Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 11 Q151-165
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Question 151:
A network engineer deploys EIGRP in a large campus network. Some routers fail to exchange routes even though they are on the same subnet. What is the most likely cause?
A) Mismatched EIGRP AS numbers
B) Passive interface is enabled
C) K values are different
D) Split-horizon is enabled
Answer: A)
Explanation:
EIGRP is a distance vector routing protocol that requires routers to form neighbor relationships before they can exchange routing information. One of the critical requirements for neighbor formation is that all routers must be configured with the same autonomous system (AS) number. The AS number is included in EIGRP hello packets and identifies the routing domain. If routers have different AS numbers, they will not recognize each other as valid neighbors, preventing route exchange. In a large campus network, this can lead to partial connectivity, routing table inconsistencies, and delayed convergence. Verification involves checking the running configuration with commands like “show running-config” and inspecting neighbor relationships using “show ip eigrp neighbors.” Correctly matching AS numbers ensures that routers recognize each other and that the EIGRP adjacency is established, allowing full route exchange and consistent network convergence.
In this scenario, routers on the same subnet fail to exchange routes, strongly indicating a mismatch in AS numbers. Resolving the AS mismatch ensures proper neighbor formation and route propagation. Network engineers must also validate interface IP addresses, network statements, and ensure that the interfaces participating in EIGRP are correctly configured. Documenting AS numbers and maintaining consistency across all routers in the enterprise network is critical, especially when multiple EIGRP domains exist.
A passive interface configuration prevents EIGRP hello packets from being sent out of a specific interface. While this could prevent neighbor formation, it usually affects only the interfaces configured as passive. In a widespread issue across multiple routers on the same subnet, passive interface configuration alone is less likely to be the root cause.
Different K values affect how EIGRP calculates its metrics and selects routes. Mismatched K values will not prevent neighbor formation; routers will still exchange updates, although route selection may be affected due to different metric calculations.
Split-horizon prevents a router from advertising routes back out the interface from which they were learned. It may impact routing in hub-and-spoke topologies, but it does not prevent neighbor formation or route exchange across multiple routers on the same subnet.
The root cause is mismatched AS numbers. Ensuring that all routers share the same AS number is essential for establishing neighbor relationships, exchanging routes, and maintaining network stability. Correct configuration supports predictable route propagation, efficient resource utilization, and seamless connectivity. Network engineers must verify AS numbers, interface configurations, and adjacency status using diagnostic commands. Proper AS number alignment guarantees full EIGRP functionality, reduces the risk of partial network segmentation, and ensures that routing tables are consistent. Understanding the significance of AS numbers in EIGRP is critical for avoiding neighbor formation failures and maintaining robust, reliable routing in large enterprise networks. By ensuring consistency in AS configuration, engineers can optimize network performance, prevent route loss, and achieve stable convergence across the entire EIGRP domain, which is essential for high-performance and resilient campus networks.
Question 152:
A network engineer deploys OSPFv3 in an IPv6 network. Some routers fail to form neighbor adjacencies after configuration. What is the most likely cause?
A) Missing link-local addresses on interfaces
B) Duplicate router IDs
C) Area authentication mismatch
D) IPv6 unicast routing is disabled globally
Answer: A)
Explanation:
OSPFv3 relies on IPv6 link-local addresses for neighbor discovery and adjacency formation. Every interface participating in OSPFv3 must have a valid link-local address because OSPFv3 uses these addresses as the source and destination of hello packets. Without link-local addresses, routers cannot identify each other as neighbors, preventing adjacency formation, halting LSA exchange, and ultimately blocking proper routing table updates. Link-local addresses are essential because OSPFv3 communicates directly on the link, allowing routers to form adjacencies even if global IPv6 addresses are not assigned or change dynamically. This mechanism ensures stable neighbor formation and consistent route propagation across the network.
In this scenario, routers fail to form adjacencies, strongly indicating that link-local addresses are missing or improperly configured. Verification includes checking interface configurations using “show ipv6 interface brief” and inspecting neighbor status with “show ipv6 ospf neighbor.” Proper configuration involves assigning link-local addresses and enabling OSPFv3 on all participating interfaces. Additionally, unique router IDs must be assigned to each OSPFv3 router to identify them uniquely in the LSDB, which complements link-local addressing to maintain stable routing.
Duplicate router IDs can cause adjacency issues, but initial hello packets are still sent and received if link-local addresses exist. Duplicate IDs generate log messages indicating conflicts, which need to be resolved to prevent LSDB inconsistencies.
Area authentication mismatches affect the acceptance of LSAs from neighbors but do not prevent adjacency formation. Hello packets can still be exchanged, allowing routers to establish initial adjacency even if authentication settings differ.
IPv6 unicast routing being disabled globally prevents the forwarding of IPv6 traffic, but does not stop OSPFv3 hello packets from being sent using link-local addresses. Full OSPFv3 operation, including LSA exchange and route propagation, requires IPv6 routing to be enabled.
