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Question 181
Which EIGRP metric component affects load balancing among multiple paths?
A) Bandwidth
B) Delay
C) Reliability
D) K-values
Answer: D
Explanation:
In Enhanced Interior Gateway Routing Protocol (EIGRP), load balancing is directly influenced by the metric calculation. EIGRP uses a composite metric formula that incorporates several parameters, including bandwidth, delay, reliability, load, and MTU, with optional K-values controlling which parameters are used in the calculation. K-values are numeric multipliers that define the weight of each metric component in determining the best path.
By default, only bandwidth (K1) and delay (K3) are considered in the metric computation, but adjusting K-values allows network engineers to include reliability (K4) and load (K5). The EIGRP metric formula is:
Metric = [(K1 * Bandwidth) + (K2 * Bandwidth/(256-Load)) + (K3 * Delay) * (K5/(Reliability+K4))] * 256
Option D) is correct because K-values directly control how the metric is calculated, affecting which paths EIGRP considers feasible for routing and load balancing. Option A), Bandwidth, and B), Delay, are individual components but cannot independently control load balancing without considering K-values. Option C), Reliability, is also part of the metric but its influence depends on the K-values configuration.
Understanding K-values is crucial for ENARSI candidates, particularly when designing enterprise networks with multiple parallel WAN links. Adjusting K-values allows engineers to favor faster or more reliable links, fine-tuning EIGRP’s Equal-Cost Multipath (ECMP) behavior. Practical configuration involves the metric weights command under EIGRP and verification via show ip eigrp topology. Misconfigured K-values can result in suboptimal routing, traffic congestion, or asymmetric load distribution, emphasizing the importance of deep metric knowledge for exam scenarios and real-world enterprise deployments.
Advanced EIGRP design often combines K-values tuning with variance configuration, allowing unequal-cost load balancing to increase bandwidth utilization. Engineers must also monitor feasible successors, as EIGRP only performs load balancing among routes that satisfy loop-free criteria. A thorough understanding of K-values, feasible successors, and variance ensures predictable routing, redundancy, and performance optimization in enterprise networks, making it a critical ENARSI concept.
Question 182
Which BGP attribute influences path selection by external neighbors?
A) Local preference
B) AS-path
C) Weight
D) MED
Answer: D
Explanation:
In Border Gateway Protocol (BGP), influencing how external neighbors select the best route to reach your network is critical for traffic engineering and multi-ISP enterprise designs. The Multi-Exit Discriminator (MED) attribute serves this purpose by providing a hint to external peers regarding which path should be preferred when multiple entry points into your AS exist.
Option D) is correct because MED is advertised to external neighbors (eBGP) and instructs them which of your AS entry points is preferable. For example, in a network connected to two ISPs, assigning a lower MED to the preferred entry point signals external ASes to use that path for inbound traffic. Option A), Local Preference, is internal to your AS and does not influence external neighbors. Option B), AS-path, affects route selection by influencing path length but may be ignored depending on the external peer’s policies. Option C), Weight, is Cisco-specific and internal to the router, having no impact on external neighbor decisions.
Understanding MED is essential for ENARSI candidates, as it directly impacts inbound traffic engineering, redundancy, and path selection in multi-homed environments. Practical configuration involves route maps with set metric or set MED commands. Verification is performed with show ip bgp to confirm MED values and the active path chosen by neighbors. Misconfigured MED values can cause unbalanced traffic, congestion, or suboptimal use of expensive WAN links, which are common exam scenarios.
Additionally, MED interacts with other attributes like Local Preference, AS-path, and communities. Engineers often combine these to implement comprehensive BGP policy control, ensuring predictable and efficient traffic flow. Mastery of MED is particularly important when designing enterprise networks with MPLS VPNs, hybrid cloud connectivity, or multi-ISP redundancy, as improper MED usage can undermine resilience and SLA compliance.
Question 183
Which OSPF area type allows limited external route redistribution?
A) Stub area
B) Totally stubby area
C) NSSA
D) Backbone area
Answer: C
Explanation:
Not-So-Stubby Area (NSSA) is a crucial OSPF concept designed for branch networks or remote sites where some external route redistribution is required, but flooding of full external LSAs is undesirable. Unlike regular stub areas, NSSAs allow Type 7 LSAs to carry external routes that are then translated to Type 5 LSAs at the ABR for propagation to the backbone.
