Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 15 Q211-225
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Question 211:
A network engineer deploys OSPF in a large enterprise network. Remote branch routers cannot reach external networks redistributed from BGP. What is the most likely cause?
A) The branch area is configured as a totally stubbed area
B) Type-3 LSAs are filtered by an ACL
C) OSPF process IDs are mismatched
D) Area 0 is down
Answer: A)
Explanation:
OSPF area types are designed to control the flow of routing information between areas and to optimize LSDB size and resource utilization. A totally stub area is configured to reduce LSDB complexity and conserve resources on routers in remote locations, such as branch offices. In a totally stub area, Type-5 LSAs, which carry external routes redistributed from protocols such as BGP, are blocked from entering the area. This design prevents routers in the area from learning external routes directly. To maintain external connectivity, the ABR injects a default route into the area. If this default route is missing or misconfigured, routers in the totally stub area cannot reach external networks. Verification involves using “show ip ospf” to confirm area type and “show ip route” to verify the presence of a default route. Proper ABR configuration ensures that branch routers maintain connectivity to external networks without increasing the LSDB size. This is critical for conserving CPU and memory resources on branch routers while still providing reliable connectivity.
In this scenario, remote branch routers cannot reach external networks despite the backbone and ABRs being operational. This strongly indicates that the area is configured as a totally stubbed area. Engineers should check the ABR configuration to ensure a default route is being advertised. Correct configuration ensures predictable routing, optimized network performance, and a minimized LSDB size. Monitoring ABR behavior, verifying default route propagation, and confirming area types are essential for stable OSPF operation and external reachability.
Filtering Type-3 LSAs via an ACL affects inter-area route propagation but does not block external routes if a default route exists. Because the problem involves external network reachability, the area type configuration is the primary cause.
If Area 0 were down, inter-area routing would fail network-wide, not just in the branch. Since only the branch is affected, the backbone is functioning properly, confirming that the branch area being a totally stub area is the root cause.
OSPF process IDs are locally significant and do not affect inter-area LSA propagation. Mismatched process IDs may trigger warnings, but do not prevent routers from receiving a default route if area types are correctly configured.
The root cause is the branch area being a totally stubbed area. Ensuring the ABR injects a default route allows routers to reach external networks. Engineers should verify area type, ABR configuration, and default route propagation. Totally stub areas reduce LSDB size, conserve resources, and maintain network stability while providing connectivity to external networks. Proper planning, monitoring, and verification of stub areas ensure stable OSPF operation, predictable convergence, and reliable external reachability. Correct deployment prevents routing issues, optimizes resource usage, and supports enterprise requirements. Understanding the behavior of totally stub areas and ABR functionality is critical for designing scalable OSPF networks. Proper configuration ensures stable routing, minimal overhead, and efficient propagation of external routes through default route injection. Continuous monitoring and verification are necessary to maintain predictable external connectivity, minimize LSDB size, and support resource-efficient OSPF operation across enterprise networks.
Question 212:
A network engineer configures BGP multipath in a multi-homed network. Multiple paths appear in the BGP table, but traffic only uses a single path. What is the most likely cause?
A) Paths differ 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 multiple equal-cost paths to be used for traffic forwarding, improving redundancy and load balancing in multi-homed networks. Multipath operation requires candidate paths to be identical in attributes such as AS path, origin type, MED, local preference, and next-hop reachability. BGP uses a deterministic path selection process to prevent routing loops, so any discrepancies in these attributes prevent multipath forwarding even if multiple paths appear in the BGP table. Verification involves using “show bgp ipv4 unicast” to compare AS path, origin type, MED, and local preference among candidate paths. Proper attribute alignment allows traffic to be distributed across multiple paths, optimizing bandwidth utilization, improving redundancy, and enhancing network performance. Correct configuration ensures predictable traffic distribution, prevents congestion on a single path, and maximizes available bandwidth.
All neighbors being in iBGP does not prevent multipath functionality. Multipath is supported with both iBGP and eBGP neighbors as long as the attributes match. Neighbor type alone does not override the need for attribute equality.
An unreachable next-hop prevents a specific path from being installed in the routing table. Since multiple paths appear in the BGP table, next-hop reachability is not the issue. Verification can be done using “show ip route” and ping tests.
