Cisco 300-410  Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam  Dumps and Practice Test Questions Set 3 Q41-60

Visit here for our full Cisco 300-410 exam dumps and practice test questions.

Question 41:

Which EIGRP metric component is used to reflect the delay of a route in the path selection process?

A) Bandwidth
B) Delay
C) Reliability
D) Load

Answer: B) Delay

Explanation:

A) Bandwidth is a key metric in EIGRP used to reflect the minimum bandwidth along a path. While it plays a significant role in the composite metric calculation, it primarily represents the speed of the slowest link in a route. Bandwidth is crucial for path selection, but it does not capture the time it takes for packets to traverse the route. High bandwidth does not necessarily imply a low-delay path, so bandwidth alone cannot be used to account for propagation and queuing delays along the path.

B) Delay is the EIGRP metric component that measures the cumulative interface delay along the path to a destination. Each interface contributes its configured delay, which is added to calculate the total route delay. This metric captures both propagation and queuing delays, making it a critical factor in determining the most efficient route. EIGRP uses the formula: composite metric = (K1 * Bandwidth + K3 * Delay) * 256, by default K1 and K3 are set to 1, emphasizing bandwidth and delay as key contributors. Delay is therefore the correct answer because it directly quantifies the time a packet requires to travel through a path, allowing EIGRP to select routes that minimize latency, not just maximize throughput.

C) Reliability reflects the historical error rate of an interface and is used optionally in the EIGRP composite metric. While reliability provides insight into path stability, it does not represent the time delay experienced along a route. Low reliability might discourage selection of a route, but it does not indicate transmission speed or latency, so it is not the metric component used to represent delay.

D) Load indicates the current utilization of an interface, providing information about congestion levels. High load may temporarily reduce performance, but it does not provide a direct measure of the path’s inherent delay. Load can fluctuate frequently, so it is not a reliable indicator for the path delay calculation used in EIGRP metric computations.

Delay is the correct answer because it explicitly measures the cumulative time for packets to traverse a route, which is essential in EIGRP’s path selection alongside bandwidth. Bandwidth, reliability, and load are supporting metrics but do not directly capture the delay.

Question 42:

Which OSPFv3 LSA type is used by an ABR to advertise networks from one area to another?

A) Type 1
B) Type 2
C) Type 3
D) Type 7

Answer: C) Type 3

Explanation:

A) Type 1 LSAs, known as router LSAs, describe the state of a router’s links and interfaces within an area. They do not propagate across areas and therefore cannot be used by an ABR to summarize or advertise networks into another area. Type 1 LSAs are internal to each area, serving to build the intra-area topology.

B) Type 2 LSAs, or network LSAs, are generated by the designated router on broadcast or non-broadcast multi-access networks. They describe all routers on the segment and exist only within the area, providing visibility to all routers on that segment. They are not designed for inter-area route advertisement, making them unsuitable for ABR summaries.

C) Type 3 LSAs, or summary LSAs, are explicitly used by ABRs to advertise networks from one area into another. When an ABR receives Type 1 and Type 2 LSAs from one area, it generates Type 3 LSAs containing summarized routes or network prefixes for other areas. Type 3 LSAs allow inter-area communication while maintaining area boundaries, preventing full flooding of intra-area LSAs. This summarization reduces LSA overhead, minimizes routing table size, and ensures scalable OSPFv3 operation across multiple areas. By advertising summary LSAs instead of individual intra-area LSAs, ABRs maintain hierarchical efficiency and optimize SPF calculations. This makes Type 3 LSAs the correct answer.

D) Type 7 LSAs are used in Not-So-Stubby Areas (NSSA) to carry external routes introduced into the NSSA. They are converted to Type 5 LSAs by ABRs before propagating to other areas. Type 7 LSAs are not used for regular inter-area network advertisement, so they are not the correct choice in this context.

Type 3 LSAs are the correct answer because they are the mechanism for ABRs to summarize and propagate routes between areas, supporting hierarchical OSPFv3 deployment. Type 1, Type 2, and Type 7 LSAs serve other functions and do not achieve inter-area advertisement.

 

Question 43:

Which BGP attribute is used to control inbound traffic from external autonomous systems?