The root cause is missing link-local addresses. Ensuring that each interface participating in OSPFv3 has a link-local address is crucial for neighbor formation, LSA exchange, and network convergence. Proper configuration guarantees stable OSPFv3 operation, predictable route propagation, and reliable network connectivity. Engineers must verify link-local addresses, enable OSPFv3, assign unique router IDs, and monitor neighbor formation to maintain operational stability. Understanding the importance of link-local addresses in IPv6 networks allows for reliable OSPFv3 deployment, uninterrupted LSA exchange, and predictable routing behavior. Correct configuration supports adjacency formation, efficient route propagation, and optimal network performance.
Question 153:
A network engineer configures MPLS LDP in a service provider network. Some routers fail to establish LDP sessions with neighbors. What is the most likely cause?
A) Missing IGP adjacency between routers
B) LDP transport addresses are mismatched
C) MPLS is not enabled globally
D) LDP hello timers are too long
Answer: A)
Explanation:
MPLS LDP relies on the underlying IGP to provide IP reachability between routers for session establishment. LDP uses TCP port 646 to exchange hello messages and label mapping messages. Routers must have IGP-provided reachability to establish LDP neighbor relationships. If routers do not have an IGP adjacency, LDP messages cannot be exchanged, preventing session formation and blocking the establishment of label-switched paths essential for MPLS forwarding. The dependency on IGP connectivity is critical because LDP does not provide its own independent discovery mechanism. Proper IGP operation ensures that routers can communicate directly, exchange labels, and set up MPLS forwarding paths.
In this scenario, LDP sessions fail to establish, indicating that IGP adjacency or reachability is missing. Verification involves checking the IGP routing table, examining adjacency status with “show ip ospf neighbor” or “show ip eigrp neighbors,” and using ping to test IP reachability. Correct IGP configuration ensures that LDP messages reach neighbors, allowing label-switched paths to be established. Engineers must ensure that interfaces participating in LDP have correct IP addressing and that the IGP is fully converged across the network.
Mismatched LDP transport addresses can prevent session formation in manually configured networks, but default deployments typically use loopback or primary interface addresses, making this scenario less common.
MPLS not being globally enabled prevents label forwarding but does not stop LDP from attempting neighbor discovery. LDP sessions can still be attempted even if MPLS is not enabled, though labels will not be used.
LDP hello timers affect the frequency of hello messages but do not prevent neighbor discovery. Longer timers may delay session formation, but do not block neighbor establishment if connectivity exists.
The root cause is missing IGP adjacency. Proper IGP configuration ensures that LDP messages can be exchanged, enabling label distribution and MPLS forwarding. Network engineers must monitor IGP adjacency, interface configurations, and LDP session status to maintain predictable and stable MPLS operations. Understanding the dependency between IGP connectivity and LDP operation is essential for successful MPLS deployment. Proper planning, monitoring, and verification prevent session failures and ensure resilient, high-performance MPLS networks. Network engineers must ensure that all IGP adjacencies are stable, providing a reliable foundation for LDP session establishment, label-switched path creation, and efficient MPLS traffic forwarding.
Question 154:
A network engineer configures EIGRP on a multi-area enterprise network. Some routers do not form neighbor relationships despite being on the same subnet. What is the most likely cause?
A) Mismatched EIGRP AS numbers
B) Passive interface is enabled
C) K values are different
D) Split-horizon is enabled
Answer: A)
Explanation:
EIGRP requires all routers within the same routing domain to share the same autonomous system (AS) number to form neighbor relationships and exchange routing information. The AS number acts as an identifier for the EIGRP domain and is included in the hello packets sent by routers. If routers on the same subnet have mismatched AS numbers, they will not recognize each other as valid neighbors, and route exchange will not occur. In a multi-area enterprise network, this can lead to partial network segmentation, inconsistent routing tables, and delayed convergence. Verification of neighbor relationships can be performed using commands such as “show ip eigrp neighbors” and “show running-config” to inspect AS number consistency across the network. Ensuring that all routers share the same AS number allows them to establish adjacencies and properly exchange routing updates, supporting consistent routing behavior and network stability.
In this scenario, routers on the same subnet fail to establish neighbor relationships, strongly suggesting that AS numbers are mismatched. Correcting the AS number ensures that routers recognize each other as neighbors, allowing full route propagation throughout the network. Network engineers must also verify network statements, interface IP addresses, and any passive interface configurations to ensure that all EIGRP-enabled interfaces are actively participating. Proper documentation and alignment of AS numbers are particularly important in large networks with multiple EIGRP domains to prevent inadvertent mismatches that disrupt routing.
Passive interfaces can prevent neighbor formation by stopping hello packets from being sent on those interfaces. However, if multiple routers on the same subnet fail to form neighbors, it is more likely due to AS mismatches rather than passive interface misconfiguration, which usually affects specific interfaces.
Different K values determine how EIGRP calculates metrics for route selection, but do not prevent neighbor formation. Routers with mismatched K values will still exchange routes, although they may disagree on feasible successors and path preference.