BGP route dampening suppresses unstable routes but does not prevent stable paths from being used for forwarding traffic. Multipath operation functions properly as long as candidate paths meet attribute equality requirements.
The root cause is differences in key attributes among candidate paths. Differences in AS path, origin type, or MED prevent multipath forwarding. Engineers must align these attributes to enable multiple paths to carry traffic. Proper alignment improves redundancy, ensures balanced traffic distribution, and optimizes bandwidth utilization. Misalignment may result in underutilization of available paths, congestion on the active path, and reduced network resilience. Understanding BGP multipath requirements is essential for multi-homed network design. Continuous verification of attributes and monitoring of traffic flow ensure proper multipath operation. A correct configuration provides predictable traffic distribution, high availability, and efficient bandwidth usage, supporting stable enterprise network performance. Proper multipath BGP deployment improves redundancy, prevents bottlenecks, and ensures consistent traffic distribution across the network.
Question 213:
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 the underlying IGP for IP connectivity between routers to establish neighbor relationships and exchange label mapping information. LDP uses TCP port 646 to send hello messages and label updates. Without IGP adjacency, routers cannot communicate with neighbors, preventing LDP session formation and blocking the creation of label-switched paths required for MPLS traffic forwarding. LDP cannot function independently; connectivity provided by the IGP is essential. Verification involves checking IGP routing tables, adjacency status using commands like “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity with ping or traceroute. Proper IGP operation ensures that LDP messages can reach neighbors, enabling session formation and label distribution. Engineers must ensure interface IP addressing, full IGP convergence, and MPLS configuration to maintain stable LDP operation. Proper IGP configuration ensures predictable label distribution, stable traffic forwarding, and efficient network performance.
Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly, but most LDP deployments use loopback or interface addresses by default, making this scenario less likely.
MPLS not being globally enabled prevents label forwarding but does not stop LDP from attempting neighbor discovery. Discovery still occurs even if MPLS is inactive, though LSPs cannot forward traffic until MPLS is enabled.
LDP hello timers define how frequently hello messages are sent, but do not prevent neighbor formation. Longer timers may delay session establishment, but do not block LDP if IGP connectivity exists.
The root cause is missing IGP adjacency. Proper IGP configuration ensures LDP messages reach neighbors, enabling label-switched path formation and MPLS forwarding. Engineers should monitor IGP adjacency, interface configuration, and LDP session status to maintain stable MPLS networks. Understanding LDP’s dependency on the IGP is critical for network design and troubleshooting. Correct planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Stable IGP adjacencies provide the foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.
Question 214:
A network engineer deploys OSPFv3 in an IPv6 enterprise network. Some routers on the same link 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 designed for IPv6 networks and relies heavily on link-local addresses to form neighbor adjacencies. Each OSPFv3-enabled interface must have a link-local address because OSPFv3 uses link-local addresses as the source and destination of hello packets and LSA exchanges. Without link-local addresses, routers cannot communicate on the same link, preventing neighbor discovery and adjacency formation. This results in incomplete LSDB synchronization and prevents proper route propagation across the network. Verification involves using “show ipv6 interface brief” to ensure each interface has a link-local address and “show ipv6 ospf neighbor” to confirm adjacency formation. Properly configured link-local addresses ensure that OSPFv3 routers can discover neighbors, form stable adjacencies, and exchange routing information, which is critical for network convergence, route propagation, and redundancy.
In this scenario, routers on the same link fail to establish neighbor adjacencies, indicating missing or misconfigured link-local addresses. Configuring correct link-local addresses allows hello packets to be exchanged, adjacencies to form, and LSDBs to synchronize across the network. Ensuring that all OSPFv3-enabled interfaces have link-local addresses is essential for predictable convergence, stable routing, and efficient IPv6 network operation. Engineers must also verify that each router has a unique router ID to avoid conflicts, although router ID conflicts do not prevent adjacency formation.
Duplicate router IDs may cause LSDB inconsistencies or LSA rejection, but they do not prevent hello packets from being exchanged. While duplicate IDs can cause database conflicts, adjacency formation is still possible if link-local addresses exist and are unique per interface.
Area authentication mismatches in OSPFv3 affect LSA acceptance rather than adjacency formation. Routers will still attempt to form adjacencies and exchange hello packets; authentication failures only prevent LSAs from being accepted.
