF area types, engineers can control routing table growth, reduce LSA flooding, and improve overall network efficiency, ensuring that branch routers focus on internal traffic while relying on backbone routers for external reachability. The decision to implement stub areas should consider future scalability, external route requirements, and potential topology changes to prevent operational disruptions.
Question 67
Which EIGRP feature enables fast convergence during link failure?
A) Feasible successor
B) Split horizon
C) Poison reverse
D) Passive interface
Answer: A
Explanation:
A feasible successor is a critical feature of EIGRP that enables rapid convergence when a primary route fails. EIGRP maintains a feasible successor list in addition to the primary successor route. These feasible successors are backup routes that satisfy the feasibility condition: their reported distance to a destination must be less than the feasible distance of the primary route. When the primary route fails, EIGRP can immediately switch to a feasible successor without waiting for the entire network to recalculate routes, minimizing downtime and maintaining network stability.
Option A) is correct because feasible successors provide loop-free backup paths, allowing instantaneous failover, which is a key reason EIGRP is considered a fast-converging distance-vector protocol. Split horizon, B), prevents routing loops but does not provide immediate backup paths. Poison reverse, C), informs neighbors of an unreachable route but does not enable immediate failover. Passive interface, D), prevents neighbor formation on an interface but has no impact on convergence speed.
Feasible successors are especially beneficial in multi-access networks where redundancy and high availability are essential. When a primary path fails, EIGRP immediately promotes a feasible successor to the successor role and recalculates new feasible successors if necessary. This mechanism significantly reduces network downtime, particularly for enterprise environments with mission-critical applications. Network engineers often design EIGRP topologies with multiple feasible successors on strategic links to ensure resilience, redundancy, and predictable network behavior.
Moreover, understanding the feasibility condition is crucial. Only routes that meet the condition can be chosen as feasible successors, guaranteeing loop-free backups. If no feasible successor exists, EIGRP triggers a diffusing computation algorithm (DUAL) to calculate a new route, which may take longer. By carefully tuning metrics, administrators can influence which paths qualify as feasible successors, ensuring optimal path selection and performance. Feasible successors, combined with DUAL, allow EIGRP to achieve rapid convergence, stable routing, and minimal disruption, making it highly suitable for enterprise networks requiring high availability and fast failover capabilities. Proper monitoring of feasible successor availability and metrics ensures the network remains resilient even under dynamic topology changes or link failures.
Question 68
Which BGP message type establishes neighbor relationships?
A) OPEN
B) UPDATE
C) NOTIFICATION
D) KEEPALIVE
Answer: A
Explanation:
The OPEN message in BGP is the initial message exchanged between peers to establish a neighbor relationship. It carries essential information, including BGP version, AS number, hold time, and BGP identifier. Upon successful negotiation, peers exchange KEEPALIVE messages to maintain the session. The OPEN message ensures both routers agree on protocol parameters before exchanging routing information.
Option A) is correct because without the OPEN message, BGP neighbors cannot form a session or exchange routes. UPDATE messages, B), are used for exchanging route advertisements, NOTIFICATION messages, C), communicate errors or session termination, and KEEPALIVE messages, D), maintain the session after it is established.
BGP sessions are critical in multi-AS and enterprise WAN environments. The OPEN message plays a foundational role, ensuring compatibility and proper neighbor configuration. Each BGP peer verifies parameters such as AS numbers to confirm whether the peer belongs to the same iBGP or external eBGP session. Misconfigurations can prevent session establishment, leading to unreachable networks and service disruptions. Proper sequencing of OPEN followed by KEEPALIVE ensures stable peering and reliable route propagation.
The importance of the OPEN message extends beyond simple session establishment. It enforces security, compatibility, and network policy compliance. Many operational issues in BGP networks arise from improper OPEN message negotiation, such as mismatched hold times or incorrect AS numbers. Administrators often enable logging of BGP OPEN messages to troubleshoot peering issues, verify session parameters, and ensure policy compliance. Understanding the BGP state machine, which begins with the OPEN message, is crucial for enterprise network design, fault isolation, and maintaining predictable routing behavior. OPEN messages, combined with UPDATE and KEEPALIVE, form the backbone of reliable BGP communications in large-scale deployments.
Question 69
Which OSPF feature allows route summarization between areas?
