Cisco 300-410  Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam  Dumps and Practice Test Questions Set 1  Q1-20 

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Question 1:

 Which routing protocol is best suited for a large enterprise network requiring fast convergence and support for Variable Length Subnet Masking (VLSM)?

A) RIP
B) OSPF
C) EIGRP
D) BGP

Answer: B) OSPF

Explanation:

A) RIP is a distance-vector routing protocol that uses hop count as its metric. It is limited to a maximum of 15 hops, making it unsuitable for large enterprise networks. RIP also converges slowly because it relies on periodic updates rather than event-driven updates, which can cause routing loops during topology changes. Additionally, RIP does not efficiently support VLSM, which can lead to wasteful IP address utilization. Due to these limitations, RIP is not ideal for large-scale or complex networks.

B) OSPF is a link-state protocol that calculates the shortest path using Dijkstra’s algorithm. It is highly scalable, supports hierarchical design with multiple areas, and converges quickly, making it ideal for large enterprise networks. OSPF fully supports VLSM, allowing efficient subnetting and optimal IP address usage. Its ability to segment large networks into areas reduces routing table size and limits unnecessary propagation of routing updates, ensuring better performance and faster convergence. These features make OSPF the correct choice for this scenario.

C) EIGRP is a Cisco-proprietary hybrid routing protocol that combines distance-vector and link-state features. It converges faster than RIP and supports VLSM, making it a good choice for Cisco-only networks. However, it is proprietary, which limits interoperability with devices from other vendors, making it less ideal for large multi-vendor enterprise networks compared to OSPF.

D) BGP is an exterior gateway protocol used primarily for interconnecting autonomous systems on the internet. While it is highly scalable and can support very large networks, BGP is designed for inter-AS routing, not internal enterprise routing. Its convergence is slower than OSPF, and it is more complex to configure. Therefore, it is not suitable for the internal routing requirements of a large enterprise network.

Question 2:

Which type of IPv6 address is used for a one-to-many communication in a network?

A) Unicast
B) Anycast
C) Multicast
D) Broadcast

Answer: C) Multicast

Explanation:

A) Unicast addresses are used for one-to-one communication. A packet sent to a unicast address reaches only the specific interface associated with that address. While unicast is essential for direct communication between two devices, it cannot efficiently deliver data to multiple receivers simultaneously.

B) Anycast addresses are assigned to multiple interfaces, often on different devices. A packet sent to an anycast address is delivered to the nearest interface according to the routing protocol’s definition of “nearest.” This is useful for load balancing and redundancy, but anycast does not support one-to-many communication. Only a single destination receives the packet.

C) Multicast addresses are explicitly designed for one-to-many communication. A packet sent to a multicast address is delivered to all interfaces that have joined the multicast group. This allows efficient distribution of data to multiple receivers without sending multiple copies, making it ideal for streaming, video conferencing, or routing updates. Multicast eliminates the inefficiencies of sending repeated unicast packets to each receiver individually.

D) Broadcast addresses are used in IPv4 to send traffic to all devices in a subnet. However, IPv6 does not implement traditional broadcasting. Instead, multicast is used for one-to-many communication, making broadcast obsolete in IPv6 networks.

Question 3:

Which technology allows a service provider to offer multiple Layer 2 VPNs over the same backbone infrastructure?

A) MPLS
B) OSPF
C) EIGRP
D) GRE

Answer: A) MPLS

Explanation:

A) MPLS (Multiprotocol Label Switching) uses labels to forward packets instead of traditional IP routing. This allows service providers to create multiple virtual private networks over a shared backbone, isolating customer traffic while maintaining scalability. MPLS supports both Layer 2 and Layer 3 VPNs, traffic engineering, and fast reroute, making it the best solution for offering multiple Layer 2 VPNs efficiently.

B) OSPF is an interior gateway protocol used for distributing routing information within an autonomous system. While it ensures rapid convergence and efficient path selection, OSPF does not provide mechanisms for isolating multiple VPNs over a shared infrastructure.

C) EIGRP is a hybrid routing protocol designed for efficient routing and fast convergence within Cisco networks. While it can distribute routes and support VLSM, it does not provide the ability to segregate multiple Layer 2 VPNs across a shared backbone.

D) GRE (Generic Routing Encapsulation) tunnels encapsulate packets for point-to-point connections and can carry different Layer 3 protocols over IP networks. However, GRE tunnels are not scalable for multiple Layer 2 VPNs because they do not inherently isolate traffic or provide label-based forwarding like MPLS.

Question 4:

Which BGP attribute is primarily used to influence outbound traffic from a local autonomous system?

