Cisco 300-410  Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam  Dumps and Practice Test Questions Set 4 Q61 -80

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Question 61:

Which EIGRP feature allows traffic to be distributed across 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 backup route maintained in the EIGRP topology table that satisfies the feasibility condition to guarantee loop-free paths. While feasible successors can serve as backup paths for immediate failover, they do not inherently control how traffic is distributed across multiple paths. Feasible successors are used primarily for fast convergence, not load balancing.

B) Variance is an EIGRP feature that enables unequal-cost load balancing. By default, EIGRP performs equal-cost load balancing across multiple successor routes. The variance command allows the router to include feasible successors with a metric less than the product of the primary route metric and the variance value. This enables traffic to be distributed across multiple paths with different costs, improving network utilization and redundancy. Variance works in combination with feasible successors to determine which backup routes are eligible for traffic forwarding. By configuring variance, network engineers can fine-tune load distribution for high-capacity paths, optimize bandwidth, and prevent overloading of a single route. This makes variance the correct answer because it explicitly controls unequal-cost traffic distribution.

C) Route Summarization reduces routing table size and advertisement overhead by aggregating multiple contiguous subnets into a single summary route. While summarization improves efficiency and reduces routing update traffic, it does not control the distribution of traffic across multiple paths with different metrics.

D) Hold Timer determines the interval a router waits before declaring a neighbor down if no hello packets are received. The hold timer affects convergence and neighbor detection but does not control load balancing or path selection.

Variance is correct because it directly enables traffic distribution across unequal-cost paths in EIGRP. Feasible successors, route summarization, and hold timers provide backup, efficiency, and stability, but they do not control traffic distribution across multiple routes.

Question 62:

Which OSPFv3 LSA type carries external route information for injection into an area?

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 networks from one area into another. Type 3 LSAs carry inter-area routes, not external routes. While they reduce LSA flooding by summarizing internal prefixes, they do not carry external route information.

B) Type 5 LSAs are external LSAs used to propagate external routes (from redistribution) into OSPF. However, Type 5 LSAs are not generated inside Not-So-Stubby Areas (NSSA). In NSSAs, Type 5 LSAs are blocked, so another mechanism is required for external route injection.

C) Type 7 LSAs are used specifically within NSSAs to carry external route information. Routers within an NSSA can redistribute routes from other protocols into OSPF using Type 7 LSAs. These LSAs are eventually translated to Type 5 LSAs by the ABR before being flooded to the rest of the OSPF domain. Type 7 LSAs allow controlled injection of external routes without flooding the entire OSPF network, making this the correct answer.

D) Type 1 LSAs describe a router’s links within an area. They are strictly intra-area LSAs and do not carry external or inter-area route information.

Type 7 LSAs are correct because they allow external route injection within NSSAs, preserving hierarchical OSPF scalability. Type 3 LSAs summarize internal routes, Type 5 LSAs are blocked in NSSAs, and Type 1 LSAs only describe intra-area topology.

Question 63:

Which BGP attribute is used to control exit points for outbound traffic across all routers in an autonomous system?

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

Answer: B) Local Preference

Explanation:

A) Weight is a Cisco-proprietary attribute that influences path selection only on the local router. While it ensures that a particular path is preferred locally, it does not propagate to other routers in the autonomous system. Weight cannot enforce consistent outbound path selection network-wide.

B) Local Preference is a well-known BGP attribute propagated throughout the AS. It allows network engineers to define which exit points should be preferred for outbound traffic. By assigning higher local preference values to certain routes, routers throughout the AS consistently select the desired path for traffic leaving the autonomous system. Local preference is crucial in multi-homed environments to enforce routing policy and ensure predictable outbound traffic behavior, making it the correct answer.

C) MED (Multi-Exit Discriminator) influences how external neighbors choose which path to use when entering your AS. MED affects inbound traffic selection rather than outbound traffic from within the AS.

D) AS Path records the autonomous systems traversed by a route. Manipulating AS Path (e.g., by prepending) affects external route selection and loop prevention, not consistent outbound traffic control within the AS.

Local Preference is correct because it allows coordinated outbound path selection across the entire AS. Weight is router-local, MED affects inbound traffic, and AS Path is for inter-AS policy and loop prevention.