Split-horizon prevents a router from advertising a route back out the interface from which it was learned. While this may affect routing in hub-and-spoke topologies, it does not prevent neighbor formation or the exchange of routes between routers on the same subnet.
The root cause in this scenario is mismatched AS numbers. Ensuring that all routers share the same AS number is crucial for establishing neighbor relationships, exchanging routes, and maintaining network stability. Proper verification of AS numbers, interface configurations, and adjacency status allows predictable route propagation and efficient network convergence. Understanding the significance of AS numbers in EIGRP is essential to avoid neighbor formation failures and partial network segmentation. Network engineers must confirm AS number alignment across all routers to achieve consistent routing, maintain robust connectivity, and optimize performance. By ensuring AS consistency, enterprises can maintain stable routing tables, efficient convergence, and predictable EIGRP behavior, which is particularly critical in large-scale, multi-area networks. Proper AS number configuration supports reliable connectivity, prevents route loss, and ensures operational stability across all EIGRP routers.
Question 155:
A network engineer deploys OSPFv3 in a large IPv6 network. Some routers fail to form neighbor adjacencies. What is the most likely cause?
A) Missing link-local addresses on interfaces
B) Duplicate router IDs
C) Area authentication mismatch
D) IPv6 unicast routing is disabled globally
Answer: A)
Explanation:
OSPFv3 is the OSPF protocol designed for IPv6 networks and requires link-local addresses on all participating interfaces. Each interface must have a valid link-local address because OSPFv3 uses these addresses as the source and destination of hello packets. Without link-local addresses, routers cannot identify each other as neighbors, preventing adjacency formation, LSA exchange, and proper routing. Link-local addresses are essential because they allow routers to communicate directly on the same link, independent of global IPv6 addresses. This ensures that routers can establish neighbor relationships even when global addresses are not fully configured or are dynamically assigned. Proper link-local addressing is a prerequisite for stable neighbor formation, consistent route propagation, and reliable network convergence.
In this scenario, routers fail to form adjacencies, indicating that link-local addresses are missing or improperly configured. Verification involves inspecting interface configurations using “show ipv6 interface brief” and monitoring neighbor status with “show ipv6 ospf neighbor.” Correct configuration requires assigning link-local addresses to all OSPFv3-enabled interfaces and enabling OSPFv3. Additionally, unique router IDs must be configured on all routers, as they serve as unique identifiers in the OSPFv3 LSDB and complement link-local addressing. Properly configured router IDs ensure database consistency and help prevent adjacency conflicts.
Duplicate router IDs can cause adjacency conflicts and database inconsistencies, but hello packets will still be exchanged if link-local addresses exist. Duplicate IDs generate warnings or logs that indicate conflicts, which must be resolved to maintain LSDB integrity.
Area authentication mismatches impact the acceptance of LSAs but do not prevent the formation of neighbor adjacencies. Hello packets will still be exchanged, and initial neighbor relationships can form, although authenticated LSAs may be rejected.
IPv6 unicast routing being disabled globally prevents the forwarding of IPv6 traffic, but does not stop hello packets from being exchanged using link-local addresses. Full OSPFv3 operation, including route propagation, requires IPv6 routing to be enabled.
The root cause is missing link-local addresses. Ensuring that every interface participating in OSPFv3 has a valid link-local address is essential for adjacency formation, LSA exchange, and network convergence. Proper configuration ensures stable OSPFv3 operation, predictable route propagation, and reliable connectivity. Network engineers must verify link-local addresses, enable OSPFv3 on all interfaces, and assign unique router IDs to maintain operational stability. Understanding the role of link-local addresses in OSPFv3 is critical for deploying IPv6 networks successfully. Correct configuration allows seamless adjacency formation, uninterrupted LSA flooding, and predictable convergence, which are essential for efficient and reliable IPv6 network operation.
Question 156:
A network engineer configures MPLS LDP in a service provider network. LDP sessions fail to establish between some routers. What is the most likely cause?
A) Missing IGP adjacency between routers
B) LDP transport addresses are mismatched
C) MPLS is not enabled globally
D) LDP hello timers are too long
Answer: A)
Explanation:
MPLS Label Distribution Protocol (LDP) depends on the underlying IGP to provide IP reachability for neighbor discovery and session establishment. LDP uses TCP port 646 to exchange hello messages and label mapping messages. Routers must have IP reachability, typically provided by an IGP such as OSPF or IS-IS, to form LDP sessions. If there is no IGP adjacency or routing path between routers, LDP messages cannot reach the intended neighbors, preventing session formation and hindering the establishment of label-switched paths required for MPLS forwarding. LDP does not provide its own discovery mechanism independent of the IGP, making connectivity between routers crucial.
In this scenario, LDP sessions fail to establish, indicating that IGP adjacency or reachability is missing. Verification involves checking the IGP routing table, inspecting adjacency status with commands like “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity using ping or traceroute. Correct IGP configuration ensures that LDP hello messages reach neighbors, allowing session formation and label distribution. Engineers must confirm that all interfaces participating in LDP have proper IP addresses and that the IGP is fully converged to provide reliable connectivity. Proper IGP configuration forms the foundation for LDP operation, enabling the establishment of label-switched paths and efficient MPLS forwarding.
