Cisco 300-410  Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam  Dumps and Practice Test Questions Set 7 Q121-140

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Question 121:

 Which EIGRP metric component reflects the cumulative time a packet takes to traverse a path?

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

Answer: B) Delay

Explanation:

A) Bandwidth is a measure of the minimum capacity along the path, determining the maximum possible throughput for a route. While high bandwidth enables faster data transmission, it does not reflect the actual time it takes for a packet to travel along multiple links. Bandwidth is useful for prioritizing high-capacity links but cannot capture queuing delays, propagation delays, or processing times, which are crucial for determining latency.

B) Delay represents the total time a packet takes to traverse a path, including propagation, transmission, queuing, and processing delays. EIGRP sums the delays of each interface along the route to calculate a cumulative value. This component is particularly important for latency-sensitive traffic such as VoIP, video conferencing, and real-time applications. Routes with lower cumulative delay are preferred over higher-delay paths, even if the higher-delay path has better bandwidth. Delay is calculated in tens of microseconds per interface and is included in EIGRP’s composite metric formula, combined with bandwidth to produce an accurate representation of path quality. Delay is dynamic and can change with interface congestion, making EIGRP responsive to network performance variations. This makes delay the correct answer because it directly reflects the traversal time of packets along a route.

C) Reliability measures the historical stability of a link by tracking error rates. While it helps EIGRP avoid unstable or frequently failing links, it does not quantify the traversal time of packets. A highly reliable link can still have high latency, so reliability is unrelated to delay calculations.

D) Load indicates the current utilization of a link. High load may introduce queuing and congestion, indirectly affecting delay, but load alone does not provide a static measure of the time packets take to traverse a path. Load is a dynamic value that can fluctuate rapidly and is not used in the baseline EIGRP composite metric to represent traversal time.

Delay is correct because it provides the cumulative time for packets to traverse a path. Bandwidth measures throughput, reliability measures stability, and load measures utilization. Only delay directly reflects end-to-end traversal time.

Question 122:

 Which OSPF area type allows redistribution of external routes while preventing Type 5 LSAs from flooding the area?

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

Answer: C) NSSA

Explanation:

A) Stub areas block Type 5 LSAs to reduce routing table size and limit SPF calculations. They only allow inter-area (Type 3) LSAs for connectivity and cannot redistribute external routes into the area. This makes stub areas unsuitable for scenarios requiring external route injection.

B) Totally Stubby Areas block both Type 3 and Type 5 LSAs. They leave only a default route for external connectivity, preventing redistribution of external routes within the area. This extreme LSA restriction is beneficial for very small or simplified branch areas but is not suitable when external route redistribution is required.

C) NSSA (Not-So-Stubby Area) provides a middle ground. It allows external routes to be introduced into the area via Type 7 LSAs while blocking Type 5 LSAs from flooding the area. The ABR then translates Type 7 LSAs into Type 5 LSAs for propagation into other areas. This enables localized redistribution of external routes without exposing the entire OSPF domain to all external LSAs. NSSAs are ideal for edge areas where external connectivity is needed, such as branch offices with BGP or EIGRP redistribution into OSPF, while still preserving hierarchical OSPF design and limiting SPF calculations. NSSA allows administrators to maintain network scalability and reduce LSA flooding, making it the correct choice for this scenario.

D) Backbone Area (Area 0) is central to OSPF and propagates all LSA types, including external and inter-area LSAs. While it allows redistribution of external routes, it does not restrict Type 5 LSAs and therefore cannot provide the controlled environment that NSSA offers.

NSSA is correct because it enables external route redistribution with Type 7 LSAs while blocking Type 5 LSAs from flooding the area. Stub areas restrict external redistribution, totally stubby areas restrict both internal and external LSAs, and the backbone area propagates all LSAs.

Question 123:

 Which BGP attribute is used to influence inbound traffic from neighboring autonomous systems?

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

Answer: C) MED

Explanation:

A) Weight is a Cisco-proprietary attribute that only affects path selection on the local router. It is used to influence outbound traffic preferences locally and does not propagate to other routers. Weight has no effect on how external ASes send traffic into your AS, making it irrelevant for controlling inbound routing.