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

Answer: D) Weight

Explanation:

A) Local Preference is a BGP attribute used to influence route selection within an autonomous system for inbound traffic. It indicates which exit point should be preferred for routes coming into the AS, but it does not control outbound traffic directly.

B) AS Path records the sequence of autonomous systems that a route has traversed. Shorter AS paths are preferred, but AS Path primarily influences external route selection and indirectly affects inbound traffic from other ASes rather than controlling outbound paths.

C) MED (Multi-Exit Discriminator) is used to suggest a preferred entry point into a neighboring AS. MED affects inbound traffic by signaling to external peers which entry point is preferred but does not control outbound traffic from the local AS.

D) Weight is a Cisco-specific attribute applied locally on a router. It is not propagated to other routers, and a higher weight makes a route more preferred for outbound traffic from the local AS. Because it provides direct control over the router’s selection of outbound paths, weight is the most effective attribute for influencing outbound traffic.

Question 5:

Which protocol provides rapid failover for a default gateway in an enterprise network?

A) HSRP
B) OSPF
C) EIGRP
D) BGP

Answer: A) HSRP

Explanation:

A) HSRP (Hot Standby Router Protocol) provides redundancy for default gateways. Multiple routers share a virtual IP address, with one router actively forwarding traffic while the others remain in standby. If the active router fails, a standby router quickly takes over, ensuring minimal disruption to end devices. This makes HSRP ideal for maintaining continuous gateway availability.

B) OSPF is a routing protocol that provides rapid convergence in response to network topology changes. While it quickly recalculates routes, it does not create a virtual default gateway for hosts. Therefore, it cannot provide seamless failover for a default gateway.

C) EIGRP also provides fast convergence within an autonomous system. However, like OSPF, it is focused on route selection and calculation, not on providing high-availability for a default gateway IP address.

D) BGP is designed for routing between autonomous systems on the internet. It is highly scalable but does not provide default gateway redundancy within an enterprise network. BGP focuses on inter-AS path selection and control, not internal gateway failover.

Question 6

Which feature of OSPF allows large networks to be segmented into smaller, more manageable parts to reduce routing overhead?

A) Area
B) Route summarization
C) Stub networks
D) LSAs

Answer: A) Area

Explanation:

A) Area in OSPF is a fundamental concept that divides a large OSPF network into smaller, logical segments. Each area maintains its own link-state database and summarizes routing information when communicating with other areas. By dividing the network into areas, OSPF reduces the size of routing tables and limits the scope of routing updates. This segmentation prevents excessive link-state update flooding across the entire network, enhancing scalability and improving convergence times. For example, Area 0, the backbone area, interconnects all other areas, ensuring efficient routing across the enterprise network. Without areas, a large OSPF network would face significant overhead, with every router needing to maintain complete topology information for the entire network.

B) Route summarization is a technique to aggregate multiple IP prefixes into a single, summarized route. While summarization reduces routing table entries and can help optimize inter-area OSPF traffic, it is not the primary mechanism used to segment a network. Areas provide structural segmentation, whereas summarization is a supplementary optimization tool. Route summarization works within the framework of areas and cannot replace the area-based hierarchical design.

C) Stub networks are OSPF designations that restrict certain types of external routing information from entering an area. While this reduces unnecessary route advertisements and simplifies routing within an area, stub networks alone do not segment the network. They are primarily a tool for controlling external route distribution within an existing area, not for overall network segmentation.

D) LSAs, or Link-State Advertisements, are the packets used by OSPF routers to communicate topology information. While LSAs are critical for OSPF operation and convergence, they are the means by which routing information is exchanged, not a mechanism for segmenting the network. LSAs are generated within areas and propagated according to OSPF rules but cannot themselves divide a network into manageable portions.

Ultimately, areas are the cornerstone of OSPF hierarchical design, allowing large networks to be broken into smaller, manageable segments. They reduce routing overhead, enhance stability, and improve convergence, making this the correct answer. The other options (route summarization, stub networks, LSAs) are supporting mechanisms but do not provide the structural segmentation that areas enable.

Question 7:

Which feature of EIGRP ensures that only incremental updates are sent after the initial topology exchange?

A) Diffusing Update Algorithm (DUAL)
B) Split horizon
C) Hello packets
D) Periodic full updates

Answer: A) Diffusing Update Algorithm (DUAL)

Explanation:

A) The Diffusing Update Algorithm (DUAL) is the core of EIGRP’s operation. It allows the protocol to maintain loop-free, optimal paths while minimizing unnecessary updates. After the initial topology exchange, EIGRP sends only incremental updates to neighboring routers whenever a topology change occurs. This efficiency reduces network bandwidth usage and enhances convergence times. DUAL ensures that only routes impacted by a change are updated, rather than sending the entire routing table repeatedly. This capability distinguishes EIGRP from traditional distance-vector protocols like RIP, which rely on periodic full updates.