A) Weight
B) Local Preference
C) AS Path
D) MED

Answer: D) MED

Explanation:

A) Weight is a Cisco-specific BGP attribute that only affects outbound path selection locally on a router. It does not influence other routers within the AS or external neighbors. Weight is therefore irrelevant when the goal is to control inbound traffic from outside autonomous systems.

B) Local Preference is used to influence outbound path selection within an autonomous system. By adjusting local preference, routers within the AS prioritize certain exit points, but this attribute is not propagated to external neighbors and does not affect inbound traffic.

C) AS Path is primarily used for loop prevention and for selecting the shortest AS path when multiple routes exist. While manipulating AS path length via prepending can indirectly influence inbound traffic, it is not a granular method for controlling inbound routing. AS path manipulation can be unpredictable and is a less reliable mechanism compared to MED.

D) MED (Multi-Exit Discriminator) is designed to influence how neighboring autonomous systems select a preferred entry point into your AS. By advertising a lower MED value for a preferred path and higher MEDs for less desirable paths, the AS informs external neighbors which route to prefer. MED is a tool for inbound traffic engineering and is communicated between autonomous systems. This attribute allows network engineers to control traffic entering their network from specific peers, making MED the correct answer.

MED is the correct answer because it provides a standardized, predictable mechanism to influence inbound path selection. Weight, local preference, and AS path affect either local outbound routing or internal decisions, not external neighbor behavior.

Question 44:

Which HSRP state indicates a router is actively forwarding traffic for the virtual IP?

A) Active
B) Standby
C) Listen
D) Init

Answer: A) Active

Explanation:

A) Active is the HSRP state in which a router is currently forwarding traffic for the virtual IP address. The router responds to ARP requests for the VIP and handles all routing for devices using the HSRP gateway. It is the operational state where the router is performing the primary forwarding function, making it the correct answer.

B) Standby is the state of a router that is ready to take over if the active router fails. While it monitors the active router, it does not forward traffic unless a failover occurs. Standby ensures redundancy but is not actively forwarding traffic, so it does not match the description in the question.

C) Listen is the state where a router is initialized and aware of HSRP, but it neither forwards traffic nor participates in active election. Routers in listen state are passive and preparing to transition to standby or active, making it unsuitable for this scenario.

D) Init is the initial state of HSRP when the router has not yet heard hello messages from neighbors. It is the startup phase and does not involve traffic forwarding or participation in election.

Active is correct because it represents the router currently responsible for forwarding packets to the virtual IP. Standby monitors, listen prepares, and init initializes, but only active performs the actual forwarding function.

Question 45:

 Which OSPF feature is used to reduce LSA flooding by limiting the advertisement of external routes into an area?

A) Stub Area
B) ABR Summary
C) Type 1 LSA
D) Backbone Area

Answer: A) Stub Area

Explanation:

A) Stub areas are OSPF areas designed to reduce routing overhead by blocking external routes (Type 5 LSAs) while allowing internal and inter-area routes (Type 3 LSAs). In a stub area, the ABR injects a default route so that routers can reach external destinations without having full visibility of all external prefixes. This significantly reduces LSA flooding, memory usage, and SPF calculation time within the area. Stub areas are ideal for edge areas where external routing information is not critical, making this the correct answer.

B) ABR Summary is used to summarize routes from one area into another, reducing routing table size and inter-area LSA flooding. While ABR summary optimizes hierarchical OSPF operation, it does not specifically block external routes within an area. Summarization controls inter-area propagation, whereas stub areas control external LSA flooding.

C) Type 1 LSAs describe the router’s own links and interfaces within an area. They do not control the propagation of external routes. Type 1 LSAs are essential for intra-area topology but do not reduce flooding of external LSAs.

D) Backbone Area (Area 0) is the central hub of OSPF and carries all types of LSAs, including external routes. It does not limit the flooding of external LSAs; in fact, it is responsible for distributing LSAs between areas.

Stub area is correct because it is the feature explicitly designed to limit the advertisement of external routes (Type 5 LSAs) into an area, reducing LSA flooding. ABR summary and LSAs have supporting roles but do not perform the same function, and the backbone area allows all LSAs.

Question 46:

Which EIGRP feature allows a router to advertise a summarized route to neighbors instead of multiple specific prefixes?