B) Local Preference is an attribute that influences outbound path selection within your autonomous system. It is propagated to all routers in the AS, ensuring consistent outbound routing decisions. However, local preference does not influence how external ASes choose which entry point to use for traffic destined to your network.

C) MED (Multi-Exit Discriminator) is the BGP attribute specifically designed to influence inbound traffic from neighboring ASes. MED is advertised to neighbors to suggest which path is preferable for entering your AS. A lower MED value indicates a preferred entry point, while a higher MED indicates a less preferred path. MED allows network administrators to control how external ASes distribute traffic among multiple entry points, optimize bandwidth usage, and prevent congestion on specific links. It is particularly useful in multi-homed environments, where inbound traffic needs to be directed along specific paths to ensure performance and load balancing. Although MED is a suggestion rather than an enforcement, it is widely respected by neighboring ASes, making it the correct answer.

D) AS Path records the sequence of autonomous systems a route has traversed. AS path prepending can influence inbound traffic by making a path appear longer and less desirable, but it is an indirect method and less precise than MED.

MED is correct because it directly influences how neighboring ASes select entry points into your AS. Weight is local-only, local preference is outbound-focused, and AS path only indirectly affects inbound traffic.

 

Question 124:

 Which HSRP state monitors hello messages from the active router and is ready to assume forwarding if the active router fails?

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

Answer: B) Standby

Explanation:

A) Active is the state in which a router is currently forwarding traffic for the HSRP virtual IP. Active routers handle ARP requests and serve as the primary gateway for hosts. They do not monitor for failover because they are already responsible for traffic forwarding.

B) Standby is the state in which a router actively monitors hello messages from the active router. If the active router fails, the standby router immediately assumes the forwarding role for the virtual IP. Standby routers maintain backup state information, allowing seamless failover with minimal network disruption. They do not forward traffic during normal operation but are fully prepared to take over when required. Standby is crucial for high availability, ensuring uninterrupted gateway functionality in enterprise networks. This makes standby the correct answer because it provides monitoring and immediate readiness for failover.

C) Listen is an intermediate state where the router is aware of the HSRP group and receives hello messages, but it does not actively participate in forwarding or standby functions. Listen is preparatory and does not provide failover readiness.

D) Init is the initial state during HSRP startup where routers have not yet received hello messages from neighbors. Routers in init cannot forward traffic or participate in failover.

Standby is correct because it represents a router monitoring the active router and ready to take over immediately if failure occurs. Active is forwarding, listen is passive, and init is initial.

Question 125:

Which MPLS feature allows multiple VPNs to coexist on the same physical infrastructure while maintaining complete isolation?

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

Answer: A) VRF

Explanation:

A) VRF (Virtual Routing and Forwarding) is the MPLS mechanism that allows multiple logical routing and forwarding instances to exist on a single physical router. Each VRF maintains independent interfaces, routing tables, and forwarding information. When combined with MPLS, VRFs ensure that traffic from one VPN cannot reach another VPN, even if both use the same physical links. VRFs allow overlapping IP address spaces for different VPNs, providing secure isolation and enabling multi-tenant environments in enterprise and service provider networks. By maintaining separate routing tables and forwarding paths, VRFs ensure traffic segregation, security, and scalability. This makes VRF the correct answer.

B) LDP (Label Distribution Protocol) is responsible for distributing MPLS labels for forwarding packets along label-switched paths. While essential for MPLS operation, LDP does not provide traffic isolation or separate routing tables for VPNs.

C) RSVP-TE establishes explicit label-switched paths with bandwidth reservation for traffic engineering purposes. RSVP-TE optimizes network performance and resource utilization but does not provide logical separation of multiple VPNs.

D) QoS (Quality of Service) manages traffic prioritization, bandwidth allocation, and performance guarantees for applications. While QoS is crucial for maintaining service quality, it does not provide isolation between VPNs.

VRF is correct because it allows multiple VPNs to share the same infrastructure while maintaining complete logical separation. LDP, RSVP-TE, and QoS enhance MPLS performance but do not enforce VPN isolation.

Question 126:

Which EIGRP feature allows multiple paths to be used for traffic forwarding, even if their metrics are not identical?