Disabling IPv6 unicast routing globally prevents the forwarding of IPv6 traffic but does not prevent hello packets from being exchanged over link-local addresses. Neighbor adjacencies can still form if link-local addresses are correctly configured, although traffic forwarding would be impacted.
The root cause is missing link-local addresses. Correct configuration ensures neighbor adjacency formation, LSA exchange, and stable OSPFv3 operation. Engineers should verify link-local addresses on all OSPFv3 interfaces, ensure unique router IDs, and confirm that interfaces participate in the correct areas. Proper link-local addressing allows seamless adjacency formation, uninterrupted LSA flooding, and predictable network convergence. Continuous verification and monitoring prevent operational issues and maintain stable OSPFv3 networks. Proper configuration ensures stable routing, reliable connectivity, and optimal IPv6 network performance. Ensuring link-local addresses are present allows OSPFv3 to function correctly, supports redundancy, and provides predictable behavior for enterprise and service provider networks. Engineers must also confirm that interfaces are correctly assigned to areas and that any IPv6 addressing conflicts are resolved to maintain stable adjacency formation.
Question 215:
A network engineer configures BGP multipath in a dual-homed network. Multiple paths appear in the BGP table, but only a single path is used for forwarding traffic. What is the most likely cause?
A) Paths differ 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 functionality allows traffic to be load-balanced across multiple equal-cost paths. For multipath to work, all candidate paths must be identical in several key attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP follows a strict deterministic selection process to avoid routing loops. Any difference in these attributes causes BGP to select only a single best path for traffic forwarding, even if multiple paths appear in the BGP table. Verification involves inspecting BGP paths using “show bgp ipv4 unicast” to compare attributes such as AS path, origin type, MED, and local preference. Aligning these attributes across paths allows traffic to be distributed evenly, optimizing bandwidth utilization, improving redundancy, and enhancing network reliability. Correct configuration guarantees predictable traffic distribution, prevents congestion on a single path, and ensures high availability.
All neighbors being in iBGP does not prevent multipath operation. Multipath is supported for both iBGP and eBGP neighbors, provided that candidate paths have identical attributes. Neighbor type does not override attribute equality requirements.
An unreachable next-hop prevents a path from being installed in the routing table. Since multiple paths appear in the BGP table, next-hop reachability is not the cause. Verification can be done with “show ip route” and ping tests to ensure reachability.
BGP route dampening suppresses unstable routes temporarily but does not prevent stable paths from being used for forwarding. Multipath works as long as paths meet the equality criteria.
The root cause is differences in attributes among candidate paths. Variations in AS path, origin type, or MED prevent multipath forwarding. Engineers must align these attributes to enable multiple paths for traffic distribution. Proper alignment improves redundancy, ensures balanced traffic distribution, and maximizes bandwidth utilization. Misalignment may lead to underutilization of available paths, congestion on the active path, and reduced network resilience. Understanding BGP multipath requirements is critical for designing multi-homed networks. Continuous verification and monitoring of BGP attributes and traffic distribution ensure proper multipath operation. CA corrects multipath configuration, provides predictable traffic distribution, high availability, and efficient bandwidth usage. Proper multipath deployment prevents congestion, ensures redundancy, and maintains consistent traffic flow across enterprise networks. Continuous monitoring of BGP attributes and path availability ensures optimal utilization of multiple links and network stability.
Question 216:
A network engineer deploys MPLS LDP in a service provider network. Certain 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 depends on the underlying IGP to provide IP connectivity between routers for neighbor discovery and label exchange. LDP uses TCP port 646 to send hello messages and label mapping updates. Without an IGP adjacency, routers cannot communicate, preventing LDP session formation. This failure blocks the creation of label-switched paths, which are essential for MPLS traffic forwarding. LDP cannot operate independently; proper connectivity through the IGP is required. Verification involves checking the IGP routing table, inspecting adjacency status with “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity using ping or traceroute. Ensuring full IGP convergence, proper interface IP addressing, and MPLS configuration is critical for stable LDP operation. A properly functioning IGP ensures that LDP messages reach neighbors, allowing sessions to form and label distribution to occur. Stable IGP adjacencies form the foundation for predictable label-switched paths, high availability, and efficient MPLS forwarding.
Mismatched LDP transport addresses may prevent session establishment if manually configured incorrectly, but most deployments use loopback or interface addresses by default, making this less likely.