Mismatched LDP transport addresses can prevent session formation if manually configured incorrectly, but default deployments often use loopback or primary interface addresses, making this scenario less common.
MPLS not being globally enabled affects label forwarding but does not stop LDP from attempting to establish sessions. LDP neighbor discovery still occurs, though label exchange will not function until MPLS is activated.
LDP hello timers influence the frequency of hello message transmission, but do not prevent session formation. Longer timers may delay neighbor detection, but do not block session establishment if connectivity exists.
The root cause is missing IGP adjacency. Ensuring proper IGP configuration allows LDP messages to reach neighbors, enabling label distribution and MPLS traffic forwarding. Network engineers must monitor IGP adjacency, interface configurations, and LDP session status to maintain predictable MPLS operations. Understanding the dependency between IGP connectivity and LDP operation is critical for successful MPLS deployment. Proper planning, monitoring, and verification prevent session failures and support resilient, high-performance MPLS networks. Engineers must ensure that IGP adjacencies are stable, providing a reliable foundation for LDP session formation, label-switched path establishment, and efficient MPLS traffic forwarding.
Question 157:
A network engineer configures RSVP-TE tunnels in an MPLS network. Despite sufficient bandwidth being available, the tunnels fail to establish. What is the most likely cause?
A) CSPF cannot find a feasible path due to link attribute constraints
B) RSVP authentication is mismatched
C) RSVP soft-state refresh timers are too long
D) IGP metrics are misconfigured
Answer: A)
Explanation:
RSVP-TE is used in MPLS networks to establish label-switched paths with explicit traffic engineering, allowing network engineers to reserve bandwidth and enforce path constraints. The Constrained Shortest Path First (CSPF) algorithm calculates feasible paths based on available bandwidth, administrative group constraints, TE colors, and other link attributes. Even when links report sufficient bandwidth, tunnels can fail to establish if CSPF cannot identify a path that satisfies all constraints. Link attribute constraints, such as missing administrative group tags, conflicting TE color settings, or misconfigured path requirements, can prevent CSPF from computing a feasible path. This leads to tunnel setup failure even though the raw bandwidth is sufficient. Proper configuration and verification of TE link attributes are critical for successful RSVP-TE deployment. Network engineers must ensure that all links along the intended path have correct TE attributes and sufficient bandwidth to satisfy tunnel requirements.
RSVP authentication mismatches affect signaling but do not influence CSPF path computation. If authentication fails, signaling messages may be rejected, preventing tunnel establishment. However, the presence of sufficient bandwidth and configured attributes would normally allow path computation. Properly configuring RSVP authentication ensures secure signaling but does not impact CSPF feasibility calculations.
RSVP soft-state refresh timers maintain tunnel state, refreshing reservations periodically to prevent tunnel timeout. While timers that are too long may delay detection of failed tunnels, they do not prevent the initial CSPF path computation or tunnel establishment. Proper timer configuration ensures consistent resource reservation but is not typically the cause of initial tunnel failure.
IGP metrics influence the SPF computation for basic routing decisions but do not override TE constraints. CSPF evaluates paths based on both IGP metrics and explicit TE constraints, including bandwidth, administrative groups, and TE colors. Even if IGP metrics are misconfigured, CSPF will reject paths that do not meet TE requirements, preventing tunnel establishment.
In this scenario, the root cause is CSPF being unable to find a feasible path due to link attribute constraints. Network engineers must verify TE link attributes, administrative groups, and TE colors to ensure that CSPF can compute valid paths. Proper configuration guarantees successful tunnel establishment, predictable traffic engineering, and efficient use of MPLS resources. Engineers should monitor tunnel status, inspect CSPF logs, and validate TE attributes using commands like “show mpls traffic-eng tunnels” or “show mpls traffic-eng topology” to confirm feasibility. Understanding the interplay between CSPF, link attributes, and RSVP-TE signaling is critical for designing and maintaining a reliable MPLS network. By ensuring proper TE configuration and attribute consistency, network engineers can achieve predictable tunnel behavior, efficient bandwidth utilization, and high availability, even in large-scale service provider networks. Misconfigurations or overlooked constraints can lead to tunnel failures, impacting traffic engineering objectives and network performance. Proper planning, verification, and monitoring prevent tunnel failures and support optimal MPLS operation, ensuring reliable service delivery, predictable path selection, and stable network convergence.
Question 158:
A network engineer deploys BGP multipath in a multi-homed enterprise network. Multiple paths appear in the BGP table, but traffic only flows through a single path. What is the most likely cause?