A) Feasible Successor
B) Variance
C) Route Summarization
D) Hold Timer

Answer: B) Variance

Explanation:

A) Feasible Successor is a backup route maintained in the EIGRP topology table. Routes that satisfy the feasibility condition are considered loop-free and can be immediately used if the primary route fails. While feasible successors provide redundancy and rapid failover, they do not inherently allow load balancing across paths with different metrics. Feasible successors are pre-computed and stored for convergence purposes but are only selected if the primary route fails.

B) Variance is the EIGRP feature that allows unequal-cost load balancing. By default, EIGRP performs equal-cost load balancing across all successors. When variance is configured, it multiplies the metric of the best path (successor) by a defined value. Feasible successors with metrics less than this multiplied value become eligible for traffic forwarding. This allows EIGRP to distribute traffic across multiple paths with different costs, improving bandwidth utilization and redundancy. Variance enables network engineers to leverage redundant paths that would otherwise remain idle due to having higher metrics than the primary path. This feature is particularly useful in networks with mixed-speed links, redundant paths, or where traffic engineering is required. Variance combined with feasible successors ensures loop-free load balancing, making variance the correct answer.

C) Route Summarization aggregates multiple network prefixes into a single advertisement. While summarization reduces routing table size and improves scalability, it does not enable traffic to use multiple unequal-cost paths. Summarization affects advertisement efficiency but does not influence load balancing.

D) Hold Timer defines the interval a router waits before declaring a neighbor down if no hello messages are received. It affects convergence timing and neighbor stability but does not influence path selection or enable unequal-cost traffic forwarding.

Variance is correct because it allows multiple feasible paths to carry traffic even when metrics differ. Feasible successors provide backup, route summarization optimizes table size, and hold timers govern neighbor detection and convergence.

Question 127:

Which OSPF LSA type is used to carry external routes redistributed into an NSSA?

A) Type 3
B) Type 5
C) Type 7
D) Type 1

Answer: C) Type 7

Explanation:

A) Type 3 LSAs are summary LSAs used to advertise routes from one area to another. They summarize internal OSPF routes but do not carry external routes redistributed from other protocols. Type 3 LSAs are used for inter-area propagation, not for external redistribution.

B) Type 5 LSAs carry external routes redistributed into OSPF from other protocols such as BGP or EIGRP. However, Type 5 LSAs are blocked from entering NSSAs to prevent flooding of external route information into stub-like areas. They are not suitable for controlled external redistribution within NSSAs.

C) Type 7 LSAs are designed specifically for NSSAs. They allow the injection of external routes (redistributed from other protocols) into an NSSA without flooding them throughout the entire OSPF domain. The ABR translates Type 7 LSAs into Type 5 LSAs for propagation into other areas if needed. Type 7 LSAs enable hierarchical OSPF design by containing external route information within the NSSA, minimizing SPF calculation overhead in other areas, and allowing controlled redistribution. This makes Type 7 LSAs the correct answer because they provide the mechanism for localized external route redistribution.

D) Type 1 LSAs describe the router’s links within an area. They provide intra-area topology information but cannot carry external routes.

Type 7 LSAs are correct because they allow external routes to enter an NSSA while restricting Type 5 flooding. Type 3 LSAs summarize internal routes, Type 5 LSAs carry external routes outside NSSAs, and Type 1 LSAs are purely intra-area.

Question 128:

Which BGP attribute is used to control outbound path selection locally on a router?

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

Answer: A) Weight

Explanation:

A) Weight is a Cisco-proprietary BGP attribute that affects path selection on the local router only. It is the first criterion evaluated in the BGP best path selection process. By assigning higher weight to a route, the local router prefers that path for outbound traffic. Weight does not propagate to other routers, allowing router-specific traffic engineering without impacting the entire AS. This is especially useful in multi-homed routers where different exit points exist for redundancy and load balancing. Weight allows precise control of outbound traffic behavior at the local router level.

B) Local Preference is propagated throughout the AS to influence outbound path selection across all routers. While effective for AS-wide policies, it cannot provide router-specific control. Changes to local preference affect all routers in the AS, making it unsuitable for fine-grained, local routing decisions.

C) MED (Multi-Exit Discriminator) is used to influence how neighboring ASes select entry points into your AS. It affects inbound traffic, not local outbound selection.

D) AS Path records the sequence of autonomous systems a route has traversed. AS path prepending can influence inbound traffic by making a path appear longer but does not provide local router-specific outbound control.