B) Split horizon is a technique used to prevent routing loops by not advertising a route back out the interface from which it was learned. While split horizon contributes to loop prevention, it does not control the sending of incremental versus full updates. It is a complementary feature but not the primary mechanism ensuring incremental updates.

C) Hello packets are used by EIGRP to establish and maintain neighbor relationships. They ensure that routers recognize each other and keep the adjacency alive, but they do not handle routing table updates directly. While essential for neighbor discovery and reliability, Hello packets are not responsible for incremental updates.

D) Periodic full updates are characteristic of older distance-vector protocols like RIP, where the entire routing table is sent at regular intervals regardless of topology changes. EIGRP does not rely on periodic full updates, which is why this option is incorrect. DUAL enables EIGRP to avoid this inefficiency entirely.

In conclusion, DUAL is the feature that allows EIGRP to send only incremental updates after the initial topology exchange. The other mechanisms (split horizon, Hello packets, periodic full updates) are either complementary or unrelated to this functionality, making them incorrect. DUAL’s design enhances network efficiency, reduces bandwidth consumption, and ensures rapid, loop-free convergence.

Question 8:

Which type of BGP neighbor relationship is established between routers in the same autonomous system to exchange internal routing information?

A) iBGP
B) eBGP
C) Confederation
D) Route reflector

Answer: A) iBGP

Explanation:

A) iBGP, or internal BGP, is used to exchange BGP routing information between routers within the same autonomous system (AS). iBGP routers share routes learned from external BGP peers (eBGP) or other iBGP routers, ensuring consistent route propagation inside the AS. Unlike eBGP, which connects different autonomous systems, iBGP maintains the integrity of routing policies within a single AS. iBGP requires a full mesh of peerings unless route reflectors or confederations are used to reduce the number of peerings. Its main purpose is to ensure that all routers within an AS have a consistent view of external routes.

B) eBGP, or external BGP, is used to exchange routing information between routers in different autonomous systems. It is critical for connecting enterprises to ISPs or interconnecting service providers. However, eBGP is not intended for exchanging routes within the same AS, making it incorrect for this scenario.

C) Confederation is a technique to subdivide a large AS into smaller sub-ASes to reduce the complexity of iBGP full-mesh peerings. While confederations facilitate scalability, they are not themselves a type of neighbor relationship. They work alongside iBGP to simplify the topology but are not the primary mechanism for exchanging internal BGP routes.

D) Route reflectors are special routers configured to redistribute iBGP routes to other iBGP peers, reducing the need for a full mesh. Although route reflectors help scale iBGP, they do not replace the iBGP neighbor relationship itself. Without iBGP, route reflectors cannot function.

Therefore, iBGP is the correct answer because it establishes the neighbor relationships necessary for exchanging internal BGP routes within the same autonomous system. The other options either operate externally (eBGP) or assist in scaling iBGP (confederation, route reflectors) but do not serve the primary function of internal BGP route exchange.

Question 9:

Which type of IPv6 OSPF LSA is used to advertise routes between areas?

A) Type 1
B) Type 2
C) Type 3
D) Type 4

Answer: C) Type 3

Explanation:

A) Type 1 LSAs, or Router LSAs, are generated by all OSPF routers to describe their interfaces and link states within an area. These LSAs remain confined to a single area and are not propagated between areas. While essential for area-level topology, Type 1 LSAs do not facilitate inter-area route advertisement, so they are incorrect for this question.

B) Type 2 LSAs, or Network LSAs, are generated by Designated Routers (DRs) to describe multi-access networks within an area. Type 2 LSAs only exist within a single area and do not cross area boundaries. They inform routers about all routers attached to a multi-access segment but are not used for inter-area route distribution.

C) Type 3 LSAs, or Summary LSAs, are generated by Area Border Routers (ABRs) to advertise networks from one OSPF area to another. These LSAs summarize routes from one area and propagate them into other areas, allowing routers in separate areas to learn about each other’s networks without having complete knowledge of the internal topology of other areas. This mechanism reduces routing table size and limits unnecessary LSA flooding across the OSPF network. Because Type 3 LSAs specifically serve the function of advertising routes between areas, this is the correct answer.

D) Type 4 LSAs, or ASBR Summary LSAs, are used to advertise the presence of an Autonomous System Boundary Router (ASBR) to other areas. They help routers locate the ASBR for external routes but are not used for general inter-area route advertisement. Type 4 LSAs are more specialized and serve a limited function compared to Type 3 LSAs.