Question 64:

Which HSRP state represents a 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 responsible for forwarding traffic for the HSRP virtual IP. The router responds to ARP requests for the VIP, handles all traffic directed to it, and is the primary gateway for hosts in the subnet. Active routers are central to maintaining uninterrupted network connectivity, making this the correct answer.

B) Standby is the state in which a router is ready to take over if the active router fails. While it monitors hello messages and maintains backup state information, it does not forward traffic unless failover occurs.

C) Listen is an initial state where a router is aware of the HSRP group and monitors hello messages but is not actively participating in forwarding or standby election.

D) Init is the initial startup state when a router has not yet received hello messages from neighbors. It is a preparatory state and does not involve traffic forwarding or election participation.

Active is correct because it represents the router currently forwarding traffic for the virtual IP. Standby is backup, listen is passive, and init is preliminary.

Question 65:

Which MPLS feature allows multiple VPNs to share the same infrastructure while keeping their traffic isolated?

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

Answer: A) VRF

Explanation:

A) VRF (Virtual Routing and Forwarding) provides logical separation of routing tables and forwarding for multiple VPNs on the same physical device. Each VRF maintains its own interfaces, routing table, and forwarding information. Combined with MPLS labels, VRFs ensure traffic from one VPN cannot interfere with traffic from another, even if the same physical links are shared. VRF allows overlapping IP addressing and complete isolation of customer traffic, making it the correct answer.

B) LDP (Label Distribution Protocol) distributes MPLS labels for forwarding along FECs. While necessary for MPLS operation, LDP does not maintain separate routing tables or provide traffic isolation for multiple VPNs.

C) RSVP-TE establishes explicit label-switched paths with traffic engineering and bandwidth reservation. It ensures predictable path usage and performance but does not provide VPN separation or logical isolation of multiple routing domains.

D) QoS manages traffic prioritization, bandwidth allocation, and delay. QoS ensures performance but does not provide isolation of traffic or routing tables for multiple VPNs.

VRF is correct because it allows multiple VPNs to share a single infrastructure while maintaining complete logical separation. LDP, RSVP-TE, and QoS support MPLS operations and performance but do not provide VPN isolation.

Question 66:

Which EIGRP metric component reflects the minimum bandwidth along a path?

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

Answer: A) Bandwidth

Explanation:

A) Bandwidth is a key metric component in EIGRP used to calculate the composite metric for path selection. EIGRP evaluates the minimum bandwidth along a route, meaning the slowest link determines the overall effective bandwidth. Bandwidth is incorporated into the metric formula to prioritize high-speed paths. For example, a route with a faster slowest link will be preferred over a route with a slower slowest link, even if other factors are similar. This ensures that EIGRP selects paths that provide higher throughput, optimizing performance. Bandwidth is especially important in enterprise networks with mixed-speed links because it allows the routing protocol to favor faster connections. Therefore, bandwidth is the correct answer because it directly reflects the limiting speed of the path.

B) Delay measures the cumulative transmission delay along a route, including propagation and queuing delays. While delay is critical for determining latency-sensitive paths, it does not indicate the capacity of the link. Delay is used alongside bandwidth in the EIGRP metric calculation, but it specifically reflects time, not throughput.

C) Reliability reflects the historical stability or error rate of a link. While a link with low reliability might be less preferred, reliability does not quantify the effective bandwidth or limit of a path. It serves as a secondary metric that can influence route selection when multiple feasible paths exist, but it is not the component used to directly reflect path speed.

D) Load measures current utilization of an interface. While load indicates potential congestion, it is dynamic and does not reflect the static characteristics of the path’s capacity. Load is considered optional in the EIGRP metric formula, and it is unsuitable for representing the minimum bandwidth along a path.

Bandwidth is correct because it identifies the slowest link in a path, which is essential for route selection in EIGRP. Delay, reliability, and load are complementary metrics that influence path choice but do not measure minimum bandwidth.

Question 67:

Which OSPF area type blocks both Type 3 and Type 5 LSAs, leaving only a default route for the area?