A) Route Summarization
B) Split Horizon
C) Feasible Successor
D) Hold Timer

Answer: A) Route Summarization

Explanation:

A) Route Summarization in EIGRP is used to aggregate multiple contiguous subnets into a single advertisement. This reduces the size of routing tables on neighboring routers and limits the number of updates propagated throughout the network. Summarization is often configured on interfaces connecting different areas or autonomous systems to optimize bandwidth and convergence efficiency. By summarizing routes, a network can prevent the unnecessary exposure of detailed internal prefixes to other routers while maintaining overall reachability. Summarization also decreases CPU and memory utilization by reducing the number of entries in routing tables and the number of updates that routers must process. This is particularly important in large-scale enterprise networks where hundreds or thousands of subnets exist. This makes route summarization the correct answer.

B) Split Horizon is a mechanism that prevents a router from advertising a route back out the interface from which it was learned. Split horizon is essential for preventing routing loops in distance-vector protocols, including EIGRP. While it affects route advertisement, it does not aggregate multiple routes into a summarized prefix. Split horizon only controls the direction of advertisement, not the granularity or summarization of routes.

C) Feasible Successor is a backup route maintained in EIGRP’s topology table to enable fast failover in case the primary successor fails. Feasible successors are precomputed loop-free backup routes, but they do not perform route summarization. Their function is to improve convergence and reliability, not reduce routing table size or advertise aggregated prefixes.

D) Hold Timer defines how long a router waits before declaring a neighbor down when no hello packets are received. While the hold timer affects convergence time and neighbor state detection, it has no impact on route summarization or advertisement.

Route Summarization is correct because it allows EIGRP to reduce routing table size and advertisement volume by sending a single summarized route instead of multiple specific prefixes. Split horizon, feasible successor, and hold timer serve entirely different functions.

Question 47:

Which OSPF area type allows external routes using Type 7 LSAs while still blocking Type 5 LSAs?

A) Stub Area
B) Totally Stubby Area
C) NSSA
D) Backbone Area

Answer: C) NSSA

Explanation:

A) Stub areas block Type 5 LSAs (external routes) to reduce routing table size but allow inter-area routes (Type 3 LSAs). Stub areas cannot carry external routes at all, so they are unsuitable for situations where external connectivity from within the area is required.

B) Totally Stubby Areas block both Type 5 and Type 3 LSAs, leaving only a default route for external and inter-area communication. Totally stubby areas do not allow external route advertisement, making them inappropriate for networks that need to inject external routes.

C) NSSA (Not-So-Stubby Area) is designed to combine the benefits of a stub area while allowing the injection of external routes via Type 7 LSAs. An NSSA allows routers within the area to redistribute external routes (e.g., from another routing protocol like EIGRP or BGP) into OSPF without flooding Type 5 LSAs throughout the OSPF domain. The ABR eventually converts Type 7 LSAs into Type 5 LSAs to propagate them outside the NSSA. This capability allows for controlled external route distribution while minimizing LSA flooding. NSSA is particularly useful at the edge of OSPF networks where stub-like behavior is desired but certain external routes must be introduced, making it the correct answer.

D) Backbone Area (Area 0) is the central area of OSPF and does not block LSAs. All types of LSAs, including Type 5 for external routes, are flooded through the backbone. Backbone areas are designed to interconnect multiple areas, not to provide stub or restricted behavior.

NSSA is correct because it allows external routes using Type 7 LSAs while blocking general Type 5 LSAs, providing controlled injection of external information without overloading the network with external LSAs. Stub and totally stubby areas block external LSAs entirely, and the backbone area does not restrict any LSAs.

Question 48:

Which BGP attribute is used to influence the preferred path within an autonomous system and is propagated to all routers in the AS?

A) Weight
B) Local Preference
C) MED
D) AS Path

Answer: B) Local Preference

Explanation:

A) Weight is a Cisco-specific BGP attribute used to influence path selection locally on a single router. It is not propagated to other routers within the AS and therefore cannot enforce a preferred path across the autonomous system. Weight is applied only locally, making it unsuitable for AS-wide path preference.