A) Paths are not equal in AS path, origin, or MED attributes
B) All BGP neighbors are configured as iBGP
C) One path has an unreachable next-hop
D) BGP route dampening is enabled
Answer: A)
Explanation:
BGP multipath allows simultaneous use of multiple equal-cost paths for load balancing and redundancy. For multipath to function, candidate paths must be identical in certain attributes, including AS path, origin type, MED, local preference, and next-hop reachability. Any discrepancy in these attributes causes BGP to select a single best path according to its deterministic path selection process, ensuring stability, loop prevention, and predictable routing behavior. When multiple paths appear in the BGP table but only a single path is used for traffic forwarding, it strongly indicates that the candidate paths are not equal in terms of AS path, origin, or MED attributes. Verification involves inspecting the BGP table with commands such as “show bgp ipv4 unicast” and comparing attributes of the candidate paths. Aligning these attributes enables multipath utilization, optimizing bandwidth and redundancy.
Configuring all BGP neighbors as iBGP does not prevent multipath operation. iBGP can support multipath, provided the paths meet equality conditions. Multipath forwarding over iBGP-learned routes is possible if local preference, AS path, and other relevant attributes match across paths.
An unreachable next-hop would prevent a specific path from being installed in the routing table. However, in this scenario, multiple paths are present in the BGP table, which indicates that next-hop reachability is not the cause. Monitoring next-hop reachability with “show ip route” can confirm which paths are viable.
BGP route dampening temporarily suppresses flapping routes to improve stability, but it does not affect the selection of multipath routes when attributes are equal and stable. While dampening can delay the use of certain paths, it is not responsible for restricting multipath use when paths are present and stable.
The root cause is unequal attributes among candidate paths, such as AS path, origin, or MED differences. Ensuring equality in these attributes is crucial for effective multipath deployment, allowing simultaneous use of multiple paths for redundancy and load balancing. Proper configuration and verification of BGP attributes enable predictable routing behavior, optimized traffic distribution, and enhanced network resilience. Network engineers must monitor BGP tables, align attribute values, and validate multipath usage to achieve efficient utilization of available network resources. Understanding BGP’s strict equality requirements is essential for multi-homed environments to ensure redundancy, load balancing, and optimal network performance. Misalignment of attributes can cause underutilization of available paths, reduce redundancy, and create potential bottlenecks. By ensuring attribute equality and verifying BGP configuration, engineers can achieve predictable multipath traffic forwarding, high availability, and efficient bandwidth utilization across enterprise networks.
Question 159:
A network engineer configures OSPF in a multi-area network. Routers in a branch area cannot reach external networks redistributed from BGP. What is the most likely cause?
A) The area is configured as a totally stubbed area
B) Type-3 LSAs are blocked by an ACL
C) Area 0 is down
D) OSPF process IDs are mismatched
Answer: A)
Explanation:
OSPF area types control the propagation of routing information and are used to optimize routing table size, reduce CPU load, and manage LSDB complexity. A totally stub area blocks Type-5 LSAs, which carry external routes such as those redistributed from BGP. It also restricts Type-3 LSAs except for the default route injected by the ABR. This design reduces routing overhead and is often deployed in remote branches with limited resources. Routers in totally stub areas rely on a default route provided by the ABR to reach external destinations rather than maintaining a full set of external or inter-area routes. Proper ABR configuration is required to inject a default route so that branch routers can reach external networks despite the restrictions imposed by the stub area configuration.
In this scenario, branch routers cannot reach external networks redistributed from BGP, strongly suggesting that the area is configured as a totally stub area. Verification involves checking the area type using commands like “show ip ospf” and ensuring that the ABR advertises a default route. Correctly configured, totally stubbed areas simplify routing, reduce overhead, and maintain connectivity to external networks via default routing. Engineers must ensure that the ABR is properly injecting a default route and that routers in the stub area are configured to accept it.
Blocking Type-3 LSAs with ACLs could also prevent inter-area route propagation, but complete external route loss in a single area more closely aligns with the behavior of a totally stub area.
If Area 0, the backbone, were down, all inter-area routing would fail, affecting multiple areas. Since only the branch area is affected, the backbone is functioning.
OSPF process IDs are locally significant and do not affect LSA propagation across areas. Mismatched process IDs would not prevent external route reception if adjacency and area type are correct.
The root cause is the branch area being configured as a totally stubbed area. Proper ABR configuration ensures that branch routers receive a default route for external connectivity. Verification includes monitoring route tables, inspecting ABR configurations, and confirming that the stub area is operational. Understanding totally stubbed areas is essential for network optimization while maintaining connectivity to external networks. Correct planning, ABR configuration, and monitoring prevent connectivity issues, reduce routing overhead, and support predictable OSPF behavior in multi-area enterprise networks.
Question 160:
A network engineer deploys OSPF in a multi-area network. Routers in a branch area cannot reach external networks redistributed from BGP. What is the most likely cause?
A) The area is configured as a totally stubbed area
B) Type-3 LSAs are blocked by an ACL
C) Area 0 is down
D) OSPF process IDs are mismatched
Answer: A)
Explanation:
OSPF area types control how routing information is propagated between areas and optimize the routing process to reduce routing table size, LSDB complexity, and CPU utilization. Totally stub areas are a special OSPF area type designed for remote branches with limited resources. In a totally stub area, Type-5 LSAs, which carry external routes such as those redistributed from BGP or other routing protocols, are blocked from entering the area. This prevents routers in the area from directly learning external networks. The ABR at the border of the totally stub area injects a default route into the area, allowing routers to reach external networks without maintaining a full external route table. This design reduces routing overhead while providing connectivity to external destinations.