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

Answer: B) Totally Stubby Area

Explanation:

A) Stub areas block Type 5 external LSAs but still allow Type 3 inter-area LSAs. This allows routers in the stub area to learn about other areas but prevents flooding of external routes. Stub areas optimize SPF calculation and routing table size but do not block inter-area summary routes.

B) Totally Stubby Areas block both Type 3 (inter-area) and Type 5 (external) LSAs. Only a default route is injected by the ABR to provide connectivity to all destinations outside the area. This design significantly reduces routing table size and SPF calculation overhead within the area. Totally stubby areas are ideal for edge areas with limited routers where detailed route knowledge is unnecessary. This makes totally stubby area the correct answer because it explicitly blocks both inter-area and external LSAs, leaving only the default route.

C) NSSA (Not-So-Stubby Area) allows external routes via Type 7 LSAs to be redistributed into OSPF while still blocking Type 5 LSAs from outside the area. NSSA is used for controlled external route injection but does not block Type 3 LSAs, so it cannot meet the criteria described.

D) Backbone Area (Area 0) is the central area of OSPF and does not block any LSA type. All LSA types are propagated through the backbone to interconnect areas, so it is unsuitable for scenarios where LSA flooding needs to be restricted.

Totally Stubby Area is correct because it prevents both external and inter-area flooding, reducing routing table size and computation. Stub, NSSA, and backbone areas do not meet these specific restrictions.

Question 68:

Which BGP attribute is used to influence path selection for traffic entering your autonomous system from neighboring ASes?

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

Answer: C) MED

Explanation:

A) Weight is a local Cisco-specific attribute used to influence path selection on the local router for outbound traffic. It is not propagated to other routers and therefore cannot affect inbound traffic selection from external autonomous systems.

B) Local Preference is propagated throughout the AS and affects outbound path selection across all routers. It is irrelevant for controlling inbound traffic entering your AS from external neighbors.

C) MED (Multi-Exit Discriminator) is used to influence the choice of entry points for neighboring autonomous systems. By advertising lower MED values for preferred entry routes and higher MEDs for less preferred routes, a network can guide external ASes to use specific paths for entering the network. MED is communicated between autonomous systems, making it the standard attribute for inbound traffic engineering. This ensures predictable traffic flow, optimal bandwidth usage, and avoidance of congested links. MED is the correct answer because it specifically controls how external ASes select routes into your network.

D) AS Path records the autonomous systems a route has traversed and is primarily used for loop prevention and selection of the shortest AS path. While prepending the AS path can influence inbound traffic, it is less precise and predictable than MED for engineering purposes.

MED is correct because it allows controlled influence over inbound traffic. Weight and local preference affect local or outbound routing, and AS path manipulation is secondary and less reliable for inbound traffic engineering.

Question 69:

Which HSRP state occurs when a router is initializing but has not yet received hello messages from neighbors?

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

Answer: D) Init

Explanation:

A) Active state is when a router is currently forwarding traffic for the virtual IP. Active routers handle all ARP requests for the VIP and perform the primary routing function.

B) Standby state is when a router is ready to take over if the active router fails. Standby routers monitor hello messages from the active router and maintain backup state, but do not forward traffic unless failover occurs.

C) Listen state occurs when a router is aware of the HSRP group and is monitoring hello messages but is neither active nor standby. Routers in listen state are preparing to participate in the HSRP election but do not forward traffic.

D) Init is the initial state when HSRP starts, and the router has not yet received hello messages from neighbors. In this state, the router is simply initializing HSRP processes and cannot forward traffic or participate in standby election. Init ensures that the router is ready to enter higher HSRP states once neighbor communication begins.

Init is correct because it represents the first stage of HSRP operation where the router is initializing and waiting for neighbor discovery. Active forwards traffic, standby is backup, and listen is intermediate monitoring.

Question 70:

Which MPLS feature enables the creation of multiple logical routing tables on the same router to support separate VPNs?