A) LSA Type 1
B) LSA Type 3
C) LSA Type 5
D) LSA Type 4
Answer: B
Explanation:
Type 3 LSAs in OSPF, also called summary LSAs, are used by Area Border Routers (ABRs) to summarize routes from one area into another. This reduces the size of the routing table in each area, conserves memory and CPU resources, and limits LSA flooding across the network. Route summarization is essential for scalability, particularly in large enterprise networks with multiple areas.
Option B) is correct because Type 3 LSAs efficiently carry summarized prefixes, allowing ABRs to advertise reachability to an entire area without propagating every individual subnet. Type 1 LSAs, A), describe router links within an area. Type 5 LSAs, C), describe external routes redistributed into OSPF. Type 4 LSAs, D), are used to describe routes to ASBRs for internal routing.
Route summarization improves network performance, scalability, and convergence. ABRs calculate summary addresses by aggregating multiple subnets into a single advertisement, reducing LSA count and simplifying the link-state database. This prevents routers in remote areas from maintaining detailed topology information that is unnecessary for routing decisions. By controlling summary boundaries, network engineers can influence routing behavior, optimize traffic flows, and enhance fault isolation.
Improper summarization can lead to routing loops, unreachable destinations, or suboptimal paths, so careful planning is required. Engineers often use discontiguous summarization to maintain logical groupings of networks while minimizing routing table growth. Additionally, Type 3 LSAs support default route injection into stub areas, further simplifying routing in branches. Understanding the role of ABRs, summary LSAs, and OSPF area design is crucial to building large-scale, resilient, and efficient enterprise networks that maintain high performance while minimizing unnecessary routing overhead.
Question 70
Which EIGRP parameter determines the maximum hop count?
A) Feasible distance
B) K-values
C) Maximum-paths
D) Hop count
Answer: D
Explanation:
The maximum hop count in EIGRP defines the longest path in terms of router hops that EIGRP will consider for routing to a destination. By default, the maximum hop count is 100, though it can be increased to 255 for very large networks. This parameter ensures that routes that are too far or impractical are ignored, preventing inefficient routing and potential loops.
Option D) is correct because it explicitly sets the limit for how many routers an EIGRP route can traverse. Feasible distance, A), determines route selection based on metric calculation. K-values, B), influence the composite metric formula, and maximum-paths, C), control load-balancing across multiple equal-cost routes.
Hop count is critical in large enterprise deployments where multiple intermediate routers exist between sites. Setting an appropriate maximum hop count ensures that distant, suboptimal paths are not selected, improving stability and predictability. Network engineers must balance hop count limits with network size and redundancy to avoid discarding valid paths. EIGRP’s combination of maximum hop count, feasible successors, and DUAL algorithm ensures loop-free, fast-converging, and reliable routing. Misconfigurations can result in unreachable networks or unexpected failovers, particularly in multi-site, WAN-connected enterprise environments. Proper documentation, monitoring, and adjustment of hop counts enable predictable routing behavior, efficient traffic distribution, and robust network performance, even in highly dynamic topologies.
Question 71
Which EIGRP metric component affects delay calculation?
A) Bandwidth
B) Delay
C) Reliability
D) Load
Answer: B
Explanation:
In EIGRP, the metric calculation is based on a composite formula that includes bandwidth, delay, reliability, load, and MTU. Among these components, delay directly contributes to the metric by reflecting the total interface delay along the path to a destination. Delay is measured in tens of microseconds and is cumulative across each hop in the path. By factoring in delay, EIGRP can evaluate not only the physical characteristics of a link but also the time it takes for packets to traverse the network, providing a more accurate representation of path efficiency.
Option B) is correct because the delay parameter is explicitly incorporated into the EIGRP metric formula:
Metric = [K1Bandwidth + (K2Bandwidth)/(256-Load) + K3*Delay]*256
Here, delay is multiplied by K3 (a configurable K-value) and contributes significantly to route selection, especially in networks where link speed is high but transmission delay is variable. Bandwidth, A), measures link speed but does not account for latency. Reliability, C), measures error rates over time. Load, D), reflects the current interface utilization.
The feasibility of a route in EIGRP depends not only on the successor route but also on feasible successors. Including delay in the metric ensures that paths with minimal delay are preferred over potentially longer or more congested paths, even if bandwidth is adequate. For example, in a WAN environment, a 1 Gbps MPLS link with high propagation delay may be less preferred than a 100 Mbps low-latency fiber link, and the delay metric accurately reflects this decision.