In this scenario, branch routers cannot reach external networks redistributed from BGP. This strongly suggests that the area is configured as a totally stub area, which inherently blocks external routes. Verification can be performed using commands like “show ip ospf” to check area types and “show ip route” to inspect route tables for the presence of a default route. Proper configuration requires the ABR to inject a default route into the totally stub area, allowing branch routers to forward traffic to external destinations even though detailed external routes are not advertised. Engineers must ensure that the ABR is functioning correctly and that branch routers are configured to accept default routes.
Blocking Type-3 LSAs with an ACL could prevent inter-area route propagation, but total loss of external routes and reliance on a default route is more characteristic of a totally stub area than selective filtering of Type-3 LSAs.
If Area 0 were down, inter-area routing would fail across multiple areas, not just the branch area. Since only the branch area is impacted, this indicates that the backbone is operational.
OSPF process IDs are locally significant and do not affect LSA propagation across areas. Mismatched process IDs would not prevent external route reception if adjacency, area type, and ABR configurations are correct.
The root cause is the branch area being configured as a totally stubbed area. Proper ABR configuration ensures the injection of a default route, enabling branch routers to reach external networks. Verification includes inspecting the ABR configuration, monitoring the routing table for default routes, and confirming the area type. Understanding totally stubbed areas is crucial for optimizing OSPF operation in large networks while maintaining connectivity. Correctly configured, totally stubbed areas reduce routing overhead, simplify route tables, and maintain predictable network behavior. Engineers must carefully design ABR configurations, validate area types, and monitor route propagation to prevent connectivity issues. Proper planning ensures that remote branch routers can access external destinations via a default route while benefiting from reduced LSDB complexity and minimal CPU usage. Totally stub areas are a best practice in scenarios where resource optimization is needed without compromising external network reachability.
Question 161:
A network engineer deploys BGP multipath in a multi-homed enterprise network. Multiple paths appear in the BGP table, but traffic flows through a single path. What is the most likely cause?
A) Paths are not equal in AS path, origin, or MED attributes
B) All BGP neighbors are configured as iBGP
C) One path has an unreachable next-hop
D) BGP route dampening is enabled
Answer: A)
Explanation:
BGP multipath allows simultaneous use of multiple equal-cost paths to optimize load balancing and redundancy in a multi-homed environment. For multipath to function, candidate paths must have identical attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP uses a strict deterministic selection process to ensure stability, avoiding routing loops and unpredictable forwarding behavior. When multiple paths appear in the BGP table but only one path is used for traffic, it usually indicates that the candidate paths are not identical in terms of AS path, origin type, or MED values. Verification involves inspecting the BGP table using commands like “show bgp ipv4 unicast” and comparing attributes among candidate paths. Equalizing these attributes allows multipath utilization, enabling simultaneous traffic forwarding through multiple paths and providing redundancy.
Configuring all BGP neighbors as iBGP does not inherently prevent multipath deployment. iBGP can support multipath as long as the paths satisfy the equality conditions regarding attributes. Multipath can operate over iBGP-learned routes provided local preference, AS path, MED, and origin attributes are aligned.
An unreachable next-hop prevents a specific path from being installed in the routing table. In this scenario, multiple paths appear in the BGP table, which indicates that next-hop reachability is not the limiting factor. Engineers may still validate reachability using “show ip route” and ping commands, but the issue is an attribute mismatch rather than connectivity.
BGP route dampening temporarily suppresses flapping routes to reduce instability, but does not block multipath use for stable routes. If the paths are stable and present in the BGP table, dampening is unlikely to be the cause.
The root cause is unequal attributes among candidate paths, such as AS path, origin type, or MED. Ensuring equality in these attributes is critical for achieving multipath forwarding. Proper configuration enables simultaneous traffic forwarding across multiple paths, optimizing bandwidth utilization and redundancy. Network engineers should monitor BGP tables, align attribute values, and validate multipath usage to achieve predictable routing and high availability. Understanding BGP’s equality requirements is essential for multi-homed networks, as misaligned attributes can prevent multipath utilization, reduce redundancy, and create potential traffic bottlenecks. By aligning attributes and confirming multipath configurations, engineers can achieve efficient load balancing, reliable path selection, and optimized network performance across enterprise environments. Correct multipath implementation improves redundancy, bandwidth efficiency, and predictable routing behavior, ensuring optimal operation in complex multi-homed network topologies.
Question 162:
A network engineer deploys MPLS LDP in a service provider network. Some routers fail to establish LDP sessions with neighbors. What is the most likely cause?