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

Answer: A) VRF

Explanation:

A) VRF (Virtual Routing and Forwarding) allows multiple logical routing tables to exist on a single physical router. Each VRF instance maintains separate interfaces, routing tables, and forwarding information. When combined with MPLS labels, VRFs ensure that traffic from one VPN cannot mix with traffic from another, even over the same physical infrastructure. VRFs enable overlapping IP addresses between VPNs while maintaining strict logical separation. This is essential for enterprise and service provider environments where multiple customers share the same network backbone. VRF is the correct answer because it provides complete isolation of VPN traffic.

B) LDP (Label Distribution Protocol) is responsible for distributing MPLS labels to enable forwarding along FECs. While LDP is essential for MPLS, it does not create separate routing tables or provide VPN isolation.

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

D) QoS manages traffic prioritization, bandwidth allocation, and delay, ensuring predictable performance for applications. However, QoS does not isolate VPN routing tables or maintain separation of multiple VPNs.

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

Question 71:

 Which EIGRP feature allows a router to immediately switch to a backup route without recalculating the topology?

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

Answer: A) Feasible Successor

Explanation:

A) Feasible Successor is a precomputed backup route maintained in EIGRP’s topology table. These routes satisfy the feasibility condition, which ensures that the neighbor’s reported distance to a destination is less than the router’s feasible distance. Because feasible successors are precomputed and loop-free, the router can immediately switch to them when the primary route (successor) fails. This eliminates the need for recalculating the entire topology and ensures near-instantaneous convergence. Feasible successors improve network reliability, reduce packet loss, and are especially important in large enterprise networks where high availability is critical. This makes feasible successor the correct answer.

B) Split Horizon is a loop-prevention mechanism that prevents a router from advertising a route back out the interface from which it was learned. While it prevents certain routing loops, it does not provide a mechanism for immediate failover or backup route usage.

C) Route Summarization reduces the number of advertised routes by combining multiple prefixes into a single aggregate route. Summarization improves scalability and reduces routing table size but does not provide immediate backup routes.

D) Hold Timer defines the interval a router waits before declaring a neighbor down if no hello messages are received. While it affects convergence speed, it does not provide precomputed backup paths for immediate failover.

Feasible Successor is correct because it ensures rapid, loop-free failover. Split horizon, route summarization, and hold timers support stability and efficiency but do not allow immediate switching to a backup route.

Question 72:

Which OSPF feature summarizes multiple subnets from one area to another?

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

Answer: B) ABR Summary

Explanation:

A) Stub areas block Type 5 external LSAs to reduce flooding of external routes but do not summarize multiple prefixes between areas. Stub areas help reduce routing table size and SPF computation but are limited to restricting external route advertisements.

B) ABR Summary is performed by Area Border Routers (ABRs) to aggregate multiple subnets from one area into a single Type 3 summary LSA for advertisement into another area. This reduces routing table size, LSA flooding, and SPF computation overhead in the receiving area. ABR summarization allows hierarchical OSPF design to scale efficiently in large networks. By summarizing multiple subnets, routers outside the originating area need fewer entries in their routing tables, which improves convergence speed and reduces CPU utilization. This makes ABR Summary the correct answer.

C) Type 1 LSAs describe router links within an area and are strictly intra-area. They do not summarize or propagate routes between areas.

D) Type 2 LSAs describe network links for broadcast or NBMA segments within an area. They do not perform inter-area summarization.

ABR Summary is correct because it allows hierarchical OSPF networks to scale efficiently by aggregating multiple subnets into a single summary route. Stub areas, Type 1 LSAs, and Type 2 LSAs serve other purposes and cannot summarize inter-area routes.

Question 73

 Which BGP attribute is used to influence path selection locally without affecting other routers?

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

Answer: A) Weight

Explanation:

A) Weight is a Cisco-specific BGP attribute that controls path selection only on the local router. It is the first criterion used in BGP best path selection and is not propagated to other routers in the autonomous system. By assigning higher weight to a route, the local router will prefer that path for outbound traffic, without affecting other routers. This makes weight ideal for router-specific path control and the correct answer.

B) Local Preference is propagated throughout the autonomous system to influence outbound path selection network-wide. While critical for enforcing AS-wide routing policy, it cannot be used for local router-specific control without impacting other routers.

C) MED (Multi-Exit Discriminator) influences how neighboring ASes choose entry points into your autonomous system. MED affects inbound traffic from external ASes, not local outbound path selection.