MPLS not being globally enabled prevents traffic forwarding but does not stop LDP from attempting neighbor discovery. LDP discovery can still occur even if MPLS is inactive, though LSPs will not carry traffic until MPLS is enabled.
LDP hello timers affect the frequency of hello messages but do not prevent neighbor formation. Longer timers may delay session establishment, but will not block LDP if IGP connectivity exists.
The root cause is missing IGP adjacency. Proper IGP configuration ensures that LDP messages reach neighbors, enabling label-switched path formation and MPLS forwarding. Engineers must monitor IGP adjacency, interface configuration, and LDP session status to maintain stable MPLS networks. Understanding LDP’s dependency on the IGP is critical for network design and troubleshooting. Correct planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Stable IGP adjacencies provide a foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.
Question 217:
A network engineer configures EIGRP across multiple routers in a large enterprise network. Some routers on the same subnet are not forming EIGRP neighbor relationships. 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 belong to the same autonomous system (AS) to form neighbor relationships. The AS number acts as an identifier for routers within a routing domain. Routers with mismatched AS numbers cannot recognize each other as valid neighbors, preventing the exchange of routing information. This failure leads to incomplete routing tables, delayed convergence, and potential network segmentation. Verification involves checking the running configuration using commands such as “show running-config” to confirm the AS number and using “show ip eigrp neighbors” to observe the neighbor status. Consistent AS numbers across routers ensure stable EIGRP adjacencies and reliable route propagation throughout the network. Proper configuration of AS numbers is critical in large networks where multiple EIGRP domains coexist because a mismatch can create isolated segments, disrupt routing, and degrade performance.
In this scenario, routers on the same subnet are unable to form neighbors, indicating a probable AS number mismatch. Correcting the AS numbers allows routers to form adjacencies, exchange routing updates, and maintain consistent routing tables. Engineers should also verify that interfaces have proper IP addressing, that passive interfaces are not inadvertently configured, and that routers can reach each other at Layer 3. Proper AS number alignment prevents neighbor formation failures, routing inconsistencies, and network segmentation.
A passive interface prevents hello packets from being sent on that interface, blocking neighbor formation on the specific interface. While this could explain a single interface issue, widespread neighbor failures across multiple routers indicate a systemic problem such as an AS number mismatch.
Differences in K values influence the calculation of EIGRP metrics but do not prevent neighbor formation. Mismatched K values affect route selection, potentially leading to suboptimal routing, but do not block adjacency establishment.
Split-horizon is a mechanism that prevents a router from advertising a route back out the interface on which it was learned. While this affects route propagation in certain topologies, it does not prevent neighbor formation on the same subnet.
The root cause is the mismatch of EIGRP AS numbers. Ensuring consistent AS numbers allows routers to form adjacencies, exchange routing information, and propagate routes efficiently. Engineers must verify AS numbers, interface configurations, and adjacency status to maintain proper EIGRP operation. Understanding the importance of AS number alignment is critical to prevent neighbor formation failures, routing inconsistencies, and network segmentation. Proper AS number configuration supports predictable route propagation, stable convergence, and reliable connectivity. Consistent AS numbers enable routers to form reliable adjacencies, maintain complete routing tables, and guarantee predictable network behavior. Continuous verification of AS numbers and monitoring of neighbor relationships ensures stable EIGRP operation, efficient routing, and robust network performance. Proper planning, deployment, and validation prevent segmentation, improve redundancy, and maintain optimal enterprise network performance.
Question 218:
A network engineer deploys OSPFv3 in an IPv6 network. Some routers fail to form neighbor adjacencies on certain links. 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 an IPv6 routing protocol that relies on link-local addresses for neighbor discovery and adjacency formation. Link-local addresses are mandatory for OSPFv3 because they serve as the source and destination for hello packets exchanged between routers on the same link. Without link-local addresses, routers cannot discover neighbors or establish adjacencies, which prevents the synchronization of link-state databases and route propagation across the network. Verification can be done using “show ipv6 interface brief” to confirm link-local addresses and “show ipv6 ospf neighbor” to inspect adjacency formation. Proper link-local addressing ensures that OSPFv3 routers can form stable adjacencies, exchange LSAs, and achieve predictable convergence. Correct configuration supports redundancy, network stability, and consistent route propagation, which are essential for enterprise and service provider IPv6 networks.