Weight is correct because it allows local outbound path selection on a single router without affecting other routers. Local preference is AS-wide, MED affects inbound traffic, and AS path is only indirectly involved.

Question 129:

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

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

Answer: A) Active

Explanation:

A) Active is the state in which a router is currently forwarding traffic for the HSRP virtual IP. Active routers respond to ARP requests and handle all data forwarding for hosts in the subnet. The active router maintains hello messages to allow standby routers to monitor its status. Active is essential for ensuring uninterrupted gateway functionality. This makes active the correct answer because it represents the router currently responsible for forwarding traffic.

B) Standby is the backup router that monitors the active router and is ready to take over if the active router fails. It does not forward traffic during normal operation.

C) Listen is an intermediate state where the router is aware of the HSRP group and receives hello messages but is not yet participating in forwarding or standby functions.

D) Init is the initial state during HSRP startup. Routers in init have not received hello messages and cannot forward traffic or participate in failover.

Active is correct because it represents the router currently performing forwarding duties. Standby monitors, listen is passive, and init is preliminary.

Question 130:

Which MPLS feature allows multiple VPNs to share the same physical infrastructure while maintaining traffic isolation?

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

Answer: A) VRF

Explanation:

A) VRF (Virtual Routing and Forwarding) allows multiple independent routing and forwarding instances to coexist on a single physical router. Each VRF maintains its own interfaces, routing table, and forwarding paths. When combined with MPLS, VRFs provide complete isolation between VPNs, even if they share the same physical links. VRFs also enable overlapping IP address spaces for different VPNs, providing secure separation for multi-tenant environments. By maintaining separate routing tables and forwarding paths, VRFs ensure that traffic from one VPN cannot accidentally reach another. VRF is essential for enterprise and service provider networks to support multiple VPNs securely and efficiently.

B) LDP (Label Distribution Protocol) distributes MPLS labels for forwarding packets along Label-Switched Paths. While necessary for MPLS operation, LDP does not provide logical isolation between VPNs.

C) RSVP-TE establishes explicit Label-Switched Paths with bandwidth reservations for traffic engineering. While useful for path optimization and performance guarantees, RSVP-TE does not separate traffic between VPNs.

D) QoS (Quality of Service) prioritizes traffic, allocates bandwidth, and ensures service levels. QoS manages performance but does not provide isolation between VPNs.

VRF is correct because it allows multiple VPNs to coexist on the same infrastructure with complete traffic isolation. LDP, RSVP-TE, and QoS improve MPLS performance but do not enforce VPN separation.

Question 131:

Which EIGRP feature ensures that backup routes are immediately available and loop-free if the primary route fails?

A) Feasible Successor
B) Variance
C) Route Summarization
D) Hold Timer

Answer: A) Feasible Successor

Explanation:

A) Feasible Successor is a critical EIGRP feature designed for rapid convergence and network stability. EIGRP maintains a topology table that contains all learned routes, including primary successors and potential backup routes. A feasible successor is a route that satisfies the feasibility condition, meaning the neighbor-reported distance is less than the feasible distance of the current primary route. This condition guarantees that the route is loop-free and can be used immediately if the primary route fails. The feasible successor provides redundancy without requiring a full recomputation of the topology, reducing network downtime and maintaining traffic flow. Because these routes are precomputed and stored in the topology table, switching to a feasible successor occurs almost instantaneously. This is especially important in large or highly redundant networks where convergence speed is critical. Feasible successors allow EIGRP to provide fast failover while ensuring routing loops do not occur, making it the correct answer.

B) Variance is an EIGRP feature that enables unequal-cost load balancing by multiplying the metric of the best path to include other feasible successors with metrics below a defined threshold. While variance allows traffic distribution across multiple paths, it does not specifically ensure immediate, loop-free backup routes. Variance primarily focuses on load balancing rather than failover readiness.

C) Route Summarization aggregates multiple routes into a single advertisement, reducing routing table size and LSA flooding in OSPF or route advertisement in EIGRP. Summarization improves scalability and efficiency but does not provide a precomputed backup path. Its primary goal is advertisement efficiency rather than rapid failover.

D) Hold Timer determines how long a router waits before declaring a neighbor down if no hello messages are received. While hold timers influence convergence time, they do not precompute backup routes or provide immediate failover. Hold timers are about neighbor detection, not route readiness.