B) Local Preference is a well-known BGP attribute propagated to all routers within an autonomous system. By assigning higher local preference to certain routes, network engineers can control which exit points are preferred for outbound traffic throughout the AS. Local preference is essential in multi-homed environments to ensure consistent routing policy across all routers. It provides a predictable, network-wide mechanism for managing path selection and traffic engineering, making it the correct answer.

C) MED (Multi-Exit Discriminator) is used to influence how external autonomous systems select entry points into your AS. While it affects inbound traffic from neighbors, MED is not propagated internally to control path selection within the AS. It cannot enforce AS-wide routing preferences.

D) AS Path records the autonomous systems a route traverses. While AS path length affects BGP best path selection, it is primarily used for loop prevention and inter-AS path selection. It is not used for controlled internal routing preference, so it cannot replace local preference for internal path selection.

Local Preference is correct because it is designed to propagate throughout the AS and enforce consistent internal path preference. Weight, MED, and AS Path affect local or external routing decisions but cannot provide AS-wide preference control.

Question 49:

Which MPLS mechanism assigns a label to packets to enable forwarding along predetermined paths?

A) LDP
B) RSVP-TE
C) VRF
D) MPLS Forwarding Table

Answer: A) LDP

Explanation:

A) LDP (Label Distribution Protocol) is the protocol used in MPLS networks to assign labels to packets based on Forwarding Equivalence Classes (FECs). LDP establishes label-switched paths between routers, ensuring that packets are forwarded according to predetermined labels rather than IP routing. LDP automatically distributes label bindings between MPLS-enabled routers, enabling efficient, scalable forwarding without manual label configuration. By assigning labels, LDP allows routers to switch packets at layer 2.5 with minimal processing, providing predictable paths through the MPLS backbone. This makes LDP the correct answer for label assignment and basic MPLS forwarding.

B) RSVP-TE is used in MPLS Traffic Engineering to create explicit label-switched paths with bandwidth reservation. While RSVP-TE can assign labels for traffic-engineered paths, it is primarily used for TE purposes rather than standard MPLS label distribution. LDP is the default protocol for label assignment in non-TE MPLS networks.

C) VRF (Virtual Routing and Forwarding) provides logical separation of routing tables for multiple VPNs. VRFs ensure traffic isolation but do not assign labels to enable packet forwarding along MPLS paths. Labels may be used in conjunction with VRFs, but VRF alone does not perform label assignment.

D) MPLS Forwarding Table is a data-plane table mapping labels to outgoing interfaces and next-hop labels. While it is essential for forwarding, it does not perform label assignment. The forwarding table relies on LDP or RSVP-TE to populate label bindings.

LDP is correct because it is the primary mechanism for assigning labels in MPLS, enabling efficient forwarding. RSVP-TE, VRF, and the forwarding table support MPLS operations but do not perform automatic label assignment for standard traffic.

 

Question 50:

 Which OSPF feature allows hierarchical routing by summarizing multiple networks from one area into another?

A) ABR Summary
B) Stub Area
C) Type 1 LSA
D) Type 2 LSA

Answer: A) ABR Summary

Explanation:

A) ABR Summary is the process by which Area Border Routers (ABRs) summarize multiple network prefixes from one area into a single Type 3 LSA for advertisement into another area. This reduces the number of routes that need to be stored in the routing table, limits LSA flooding, and optimizes SPF calculation performance. ABR summarization is essential in large OSPF networks to maintain scalability, reduce CPU load, and improve convergence efficiency. By aggregating prefixes, ABRs prevent unnecessary detail from propagating to other areas, allowing routers in other areas to maintain smaller routing tables and process fewer LSAs. This hierarchical summarization ensures OSPF can scale to large enterprise environments.

B) Stub Area is used to block Type 5 LSAs and reduce routing overhead but does not perform inter-area summarization. Stub areas reduce flooding of external routes but do not aggregate multiple prefixes for advertisement between areas.

C) Type 1 LSAs describe the router’s links and interfaces within an area. They do not provide inter-area summarization; they are strictly intra-area LSAs used for building the area topology.

D) Type 2 LSAs are network LSAs generated by a DR on broadcast or NBMA segments. They advertise all attached routers in the segment within an area. Type 2 LSAs exist only within an area and are not used for inter-area summarization.