In this scenario, routers fail to form adjacencies on certain links, suggesting missing or misconfigured link-local addresses. Configuring valid link-local addresses enables hello packets to be exchanged, adjacencies to form, and LSDBs to synchronize, ensuring reliable OSPFv3 operation. Engineers must also verify that each router has a unique router ID, although router ID conflicts do not prevent adjacency formation.
Duplicate router IDs can cause LSDB conflicts or LSA rejection, but do not prevent hello packets from being exchanged or adjacencies from forming. The problem in this scenario is the inability to form neighbors, pointing to link-local address issues rather than router ID conflicts.
Area authentication mismatches affect LSA acceptance but do not prevent adjacency formation. Routers will still exchange hello packets and attempt to form adjacencies; authentication failures only impact the acceptance of LSAs.
Disabling IPv6 unicast routing globally prevents IPv6 packet forwarding but does not stop hello packets from being exchanged over link-local addresses. Adjacency formation can still occur if link-local addresses are configured correctly.
The root cause is missing link-local addresses. Ensuring that all OSPFv3-enabled interfaces have link-local addresses allows adjacency formation, LSA exchange, and stable network operation. Engineers should verify interface configuration, link-local addresses, and router IDs. Proper link-local addressing enables seamless adjacency formation, uninterrupted LSA flooding, and predictable convergence. Continuous monitoring and verification prevent operational issues and maintain stable OSPFv3 networks. Proper configuration ensures stable routing, reliable connectivity, and optimal IPv6 network performance. Engineers must also confirm that interfaces are assigned to the correct OSPFv3 areas and resolve any addressing conflicts to maintain stable neighbor relationships and predictable network behavior.
Question 219:
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 requires an underlying IGP to provide IP connectivity between routers for neighbor discovery and label exchange. LDP uses TCP port 646 to send hello messages and label updates. Without IGP adjacency, routers cannot reach neighbors, preventing LDP session formation. This blocks the creation of label-switched paths, which are necessary for MPLS traffic forwarding. LDP cannot operate independently; proper connectivity provided by the IGP is essential. Verification involves inspecting IGP routing tables, checking adjacency status using “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity using ping or traceroute. Ensuring full IGP convergence, correct interface addressing, and MPLS configuration is critical for stable LDP operation. Proper IGP configuration guarantees that LDP messages can reach neighbors, enabling session formation, label distribution, and predictable traffic forwarding. Stable IGP adjacencies form the foundation for reliable MPLS operation, high availability, and efficient forwarding.
Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly, but most deployments use loopback or interface addresses by default.
MPLS not being enabled globally prevents forwarding, but does not stop LDP from attempting neighbor discovery. LDP discovery can occur even if MPLS is inactive, though LSPs will not carry traffic until MPLS is enabled.
LDP hello timers define the interval of hello messages, but do not prevent neighbor formation. Longer timers may delay session establishment, but will not block LDP if IGP connectivity exists.
The root cause is missing IGP adjacency. Proper IGP configuration ensures that LDP messages reach neighbors, enabling label-switched path formation and MPLS forwarding. Engineers should monitor IGP adjacency, interface configuration, and LDP session status. Understanding LDP’s dependency on the IGP is essential for network design and troubleshooting. Correct planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Stable IGP adjacencies provide the foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.
Question 220:
A network engineer configures OSPF in a multi-area network. Branch routers cannot reach external networks redistributed from BGP. What is the most likely cause?
A) The branch area is configured as a totally stubbed area
B) Type-3 LSAs are filtered by an ACL
C) OSPF process IDs are mismatched
D) Area 0 is down
Answer: A)
Explanation:
OSPF uses area types to control the scope and flow of routing information, optimizing LSDB size and router resources. A totally stubbed area is designed for remote or branch locations to minimize the amount of LSDB entries and simplify routing tables. In a totally stub area, Type-5 LSAs, which carry external routes such as those redistributed from BGP, are blocked. Instead, the ABR injects a default route into the area, allowing routers to reach external networks without learning detailed external routes. If the default route is missing or misconfigured, branch routers cannot reach external destinations, causing connectivity issues. Verification involves using “show ip ospf” to confirm the area type and “show ip route” to check for a default route. Correct configuration of ABRs ensures that branch routers receive the default route, maintain external connectivity, and avoid increasing the LSDB size unnecessarily. This approach conserves CPU and memory on branch routers while maintaining network stability.