Network engineers must carefully consider delay values when designing enterprise networks. Interfaces with artificially high delay, misconfigured for testing or lab purposes, may unintentionally make certain paths less preferred, disrupting traffic flows. Conversely, accurate delay metrics help EIGRP choose paths that minimize latency-sensitive traffic for applications like VoIP, video conferencing, and financial transactions. Understanding the role of delay in the composite metric is critical for predictable routing, efficient failover, and optimal network performance, particularly in large-scale multi-site enterprise topologies. Correctly configuring delay ensures that EIGRP does not merely route on bandwidth but accounts for overall path efficiency, improving both convergence speed and network reliability.
Question 72
Which OSPF type describes an ASBR within an area?
A) Type 1
B) Type 2
C) Type 4
D) Type 5
Answer: C
Explanation:
In OSPF, a Type 4 LSA, or ASBR summary LSA, is used to advertise the location of an Autonomous System Boundary Router (ASBR) within the network. ASBRs are routers that redistribute routes from other protocols or external networks into OSPF. Type 4 LSAs are generated by Area Border Routers (ABRs) to inform internal routers about the presence and reachability of an ASBR located in another area, ensuring proper routing to external destinations.
Option C) is correct because Type 4 LSAs specifically describe ASBRs. Type 1 LSAs, A), describe router links within an area. Type 2 LSAs, B), describe networks in a broadcast or non-broadcast multi-access segment. Type 5 LSAs, D), describe external routes redistributed into OSPF but not the ASBR location itself.
The purpose of Type 4 LSAs is to maintain loop-free routing and efficient reachability to external networks. Routers within an OSPF area need to know how to reach ASBRs, not just the external destinations they advertise. Without Type 4 LSAs, internal routers may incorrectly route traffic destined for external networks, potentially causing blackholing or suboptimal paths.
Type 4 LSAs are particularly important in multi-area OSPF deployments. They allow routers in non-backbone areas to have awareness of ASBRs in other areas, facilitating proper redistribution and reachability. This LSA, combined with Type 3 (summary) and Type 5 (external) LSAs, provides a comprehensive mechanism for scalable and hierarchical routing, reducing flooding and simplifying link-state databases. Proper configuration ensures that traffic destined for external networks follows predictable, loop-free paths and optimizes performance. Misconfigurations, such as incorrect area assignments for ASBRs, can result in missing Type 4 LSAs, leading to failed routing to external networks. By understanding the role of Type 4 LSAs, network engineers can design highly available, efficient, and resilient OSPF topologies, especially in large-scale enterprise WAN deployments where external connectivity and route redistribution are critical.
Question 73
Which BGP path attribute prevents routing loops in AS?
A) MED
B) Local preference
C) AS-PATH
D) Next-hop
Answer: C
Explanation:
The AS-PATH attribute in BGP is fundamental for preventing routing loops across autonomous systems. Every BGP router prepends its AS number to the AS-PATH when advertising routes to external peers. When a BGP router receives a route advertisement, it examines the AS-PATH; if its own AS number is already present, it rejects the route, thus preventing loops. This mechanism is critical in multi-AS environments where routes traverse multiple networks before reaching their destination.
Option C) is correct because AS-PATH provides both loop prevention and path selection information. MED, A), is used to influence incoming traffic preference between multiple entry points. Local preference, B), controls outbound route preference within an AS. Next-hop, D), identifies the next-hop router for routing but does not prevent loops.
AS-PATH is integral to BGP’s policy-based routing. Network engineers use AS-PATH filtering and prepending techniques to influence path selection, distribute traffic efficiently, and maintain hierarchical control over route propagation. For example, prepending the AS multiple times on a particular path makes it less preferred to remote BGP peers, while shorter AS-PATHs are generally preferred. This provides a mechanism to shape traffic flows while ensuring loop-free interdomain routing.
Understanding AS-PATH is critical for enterprise WAN and internet edge design. In multi-provider scenarios, incorrect filtering or missing AS-PATH verification can cause routing loops, suboptimal paths, and convergence delays. Implementing AS-PATH inspection, combined with route maps and prefix lists, allows network engineers to maintain predictable routing behavior, control inbound/outbound traffic, and optimize performance. AS-PATH is especially important in redundant and multi-homed networks, ensuring that despite multiple connections to external networks, BGP consistently avoids loops while supporting high availability, resilience, and policy-driven traffic engineering. Proper AS-PATH configuration is a cornerstone of robust, scalable BGP deployments in enterprise and service provider environments.