Option C) is correct because NSSA provides a balance between stub functionality and controlled redistribution, preventing excessive routing table growth while allowing limited external connectivity. Option A), Stub Area, blocks all external LSAs. Option B), Totally Stubby Area, blocks both external and inter-area routes. Option D), Backbone Area, carries all LSAs and is unsuitable for controlling redistribution.
ENARSI candidates must understand NSSA design for hierarchical OSPF networks, where remote branch routers or VPN connections require controlled external route injection. Configuration involves area x nssa on the ABR and internal routers, with verification via show ip ospf database and show ip route ospf. NSSA also interacts with virtual links, summarization, and route redistribution, making it a foundational skill for large-scale OSPF deployments. Proper design ensures stable convergence, reduced SPF recalculation, and efficient resource utilization.
Question 184
Which feature allows EIGRP to load balance over unequal-cost paths?
A) Feasible successor
B) Variance
C) K-values
D) Route summarization
Answer: B
Explanation:
EIGRP supports unequal-cost load balancing through the variance feature, allowing multiple feasible paths to carry traffic proportionally. Variance multiplies the feasible successor metric by a value, including paths with metrics within the defined ratio of the best path.
Option B) is correct because variance enables engineers to utilize secondary paths, increasing bandwidth efficiency while maintaining loop-free routing. Option A), Feasible successor, represents backup routes but does not enable traffic sharing unless variance is applied. Option C), K-values, define metric calculation but not path balancing directly. Option D), Route summarization, affects routing table size but not traffic distribution.
For ENARSI exam scenarios, variance allows optimization of multiple WAN links without complex redistribution or policy-based routing. Commands like variance 2 under EIGRP and verification via show ip eigrp topology confirm that feasible successors are properly used. Understanding variance ensures predictable, high-performance routing and redundancy in enterprise WANs, critical for advanced EIGRP design.
Question 185
Which command verifies MPLS forwarding for labeled packets?
A) show mpls forwarding-table
B) show mpls ldp neighbor
C) show ip route mpls
D) show mpls interfaces
Answer: A
Explanation:
MPLS networks use labels instead of IP routing to forward packets efficiently. The MPLS forwarding table contains the mapping of incoming labels to outgoing labels and interfaces, enabling routers to make label-switched forwarding decisions.
Option A) is correct because show mpls forwarding-table displays the label information, interfaces, and next-hop for labeled traffic. Option B) shows LDP neighbor states, not forwarding. Option C) shows routes but not actual label operations. Option D) shows interfaces participating in MPLS but not label mappings.
For ENARSI candidates, verifying MPLS forwarding is essential in enterprise WANs using MPLS VPNs, LSPs, or traffic-engineered paths. Misconfigured forwarding can result in dropped traffic, routing loops, or failed VPN connections. Engineers should understand label distribution, LDP operation, and forwarding verification as part of MPLS troubleshooting and exam mastery. Commands like show mpls forwarding-table combined with show mpls ldp bindings help verify end-to-end label propagation and ensure network reliability and predictable performance.
Question 186
A network engineer is configuring OSPF on a multi-area network. The engineer needs to ensure that traffic destined for external networks is efficiently distributed among multiple ABRs. Which configuration should the engineer apply to achieve optimal inter-area routing?
A) Configure OSPF NSSA on all areas
B) Implement OSPF default-information originate on all ABRs
C) Use OSPF stub areas with no-summary option
D) Enable OSPF route summarization on all ABRs
Answer: D
Explanation:
In OSPF networks that span multiple areas, the handling of inter-area routing can significantly affect both routing efficiency and network stability. OSPF, being a link-state protocol, maintains a database of the entire topology, but this can become unwieldy in larger networks. To address this, OSPF allows route summarization on Area Border Routers (ABRs), which aggregates multiple specific routes into a single summary route when advertising into other areas. This reduces the size of the routing table, lowers memory consumption on internal routers, and ensures that traffic destined for external or inter-area networks is optimally distributed. Option A), configuring NSSA, is primarily used to inject external routes into OSPF in a controlled manner but does not directly optimize inter-area routing. Option B), default-information originate, pushes a default route to other areas but does not summarize existing network routes, which can still lead to large routing tables and suboptimal path selection. Option C), configuring stub areas with the no-summary option, is intended to limit the propagation of external routes into specific areas but at the expense of route visibility; it reduces routing information but may hinder traffic distribution if multiple ABRs are present. Option D) ensures that multiple ABRs can advertise summarized routes, allowing routers in other areas to efficiently select the shortest or most appropriate path to external destinations. Summarization also minimizes routing flaps and improves convergence by containing link-state updates within individual areas rather than propagating detailed changes network-wide. This configuration is particularly essential for enterprise networks with hundreds or thousands of subnets across different geographical locations. Therefore, enabling OSPF route summarization on ABRs is the recommended approach for maintaining optimal inter-area routing efficiency, reducing unnecessary route advertisements, and enhancing overall network performance.