In the scenario described, branch routers cannot access external networks even though the ABRs and backbone are operational. This strongly indicates that the branch area is a totally stub area. Engineers need to ensure the ABR correctly advertises the default route to enable reachability to external networks. Correct configuration ensures predictable routing, minimal LSDB size, and optimized network performance. Monitoring ABR behavior, verifying default route propagation, and confirming area types are critical for stable OSPF operation.
Filtering Type-3 LSAs with an ACL affects inter-area routing but does not prevent external network reachability if a default route exists. Since the problem involves external connectivity, the area type configuration is the main factor.
If Area 0 were down, inter-area routing would fail across the network, not just in a branch area. Since the backbone is operational, the cause is localized to the branch area, which is a totally stubbed area.
OSPF process IDs are locally significant and do not affect the propagation of LSAs across areas. Mismatched process IDs might generate local warnings, but do not prevent a router from receiving a default route if the area type is correctly configured.
The root cause is the branch area being a totally stubbed area. Ensuring the ABR injects a default route allows branch routers to reach external networks. Engineers should verify the area type, ABR configuration, and default route propagation. Totally stub areas reduce LSDB size, conserve router resources, and maintain network stability while still providing essential connectivity to external networks. Proper planning, monitoring, and verification of stub areas ensure predictable convergence, stable OSPF operation, and reliable external reachability. Correct deployment prevents routing issues, optimizes resources, and supports enterprise or service provider network requirements. Understanding totally stubbed areas and ABR behavior is critical for designing scalable OSPF networks. Proper configuration ensures stable routing, minimal overhead, and efficient external route propagation through default route injection. Continuous monitoring is necessary to maintain predictable external connectivity, reduce LSDB size, and ensure resource-efficient OSPF operation across enterprise networks.
Question 221:
A network engineer configures BGP multipath in a dual-homed network. Multiple paths appear in the BGP table, but only one path is actively used for forwarding traffic. What is the most likely cause?
A) Paths differ 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 the simultaneous use of multiple equal-cost paths to increase redundancy and load balancing. For multipath to function, all candidate paths must be identical in key attributes such as AS path, origin type, MED, local preference, and next-hop reachability. BGP follows a deterministic best-path selection process to prevent loops. If there are differences in these attributes, only one path is selected for forwarding traffic, even if multiple paths are visible in the BGP table. Verification involves using “show bgp ipv4 unicast” to inspect AS path, origin type, MED, and local preference values. Proper alignment of these attributes allows traffic to be distributed evenly across multiple paths, optimizing bandwidth utilization, improving redundancy, and ensuring high availability. Correct configuration guarantees predictable traffic distribution, prevents congestion on a single path, and maximizes available bandwidth.
All neighbors being in iBGP does not prevent multipath. Multipath is supported for both iBGP and eBGP neighbors as long as candidate paths have identical attributes. Neighbor type alone does not override the requirement for attribute equality.
An unreachable next-hop prevents a path from being installed in the routing table. Since multiple paths appear in the BGP table, next-hop reachability is not the issue. Verification can be done using “show ip route” and ping tests to ensure reachability.
BGP route dampening suppresses unstable routes temporarily but does not prevent stable paths from being used for traffic forwarding. Multipath works as long as paths meet the attribute equality requirements.
The root cause is differences in key attributes among candidate paths. Differences in AS path, origin type, or MED prevent multipath forwarding. Engineers must align these attributes to enable multiple paths to carry traffic. Proper alignment improves redundancy, ensures balanced traffic distribution, and optimizes bandwidth usage. Misalignment may result in underutilization of available paths, congestion on the active path, and reduced network resilience. Understanding BGP multipath requirements is critical in multi-homed network design. Continuous verification of attributes and monitoring traffic flow ensures proper multipath operation. A correct multipath configuration provides predictable traffic distribution, high availability, and efficient bandwidth usage. Proper multipath deployment prevents congestion, ensures redundancy, and maintains consistent traffic flow across enterprise networks.