A) Missing IGP adjacency between routers
B) LDP transport addresses are mismatched
C) MPLS is not enabled globally
D) LDP hello timers are too long
Answer: A)
Explanation:
MPLS LDP relies on underlying IGP connectivity to provide reachability between routers for neighbor discovery and label distribution. LDP uses TCP port 646 to exchange hello messages and label mapping messages, requiring IP reachability for session establishment. Without IGP adjacency, routers cannot discover neighbors or exchange LDP messages, preventing session formation and hindering the establishment of label-switched paths essential for MPLS forwarding. LDP does not operate independently of the IGP; connectivity between routers is critical for proper operation. Verification involves checking the IGP routing table, inspecting adjacency status using commands like “show ip ospf neighbor” or “show ip eigrp neighbors,” and confirming connectivity with ping or traceroute. Proper IGP configuration ensures that LDP hello messages reach neighbors, enabling session establishment and label exchange. Network engineers must confirm that all interfaces participating in LDP have correct IP addressing and that the IGP is fully converged. Proper IGP configuration provides a foundation for LDP operation, enabling predictable MPLS forwarding.
Mismatched LDP transport addresses can prevent session formation if configured incorrectly, but default LDP deployments usually use loopback or primary interface addresses, making this scenario less common.
MPLS not being globally enabled prevents label forwarding but does not stop LDP from attempting neighbor discovery. LDP discovery still occurs even if MPLS is not activated, although labels cannot be used until MPLS is enabled.
LDP hello timers affect how frequently hello messages are sent, but do not prevent neighbor formation. Longer timers may delay session establishment, but connectivity will eventually be detected if the underlying IGP adjacency exists.
The root cause is missing IGP adjacency. Ensuring proper IGP configuration allows LDP messages to reach neighbors, enabling label distribution and MPLS traffic forwarding. Network engineers must monitor IGP adjacency, interface configuration, and LDP session status to maintain stable MPLS operations. Understanding the dependency between IGP connectivity and LDP operation is essential for successful MPLS deployment. Proper planning, monitoring, and verification prevent session failures, support resilient MPLS networks, and enable efficient label-switched path establishment. Stable IGP adjacencies form a reliable foundation for LDP operation, ensuring predictable MPLS traffic forwarding, optimal network performance, and high availability in service provider networks.
Question 163:
A network engineer configures MPLS TE tunnels with RSVP-TE across an enterprise network. Despite adequate bandwidth being available, the tunnels fail to establish. What is the most likely cause?
A) CSPF cannot find a feasible path due to link attribute constraints
B) RSVP authentication is mismatched
C) RSVP soft-state refresh timers are too long
D) IGP metrics are misconfigured
Answer: A)
Explanation:
RSVP-TE is used to establish traffic-engineered label-switched paths (LSPs) across MPLS networks, allowing explicit paths and bandwidth reservations for traffic flows. The Constrained Shortest Path First (CSPF) algorithm calculates paths that meet specific constraints, including available bandwidth, administrative group assignments, and TE colors. Even when links indicate sufficient bandwidth, tunnels may fail to establish if CSPF cannot identify a feasible path that satisfies all configured constraints. Constraints can include administrative group mismatches, conflicting TE color settings, or inconsistent bandwidth reservations along a potential path. When CSPF cannot compute a valid path, the RSVP-TE tunnel setup fails, and LSPs are not created. This situation is common in complex networks where multiple TE attributes are applied across various links. Verification involves using commands like “show mpls traffic-eng tunnels” or “show mpls traffic-eng topology” to confirm CSPF calculations and evaluate link attributes. Ensuring the correct configuration of link attributes, TE colors, and bandwidth reservations is essential for successful tunnel establishment and predictable MPLS traffic engineering.
RSVP authentication mismatches impact signaling security. While authentication issues can prevent tunnels from being established, they do not explain CSPF’s inability to find a feasible path. Authentication must be configured consistently across routers to ensure secure signaling, but a properly configured CSPF requires valid link attributes to calculate feasible paths.
RSVP soft-state refresh timers maintain tunnel state by periodically refreshing resource reservations. While timers that are too long may delay failure detection or recovery, they do not affect initial path computation or tunnel establishment. Soft-state refresh timers are critical for maintaining LSP integrity over time, but they are not a primary factor in initial CSPF failures.
IGP metrics influence the basic shortest path calculation but do not override explicit TE constraints. CSPF evaluates all paths against configured constraints in addition to IGP metrics, meaning that even if IGP metrics are correctly set, CSPF may reject paths that do not satisfy TE requirements. Proper IGP configuration complements CSPF but does not replace the need for valid TE attributes.
In this scenario, the root cause is CSPF’s inability to find a feasible path due to link attribute constraints. Network engineers must ensure consistency of TE colors, administrative groups, and bandwidth reservations across all links along the intended path. Correctly configuring and verifying TE attributes ensures that CSPF can compute valid paths and RSVP-TE tunnels can be established successfully. Engineers should monitor tunnel status, inspect CSPF logs, and validate TE configurations using network management tools to confirm path feasibility. Understanding the interaction between CSPF, RSVP-TE, and TE link attributes is essential for deploying and maintaining a reliable MPLS traffic-engineered network. Proper planning, attribute alignment, and verification prevent tunnel setup failures, support predictable traffic engineering, and optimize network resource utilization. Misconfigured link attributes can prevent LSP establishment, reduce redundancy, and impact network performance, highlighting the importance of comprehensive configuration and verification. Ensuring CSPF feasibility supports robust, high-performance MPLS networks, predictable traffic paths, and efficient bandwidth usage.