Question 187
A network administrator is tasked with deploying BGP in an enterprise network. The requirement is that certain prefixes learned from a BGP peer must be preferred over others regardless of the AS path length. Which BGP attribute should be manipulated to achieve this requirement?
A) Local Preference
B) Weight
C) MED (Multi-Exit Discriminator)
D) AS Path
Answer: A
Explanation:
In Border Gateway Protocol (BGP), route selection is determined by a series of attributes, which influence the path a router chooses to reach a destination. The Local Preference attribute is used within an autonomous system (AS) to prefer one exit point over another when multiple paths exist to reach external networks. Unlike the AS path, which considers the number of autonomous systems traversed, Local Preference allows network administrators to prioritize certain prefixes for internal routing decisions, ensuring traffic exits the network through the preferred path. Option B), Weight, is a Cisco-proprietary attribute applied only locally to the router on which it is configured; it does not propagate to other routers, making it less suitable for enterprise-wide routing policies. Option C), MED, is used to influence how external neighbors prefer certain entry points into your AS, but it is considered only when comparing routes from different ASs and does not have priority over Local Preference. Option D), AS Path, affects path selection by choosing the route with the shortest AS path, which is less flexible for internal administrative control. By configuring Local Preference, an administrator can ensure that traffic to certain prefixes is consistently routed through the preferred exit points, allowing fine-grained control over network traffic flow. For example, in large multi-homed enterprise environments, this configuration is critical to maintain bandwidth efficiency, reduce latency, and adhere to organizational traffic engineering policies. Local Preference can be set using route maps based on prefix lists, community tags, or other matching criteria, giving network engineers the ability to influence routing without disrupting overall BGP convergence. The use of Local Preference is a cornerstone for internal path selection in BGP, providing a reliable mechanism to enforce administrative routing preferences across an entire autonomous system.
Question 188
An enterprise network requires high availability for critical applications. The network engineer is implementing HSRP across multiple distribution switches. Which HSRP configuration ensures load sharing while maintaining a single virtual IP for client devices?
A) Configure HSRP with a single active router and multiple standby routers
B) Use HSRP with unequal priority values across routers
C) Implement HSRP on multiple VLANs with different active routers per VLAN
D) Deploy HSRP with preempt disabled on all routers
Answer: C
Explanation:
HSRP (Hot Standby Router Protocol) is designed to provide redundancy and ensure high availability of IP networks. It achieves this by allowing multiple routers to share a single virtual IP address while designating one router as the active gateway and another as standby. However, in scenarios where enterprises require both redundancy and traffic load balancing, HSRP can be deployed across multiple VLANs with different routers configured as active for each VLAN. Option A), configuring a single active router with multiple standbys, provides redundancy but does not balance traffic, resulting in a single router handling all client traffic while the others remain idle until failover occurs. Option B), using unequal priority values, can influence the election process but does not inherently provide balanced traffic distribution across VLANs or segments. Option D), disabling preemption, ensures that the current active router remains active even if a higher-priority router comes online, which can prevent proper failover and load distribution. Option C) allows each VLAN to have a different active router while all VLANs still share the HSRP virtual IP addresses within their respective segments. This configuration effectively distributes traffic load across multiple routers while maintaining a consistent gateway address for client devices. By strategically assigning HSRP active roles per VLAN, enterprises can optimize bandwidth utilization, minimize bottlenecks, and provide seamless failover in case of router failure. This design is critical in environments with multiple distribution switches supporting hundreds of client devices, where maintaining high availability and consistent performance is essential. The approach also simplifies client configurations because each VLAN continues to use the same virtual IP, while the network benefits from balanced traffic handling and increased fault tolerance.
Question 189
A network engineer is configuring route redistribution between OSPF and EIGRP. Certain routes from EIGRP should appear in OSPF with a specific metric type to control path selection. Which configuration ensures that the redistributed EIGRP routes maintain predictable OSPF metrics?