Question 222:
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 the underlying IGP to provide IP connectivity between routers in order to establish neighbor relationships and exchange label mapping information. LDP uses TCP port 646 to send hello messages and label updates. Without an IGP adjacency, routers cannot communicate, preventing LDP session formation and blocking the creation of label-switched paths required for MPLS traffic forwarding. LDP cannot operate independently; proper IGP connectivity is essential. Verification involves inspecting the IGP routing table, checking neighbor adjacency status using “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity using ping or traceroute. Ensuring full IGP convergence, proper interface addressing, and MPLS configuration is critical for stable LDP operation. Proper IGP configuration guarantees that LDP messages reach neighbors, allowing session formation, label distribution, and predictable MPLS forwarding. Stable IGP adjacencies form the foundation for reliable label-switched paths, optimal performance, high availability, and robust traffic forwarding.
Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly, but most deployments use loopback or interface addresses by default.
MPLS not being globally enabled prevents label forwarding but does not stop LDP from attempting neighbor discovery. LDP discovery can occur even if MPLS is inactive, though LSPs will not forward traffic until MPLS is enabled.
LDP hello timers define how frequently hello messages are sent, but do not prevent neighbor formation. Longer timers may delay session establishment, but will not block LDP if IGP connectivity exists.
The root cause is missing IGP adjacency. Proper IGP configuration ensures LDP messages reach neighbors, enabling label-switched path formation and MPLS forwarding. Engineers should monitor IGP adjacency, interface configuration, and LDP session status. Understanding LDP’s dependency on the IGP is essential for network design and troubleshooting. Correct planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Stable IGP adjacencies provide the foundation for reliable LDP operation, optimal performance, high availability, and robust traffic forwarding in service provider networks.
Question 223:
A network engineer configures OSPF in a multi-area enterprise network. Some branch routers cannot reach external networks redistributed from BGP. What is the most likely cause?
A) The branch area is configured as a totally stubbed area
B) Type-3 LSAs are filtered by an ACL
C) OSPF process IDs are mismatched
D) Area 0 is down
Answer: A)
Explanation:
OSPF uses different area types to control the flow of routing information, optimize LSDB size, and conserve router resources. A totally stubbed area is designed for remote or branch locations to minimize LSDB entries and reduce CPU and memory consumption. In a totally stub area, Type-5 LSAs, which carry external routes such as those redistributed from BGP, are blocked. Instead, the ABR injects a default route into the area to allow routers to reach external destinations without learning detailed external routes. If the default route is missing or misconfigured, branch routers cannot reach external networks, causing connectivity issues. Verification involves using “show ip ospf” to confirm the area type and “show ip route” to check for the presence of a default route. Correct ABR configuration ensures that branch routers receive the default route, maintain connectivity, and avoid increasing LSDB size unnecessarily. This is critical for conserving CPU and memory on branch routers while maintaining network stability.
In this scenario, branch routers cannot access external networks even though the ABRs and backbone are operational. This strongly suggests that the branch area is totally stubbed. Engineers should ensure the ABR correctly advertises the default route to allow reachability to external networks. Correct configuration guarantees predictable routing, minimal LSDB size, and optimized network performance. Monitoring ABR behavior, verifying default route propagation, and confirming area types are crucial for stable OSPF operation.
Filtering Type-3 LSAs with an ACL affects inter-area routing but does not prevent external network reachability if a default route exists. Since the issue involves external connectivity, the area type configuration is the primary factor.
If Area 0 were down, inter-area routing would fail network-wide, not just in a branch area. Since the backbone is functional, the root cause is likely the branch area being a totally stubbed area.
OSPF process IDs are locally significant and do not affect LSA propagation between areas. Mismatched process IDs may trigger warnings locally, but do not prevent routers from receiving a default route if the area type is correctly configured.
The root cause is the branch area being a totally stubbed area. Ensuring the ABR injects a default route allows branch routers to reach external networks. Engineers should verify area type, ABR configuration, and default route propagation. Totally stub areas reduce LSDB size, conserve router resources, and maintain stability while providing connectivity to external networks. Proper planning, monitoring, and verification of stub areas ensure predictable convergence, stable OSPF operation, and reliable external reachability. Correct deployment prevents routing issues, optimizes resource usage, and supports enterprise network requirements. Understanding totally stubbed areas and ABR behavior is essential for scalable OSPF design. Proper configuration ensures stable routing, minimal overhead, and efficient external route propagation through default route injection. Continuous monitoring is necessary to maintain predictable external connectivity, reduce LSDB size, and ensure resource-efficient OSPF operation across enterprise networks.