ABR Summary is correct because it enables hierarchical routing by aggregating prefixes between areas, reducing LSA flooding and routing table size. Stub areas and LSAs serve supporting roles but do not perform inter-area summarization.

Question 51:

Which EIGRP feature ensures that backup routes are immediately available when the primary route fails?

A) Feasible Successor
B) Split Horizon
C) Route Summarization
D) Hold Timer

Answer: A) Feasible Successor

Explanation:

A) Feasible Successor is a core feature of EIGRP that ensures high availability and rapid convergence. EIGRP maintains a topology table containing all known routes to a destination, not just the primary route (successor). Feasible successors are backup routes that satisfy the feasibility condition, which states that a neighbor’s reported distance to a destination must be less than the current feasible distance from the router itself. This ensures loop-free backup paths. When the primary route fails, the router can immediately switch to a feasible successor without recalculating the entire topology or waiting for updates from neighbors. This results in near-instantaneous failover, minimizing packet loss and downtime. Feasible successors are therefore critical in enterprise networks where uptime and performance are essential.

B) Split Horizon is a loop prevention mechanism that stops a router from advertising a route back out the interface from which it was learned. While split horizon is important for preventing routing loops, it does not provide backup routes or improve convergence time when a primary route fails. Split horizon is a preventative mechanism, not a failover solution.

C) Route Summarization reduces the number of advertised routes by combining multiple prefixes into a single aggregate route. While it reduces routing table size and update overhead, summarization does not provide backup routes for immediate use if the primary route fails. Its primary benefit is scalability and efficiency, not high availability.

D) Hold Timer defines the interval a router waits before declaring a neighbor down if no hello packets are received. Although hold timers affect convergence time, they do not provide precomputed backup routes. Relying solely on hold timers would delay failover until the neighbor is declared down, which is much slower than using feasible successors.

Feasible Successor is the correct answer because it ensures immediate availability of backup routes when the primary path fails. Split horizon, route summarization, and hold timer support network stability and efficiency but do not provide immediate failover capability.

Question 52:

Which OSPF feature allows inter-area route summarization to reduce routing table size?

A) Stub Area
B) ABR Summary
C) Type 1 LSA
D) Type 2 LSA

Answer: B) ABR Summary

Explanation:

A) Stub areas block Type 5 LSAs to reduce flooding of external routes within an area but do not summarize multiple network prefixes from one area into another. Stub areas optimize routing table size by limiting external routes, but they do not provide inter-area summarization.

B) ABR Summary is the process where Area Border Routers (ABRs) generate summary LSAs (Type 3) to advertise multiple network prefixes from one area into another as a single aggregated route. This reduces the number of routes that need to be stored in routers outside the originating area, minimizing routing table size and improving SPF calculation efficiency. ABR summarization ensures hierarchical OSPF design, enhances scalability, and reduces LSA flooding, making it the correct answer. It is particularly important in large enterprise networks with numerous subnets, as it prevents excessive routing table growth and conserves CPU and memory resources.

C) Type 1 LSAs, also known as router LSAs, describe a router’s interfaces and the state of its links within an area. Type 1 LSAs are essential for building the area topology but are strictly intra-area. They do not summarize routes across areas and cannot reduce inter-area routing table entries.

D) Type 2 LSAs, or network LSAs, are generated by the designated router on broadcast or NBMA networks. They describe all routers on a segment but exist only within the area. Type 2 LSAs do not provide inter-area summarization or reduce routing table size outside the area.

ABR Summary is correct because it allows hierarchical OSPF networks to scale efficiently by aggregating routes between areas. Stub areas, Type 1 LSAs, and Type 2 LSAs do not perform inter-area summarization and serve other routing purposes.

Question 53:

Which BGP attribute is used to influence path selection for inbound traffic from external autonomous systems?

A) Weight
B) Local Preference
C) AS Path
D) MED

Answer: D) MED

Explanation:

A) Weight is a Cisco-specific BGP attribute that influences local outbound path selection on a single router. It is not propagated to other routers in the AS and therefore cannot influence inbound traffic from external autonomous systems.

B) Local Preference is propagated throughout the AS to influence outbound path selection for all routers within the autonomous system. While effective for controlling exit points, it does not affect how external neighbors choose to send traffic into your AS. Local Preference is purely internal to the AS.