A) Redistribute EIGRP into OSPF using metric-type 2
B) Use a default metric with type 1
C) Configure OSPF redistribute command with a route map modifying tags
D) Enable OSPF external route filtering
Answer: B
Explanation:
Route redistribution between different routing protocols requires careful metric configuration to ensure predictable path selection and avoid routing loops. In OSPF, redistributed external routes can be advertised as Type 1 or Type 2 external routes. Type 1 metrics are additive, meaning the metric combines the external metric with the internal OSPF cost to the ASBR (Autonomous System Boundary Router), providing more precise and predictable path selection across multiple areas. Type 2 metrics, in contrast, use only the external metric, which can lead to situations where OSPF internal path costs are ignored, potentially causing suboptimal routing decisions. Option A), using metric-type 2, would not allow for accurate internal OSPF cost calculation, reducing control over path selection. Option C), applying a route map with tags, is useful for filtering and policy-based routing but does not inherently ensure predictable OSPF metrics. Option D), enabling external route filtering, can limit which routes are advertised but does not influence metric calculation or path preference. By using Type 1 metrics with a default metric, redistributed EIGRP routes maintain consistency with OSPF’s internal cost calculations, ensuring that traffic flows along the most efficient paths as determined by OSPF’s Dijkstra algorithm. This approach is particularly beneficial in enterprise networks where multiple routing protocols coexist, as it allows for smoother integration, prevents routing loops, and ensures that redistributed routes do not inadvertently bypass more optimal internal paths. Proper metric assignment is a critical step in route redistribution, as it directly impacts network performance, convergence times, and overall reliability.
Question 190
An enterprise is deploying MPLS Layer 3 VPNs across multiple sites. The network engineer wants to ensure that customer traffic remains isolated and reachable without impacting the service provider backbone. Which MPLS configuration is essential to achieve this goal?
A) Implement MP-BGP with VRF and route-targets
B) Configure LDP between all customer edge devices
C) Enable OSPF on the provider core only
D) Use static routes on all provider edge routers
Answer: A
Explanation:
MPLS Layer 3 VPNs allow service providers to offer scalable, secure, and isolated connectivity for multiple customer networks over a shared backbone infrastructure. The key to achieving traffic isolation and inter-site connectivity lies in the proper configuration of VRFs (Virtual Routing and Forwarding instances) on provider edge (PE) routers. Each VRF maintains separate routing tables for each customer, ensuring that traffic is logically segregated even though it traverses the same physical infrastructure. MP-BGP (Multiprotocol Border Gateway Protocol) is used to exchange routes between PE routers, and route-targets control which VRFs import or export specific prefixes. Option B), configuring LDP (Label Distribution Protocol) between customer edge devices, is unnecessary and does not provide the required segmentation; LDP is primarily used for label distribution in the MPLS core. Option C), enabling OSPF on the provider core only, ensures backbone routing but does not provide per-customer isolation. Option D), using static routes on all PE routers, is not scalable for large deployments and is error-prone. By combining MP-BGP with VRFs and route-targets, the service provider can advertise customer routes securely and selectively, maintain backbone stability, and allow each customer to operate independently. This configuration also supports route filtering, segmentation, and policy-based routing, providing enterprises with robust connectivity across sites. Furthermore, MPLS VPNs ensure that customer traffic is encapsulated with labels, maintaining isolation from other customers and preserving service quality. Proper planning of route-targets and VRFs is critical to achieving scalability, security, and seamless integration within a multi-tenant network environment. The architecture allows for consistent end-to-end reachability, supports redundancy, and provides a framework for future expansion without affecting other services or the underlying MPLS infrastructure.
Question 191
A network engineer is troubleshooting OSPFv3 in an IPv6-enabled enterprise network. Certain links are not appearing in the OSPF database despite proper interface configuration. Which OSPFv3 configuration step is most likely missing?