In conclusion, Type 3 LSAs are the correct choice because they are explicitly designed to carry routing information between OSPF areas. The other LSA types (Type 1, Type 2, Type 4) either operate within a single area or have specialized purposes that do not include general inter-area route advertisement.

Question 10:

Which mechanism in HSRP determines which router becomes the active router in a group?

A) Priority
B) IP Address
C) Hello Timer
D) Preemption

Answer: A) Priority

Explanation:

A) Priority is the primary mechanism used in HSRP to determine which router becomes the active router. Each HSRP router is assigned a priority value ranging from 0 to 255. The router with the highest priority becomes the active router, managing traffic for the virtual IP address. If multiple routers have the same priority, the router with the highest IP address becomes the tiebreaker. Priority allows network administrators to control failover behavior and ensure that the most capable router handles the traffic.

B) IP Address is used as a tiebreaker if multiple routers have the same priority. While the IP address can influence the selection in rare cases of equal priority, it is not the primary determinant for active router selection. Its role is secondary and only comes into play under specific circumstances.

C) Hello Timer determines how frequently HSRP routers send hello messages to communicate status and maintain adjacency. While hello timers affect the detection of failures and convergence times, they do not determine which router is initially selected as active. Hello timers are important for responsiveness but not for priority-based selection.

D) Preemption allows a router with higher priority to take over as the active router if it comes online after a lower-priority router is already active. Preemption ensures that the desired router becomes active when available, but it is dependent on priority values. Without priority configuration, preemption alone cannot determine which router should be active.

Therefore, priority is the correct answer because it is the primary factor that designates the active router in an HSRP group. The other mechanisms either play supporting roles or act as tiebreakers, making them insufficient to determine active router selection on their own.

Question 11:

Which technology is primarily used to prevent routing loops in a Layer 3 VPN deployment?

A) Route Target
B) Split Horizon
C) BGP Confederation
D) Route Reflector

Answer: B) Split Horizon

Explanation:

A) Route Target is an extended BGP community used in MPLS Layer 3 VPNs to control the import and export of routes between different VRFs (Virtual Routing and Forwarding instances). While route targets are essential for segregating customer routes and ensuring that VPN routes are propagated correctly between provider edge routers, they do not inherently prevent routing loops. Route targets dictate policy and route distribution but cannot independently enforce loop prevention. In a scenario where multiple VRFs or overlapping routes exist, improper configuration of route targets could still lead to routing inconsistencies, making them unsuitable as a standalone loop-prevention mechanism.

B) Split Horizon is a loop-prevention technique widely used in both distance-vector routing protocols and MPLS VPN deployments. In the context of Layer 3 VPNs, split horizon prevents a router from advertising a route back out the interface through which it was learned. This prevents routing loops in complex topologies where multiple paths exist between provider edge routers and customer sites. By ensuring that a learned route is not re-advertised back to its source, split horizon maintains loop-free forwarding and ensures stable convergence within the VPN. Split horizon is particularly important in scenarios involving redundant provider edge routers or multiple MPLS paths. Unlike other mechanisms, it provides a simple yet highly effective method of preventing loops at the data-plane level, which is why it is the correct answer.

C) BGP Confederation is a technique used to divide a large autonomous system into smaller sub-ASes to reduce the complexity of iBGP full-mesh peerings. Confederations improve scalability and manageability, but they are not inherently designed to prevent routing loops in VPN deployments. While they do reduce the number of iBGP sessions required and can simplify path selection, loop prevention within VPNs relies on mechanisms such as split horizon and route reflection rules rather than the confederation itself.

D) Route Reflector is a method in iBGP to reduce the need for a full-mesh peering. A route reflector allows a central router to reflect iBGP routes to other routers in the same AS. While route reflectors improve scalability and can reduce administrative overhead, they do not inherently prevent routing loops. Improper configuration of route reflectors, such as reflecting routes back to the original advertiser, can actually introduce loops. Loop prevention is maintained through additional configuration rules and route reflection policies, not by the route reflector feature alone.

In conclusion, split horizon is the primary mechanism used in Layer 3 VPN deployments to prevent routing loops. Route target, BGP confederation, and route reflectors each play critical roles in policy control, scalability, or route distribution but do not serve as the core loop-prevention method. Split horizon operates directly at the routing advertisement level, ensuring that learned routes are never re-advertised along the same path, which is essential for loop-free forwarding in VPN networks.

Question 12:

 Which OSPF network type requires a designated router (DR) and a backup designated router (BDR)?