Question 74
Which EIGRP feature balances traffic over unequal-cost paths?
A) Variance
B) Maximum-paths
C) Feasible successor
D) Split horizon
Answer: A
Explanation:
The variance command in EIGRP allows traffic to be load-balanced over unequal-cost paths. By default, EIGRP only uses equal-cost paths for routing, which may underutilize available network resources. The variance parameter multiplies the metric of the primary route, and any route whose metric is less than or equal to the result qualifies for inclusion in the routing table. This allows multiple routes with varying metrics to be actively used, improving bandwidth utilization and redundancy.
Option A) is correct because variance directly enables unequal-cost load balancing. Maximum-paths, B), controls the number of equal-cost paths used. Feasible successor, C), provides backup routes for fast failover but does not balance traffic. Split horizon, D), prevents routing loops in distance-vector protocols.
Variance is particularly useful in enterprise WAN networks with multiple links of different speeds or costs. For example, a primary MPLS link may be faster, while a secondary internet VPN may be slower. By setting an appropriate variance, EIGRP can send traffic over both paths proportionally, increasing efficiency and resilience. Network engineers must carefully configure variance to avoid routing loops; only routes meeting the feasibility condition are eligible.
Traffic balancing using variance also supports quality-of-service considerations, ensuring that latency-sensitive applications primarily use the fastest paths while lower-priority traffic can traverse secondary routes. Variance works hand-in-hand with feasible successors and DUAL to ensure that unequal-cost load balancing does not compromise network stability. By strategically applying variance, enterprises can achieve efficient bandwidth utilization, failover capability, and optimized network performance, even in complex multi-site topologies with heterogeneous link characteristics. Proper monitoring is essential to ensure that traffic distribution aligns with business priorities and avoids congestion or overutilization of slower links.
Question 75
Which OSPF feature reduces LSA flooding in large networks?
A) Stub area
B) Area summarization
C) Type 1 LSA
D) Type 5 LSA
Answer: B
Explanation:
Area summarization in OSPF reduces LSA flooding in large enterprise networks by aggregating multiple subnets into a single advertisement when passing routes between areas. This minimizes the number of LSAs that routers must process and store in their link-state database, reducing CPU and memory usage, and improving network convergence. Summarization is performed at Area Border Routers (ABRs) and is critical in hierarchical OSPF design for scalable enterprise deployments.
Option B) is correct because summarization limits the propagation of detailed routing information outside an area. Stub areas, A), block Type 5 LSAs but do not aggregate routes. Type 1 LSAs, C), describe router links within an area. Type 5 LSAs, D), describe external routes but are not directly related to reducing flooding within internal areas.
Area summarization provides multiple benefits, including optimized resource usage, faster convergence, and simplified troubleshooting. In large multi-area networks, without summarization, each router would maintain detailed routes for every subnet in other areas, creating unnecessary complexity. By using summarized prefixes, ABRs allow routers to maintain smaller link-state databases and focus on internal routing decisions while still reaching external areas efficiently.
Effective summarization requires careful planning to avoid creating routing black holes or suboptimal paths. Network engineers often analyze IP addressing schemes and subnet hierarchies to design summaries that align with organizational topology. Combining summarization with stub areas and route filtering allows enterprises to build hierarchical OSPF networks that scale to hundreds of routers without compromising stability, convergence time, or performance. Proper implementation ensures that LSA flooding is minimized, memory and CPU usage is optimized, and network operations remain predictable and resilient even as the enterprise grows or network topologies change. Summarization is thus a cornerstone of large-scale OSPF network design, balancing efficiency with maintainability and high availability.
Question 76
Which BGP attribute controls outbound traffic within an AS?
A) MED
B) Local preference
C) AS-PATH
D) Next-hop
Answer: B
Explanation:
The Local Preference (Local Pref) attribute in BGP is used to influence outbound traffic from within an Autonomous System (AS). It allows network engineers to assign preference values to routes advertised internally via iBGP peers, enabling a predictable routing choice for traffic leaving the AS. The higher the Local Preference value, the more preferred the route becomes.