A) Enable OSPFv3 on the interfaces using the ‘ipv6 ospf’ command
B) Configure OSPFv3 router ID on all routers
C) Enable OSPFv3 redistribution from RIPng
D) Configure OSPFv3 passive interfaces
Answer: A
Explanation:
OSPFv3 is the IPv6 variant of OSPF and differs from OSPFv2 in several key aspects. One major distinction is that OSPFv3 requires explicit activation on each interface, unlike OSPFv2, which can be network-wide through the ‘network’ statement. Without enabling OSPFv3 on the interface using the ipv6 ospf <process-id> area <area-id> command, the router will not advertise the network, and links will not appear in the OSPF database. Option B), configuring a router ID, is necessary for OSPFv3 operation, but if the interfaces are not enabled for OSPFv3, the router ID alone will not populate the link-state database. Option C), redistributing routes from RIPng, is unrelated to the visibility of directly connected OSPFv3 links; it only affects the advertisement of external routes. Option D), configuring passive interfaces, would prevent OSPF from sending Hello packets and forming neighbor relationships, which is counterproductive for link advertisement but is not the primary cause when interfaces are entirely missing from the database. Proper interface-level activation ensures that OSPFv3 neighbor adjacencies can form, Hello packets are exchanged, and the link-state database accurately reflects all IPv6 networks. In larger enterprise networks, overlooking this step is a common misconfiguration that leads to routing inconsistencies, suboptimal paths, and troubleshooting challenges. By enabling OSPFv3 per interface, engineers guarantee that each segment of the network participates in the link-state exchange, contributing to convergence, optimal path selection, and reliable inter-area routing. In addition, interface-specific activation allows for finer control over routing policies, authentication, and area assignment, which is critical in multi-area OSPFv3 deployments. Understanding this difference from OSPFv2 is vital for exam success and real-world enterprise network deployments.
Question 192
A network engineer needs to implement route summarization between two EIGRP autonomous systems. The engineer wants to reduce the size of the routing table without affecting convergence times. Which configuration achieves this goal?
A) Apply manual summary at the ABR using the ip summary-address eigrp command
B) Enable automatic route summarization across all EIGRP interfaces
C) Redistribute EIGRP routes into OSPF and summarize in OSPF
D) Use route filtering with distribute lists
Answer: A
Explanation:
In EIGRP, route summarization is a technique used to aggregate multiple contiguous networks into a single summary route. This reduces the size of the routing table, lowers memory and CPU usage on routers, and improves network stability. Option A), applying a manual summary using the ip summary-address eigrp command, allows precise control over which networks are summarized, ensuring that only the desired prefixes are aggregated without affecting unrelated subnets. This is particularly important when implementing summarization between autonomous systems or across wide enterprise networks. Option B), enabling automatic summarization, can lead to unintended routing behavior, especially in discontiguous networks, because EIGRP will summarize at major network boundaries automatically, which might cause traffic blackholing or suboptimal routing. Option C), redistributing into OSPF for summarization, adds unnecessary complexity and may introduce routing loops if not carefully managed. Option D), using distribute lists, allows route filtering but does not perform summarization; it simply blocks or permits specific prefixes. Manual EIGRP summarization provides a predictable and controlled approach, ensuring reduced routing table size while maintaining fast convergence, since EIGRP recalculates routes incrementally rather than flooding the network with entire routing tables. In large-scale enterprise environments with hundreds of subnets, summarization also minimizes link-state fluctuations and allows ABRs or ASBRs to advertise fewer, more generalized routes, which simplifies troubleshooting and enhances the network’s scalability. By implementing manual summary addresses on the ABR or redistribution point, the engineer ensures efficient routing, reduced resource utilization, and optimal path selection across multiple EIGRP autonomous systems.
Question 193
An enterprise network uses BGP for multi-homed connectivity to two ISPs. The engineer wants to ensure that outbound traffic prefers ISP1 for certain prefixes while inbound traffic from the internet prefers ISP2. Which configuration combination achieves this behavior?