A) Broadcast
B) Point-to-Point
C) Point-to-Multipoint
D) Non-Broadcast Multi-Access (NBMA)

Answer: A) Broadcast

Explanation:

A) Broadcast networks in OSPF include Ethernet LANs where multiple routers can be connected on the same physical network segment. In this type of network, a Designated Router (DR) and Backup Designated Router (BDR) are elected to reduce the number of LSAs (Link-State Advertisements) exchanged. The DR serves as the central point for distributing LSAs, ensuring that routers do not have to form full adjacencies with every other router on the network. The BDR serves as a standby to take over if the DR fails. This election reduces the overall OSPF overhead, optimizes LSA propagation, and ensures network stability. Without a DR/BDR, the number of OSPF adjacencies would grow quadratically with the number of routers, leading to unnecessary resource consumption.

B) Point-to-Point networks involve a direct link between two routers. Because there are only two routers on the segment, there is no need for a DR or BDR. Each router can form a full adjacency with the other directly, and LSAs can be exchanged without an intermediary. Point-to-point networks inherently have minimal overhead due to the small number of participants, so DR/BDR elections are unnecessary.

C) Point-to-Multipoint networks simulate multiple point-to-point connections over a single physical interface. In this design, routers treat each connection as a separate logical point-to-point link. OSPF does not require a DR/BDR for point-to-multipoint networks because LSAs can be exchanged directly between routers, just as in point-to-point scenarios. Although OSPF can operate in broadcast mode over some point-to-multipoint networks, the DR/BDR requirement is not inherent to this network type.

D) Non-Broadcast Multi-Access (NBMA) networks include technologies such as Frame Relay or ATM, where multiple routers share the same network but do not support native broadcast. NBMA networks require manual configuration of neighbors and may also use DR/BDR elections to optimize LSA distribution. However, unlike broadcast networks, the election process is more complex and requires additional configuration steps. The DR/BDR requirement is conditional, depending on the deployment scenario, whereas broadcast networks automatically trigger DR/BDR elections.

The correct answer is broadcast networks because they automatically require a DR and BDR to reduce adjacency overhead and streamline LSA flooding. Point-to-point and point-to-multipoint networks do not require these roles due to their inherent topology, and NBMA networks require additional manual configuration, making them less straightforward.

Question 13:

 Which routing protocol supports route tagging to prevent routing loops in redistribution scenarios?

A) OSPF
B) EIGRP
C) BGP
D) RIP

Answer: B) EIGRP

Explanation:

A) OSPF does not inherently support route tagging in redistribution scenarios. While OSPF allows external routes to be redistributed and can mark them as E1 or E2 for cost calculations, it lacks a native mechanism to tag routes to prevent redistribution loops. Without additional techniques such as route maps, OSPF alone cannot provide fine-grained loop prevention when exchanging routes with other protocols.

B) EIGRP supports route tagging, a feature that allows administrators to mark redistributed routes with unique identifiers. These tags can then be used to control the flow of routes during redistribution between multiple protocols or autonomous systems. For example, when redistributing routes from OSPF into EIGRP and then back into OSPF, route tags can prevent loops by identifying routes that originated externally and avoiding re-injection into the source protocol. This capability is critical in complex networks where multiple redistribution points exist. EIGRP’s support for route tagging ensures that network designers can maintain loop-free topologies and enforce precise routing policies, making it the correct answer.

C) BGP does not use route tagging in the same way as EIGRP. Instead, BGP relies on AS Path attributes, communities, and policy-based filtering to prevent routing loops. While these mechanisms are effective for inter-AS routing, they are not equivalent to the route tagging feature in EIGRP that is used during redistribution between different protocols or administrative domains. BGP handles loop prevention through path attributes rather than explicit route tags.

D) RIP lacks a native route tagging mechanism. When routes are redistributed into or out of RIP, additional configuration using route maps may be necessary to avoid loops, but RIP itself does not provide a built-in feature for marking and tracking redistributed routes. As a simple distance-vector protocol, RIP’s capabilities are limited, and it is prone to loops without careful manual configuration.

EIGRP is the correct answer because it allows administrators to tag routes during redistribution, providing an automated and reliable method to prevent routing loops. OSPF and RIP require additional configuration to achieve similar results, while BGP relies on different mechanisms that do not operate in the same context.

Question 14:

Which type of MPLS VPN separates customer routing tables and ensures that overlapping IP addresses do not cause conflicts?

A) Layer 2 VPN
B) Layer 3 VPN
C) VPLS
D) GRE Tunnel

Answer: B) Layer 3 VPN

Explanation:

A) Layer 2 VPNs extend a customer’s Layer 2 network across a service provider backbone. They allow customers to connect geographically separated sites as if they were on the same LAN. While Layer 2 VPNs provide transparency and isolation at Layer 2, they do not inherently separate routing tables at Layer 3. Overlapping IP addresses in different customer sites could lead to conflicts unless additional mechanisms like VLAN segmentation are implemented.