Option B) is correct because Local Preference is propagated throughout the AS and affects the selection of the best path among multiple available routes to external destinations. MED, A), influences incoming traffic from neighboring ASes, not internal route selection. AS-PATH, C), is used primarily for loop prevention and path selection, while Next-hop, D), determines the IP address of the next router for forwarding packets.
Local Preference is critical in multi-homed enterprise networks connected to multiple ISPs. For example, if an enterprise has two providers, the administrator can set a higher Local Preference on routes learned from the preferred ISP so that outbound traffic is consistently routed through that ISP. This ensures optimized performance, predictable routing, and policy enforcement.
In addition, Local Preference simplifies traffic engineering within an AS without affecting external BGP neighbors. By adjusting Local Preference, network engineers can achieve load balancing, avoid congestion on certain paths, and maintain high availability. When combined with other BGP attributes, such as AS-PATH, MED, and communities, Local Preference provides a robust mechanism to enforce organizational routing policies while maintaining loop-free and resilient network operations. Proper implementation of Local Preference also allows for seamless failover between ISPs: if the primary path becomes unavailable, the lower-preference route automatically takes over, ensuring continuous connectivity.
In large-scale environments, understanding Local Preference is crucial for routing predictability and enterprise-wide policy consistency. Misconfigurations can lead to suboptimal routing, increased latency, or traffic congestion. Therefore, Local Preference is a fundamental tool for enterprise network architects designing multi-homed, redundant, and high-performance WAN topologies.
Question 77
Which EIGRP mechanism prevents routing loops?
A) DUAL
B) Split horizon
C) Hold-down timer
D) Poison reverse
Answer: A
Explanation:
DUAL (Diffusing Update Algorithm) is the core mechanism in EIGRP responsible for loop-free routing and fast convergence. DUAL calculates the best path to each destination and maintains feasible successors as backup paths, ensuring that if the primary path fails, traffic can immediately reroute without creating loops. DUAL uses feasible distance (FD) and reported distance (RD) to evaluate the loop-free condition.
Option A) is correct because DUAL provides the mathematical foundation for EIGRP’s loop prevention. Split horizon, B), prevents routing information from being advertised back out of the interface from which it was learned, while Hold-down timers, C), delay route acceptance but do not inherently prevent loops. Poison reverse, D), advertises an infinite metric back to the origin router, also mitigating loops, but DUAL is the underlying algorithm that ensures loop-free path computation.
EIGRP calculates the feasibility condition: a neighbor’s reported distance must be less than the router’s feasible distance for a route to qualify as a feasible successor. Only feasible successors are eligible for immediate failover, which guarantees that backup paths will not form loops. This mathematical approach differentiates EIGRP from traditional distance-vector protocols like RIP, which rely solely on timers and hop counts.
In large enterprise networks, DUAL ensures high availability and rapid convergence. When a link fails, routers using DUAL send queries to neighbors to locate an alternate path. If a feasible successor exists, it immediately becomes the successor route; otherwise, DUAL continues the search while maintaining loop-free operation. This enables EIGRP to scale effectively across WAN environments, multiple redundant paths, and complex topologies, providing predictable, stable routing without the risk of loops.
DUAL’s use of successors, feasible successors, and the diffusing update process also reduces unnecessary route flaps and network instability. Network engineers leveraging DUAL can implement complex traffic engineering strategies, unequal-cost load balancing, and hierarchical routing while ensuring reliable, loop-free network operation. Understanding DUAL is essential for anyone preparing for the ENARSI exam and for designing robust, high-performance enterprise networks.
Question 78
Which OSPF type is used for external routes in non-backbone areas?
A) Type 3
B) Type 4
C) Type 5
D) Type 7
Answer: C
Explanation:
Type 5 LSAs in OSPF are used to advertise external routes (routes learned from other protocols or external ASes) into OSPF areas that are not the backbone (Area 0). These LSAs are generated by Autonomous System Boundary Routers (ASBRs) and flooded throughout the OSPF network except into stub areas, which block Type 5 LSAs.
Option C) is correct because Type 5 LSAs describe external destinations. Type 3 LSAs, A), summarize networks between areas. Type 4 LSAs, B), describe ASBR locations. Type 7 LSAs, D), are used within not-so-stubby areas (NSSA) to carry external routes into OSPF.