Feasible Successor is correct because it ensures precomputed, loop-free backup paths are immediately available. Variance allows unequal-cost load balancing, summarization reduces routing table size, and hold timer governs neighbor detection and convergence.

Question 132:

Which OSPF LSA type carries external routes redistributed into an NSSA without flooding the entire domain?

A) Type 3
B) Type 5
C) Type 7
D) Type 1

Answer: C) Type 7

Explanation:

A) Type 3 LSAs, also called summary LSAs, are generated by Area Border Routers (ABRs) to advertise inter-area routes. They summarize internal OSPF networks between areas but cannot carry external routes redistributed from other protocols. Type 3 LSAs do not facilitate external route injection into NSSAs, making them unsuitable for controlled external route propagation.

B) Type 5 LSAs carry external routes redistributed into OSPF from other protocols such as BGP or EIGRP. However, Type 5 LSAs are blocked from entering NSSAs to prevent flooding external routes into stub-like areas. While essential for standard OSPF areas, Type 5 LSAs cannot provide localized redistribution within NSSAs.

C) Type 7 LSAs are specifically designed for NSSAs. They carry external routes redistributed into the NSSA without propagating them throughout the entire OSPF domain. The ABR translates Type 7 LSAs into Type 5 LSAs if needed for wider propagation. This controlled approach ensures that external route information is contained within the NSSA, reducing SPF computation overhead in other areas while still allowing external connectivity. Type 7 LSAs maintain the hierarchical design of OSPF, enabling branch offices or edge areas to inject external routes without flooding the backbone. This makes Type 7 LSAs the correct answer.

D) Type 1 LSAs describe a router’s links within an area. They provide intra-area topology information but cannot carry external routes, making them unsuitable for NSSA redistribution.

Type 7 LSAs are correct because they enable localized external route injection in NSSAs. Type 3 LSAs summarize internal routes, Type 5 LSAs carry external routes outside NSSAs, and Type 1 LSAs are purely intra-area.

Question 133:

 Which BGP attribute is used to control outbound path selection on a local router?

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

Answer: A) Weight

Explanation:

A) Weight is a Cisco-proprietary BGP attribute that affects only the local router. It is the first criterion in BGP best path selection and allows the administrator to prefer a specific path for outbound traffic. By assigning higher weight to a route, the router will choose that path over others, even if other routes have better metrics or attributes. Weight does not propagate to other routers, making it ideal for router-specific traffic engineering and fine-grained control. This is especially useful in multi-homed environments or where certain exit links are preferred for performance or policy reasons. Weight ensures deterministic outbound routing behavior on the local router without impacting the rest of the autonomous system.

B) Local Preference is propagated throughout the AS and affects outbound path selection across all routers. It cannot provide local, router-specific control, making it unsuitable for scenarios where only one router’s preference should differ.

C) MED (Multi-Exit Discriminator) influences how neighboring autonomous systems choose entry points into your AS. It affects inbound traffic, not local outbound routing.

D) AS Path records the sequence of autonomous systems traversed by a route. AS path prepending can influence inbound traffic by making a path appear longer, but it does not provide precise local outbound control.

Weight is correct because it allows router-specific outbound path selection. Local Preference is AS-wide, MED influences inbound traffic, and AS Path is an indirect factor.

Question 134:

Which HSRP state monitors hello messages and is ready to take over traffic if the active router fails?

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

Answer: B) Standby

Explanation:

A) Active is the state in which a router forwards traffic for the HSRP virtual IP. Active routers respond to ARP requests and handle data forwarding for hosts. They are not in backup mode and therefore do not monitor failover readiness.

B) Standby is the state in which a router monitors hello messages from the active router and is prepared to take over immediately if the active router fails. Standby routers maintain state information, allowing seamless failover with minimal network disruption. During normal operation, standby routers do not forward traffic but actively monitor the active router’s health. This high-availability mechanism ensures that hosts maintain uninterrupted connectivity even if the primary router fails. Standby is crucial for enterprise networks requiring fault tolerance and uninterrupted gateway functionality, making it the correct answer.

C) Listen is an intermediate state where the router is aware of the HSRP group and receives hello messages but is not actively participating in traffic forwarding or standby readiness. Listen is preparatory and not fully functional for failover.