B) Layer 3 VPNs use MPLS and VRFs (Virtual Routing and Forwarding) to maintain separate routing tables for each customer. VRFs ensure that overlapping IP addresses do not interfere with each other because each customer’s routes are isolated from other VRFs. MPLS labels are used to forward traffic across the provider backbone without exposing customer routing information. Layer 3 VPNs provide both traffic isolation and scalable IP routing for multiple customers over the same physical infrastructure, making this the correct answer.

C) VPLS (Virtual Private LAN Service) is a Layer 2 VPN technology that emulates a LAN across geographically dispersed customer sites. VPLS provides Layer 2 connectivity but does not inherently provide separate routing tables at Layer 3. It focuses on Ethernet frame delivery rather than IP routing isolation, so overlapping IP addresses may still require additional configuration to avoid conflicts.

D) GRE tunnels encapsulate traffic between endpoints, allowing the transport of different Layer 3 protocols over an IP network. While GRE provides connectivity and can segment traffic, it does not inherently isolate routing tables or prevent IP address conflicts between multiple customers. GRE tunnels require careful manual configuration for separation and do not scale as efficiently as MPLS Layer 3 VPNs.

Layer 3 VPNs are the preferred solution for separating customer routing tables and ensuring overlapping IP addresses do not cause conflicts. The other technologies either operate at Layer 2 (Layer 2 VPNs, VPLS) or require manual configuration for separation (GRE), making them less suitable for scalable, isolated customer routing.

Question 15:

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

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

Answer: C) MED

Explanation:

A) Weight is a Cisco-specific BGP attribute that is used to influence outbound traffic selection from a local router. It is not propagated to other routers and has no effect on inbound traffic from external autonomous systems. While it is a powerful tool for controlling local routing decisions, it is irrelevant for influencing how other autonomous systems send traffic to you.

B) Local Preference is a well-known BGP attribute that affects route selection within an autonomous system. It is used to influence outbound traffic decisions and preference for specific exit points, but it does not affect how neighboring autonomous systems send traffic to your AS. Therefore, local preference is not applicable for controlling inbound traffic.

C) MED (Multi-Exit Discriminator) is an optional BGP attribute used to signal preferred entry points into your autonomous system from an external AS. A lower MED value indicates a more preferred path for inbound traffic. Unlike weight or local preference, MED is communicated to external neighbors and influences their route selection, making it the correct mechanism for controlling how external autonomous systems route traffic into your network.

D) AS Path records the autonomous systems a route has traversed and is used primarily for loop prevention and path selection. While prepending AS numbers can influence inbound traffic by making a path appear longer, it is a coarse mechanism compared to MED and is generally used in combination with MED for precise traffic engineering. AS Path alone is not the primary attribute for inbound traffic control.

MED is the correct answer because it explicitly provides a method for influencing inbound traffic from external autonomous systems. The other attributes either control local router behavior or provide indirect influence, making them less suitable for fine-grained inbound traffic engineering.

Question 16:

Which protocol is used by Cisco to provide fast convergence for IPv6 routing in an enterprise network?

A) OSPFv3
B) EIGRP for IPv6
C) RIPng
D) BGP

Answer: B) EIGRP for IPv6

Explanation:

A) OSPFv3 is the IPv6 version of OSPF. It is a link-state routing protocol that supports hierarchical design with areas, provides rapid convergence, and scales well in large enterprise networks. OSPFv3 uses the same Dijkstra SPF algorithm as OSPFv2 and provides efficient route calculation and loop-free paths. While OSPFv3 is highly reliable and supports IPv6, its convergence speed is slightly slower than EIGRP for IPv6 in scenarios where immediate neighbor updates and topology changes need rapid recovery, because OSPFv3 requires recalculating SPF trees and flooding LSAs throughout the area hierarchy, which can add overhead in large-scale networks.

B) EIGRP for IPv6 is a Cisco-proprietary routing protocol designed to provide fast convergence in IPv6 networks. It combines the benefits of distance-vector and link-state protocols, maintaining a topology table and using the Diffusing Update Algorithm (DUAL) to compute loop-free paths. EIGRP for IPv6 sends only incremental updates after the initial topology exchange, which reduces bandwidth consumption and allows very fast adaptation to network changes. Because of its incremental update mechanism, neighbor discovery, and optimized topology maintenance, EIGRP for IPv6 converges more quickly than link-state protocols in many enterprise deployments. Its support for VLSM, summarized routing, and multiple metrics (bandwidth, delay, reliability, load) makes it a highly flexible choice for enterprise networks requiring fast recovery from failures. For these reasons, EIGRP for IPv6 is the best answer.