D) AS Path records the autonomous systems a route traverses and is primarily used for loop prevention and shortest path selection. While AS path prepending can influence external routing decisions, it does not provide router-local control for outbound traffic.

Weight is correct because it provides local path control without impacting other routers. Local Preference, MED, and AS Path influence network-wide or external routing decisions.

Question 74:

Which HSRP state monitors hello messages but does not forward traffic unless the active router fails?

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

Answer: B) Standby

Explanation:

A) Active state occurs when a router is currently forwarding traffic for the virtual IP. Active routers handle all ARP requests and act as the primary gateway, so they do not wait to take over—they are already responsible for traffic.

B) Standby is the state in which a router monitors hello messages from the active router and is prepared to take over forwarding traffic if the active router fails. Standby routers maintain backup state information and ensure fast failover, but they do not forward traffic under normal operation. This readiness and monitoring function makes standby the correct answer.

C) Listen is an intermediate state where a router is aware of the HSRP group and receives hello messages but is neither forwarding traffic nor in standby election. Listen is a preparatory state rather than an active backup state.

D) Init is the initial state where the router is starting HSRP processes and has not yet received hello messages from neighbors. It cannot forward traffic or participate in standby election.

Standby is correct because it represents a router ready to take over if the active router fails. Active forwards traffic, listen is passive, and init is preliminary.

Question 75:

 Which MPLS feature isolates multiple VPNs on the same physical infrastructure?

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

Answer: A) VRF

Explanation:

A) VRF (Virtual Routing and Forwarding) allows multiple logical routing tables to exist on a single router, providing complete separation of VPNs. Each VRF instance has its own routing table, interfaces, and forwarding information. When used with MPLS, VRFs allow overlapping IP addresses and ensure that traffic from one VPN cannot reach another VPN. VRFs are widely used in enterprise and service provider environments to provide secure, isolated VPN services over shared infrastructure, making VRF the correct answer.

B) LDP (Label Distribution Protocol) distributes MPLS labels to enable forwarding along FECs. While necessary for MPLS operation, LDP does not maintain separate routing tables or provide VPN isolation.

C) RSVP-TE establishes explicit label-switched paths with bandwidth reservation for traffic engineering. It optimizes path usage and ensures predictable performance, but it does not provide logical separation of VPNs.

D) QoS prioritizes traffic, manages bandwidth, and ensures performance for applications. While important for performance, QoS does not provide logical separation of VPN routing tables or isolation between multiple VPNs.

VRF is correct because it provides isolated routing and forwarding environments for multiple VPNs on the same physical device. LDP, RSVP-TE, and QoS support MPLS and performance but do not isolate VPN traffic.

Question 76:

Which EIGRP feature ensures loop-free routing by verifying that the neighbor’s reported distance is less than the feasible distance?

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

Answer: C) DUAL Algorithm

Explanation:

A) Feasible Successor is a precomputed backup route stored in the EIGRP topology table that satisfies the feasibility condition. While feasible successors provide immediate failover and contribute to loop-free routing, they do not themselves enforce loop prevention; their validity depends on the underlying algorithm, DUAL.

B) Split Horizon prevents a router from advertising a route back out the interface from which it was learned. Although this avoids simple routing loops on point-to-point or broadcast networks, it cannot guarantee loop-free operation in complex topologies with multiple paths. Split horizon is a passive prevention method rather than a dynamic calculation.

C) The DUAL (Diffusing Update Algorithm) is the core EIGRP mechanism responsible for loop-free routing. DUAL evaluates each route in the topology table and ensures that the feasibility condition is met: the neighbor’s reported distance must be less than the current feasible distance. This guarantees that any path chosen will not create a loop. Additionally, DUAL recalculates metrics dynamically in response to topology changes, selects successors, and manages feasible successors. By maintaining loop-free paths and supporting unequal-cost load balancing, DUAL provides both network stability and optimal resource utilization. This makes DUAL the correct answer because it is the algorithm that actively enforces loop-free routing in EIGRP.