Type 5 LSAs are crucial in enterprise networks that redistribute routes from protocols such as BGP, RIP, or static routes into OSPF. They allow internal OSPF routers to reach destinations outside the OSPF autonomous system while maintaining hierarchical routing. Without Type 5 LSAs, traffic destined for external networks would not be visible, causing connectivity failures.
When designing multi-area OSPF topologies, careful attention is required to LSA filtering and summarization. Type 5 LSAs can increase link-state database size and flooding overhead, so they are often combined with area summarization and stub area configurations to reduce processing load on routers and improve convergence times. For example, summarizing external prefixes at ABRs reduces the number of Type 5 LSAs flooded into other areas, optimizing memory and CPU usage on internal routers.
In networks with NSSAs, Type 7 LSAs can be translated into Type 5 at ABRs, ensuring external reachability while still limiting flooding in stub-like areas. By leveraging Type 5 LSAs effectively, enterprise engineers achieve scalable OSPF deployments, predictable routing, and controlled LSA propagation, which are essential for maintaining performance, stability, and resilience in large-scale enterprise WAN environments.
Question 79
Which BGP attribute is used for path selection between multiple ASes?
A) MED
B) Local preference
C) AS-PATH
D) Next-hop
Answer: A
Explanation:
The Multi-Exit Discriminator (MED) in BGP is used to influence inbound traffic from neighboring Autonomous Systems. MED indicates a preferred path when multiple entry points exist between two ASes. Unlike Local Preference, which affects internal outbound traffic, MED communicates preference to external neighbors, allowing them to select the route the originating AS prefers for ingress traffic.
Option A) is correct because MED explicitly signals the desired entry point for external BGP neighbors. Local Preference, B), only affects internal iBGP route selection. AS-PATH, C), is for loop prevention and path length evaluation. Next-hop, D), identifies the next router for forwarding packets but does not convey path preference between ASes.
MED is particularly useful in multi-homed enterprise networks connected to multiple ISPs. By advertising a lower MED for a preferred link, the AS signals to neighbors which path should be used first, allowing for optimized traffic flow, balanced utilization, and predictable routing. MED is non-transitive by default, meaning it is only compared by the immediate neighboring AS and is not propagated further across other ASes unless explicitly configured.
Effective MED configuration helps enterprise engineers implement ingress traffic engineering, balancing loads across multiple WAN links while avoiding congestion. It also supports redundancy and failover by automatically allowing alternative entry points if the preferred link becomes unavailable. In addition, MED can be combined with route filtering, communities, and weight attributes to enforce granular control over inbound traffic policies. Misconfigurations can lead to suboptimal routing, asymmetric traffic flows, or congestion at certain entry points, making understanding MED critical for robust, scalable BGP designs in enterprise and multi-provider environments.
Question 80
Which EIGRP feature maintains backup routes for fast convergence?
A) Feasible successor
B) Successor
C) Variance
D) Maximum-paths
Answer: A
Explanation:
A feasible successor in EIGRP is a backup route that meets the feasibility condition, ensuring it is loop-free and can immediately replace the primary path if the successor fails. EIGRP maintains these feasible successors in the topology table, allowing for near-instantaneous convergence without needing to query other routers for alternative paths.
Option A) is correct because feasible successors provide redundancy and fast failover. Successor, B), is the primary route currently installed in the routing table. Variance, C), enables unequal-cost load balancing but does not inherently provide backup routes. Maximum-paths, D), sets the limit for equal-cost paths but does not guarantee loop-free backups.
The feasibility condition requires that the reported distance from a neighbor must be less than the feasible distance of the current successor. This ensures that feasible successors are always loop-free. By maintaining multiple feasible successors, EIGRP achieves high availability and rapid convergence, critical in enterprise WAN topologies with redundant links or diverse routing paths.
Feasible successors are particularly important in networks with variable link costs, WAN connections, or high availability requirements. They prevent downtime during link or router failures by providing ready-to-use backup routes. Additionally, when combined with variance, feasible successors can participate in unequal-cost load balancing, optimizing bandwidth usage while preserving loop-free operation. Understanding feasible successors is essential for ENARSI exam preparation and real-world network design, as it underpins EIGRP’s reliability, redundancy, and fast convergence, ensuring minimal disruption during failures and predictable routing behavior in complex enterprise environments.