C) RIPng is the IPv6 version of RIP. It is a distance-vector protocol that relies on hop count as its metric and has a maximum hop limit of 15. RIPng sends periodic updates every 30 seconds and lacks sophisticated loop-prevention mechanisms found in EIGRP or OSPF. Due to its slow convergence and limited scalability, RIPng is generally unsuitable for large or highly dynamic enterprise networks. It can lead to routing loops or temporary black holes during network changes, making it a poor choice for fast convergence.

D) BGP, while supporting IPv6 and essential for inter-AS routing, is designed for wide-area networks and large-scale internet connectivity. Its convergence is slower than interior routing protocols because path selection is based on policy, AS Path attributes, and other complex rules. BGP is not optimized for rapid convergence in an enterprise LAN/WAN environment where quick failover and minimal downtime are critical. Its complexity and slower reaction to topology changes make it unsuitable for the specific scenario of fast IPv6 convergence within an enterprise.

In EIGRP for IPv6 provides the fastest convergence in typical enterprise IPv6 environments due to its incremental updates, DUAL algorithm, and loop-free topology maintenance. OSPFv3, RIPng, and BGP either converge more slowly or are unsuitable for rapid recovery in such scenarios, making EIGRP the most appropriate choice.

Question 17:

Which mechanism allows BGP routers to select the best path when multiple routes to the same destination exist?

A) Best Path Algorithm
B) DUAL Algorithm
C) SPF Algorithm
D) Route Tag

Answer: A) Best Path Algorithm

Explanation:

A) The Best Path Algorithm is the procedure BGP uses to select the single best path to reach a destination when multiple routes exist. BGP evaluates paths based on multiple attributes in a hierarchical order: Weight (Cisco-specific), Local Preference, Local Origination, AS Path length, Origin type, MED, eBGP over iBGP, IGP cost to next hop, router ID, and other tie-breakers. This algorithm ensures that routers consistently choose the most preferred path while maintaining policy control and loop-free routing across autonomous systems. It allows precise influence over outbound and inbound traffic through manipulation of BGP attributes. Without this mechanism, BGP routers would have no systematic way to determine which path to install in the routing table.

B) DUAL Algorithm is specific to EIGRP. It calculates loop-free paths and ensures rapid convergence within an EIGRP network. While DUAL is highly effective for EIGRP, it does not apply to BGP. BGP relies on policy-based selection and the Best Path Algorithm rather than DUAL to choose routes. Therefore, DUAL is not relevant for BGP path selection.

C) SPF (Shortest Path First) Algorithm is used in link-state routing protocols such as OSPF and IS-IS. SPF calculates the shortest path tree for each router using Dijkstra’s algorithm and ensures loop-free paths within the network. While SPF is essential for OSPF, it is not used in BGP because BGP is a path-vector protocol that relies on policy and attributes rather than pure link-state metrics.

D) Route Tag is used primarily in redistribution scenarios to mark routes for identification or policy control, especially in EIGRP and sometimes OSPF. Route tags can help prevent routing loops during redistribution but are not a mechanism for selecting the best path in BGP. They provide metadata for administrative purposes rather than determining the routing decision itself.

The correct answer is the Best Path Algorithm because it is the built-in procedure BGP uses to consistently evaluate and select the most preferred path from multiple candidates. DUAL and SPF belong to other protocols, and route tags are administrative tools rather than path selection mechanisms.

Question 18:

Which technology enables the extension of a VLAN across multiple switches without spanning tree loops?

A) VTP
B) EtherChannel
C) MSTP
D) VLAN Trunking

Answer: C) MSTP

Explanation:

A) VTP (VLAN Trunking Protocol) is a Cisco protocol that propagates VLAN configuration across multiple switches. While VTP ensures consistent VLAN databases across a network, it does not prevent spanning tree loops or optimize path selection for VLAN traffic. VTP is strictly a VLAN management protocol and cannot solve the loop-prevention problem inherent in Ethernet networks. Without proper spanning tree mechanisms, VTP alone cannot safely extend VLANs across multiple switches.

B) EtherChannel is a method to bundle multiple physical links into a single logical link to increase bandwidth and provide redundancy. While EtherChannel can prevent certain layer-2 failures and improve bandwidth utilization, it does not inherently extend VLANs across switches or prevent spanning tree loops across multiple VLANs. It is a link aggregation mechanism rather than a loop-prevention or VLAN-extension protocol.