D) Hold Timer determines how long a router waits before declaring a neighbor down if no hello messages are received. Although it affects convergence time, it does not actively enforce loop-free routing. Its function is neighbor monitoring rather than path verification.

DUAL Algorithm is correct because it enforces loop-free paths, validates feasible successors, and dynamically recalculates routes. Feasible successors, split horizon, and hold timers assist in stability and convergence but do not directly perform loop verification.

Question 77:

Which OSPF LSA type is generated by an ABR to advertise summary routes between areas?

A) Type 1
B) Type 2
C) Type 3
D) Type 5

Answer: C) Type 3

Explanation:

A) Type 1 LSAs describe a router’s links within an OSPF area. These LSAs are intra-area only and do not propagate routing information between areas. Type 1 LSAs are essential for SPF calculations within an area but do not summarize inter-area routes.

B) Type 2 LSAs describe network segments on broadcast or NBMA networks within an area. They contain router information but remain intra-area. Type 2 LSAs are not used for inter-area summarization.

C) Type 3 LSAs, also known as summary LSAs, are generated by Area Border Routers (ABRs) to advertise routes from one area into another. ABRs aggregate multiple routes into summary LSAs to reduce routing table size and SPF computation in the receiving area. Type 3 LSAs ensure hierarchical OSPF scalability and efficient propagation of network reachability between areas. This makes Type 3 the correct answer because it is specifically designed for inter-area route summarization.

D) Type 5 LSAs are external LSAs used to carry routes redistributed from other protocols into OSPF. While Type 5 LSAs propagate external routes, they are not used for summarizing routes between OSPF areas.

Type 3 LSAs are correct because they allow ABRs to summarize multiple subnets and advertise them efficiently between areas. Type 1 and Type 2 are intra-area, and Type 5 carries external routes.

Question 78

 Which BGP attribute is used to control path selection for traffic leaving your autonomous system on all routers?
A) Weight
B) Local Preference
C) MED
D) AS Path

Answer: B) Local Preference

Explanation:

A) Weight is a Cisco-proprietary BGP attribute that is used only on the local router. It is not advertised to any other routers in the autonomous system (AS) or to external peers. Weight is a numerical value assigned to routes to influence the router’s path selection process. A higher weight makes a route more preferred, and the router will prioritize it for outbound traffic from that device. While weight is extremely useful for fine-tuning routing on a single router, it cannot enforce consistent routing decisions across multiple routers within the same AS because it is local-only. For example, if an organization wants a specific exit path to be used for outbound traffic from all routers in a multi-homed AS, weight cannot achieve this because each router would need independent configuration. This limitation makes weight unsuitable for network-wide outbound path control, and it is primarily used when local overrides are required for specific routers.

B) Local Preference is a well-known discretionary BGP attribute that is propagated throughout the entire autonomous system. Unlike weight, local preference is advertised to all internal BGP (iBGP) peers and is therefore used to influence outbound path selection consistently across all routers within the AS. By assigning a higher local preference value to a preferred route, administrators can ensure that all routers in the AS choose that exit path for traffic destined to external networks. The default local preference value is typically 100, but network administrators can adjust this to prioritize certain routes.

Local preference provides network-wide policy enforcement, making it ideal for multi-homed environments, where an organization is connected to multiple ISPs. For instance, if a company wants to use ISP1 as the primary exit point for all outbound traffic and ISP2 as a backup, the local preference for routes learned via ISP1 can be set higher than for ISP2. All routers within the AS will then consistently select ISP1 as the preferred exit for outgoing traffic. Local preference also supports complex routing policies, such as traffic engineering, load balancing, or selective route preferences based on business or operational requirements.

Local preference is particularly important in enterprise networks where predictable routing behavior is critical. For example, in large-scale networks with multiple data centers, an organization may want to ensure that traffic destined for specific destinations leaves the AS through certain edge routers to optimize performance, comply with contractual agreements with ISPs, or enforce security policies. By adjusting local preference, network engineers can control the outbound flow of traffic centrally, without needing to configure weight individually on each router, thus reducing configuration complexity and operational overhead.