C) AS Path is used primarily for loop prevention and for selecting the shortest AS path among multiple routes. Although AS path prepending can indirectly influence inbound traffic by making a path appear longer, it is less granular and predictable than MED. AS path manipulation is also global in effect and not as precise for traffic engineering.

D) MED (Multi-Exit Discriminator) is specifically designed to influence how external neighbors select a preferred entry point into your autonomous system. By advertising a lower MED for a preferred entry point and higher MEDs for alternative paths, an AS can guide inbound traffic in a predictable manner. MED is sent to neighboring autonomous systems and helps control which external routes are preferred without affecting internal BGP path selection. This makes MED the correct answer.

MED is correct because it is the standardized BGP attribute used to manage inbound traffic from external ASes. Weight, local preference, and AS path either influence local decisions or are less precise for controlling inbound traffic.

Question 54:

Which HSRP state indicates a router is ready to take over if the active router fails but is not currently forwarding traffic?

A) Active
B) Standby
C) Listen
D) Init

Answer: B) Standby

Explanation:

A) Active state indicates that a router is currently forwarding traffic for the virtual IP. It responds to ARP requests for the VIP and handles the primary routing role. Active routers are engaged in actual packet forwarding, so this does not match the description of a backup-ready router.

B) Standby state represents a router that is prepared to take over if the active router fails. While in standby, the router monitors hello messages from the active router but does not forward traffic unless failover occurs. The standby router participates in the HSRP election process and maintains state information, ready to assume the active role immediately if necessary. This makes standby the correct answer.

C) Listen state indicates a router is aware of the HSRP group and receives hello messages, but it is not forwarding traffic and is not actively participating in election. Listen is a passive state preceding standby or active, so it does not match the description in the question.

D) Init is the initial state when HSRP has started but the router has not yet received hello messages from neighbors. Init routers are initializing and not ready to take over or forward traffic.

Standby is correct because it describes a router ready to assume the active role in case of failure. Active forwards traffic, listen is passive, and init is preliminary.

Question 55:

 Which MPLS feature ensures multiple VPNs can share the same infrastructure while keeping traffic logically separate?

A) VRF
B) LDP
C) RSVP-TE
D) QoS

Answer: A) VRF

Explanation:

A) VRF (Virtual Routing and Forwarding) allows multiple routing tables to coexist on the same physical device, providing complete separation of customer or VPN traffic. Each VRF maintains its own interfaces, routing information, and forwarding table. When combined with MPLS labels, VRF ensures that traffic from one VPN cannot mix with traffic from another, even on a shared backbone. This is the fundamental mechanism for providing isolated Layer 3 VPN services in service provider networks. VRF allows overlapping IP addresses across different VPNs while maintaining strict logical separation, making it the correct answer.

B) LDP (Label Distribution Protocol) assigns labels to Forwarding Equivalence Classes (FECs) to enable MPLS forwarding. While essential for MPLS operation, LDP does not provide logical separation of multiple VPNs. LDP assigns labels for routing but does not maintain separate routing tables for different customers.

C) RSVP-TE (Resource Reservation Protocol – Traffic Engineering) is used to establish explicit label-switched paths with bandwidth reservation and QoS. It optimizes traffic engineering but does not inherently isolate VPN traffic or maintain multiple routing tables.

D) QoS manages traffic prioritization, bandwidth allocation, and delay, but it does not provide logical separation of traffic between different VPNs. QoS ensures performance but cannot segregate routes or forwarding between VPNs.

VRF is correct because it enables multiple VPNs to share the same infrastructure while keeping routing tables and traffic logically separate. LDP, RSVP-TE, and QoS support MPLS operations and performance but do not provide VPN isolation.

Question 56:

 Which EIGRP mechanism is responsible for preventing routing loops while allowing unequal-cost load balancing?

A) Feasible Successor
B) Split Horizon
C) DUAL Algorithm
D) Hold Timer

Answer: C) DUAL Algorithm

Explanation:

A) Feasible Successor is a precomputed backup route maintained in the EIGRP topology table. While it ensures fast failover when the primary route fails and supports unequal-cost load balancing in conjunction with the variance command, it does not actively prevent routing loops. Feasible successors satisfy the feasibility condition, which is verified by DUAL, but their existence alone does not guarantee loop prevention; the underlying algorithm is responsible.