A) Use BGP local preference for outbound and MED for inbound traffic
B) Configure BGP weight for inbound and AS-path prepending for outbound
C) Implement route maps to filter prefixes from ISP2 only
D) Use static routes for outbound traffic and redistribute into BGP for inbound
Answer: A
Explanation:
BGP provides granular control over path selection using multiple attributes. To influence outbound traffic originating from an autonomous system, the local preference attribute is the most effective tool. Setting a higher local preference for certain prefixes learned from ISP1 ensures that these routes are preferred within the AS, directing outbound traffic to follow the desired ISP path. For inbound traffic, the autonomous system cannot directly dictate routing decisions in the ISP network, but it can influence them using the MED (Multi-Exit Discriminator) attribute or AS-path prepending. MED signals to the neighboring AS which entry point into your network is preferred, allowing ISPs to make informed decisions for routing traffic back to your network. Option B), using weight for inbound traffic, is ineffective because weight is local to the router and does not propagate outside the AS; AS-path prepending influences inbound traffic but does not manage internal outbound routing. Option C), filtering prefixes, may selectively allow or deny routing announcements but does not control path preference or load balancing. Option D), using static routes, is unsuitable in multi-homed BGP environments because static routing lacks the dynamic path selection and policy control that BGP attributes provide. By combining local preference for outbound traffic and MED for inbound traffic, the engineer can achieve predictable traffic patterns: internal routers favor ISP1 for specific destinations, while ISPs are guided to direct incoming traffic primarily through ISP2. This combination is particularly valuable in enterprise networks with critical applications that require optimized bandwidth utilization, low latency, and redundancy across multiple upstream providers. Properly leveraging BGP attributes also enhances scalability and ensures consistent performance during link failures or topology changes.
Question 194
A network engineer is designing an HSRP deployment across multiple distribution switches. Some VLANs need different active routers to optimize bandwidth usage. Which HSRP feature allows this configuration without changing the virtual IP address for client devices?
A) Configure HSRP groups per VLAN with different active routers
B) Enable HSRP tracking for interface load
C) Use HSRP priority only on the primary VLAN
D) Disable preemption on all routers
Answer: A
Explanation:
HSRP provides high availability by designating an active router and a standby router that share a virtual IP address for client default gateways. When enterprises require traffic load balancing across multiple distribution switches, the key is to configure different HSRP groups per VLAN, with each VLAN assigned a unique active router. This method allows client devices in each VLAN to continue using a consistent virtual IP address, while traffic is distributed across multiple routers to optimize bandwidth utilization and avoid overloading a single distribution switch. Option B), HSRP tracking, can adjust router priority dynamically based on interface status but does not inherently allow VLAN-based active router selection. Option C), using priority only on the primary VLAN, influences a single VLAN but cannot balance traffic across multiple VLANs effectively. Option D), disabling preemption, prevents a higher-priority router from becoming active automatically, which is counterproductive in load balancing scenarios. By leveraging HSRP groups per VLAN, the network engineer can maintain high availability while optimizing traffic flow. This approach ensures seamless failover in the event of a router failure and maximizes network efficiency across distribution switches. For enterprises with multiple VLANs and high-density traffic, HSRP VLAN-specific groups provide a flexible, scalable, and fault-tolerant solution, allowing administrators to tailor active and standby roles according to traffic demands and network topology. Additionally, it reduces potential congestion on a single active router, enhances redundancy, and maintains consistent default gateway addresses for clients, simplifying both deployment and ongoing management.
Question 195
An enterprise uses MPLS Layer 3 VPN to interconnect multiple branch offices. Some customers require route filtering so that specific prefixes are not shared across sites. Which MPLS feature allows selective route advertisement while maintaining traffic isolation?
A) Configure VRFs with import/export route-targets
B) Enable LDP between all PE and CE routers
C) Use OSPF across the provider backbone
D) Deploy static routes on PE routers for each customer
Answer: A
Explanation:
MPLS Layer 3 VPNs allow service providers to create logically separate routing domains for each customer using VRFs (Virtual Routing and Forwarding). A VRF maintains a distinct routing table for each customer, ensuring traffic isolation across the shared MPLS backbone. To selectively advertise routes between VRFs or sites, import and export route-targets are applied. Export route-targets define which prefixes from a VRF are advertised to other VRFs, while import route-targets determine which external prefixes a VRF can import. Option B), enabling LDP, is related to label distribution in the MPLS core but does not provide selective route advertisement or traffic isolation. Option C), using OSPF in the backbone, manages internal routing but does not control customer-specific VPN routing policies. Option D), static routes, is highly unscalable and error-prone for multi-site VPN deployments. By carefully assigning route-targets, the service provider can control which routes are visible to each customer, allowing some prefixes to remain local while others are propagated to specific sites. This approach maintains strict traffic isolation while providing flexibility in inter-site connectivity, route filtering, and policy enforcement. VRF-based MPLS VPNs also enable customers to extend their networks across multiple locations securely, leveraging a shared backbone without the risk of inter-customer traffic leakage. This configuration is critical for enterprises with multiple tenants, compliance requirements, and complex network segmentation needs. It ensures predictable, secure, and efficient routing while retaining the scalability benefits of MPLS.