C) MED (Multi-Exit Discriminator) is a BGP attribute used to influence how external autonomous systems select entry points into your AS. MED is advertised to external BGP (eBGP) peers and provides a hint about the preferred path into your network. Unlike local preference, MED affects inbound traffic, not outbound traffic. While MED can indicate to an external AS which of your multiple links is preferred for their traffic to enter your network, it does not control how your own routers select exit points for outbound traffic. In other words, MED helps external neighbors choose entry paths but does not enforce consistent outbound routing within your AS.

D) AS Path is a fundamental BGP attribute used primarily for loop prevention and for selecting the shortest path to external destinations. The AS path is a list of autonomous systems that a route has traversed. While techniques like AS path prepending can influence external peers by making a path appear longer, thereby discouraging its selection, AS path does not provide uniform outbound path selection across all routers in your AS. AS path affects how external networks perceive your routes, not how your internal routers make routing decisions for outbound traffic.

 Local Preference is the only attribute that enables coordinated and predictable outbound path selection across all routers in an AS. By contrast, weight is router-local and cannot propagate network-wide; MED influences inbound traffic from external networks, not internal routing decisions; and AS path is primarily used for loop prevention and influencing external path selection.

Understanding the distinction between these attributes is essential for BGP traffic engineering:

Local Preference: Controls network-wide outbound path selection, propagated via iBGP to all routers in the AS. High value = preferred route.

Weight: Router-specific, local-only attribute; overrides local preference locally but not on other routers.

MED: Influences how external ASes choose paths into your AS; used for inbound traffic optimization.

AS Path: Ensures loop-free routing, allows external peers to prefer shorter paths; primarily external influence.

In practice, local preference is widely used by enterprises and service providers to implement consistent routing policies, enforce primary and backup links, balance traffic across multiple egress points, and optimize performance based on business objectives. By propagating local preference throughout the AS, network operators ensure that all routers select the same preferred path for outbound traffic, avoiding routing inconsistencies, inefficient paths, or unintended use of backup links.

 A company with two ISPs, ISP-A and ISP-B, wants all outbound Internet traffic to exit via ISP-A. Local preference can be set to 200 for ISP-A routes and 100 for ISP-B routes. This ensures all routers within the AS prioritize ISP-A consistently, while ISP-B serves as a failover. Weight could only influence a single router, MED would influence inbound traffic, and AS path prepending would affect external peers’ decisions—not the internal choice of exit point.

Local Preference is correct because it enables network-wide outbound path selection, providing centralized control, predictability, and efficient traffic engineering across all routers in the autonomous system. Weight, MED, and AS Path have limited scope or influence primarily external behavior and cannot enforce uniform outbound routing.

Question 79

Which HSRP state represents a router actively forwarding traffic for the virtual IP?
A) Active
B) Standby
C) Listen
D) Init

Answer: A) Active

Explanation:

A) The Active state in HSRP (Hot Standby Router Protocol) signifies that the router is currently responsible for forwarding traffic destined for the HSRP virtual IP (VIP) address. In this role, the Active router responds to ARP requests for the VIP and effectively acts as the default gateway for all hosts in the subnet. When traffic arrives at the gateway, it is the Active router that processes and forwards packets to their destination within the local subnet or towards external networks. The Active router maintains all necessary routing and forwarding tables to ensure seamless traffic delivery.

Being in the Active state requires the router to continuously monitor the network and participate in the HSRP election process. The election is typically determined by the HSRP priority value, with the highest priority router becoming Active. If multiple routers share the same priority, the one with the highest IP address becomes Active. This ensures deterministic and predictable failover behavior in the network.

Active routers also send hello messages to the HSRP group at regular intervals, which are used to notify other routers of their presence and current state. These hello messages allow Standby routers to detect failures quickly and assume the Active role if necessary. In enterprise networks, ensuring redundancy through HSRP is critical for minimizing downtime and providing high availability for gateway services.

B) Standby is the state of a router that is prepared to take over forwarding responsibilities if the Active router fails. The Standby router continuously monitors hello messages from the Active router and maintains a synchronized state. Although it is ready to transition into the Active state, the Standby router does not forward traffic under normal conditions. It serves as a backup to ensure continuity of service in case the primary router fails, providing high availability without disrupting network operations.