B) Split Horizon is a simple loop-prevention technique that prevents a router from advertising a route back out the same interface from which it was learned. While it helps prevent some routing loops in distance-vector protocols, it is limited in scope and does not provide the sophisticated loop-free guarantees or support for unequal-cost load balancing that EIGRP requires. Split horizon is a preventive mechanism rather than a dynamic loop-control algorithm.

C) The DUAL (Diffusing Update Algorithm) is the core of EIGRP responsible for both loop-free operation and rapid convergence. DUAL evaluates all routes in the topology table, verifies the feasibility condition, and selects successors and feasible successors. By requiring that the reported distance from a neighbor to a destination is less than the current feasible distance, DUAL guarantees loop-free paths. DUAL also supports unequal-cost load balancing by allowing traffic to be distributed across multiple feasible successors when the variance command is applied. It recalculates metrics dynamically in response to topology changes, ensuring network stability while optimizing traffic distribution. This makes DUAL the correct answer because it directly controls loop prevention and supports advanced EIGRP features like unequal-cost load balancing.

D) Hold Timer determines how long a router waits before declaring a neighbor down if no hello packets are received. While the hold timer influences convergence time and neighbor detection, it has no role in loop prevention or load balancing. Its function is to monitor neighbor status, not to control routing paths.

DUAL Algorithm is correct because it ensures that EIGRP maintains loop-free paths while supporting features such as unequal-cost load balancing. Feasible successors, split horizon, and hold timers assist in stability and failover, but they do not enforce the loop-free calculations that DUAL provides.

Question 57

Which OSPF area type prevents the flooding of Type 5 external LSAs while allowing injection of internal summary routes?

A) Stub Area
B) Totally Stubby Area
C) NSSA
D) Backbone Area

Answer: A) Stub Area

Explanation:

A) Stub areas are designed to optimize OSPF performance by blocking Type 5 LSAs, which carry external route information. By preventing the flooding of these LSAs, stub areas reduce the size of routing tables and minimize SPF calculation overhead. However, stub areas still allow inter-area summary LSAs (Type 3) to propagate into the area. This ensures that routers within the stub area can reach destinations in other areas without needing to learn every external route individually. The ABR injects a default route into the stub area, allowing external connectivity without flooding detailed external routes. This makes stub areas the correct answer because they specifically block external LSAs while permitting inter-area summary route propagation.

B) Totally Stubby Areas go further than stub areas by blocking both Type 3 (inter-area) and Type 5 (external) LSAs, leaving only a default route for all destinations outside the area. While totally stubby areas reduce routing table size even more, they do not allow summary routes from other areas, which conflicts with the requirement of allowing internal summary injection.

C) NSSA (Not-So-Stubby Area) allows external routes using Type 7 LSAs to be injected into the OSPF network from within the area. These are eventually converted to Type 5 LSAs by the ABR. While NSSAs do reduce flooding of external routes from outside the area, their main purpose is to provide external route injection rather than merely blocking Type 5 LSAs while allowing inter-area summaries.

D) Backbone Area (Area 0) is the central hub in OSPF and carries all types of LSAs, including Type 5 external LSAs. It does not block flooding and therefore does not optimize routing table size in the same way stub areas do.

Stub Area is correct because it blocks Type 5 external LSAs while allowing internal summary routes to propagate from the ABR. Totally stubby, NSSA, and backbone areas either block more than necessary or do not block external LSAs at all.

Question 58:

Which BGP attribute is used to select the best path for outbound traffic from a router locally without affecting other routers?

A) Weight
B) Local Preference
C) MED
D) AS Path

Answer: A) Weight

Explanation:

A) Weight is a Cisco-proprietary BGP attribute used exclusively on the local router to influence outbound path selection. It is the first attribute considered in the BGP best path selection process, and it is not propagated to any other router in the autonomous system. A higher weight guarantees that the associated path will be chosen locally, regardless of other BGP attributes. This makes weight an ideal tool for controlling local router behavior without impacting the routing decisions of other routers in the AS, making it the correct answer.