Question 224:
A network engineer configures BGP multipath in a dual-homed network. Multiple paths appear in the BGP table, but traffic only uses one path. What is the most likely cause?
A) Paths differ 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 increase redundancy and load balancing. For multipath to work, all candidate paths must be identical in key attributes, including AS path, origin type, MED, local preference, and next-hop reachability. BGP follows a strict deterministic path selection process to avoid routing loops. Any differences in these attributes cause BGP to select a single best path for traffic forwarding, even if multiple paths appear in the BGP table. Verification involves using “show bgp ipv4 unicast” to inspect AS path, origin type, MED, and local preference of candidate paths. Aligning these attributes enables even traffic distribution across multiple paths, optimizing bandwidth, enhancing redundancy, and improving network performance. Correct configuration ensures predictable traffic distribution, prevents congestion on a single path, and maximizes available bandwidth.
All neighbors being in iBGP does not prevent multipath operation. Multipath works with both iBGP and eBGP neighbors as long as the attributes match. Neighbor type alone does not override attribute equality requirements.
An unreachable next-hop prevents a path from being installed in the routing table. Since multiple paths appear in the BGP table, next-hop reachability is not the problem. Verification can be performed with “show ip route” and ping tests.
BGP route dampening suppresses unstable routes but does not prevent stable paths from being used. Multipath functions properly as long as candidate paths meet the equality criteria.
The root cause is differences in attributes among candidate paths. Differences in AS path, origin type, or MED prevent multipath forwarding. Engineers must align these attributes to enable multiple paths to carry traffic. Proper alignment improves redundancy, ensures balanced traffic distribution, and optimizes bandwidth usage. Misalignment may lead to underutilization of available paths, congestion on the active path, and reduced resilience. Understanding BGP multipath requirements is essential in dual-homed network design. Continuous verification of attributes and monitoring of traffic flow ensure proper multipath operation. CA’s correct configuration provides predictable traffic distribution, high availability, and efficient bandwidth usage. Proper multipath deployment prevents congestion, ensures redundancy, and maintains consistent traffic flow across enterprise networks. Continuous monitoring of BGP attributes and path availability ensures optimal utilization of multiple links and network stability.
Question 225:
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 the underlying IGP to provide IP connectivity between routers for neighbor discovery and label distribution. LDP uses TCP port 646 to exchange hello messages and label mapping updates. Without an IGP adjacency, routers cannot communicate, preventing LDP session formation and blocking the creation of label-switched paths required for MPLS traffic forwarding. LDP cannot operate independently; proper connectivity through the IGP is mandatory. Verification involves checking the IGP routing table, reviewing adjacency status with “show ip ospf neighbor” or “show ip eigrp neighbors,” and testing connectivity using ping or traceroute. Full IGP convergence, correct interface addressing, and MPLS configuration are critical for stable LDP operation. Correct IGP configuration ensures that LDP messages reach neighbors, allowing session formation, label distribution, and predictable traffic forwarding. Stable IGP adjacencies form the foundation for reliable label-switched paths, optimal network performance, high availability, and robust MPLS traffic forwarding.
Mismatched LDP transport addresses can prevent session establishment if manually configured incorrectly, but default LDP deployments typically use loopback or interface addresses, making this less likely.
MPLS not being globally enabled prevents forwarding, but does not stop LDP from attempting neighbor discovery. LDP discovery occurs even if MPLS is inactive, although LSPs will not carry traffic until MPLS is enabled.
LDP hello timers define the interval between hello messages, but do not prevent neighbor formation. Longer timers may delay session establishment, but will not block LDP if IGP connectivity exists.
The root cause is missing IGP adjacency. Correct IGP configuration ensures LDP messages reach neighbors, enabling label-switched path formation and MPLS forwarding. Engineers should monitor IGP adjacency, interface configuration, and LDP session status. Understanding LDP’s dependency on the IGP is critical for network design and troubleshooting. Proper planning, configuration, and verification prevent session failures, ensure predictable label distribution, and support efficient MPLS forwarding. Stable IGP adjacencies provide the foundation for reliable LDP operation, high availability, optimal performance, and robust traffic forwarding in service provider networks.