C) Listen is a transitional state in which the router is aware of the HSRP group and receives hello messages from other routers but is neither Active nor Standby. The Listen state allows a router to synchronize information with the HSRP group, preparing for potential election participation or standby readiness. Routers in Listen state do not forward traffic, and this state is generally temporary during initial protocol operation or during configuration changes.

D) Init is the initial state of HSRP when a router first starts the protocol. In this phase, the router has not yet received any hello messages from other HSRP routers in the group and therefore has no knowledge of the network topology. Routers in Init state cannot forward traffic or participate in the election until they transition to other states like Listen or Standby. This state ensures that HSRP routers do not prematurely take on roles before the network is fully recognized.

The Active state is the only HSRP state in which a router actively forwards traffic for the virtual IP. Standby acts as a backup, Listen monitors and prepares, and Init represents the startup phase. Understanding these states is critical for designing high-availability networks, ensuring redundancy, and maintaining uninterrupted gateway services. Proper HSRP configuration and monitoring help prevent network outages, support failover scenarios, and maintain seamless connectivity for end users.

Question 80

 Which MPLS mechanism enables multiple VPNs to share the same physical infrastructure while keeping traffic logically separate?
A) VRF
B) LDP
C) RSVP-TE
D) QoS

Answer: A) VRF

Explanation:

A) VRF (Virtual Routing and Forwarding) is a fundamental MPLS feature that allows multiple logical routing and forwarding domains to exist on a single physical router. Each VRF maintains its own routing table, interfaces, and forwarding logic, effectively isolating traffic from other VRFs even though the underlying infrastructure is shared. This allows multiple VPNs, often belonging to different customers or departments, to coexist securely on the same network hardware without interfering with each other.

VRFs also allow the use of overlapping IP address spaces, which is essential when different customers or organizational units use identical private addressing schemes. By assigning each VPN to its own VRF, MPLS routers ensure that traffic remains isolated and that there is no leakage between VPNs. This logical separation is achieved through the combination of VRF instances and MPLS labels, which direct packets to the correct forwarding table and prevent cross-VRF traffic contamination.

The implementation of VRF provides scalability and efficiency for service providers and large enterprises. Multiple VPNs can leverage a single set of core routers, reducing hardware costs and simplifying network management. Additionally, VRFs integrate seamlessly with other MPLS technologies, such as MP-BGP (Multiprotocol BGP) for distributing routes between VPNs and core routers. This allows dynamic exchange of routes between VRFs and ensures end-to-end connectivity for each VPN while maintaining strict separation.

B) LDP (Label Distribution Protocol) is a protocol used to distribute MPLS labels across routers in a network, enabling the creation of Label Switched Paths (LSPs). LDP facilitates packet forwarding based on labels rather than IP addresses, which increases speed and efficiency. However, while LDP is essential for MPLS operations, it does not inherently provide VPN separation or isolated routing tables. LDP focuses on establishing paths for traffic within MPLS networks, not on the logical separation of multiple VPNs.

C) RSVP-TE (Resource Reservation Protocol – Traffic Engineering) is used to establish explicit LSPs with bandwidth guarantees and traffic engineering capabilities. It allows network operators to optimize resource utilization, balance loads, and reserve bandwidth for critical applications. While RSVP-TE enhances MPLS performance and supports traffic engineering objectives, it does not provide logical separation of VPNs or maintain isolated routing tables like VRFs do.

D) QoS (Quality of Service) manages traffic prioritization, bandwidth allocation, and latency control for applications. QoS ensures that mission-critical traffic, such as voice or video, is prioritized over less important traffic. While QoS improves performance and reliability, it does not provide logical isolation for VPN traffic. QoS controls how traffic is handled, not how it is separated or routed securely between VPNs.

VRF is the correct answer because it allows multiple VPNs to share the same physical network while maintaining complete logical separation. LDP, RSVP-TE, and QoS enhance MPLS functionality and network performance but do not provide the VPN isolation and independent routing domains that VRFs deliver. Understanding VRFs is critical for designing multi-tenant environments, enterprise VPNs, and service provider networks, ensuring security, scalability, and efficient resource utilization while supporting overlapping IP schemes and isolated routing domains.