Question 164:
A network engineer deploys BGP multipath in a multi-homed enterprise network. Multiple routes are visible in the BGP table, but only a single path is used for forwarding. What is the most likely cause?
A) Paths are not equal in AS path, origin, or MED attributes
B) All BGP neighbors are configured as iBGP
C) One path has an unreachable next-hop
D) BGP route dampening is enabled
Answer: A)
Explanation:
BGP multipath enables simultaneous use of multiple equal-cost paths for redundancy and load balancing. For multipath forwarding to occur, all candidate paths must be identical in critical attributes such as AS path, origin type, MED, local preference, and next-hop reachability. BGP follows a strict deterministic path selection process to prevent routing loops and maintain stable network operation. When multiple paths appear in the BGP table but only one path is used for forwarding, it usually indicates that the candidate paths are not equal in terms of AS path, origin type, or MED. Verification involves checking the BGP table using “show bgp ipv4 unicast” and comparing attributes across all candidate paths. Ensuring that these attributes match allows BGP multipath to function as intended, enabling traffic to be forwarded over multiple paths simultaneously and providing both redundancy and optimized bandwidth utilization.
All neighbors configured as iBGP do not inherently prevent multipath. iBGP allows multipath as long as all paths satisfy equality requirements. Multipath operation is independent of whether neighbors are iBGP or eBGP, provided attributes align.
An unreachable next-hop prevents a specific path from being installed in the routing table. In this scenario, multiple paths exist in the BGP table, indicating next-hop reachability is not the limiting factor. Engineers can verify reachability using “show ip route” and ping commands.
BGP route dampening temporarily suppresses routes to reduce instability. While dampening can prevent the use of unstable paths, it does not affect multipath forwarding for stable, visible paths in the BGP table.
The root cause is unequal attributes among candidate paths, such as differing AS paths, origin types, or MED values. Aligning these attributes is crucial for successful multipath forwarding, redundancy, and load balancing. Network engineers should monitor BGP tables, adjust attribute values where necessary, and validate multipath usage to ensure efficient traffic distribution. Misalignment can lead to underutilization of available paths, reduced redundancy, and potential congestion on the single active path. Understanding BGP’s strict attribute equality requirements is essential for multi-homed network design and operations. Correct multipath deployment supports optimal bandwidth utilization, predictable traffic flow, and high availability, ensuring that enterprise networks can effectively leverage multiple paths for improved reliability and performance. Proper monitoring and configuration enable load balancing, redundancy, and efficient use of network resources, critical for enterprise and service provider networks.
Question 165:
A network engineer deploys MPLS LDP in a service provider network. Some routers fail to form LDP sessions with neighbors. What is the most likely cause?
A) Missing IGP adjacency between routers
B) LDP transport addresses are mismatched
C) MPLS is not enabled globally
D) LDP hello timers are too long
Answer: A)
Explanation:
MPLS LDP depends on the underlying IGP to provide IP reachability between routers for neighbor discovery and label distribution. LDP communicates over TCP port 646, using hello messages and label mapping exchanges. Without IGP adjacency, routers cannot exchange LDP messages, preventing session formation and blocking the creation of label-switched paths essential for MPLS forwarding. LDP does not provide its own neighbor discovery independent of the IGP; connectivity is critical. Verification involves checking the IGP routing table, examining adjacency status with commands like “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity using ping or traceroute. Ensuring proper IGP operation allows LDP messages to reach neighbors and enables session establishment. Network engineers must confirm interface IP addressing and full IGP convergence to maintain reliable MPLS operations.
Mismatched LDP transport addresses can prevent session formation if manually configured incorrectly, but default LDP deployments typically rely on loopback or primary interface addresses, making this less common.
MPLS not being enabled globally prevents label forwarding but does not stop LDP from attempting to discover neighbors. LDP discovery occurs even if MPLS is inactive, though labels cannot be used until MPLS is enabled.
LDP hello timers determine the frequency of hello messages, but do not prevent neighbor formation. Longer timers may delay session establishment, but will not block LDP operation if connectivity exists.
The root cause is missing IGP adjacency. Ensuring proper IGP configuration allows LDP messages to reach neighbors, enabling label-switched path formation and MPLS forwarding. Engineers should monitor IGP adjacencies, interface configurations, and LDP session status to maintain stable MPLS networks. Understanding the dependency of LDP on IGP is critical for network design and operations. Proper planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Maintaining stable IGP adjacencies provides a foundation for LDP operation, enabling reliable traffic forwarding, optimal performance, and high availability in service provider networks.