D) Init is the initial state during HSRP startup where hello messages have not yet been received. Routers in init cannot forward traffic or participate in failover.

Standby is correct because it provides monitoring and immediate readiness to take over traffic forwarding. Active is forwarding, listen is passive, and init is preliminary.

Question 135:

 Which MPLS feature allows multiple VPNs to share the same physical infrastructure while maintaining isolation?

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

Answer: A) VRF

Explanation:

A) VRF (Virtual Routing and Forwarding) is the MPLS mechanism that allows multiple logical routing and forwarding instances to coexist on the same physical router. Each VRF instance has independent interfaces, routing tables, and forwarding paths. VRFs enable complete traffic isolation between VPNs even when sharing the same physical infrastructure. They also allow overlapping IP address spaces for different VPNs, making VRF essential in enterprise and service provider environments for multi-tenant networks. By separating routing tables and forwarding paths, VRFs prevent accidental leakage of traffic between VPNs and provide a scalable, secure architecture. This makes VRF the correct answer.

B) LDP (Label Distribution Protocol) distributes MPLS labels to enable forwarding along Label-Switched Paths. While critical for MPLS operation, LDP does not provide VPN separation or independent routing tables.

C) RSVP-TE establishes explicit LSPs with bandwidth reservations for traffic engineering. While it optimizes path selection and performance, RSVP-TE does not separate VPN traffic.

D) QoS (Quality of Service) prioritizes traffic and manages bandwidth allocation. QoS ensures performance for critical applications but does not provide traffic isolation between VPNs.

VRF is correct because it allows multiple VPNs to coexist securely on shared infrastructure. LDP, RSVP-TE, and QoS enhance MPLS performance but do not enforce VPN separation.

Question 136:

Which EIGRP feature allows traffic to be distributed over multiple paths with different metrics?

A) Feasible Successor
B) Variance
C) Route Summarization
D) Hold Timer

Answer: B) Variance

Explanation:

A) Feasible Successor is a precomputed backup route in EIGRP that satisfies the feasibility condition, ensuring it is loop-free. Feasible successors provide immediate failover capability if the primary path fails. While they allow traffic to switch quickly to backup routes, feasible successors do not inherently enable traffic distribution over unequal-cost paths. Their primary function is redundancy and fast convergence rather than load balancing across multiple paths with different metrics.

B) Variance is the EIGRP mechanism that allows unequal-cost load balancing. By multiplying the metric of the primary path by a defined variance value, EIGRP includes additional feasible successors with metrics within that threshold as eligible paths for traffic forwarding. This allows multiple paths, even with differing metrics, to be used simultaneously, optimizing bandwidth utilization and improving redundancy. Variance is particularly useful in networks with links of different speeds or capacities, where equal-cost load balancing would leave higher-cost but still viable paths unused. With variance, network engineers can distribute traffic across multiple paths efficiently, making it the correct answer.

C) Route Summarization aggregates multiple network prefixes into a single advertisement. Summarization reduces routing table size and improves network scalability but does not allow traffic to be forwarded over multiple unequal-cost paths. Summarization is primarily concerned with advertisement efficiency rather than load balancing.

D) Hold Timer determines how long a router waits before declaring a neighbor down if no hello messages are received. While hold timers affect convergence timing, they do not impact path selection or load balancing.

Variance is correct because it allows traffic to be distributed over multiple feasible paths with different metrics. Feasible successors provide failover, summarization reduces table size, and hold timers control neighbor detection.

Question 137:

Which OSPF LSA type carries external routes redistributed into an NSSA without flooding the entire OSPF domain?

A) Type 3
B) Type 5
C) Type 7
D) Type 1

Answer: C) Type 7

Explanation:

A) Type 3 LSAs are summary LSAs generated by ABRs to advertise inter-area routes. They summarize internal OSPF networks but cannot carry external routes. Type 3 LSAs do not allow redistribution of external routes into NSSAs, so they are unsuitable for controlled external route propagation.

B) Type 5 LSAs carry external routes from other protocols like BGP or EIGRP but are blocked from entering NSSAs. They are used for standard OSPF areas, and allowing them into NSSAs would flood external routes into areas that are intended to restrict LSA propagation.