C) MSTP (Multiple Spanning Tree Protocol) is an evolution of STP and RSTP that allows multiple VLANs to be mapped to separate spanning tree instances. MSTP prevents spanning tree loops while enabling VLANs to traverse multiple switches efficiently. By creating multiple spanning tree instances, MSTP allows VLANs to follow optimized paths, reduces unnecessary blocked ports, and provides load balancing across redundant links. MSTP ensures that VLANs can be extended across switches without causing broadcast storms or loops, making it the correct choice.

D) VLAN Trunking is a method to carry multiple VLANs over a single physical link using tagging (802.1Q). While VLAN trunking enables VLAN extension across switches, it does not prevent spanning tree loops by itself. Without a spanning tree mechanism, trunking could allow loops and broadcast storms. Therefore, VLAN trunking alone is insufficient for safe VLAN extension across multiple switches.

MSTP is the correct answer because it combines loop prevention, VLAN segregation, and path optimization to safely extend VLANs across multiple switches. VTP and VLAN trunking handle configuration and tagging but not loops, and EtherChannel provides link aggregation rather than spanning tree management.

Question 19:

 Which method is used in MPLS to forward packets based on labels rather than IP addresses?

A) LDP
B) RSVP-TE
C) Label Switching
D) GRE

Answer: C) Label Switching

Explanation:

A) LDP (Label Distribution Protocol) is used in MPLS to distribute labels between routers. It communicates which label corresponds to which FEC (Forwarding Equivalence Class). While LDP enables the establishment of label mappings, it is not the actual process by which packets are forwarded. LDP is a control plane mechanism for label distribution, not the forwarding method itself.

B) RSVP-TE (Resource Reservation Protocol – Traffic Engineering) is used in MPLS networks to establish explicitly routed label-switched paths (LSPs) for traffic engineering. RSVP-TE signals label assignments along a path but does not constitute the forwarding method itself. It provides more control than LDP for QoS and reserved bandwidth scenarios but still relies on label switching for packet forwarding.

C) Label Switching is the fundamental data-plane mechanism in MPLS. Instead of examining the IP header for each hop, MPLS routers use the label to make forwarding decisions. Each packet is assigned a label at the ingress router, and intermediate routers (LSRs) use the label to forward packets efficiently along the pre-established LSP. This method dramatically reduces lookup time, enables traffic engineering, and supports VPN isolation. Label switching is the core of MPLS operation and is the correct answer.

D) GRE (Generic Routing Encapsulation) is a tunneling protocol used to encapsulate various Layer 3 protocols inside IP packets. GRE can carry MPLS or other traffic across networks but does not perform label-based forwarding itself. GRE is a tunnel mechanism, not an MPLS forwarding mechanism.

Label switching is correct because it is the core MPLS forwarding mechanism, allowing routers to forward packets based on labels rather than performing full IP lookups. LDP and RSVP-TE are control-plane signaling protocols, and GRE is an encapsulation method, not a forwarding mechanism.

Question 20:

Which OSPF feature allows summarization of routes between areas to reduce routing table size?

A) Area Border Router
B) Type 3 LSA
C) Stub Area
D) ABR Summary

Answer: D) ABR Summary

Explanation:

A) Area Border Routers (ABRs) are routers that connect one OSPF area to another, particularly to the backbone area (Area 0). ABRs maintain separate link-state databases for each area and propagate routing information between areas. While ABRs are essential for hierarchical OSPF design, the ABR itself is a router, not a specific mechanism for summarization.

B) Type 3 LSAs, or summary LSAs, are generated by ABRs to advertise networks from one area to another. Type 3 LSAs carry the summarized route information, but the act of summarization must be configured on the ABR. Simply having a Type 3 LSA does not automatically reduce routing table size unless summarization is applied. Type 3 LSAs are part of the process, but the feature enabling summarization is the ABR’s configuration.

C) Stub Areas are OSPF areas that restrict external route advertisements to simplify routing and reduce resource consumption. While stub areas reduce external routing information, they do not perform summarization between areas. Their function is limited to filtering external routes, not aggregating multiple prefixes for efficiency.

D) ABR Summary is the specific feature where the Area Border Router summarizes multiple routes from one area into a single advertisement for another area. This reduces the size of routing tables, minimizes the number of LSAs propagated, and improves network scalability. By summarizing routes, ABRs prevent unnecessary routing detail from propagating outside an area, allowing large networks to maintain efficient routing tables. This is the correct answer because it explicitly refers to the summarization feature, not just the LSAs or area types involved.

ABR summary reduces routing table size and controls LSA flooding efficiently, whereas the other choices describe routers, LSAs, or area types without directly providing the summarization feature.