Question 196
A network engineer is configuring EIGRP for IPv6 across an enterprise network. After enabling EIGRP on the interfaces, the routers are not forming neighbor relationships. What is the most likely cause of this issue?
A) The router ID has not been configured on each router
B) The no shutdown command was not issued on the interfaces
C) EIGRP for IPv6 is enabled globally but not on the link-local addresses
D) OSPF is running on the same interfaces
Answer: A
Explanation:
EIGRP for IPv6 requires several configuration steps distinct from EIGRP for IPv4. One crucial requirement is that each EIGRP for IPv6 router must have a router ID configured. The router ID is a 32-bit value that uniquely identifies the router in the EIGRP domain. Without a router ID, EIGRP for IPv6 cannot establish neighbor relationships, and the routing process will not function correctly. While the router can have EIGRP enabled on interfaces (using ipv6 eigrp <AS-number>), the absence of a router ID prevents EIGRP from initiating the neighbor discovery process. Option B), not issuing no shutdown on interfaces, would indeed prevent interface-level communication, but typically interfaces are already administratively up during basic configurations, making this less likely. Option C) refers to link-local addresses, which are automatically assigned in IPv6, and EIGRP for IPv6 relies on these addresses for neighbor communication; however, if the router ID is missing, EIGRP cannot proceed regardless of link-local configuration. Option D), running OSPF on the same interfaces, does not inherently block EIGRP operation because multiple routing protocols can coexist on a single interface; the critical factor is correct EIGRP IPv6 configuration and router ID assignment. Understanding this distinction is vital for enterprise network troubleshooting and for exam preparation. Configuring a unique router ID ensures neighbor adjacency formation, proper topology exchange, and accurate route computation, which are essential for stable IPv6 network operation. This step is commonly overlooked in IPv6 deployments, leading to perplexing troubleshooting scenarios. For large-scale networks, consistently assigning router IDs also aids in topology visualization, network monitoring, and rapid fault isolation. Therefore, setting a router ID is a non-negotiable step in enabling EIGRP for IPv6 on any enterprise-grade network.
Question 197
An enterprise network uses BGP with multiple external peers. Some internal applications require routing via specific BGP paths. Which attribute should the engineer manipulate to control outbound traffic from the autonomous system?
A) Weight
B) MED
C) Local Preference
D) AS-path
Answer: C
Explanation:
BGP path selection is influenced by multiple attributes, each serving a specific function. To control outbound traffic originating from within the autonomous system, the most effective attribute is Local Preference. Local Preference is propagated within an autonomous system and determines which path internal routers prefer when advertising BGP routes to external peers. By assigning a higher local preference to routes learned from a preferred eBGP neighbor, internal routers will route traffic through the desired exit point. Option A), Weight, is a Cisco-specific attribute that is local to the router on which it is configured and is not propagated to other routers, making it ineffective for controlling AS-wide outbound traffic. Option B), MED (Multi-Exit Discriminator), influences inbound traffic by signaling the preferred entry point into the AS from an external AS but does not affect outbound routing decisions. Option D), AS-path, can be used to influence inbound traffic by making a path appear longer or shorter, but again it does not provide granular outbound control. Proper use of Local Preference allows enterprises to ensure predictable routing for critical applications, optimize bandwidth utilization, and maintain redundancy by dynamically adjusting priorities in multi-homed BGP environments. This approach is essential in large-scale deployments where multiple exit points exist, and specific traffic patterns must be enforced without disrupting overall network stability. Understanding and manipulating BGP attributes effectively is a fundamental skill for network engineers preparing for the 300-410 ENARSI exam and for designing scalable, resilient enterprise networks. Additionally, configuring route maps in conjunction with Local Preference allows fine-tuned control over specific prefixes, providing an additional layer of routing policy customization for complex application requirements.
Question 198
A network engineer is configuring HSRP on a pair of distribution switches for high availability. Users report intermittent connectivity issues when the primary HSRP router fails. Which HSRP feature should be verified to ensure seamless failover?