B) Local Preference influences path selection for all routers within the autonomous system. Unlike weight, it is propagated throughout the AS, affecting outbound traffic decisions network-wide. While local preference is critical for coordinated AS-wide policy, it does not allow router-specific control without affecting other routers.

C) MED (Multi-Exit Discriminator) is used to influence how neighboring autonomous systems select the preferred entry point into your AS. MED does not control local router selection for outbound traffic within the AS; it is primarily used for inbound traffic engineering.

D) AS Path records the sequence of autonomous systems a route has traversed. Manipulating AS Path (e.g., by prepending) can influence path selection by external peers, but it is not a reliable local outbound traffic control mechanism. AS Path mainly serves loop prevention and external path preference, not router-local decisions.

Weight is correct because it directly controls path selection on a single router without affecting other routers. Local preference, MED, and AS Path influence network-wide or external behavior rather than purely local outbound routing.

Question 59:

Which MPLS mechanism establishes explicit label-switched paths with bandwidth reservation for traffic engineering?

A) LDP
B) RSVP-TE
C) VRF
D) MPLS Forwarding Table

Answer: B) RSVP-TE

Explanation:

A) LDP (Label Distribution Protocol) distributes labels between MPLS routers to enable standard MPLS forwarding along FECs. LDP does not provide traffic engineering capabilities or bandwidth reservation. It simply assigns labels to facilitate forwarding along existing routes.

B) RSVP-TE (Resource Reservation Protocol – Traffic Engineering) is explicitly designed to establish label-switched paths (LSPs) with traffic engineering in mind. RSVP-TE allows network engineers to create explicit paths that consider available bandwidth, link utilization, and policy constraints. It signals LSPs across the network, reserving resources and ensuring that traffic follows predetermined paths that optimize network utilization. This mechanism enables load balancing, avoidance of congested links, and SLA compliance for latency-sensitive applications. RSVP-TE is widely used in service provider networks where predictable performance, QoS, and traffic optimization are critical, making it the correct answer.

C) VRF (Virtual Routing and Forwarding) provides logical separation of routing tables for multiple VPNs, allowing overlapping IP addresses and isolated traffic. While VRF supports MPLS VPNs, it does not create explicit LSPs or reserve bandwidth for traffic engineering.

D) MPLS Forwarding Table is a data-plane table that maps labels to outgoing interfaces and next-hop labels. While essential for forwarding labeled packets, it does not establish LSPs or reserve bandwidth. It relies on protocols like LDP or RSVP-TE for label assignment.

RSVP-TE is correct because it allows explicit path creation and resource reservation for traffic engineering. LDP, VRF, and the forwarding table support MPLS operations but do not provide traffic engineering capabilities.

Question 60:

Which OSPF feature allows external routes to be redistributed into an OSPF NSSA without flooding the entire domain?

A) Stub Area
B) Totally Stubby Area
C) Type 7 LSA
D) ABR Summary

Answer: C) Type 7 LSA

Explanation:

A) Stub areas block external routes (Type 5 LSAs) entirely, preventing external redistribution. They do not allow external routes to be introduced within the area, making them unsuitable for external route injection.

B) Totally Stubby Areas block both inter-area (Type 3) and external (Type 5) LSAs, leaving only a default route. These areas cannot redistribute external routes into OSPF.

C) Type 7 LSAs are specifically designed for Not-So-Stubby Areas (NSSA). They allow routers within the NSSA to redistribute external routes (from protocols like EIGRP, BGP, or static routes) into OSPF without flooding Type 5 LSAs into the entire OSPF domain. The ABR eventually converts Type 7 LSAs to Type 5 LSAs when propagating external routes into other areas. This controlled redistribution ensures external routes are visible where needed while limiting LSA flooding, making Type 7 LSA the correct answer.

D) ABR Summary (Type 3 LSA) is used to summarize inter-area routes between OSPF areas. It does not redistribute external routes into an NSSA or any area. Its purpose is to reduce routing table size and LSA overhead for inter-area communication, not external route injection.

Type 7 LSA is correct because it allows controlled injection of external routes into NSSAs without affecting the entire OSPF domain. Stub areas, totally stubby areas, and ABR summary serve other functions and cannot achieve this redistribution.