C) Type 7 LSAs are specifically designed for NSSAs. They allow external routes to be injected into the NSSA without propagating them across the entire OSPF domain. The ABR can translate Type 7 LSAs into Type 5 LSAs if wider propagation is required. This mechanism ensures hierarchical OSPF design is maintained, minimizing SPF calculation overhead while allowing branch or edge areas to redistribute external routes. Type 7 LSAs contain external route information within the NSSA, making it scalable and efficient. This controlled redistribution is critical in enterprise networks with branch offices that need external connectivity but should not flood the backbone with unnecessary LSAs. Type 7 LSAs are the correct answer because they provide localized external redistribution while maintaining OSPF hierarchy and performance.

D) Type 1 LSAs describe a router’s links within an area. They provide intra-area topology information but cannot carry external routes or facilitate redistribution.

Type 7 LSAs are correct because they enable external routes in NSSAs while restricting propagation beyond the area. Type 3 LSAs summarize internal routes, Type 5 LSAs carry external routes outside NSSAs, and Type 1 LSAs are purely intra-area.

Question 138:

Which BGP attribute is used to influence outbound path selection on a single router?

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 control outbound path selection. It is the first criterion evaluated in the BGP best path selection process. Assigning a higher weight to a path ensures that the router will prefer it over other available paths, regardless of other attributes like local preference or AS path. Weight does not propagate to other routers, enabling router-specific traffic engineering. This attribute is particularly valuable in multi-homed environments where traffic must be steered along specific links for performance or policy reasons. Weight provides deterministic control over outbound traffic without impacting the rest of the AS, making it the correct answer.

B) Local Preference is propagated throughout the AS to control outbound path selection across all routers. While useful for AS-wide policies, it cannot be applied locally on a single router without affecting others.

C) MED (Multi-Exit Discriminator) influences how neighboring ASes choose entry points into your AS. MED is inbound-focused and does not affect local outbound path selection.

D) AS Path records the sequence of ASes traversed by a route. AS path prepending can influence inbound traffic by making a route appear longer, but it does not provide local outbound routing control.

Weight is correct because it allows router-specific outbound path selection. Local Preference is AS-wide, MED is inbound-focused, and AS Path is indirect.

Question 139:

Which HSRP state monitors the active router and is ready to take over forwarding if the active router fails?

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

Answer: B) Standby

Explanation:

A) Active is the state in which a router is currently forwarding traffic for the HSRP virtual IP. Active routers respond to ARP requests and serve as the primary gateway. They are not in a backup monitoring role.

B) Standby is the state in which a router monitors hello messages from the active router and is ready to assume traffic forwarding immediately if the active router fails. Standby routers maintain state information to ensure seamless failover, minimizing disruption to hosts. They do not forward traffic under normal operation but are fully prepared to take over when required. This provides high availability and fault tolerance, which is essential for enterprise networks where uninterrupted connectivity is critical. Standby ensures that the virtual IP remains accessible even in the event of a primary router failure, making it the correct answer.

C) Listen is an intermediate state in which a router is aware of the HSRP group and receives hello messages but is not actively participating in forwarding or standby readiness.

D) Init is the initial state during HSRP startup. Routers in init have not received hello messages and cannot forward traffic or participate in failover.

Standby is correct because it provides monitoring and immediate readiness to take over forwarding. Active is forwarding, listen is passive, and init is preliminary.

Question 140

Which MPLS feature allows multiple VPNs to share the same physical infrastructure while maintaining traffic separation?
A) VRF
B) LDP
C) RSVP-TE
D) QoS

Answer: A) VRF

Explanation:

A) VRF (Virtual Routing and Forwarding) is a critical MPLS feature that allows multiple logical routing and forwarding instances to coexist on the same physical router. Each VRF operates as an independent routing domain, maintaining its own interfaces, routing tables, and forwarding paths. This separation ensures that traffic from one VPN cannot traverse into another VPN, providing complete traffic isolation even though all VPNs share the same underlying physical infrastructure. VRFs essentially allow a single router to function as multiple virtual routers simultaneously, each with its own set of routing policies, interfaces, and traffic flows.