A) Preemption is enabled on both routers
B) The virtual MAC address is unique per VLAN
C) Timers are set to default values
D) Interface IP addresses match the virtual IP
Answer: A
Explanation:
HSRP (Hot Standby Router Protocol) provides high availability by creating a virtual router with a shared IP address and MAC address that clients use as their default gateway. The preemption feature is critical for ensuring seamless failover. Preemption allows a router with a higher priority to take over as the active router when it becomes available. If preemption is disabled, the previously active router will remain in charge even after recovering from failure, causing suboptimal routing and potential connectivity issues. Option B), ensuring a unique virtual MAC address per VLAN, is important for avoiding MAC conflicts but typically HSRP generates this automatically; conflicts are rare unless misconfigured. Option C), timer values, affects how quickly failover occurs, but even with default timers, the absence of preemption can prevent optimal active router selection. Option D), interface IP addresses, are separate from the virtual IP, which is what clients use as the default gateway; mismatched IPs on physical interfaces do not prevent HSRP failover. Enabling preemption ensures that the router with the highest configured priority becomes active, maintaining predictable traffic flows and preventing potential outages during failover events. In enterprise networks with multiple VLANs and high traffic density, preemption combined with optimized HSRP timers guarantees minimal disruption during failures and recovery. This is particularly vital in environments where real-time applications, such as VoIP or video conferencing, rely on uninterrupted gateway availability. Engineers should also verify HSRP priority values and ensure consistency across redundant devices to avoid unexpected active/standby behavior, thereby maintaining high network reliability.
Question 199
A company is deploying MPLS Layer 3 VPNs to connect multiple branch offices. Certain branches should not receive all VPN routes to maintain traffic isolation. Which mechanism provides this selective route advertisement?
A) Import and export route-targets in VRFs
B) Static routes on PE routers
C) Enable LDP across the MPLS core
D) Use OSPF redistribution into MPLS VPN
Answer: A
Explanation:
MPLS Layer 3 VPNs use VRFs (Virtual Routing and Forwarding) to isolate routing tables for different customers or segments. Each VRF maintains its own routing table, ensuring that traffic is logically separated across the shared MPLS backbone. To selectively control which routes are advertised or received by a VRF, engineers use import and export route-targets. Export route-targets determine which prefixes are shared from the VRF, while import route-targets control which external prefixes the VRF can import. Option B), using static routes, is unscalable for multi-branch deployments and does not provide dynamic route exchange. Option C), enabling LDP, is necessary for label distribution but does not provide selective route advertisement. Option D), redistributing OSPF into MPLS, may share routes but cannot enforce customer-specific isolation policies. By configuring route-targets, network engineers can enforce granular route control, maintain traffic segregation, and prevent unintentional route leakage, which is essential for compliance, security, and operational efficiency. Proper VRF and route-target configuration enables scalable and predictable MPLS VPN operations, allowing selective route propagation, inter-branch connectivity, and controlled sharing of resources. This mechanism is widely used in service provider and enterprise environments to support multiple tenants or departments over a single infrastructure without compromising security or routing integrity.
Question 200
An engineer is implementing OSPFv3 across multiple areas in an IPv6 enterprise network. The link-state database shows missing routes in certain areas. Which configuration ensures that all areas have complete routing information without unnecessary external routes?
A) Configure OSPFv3 area type as NSSA for stub areas requiring external routes
B) Use OSPFv3 virtual links between ABRs
C) Enable OSPFv3 redistribution from EIGRP
D) Set all areas as normal and allow default routes
Answer: A
Explanation:
OSPFv3, like OSPFv2, uses areas to segment the network and optimize link-state database size. When some areas need to reduce routing table size while still receiving certain external routes, NSSA (Not-So-Stubby Area) is the ideal solution. NSSA allows the area to inject limited external routes via Type 7 LSAs, which are then converted to Type 5 LSAs by the ABR for inter-area propagation. Option B), virtual links, are necessary only when there is a connectivity gap between the backbone and an area; they do not directly solve selective route advertisement or missing internal routes. Option C), redistributing from EIGRP, introduces external routes but may lead to unnecessary flooding and complexity, contrary to the requirement of controlled routing. Option D), setting all areas as normal, results in large routing tables and unnecessary propagation of external routes, defeating the purpose of area segmentation. NSSA allows an area to receive essential external routes without bloating the routing table, ensuring optimal memory and CPU usage on routers. Properly configuring NSSA improves convergence time, prevents suboptimal routing, and maintains network scalability, especially in large IPv6 deployments where internal link-state propagation must be tightly controlled. By understanding the interplay of OSPFv3 area types, engineers can ensure that all areas maintain complete routing information for internal and selective external prefixes while avoiding unnecessary overhead, ensuring predictable and stable enterprise network behavior.