When combined with MPLS, VRFs use labels to identify and segregate traffic for each VPN. MPLS labels act as a tag, instructing routers to forward traffic according to the appropriate VRF table. This approach ensures that even if multiple VPNs share the same backbone links, the traffic is logically isolated, maintaining both security and operational independence. By using VRFs, network operators can deploy multiple Layer 3 VPNs (L3VPNs) or virtual routing domains for different tenants, departments, or customers without needing dedicated physical routers for each VPN.

Another key benefit of VRFs is their ability to support overlapping IP address spaces. In enterprise and multi-tenant environments, it is common for different VPNs or departments to use the same private IP ranges. With VRFs, each routing table is independent, so there is no conflict between overlapping subnets. For example, two tenants may both use the 10.0.0.0/24 subnet. Because each tenant’s traffic is handled within a separate VRF, their networks remain fully isolated and function as if they were separate physical networks. This eliminates the need for complex NAT configurations, allows simpler IP management, and reduces operational complexity.

VRFs also enhance security and policy enforcement. Since each VRF maintains independent routing tables, policies such as firewall rules, access control lists (ACLs), and monitoring can be applied on a per-VPN basis. This ensures that a misconfiguration, security breach, or traffic anomaly in one VPN does not impact other VPNs. Enterprises can also deploy VRFs to segment internal traffic by department, function, or application type, creating isolated routing domains that maintain compliance with internal security policies or regulatory requirements.

In addition to security, VRFs enable scalability. Service providers can host hundreds or thousands of VPNs on the same backbone infrastructure, while enterprises can deploy multiple VRFs for internal segmentation without purchasing additional hardware. VRFs integrate seamlessly with BGP (Border Gateway Protocol), particularly MP-BGP (Multiprotocol BGP), which allows routes to be exchanged securely between VRFs and the provider or core network. This provides flexible inter-VRF connectivity while maintaining isolation. VRFs are essential for creating multi-tenant environments, where multiple customers or organizational units require separate networks that coexist securely on shared physical infrastructure.

B) LDP (Label Distribution Protocol) is a protocol that distributes MPLS labels to routers to facilitate forwarding along Label-Switched Paths (LSPs). LDP ensures that MPLS routers can forward packets based on their assigned labels, enabling efficient traffic transport across the network. While LDP is a fundamental component of MPLS, it does not provide traffic isolation or independent routing domains. LDP focuses on label distribution and packet forwarding, not on separating traffic between multiple VPNs or tenants. Therefore, LDP alone cannot enforce VPN separation or maintain multiple independent routing tables.

C) RSVP-TE (Resource Reservation Protocol – Traffic Engineering) is used to establish explicit LSPs with bandwidth reservation to optimize traffic flows and network performance. RSVP-TE enables network operators to guarantee bandwidth for critical applications, avoid congestion, and control routing paths for predictable performance. While RSVP-TE is essential for traffic engineering and ensuring efficient utilization of MPLS networks, it does not provide logical separation between multiple VPNs. RSVP-TE is focused on path optimization and quality of service but cannot isolate traffic between different routing domains or prevent one VPN from interacting with another.

D) QoS (Quality of Service) prioritizes traffic, enforces bandwidth allocation, and ensures that performance requirements are met for applications like voice, video, or transactional services. QoS is critical for managing network congestion and delivering predictable application performance. However, QoS does not create independent routing tables or provide traffic isolation between VPNs. While it can influence how traffic is treated within the network, it cannot prevent one VPN’s traffic from reaching another, making it unsuitable for multi-tenant isolation.

 VRF is the correct answer because it allows multiple VPNs to coexist securely on the same physical infrastructure while maintaining complete traffic separation. Each VRF instance has independent routing tables, forwarding paths, and interfaces, ensuring that traffic cannot leak between VPNs. VRFs also support overlapping IP address spaces, multi-tenancy, scalable deployment, and granular security and policy enforcement.

LDP, RSVP-TE, and QoS improve MPLS efficiency, traffic forwarding, and performance, but none of these features provide logical isolation between multiple VPNs. VRF is the key feature that enables secure, scalable, and isolated multi-tenant or multi-department network deployments. It ensures that each VPN behaves as if it has its own dedicated physical network, even when all VPNs share the same backbone. By combining VRFs with MPLS labeling and BGP integration, operators can deliver reliable, secure, and scalable VPN services without additional hardware, making VRF essential for modern enterprise and service provider networks.