Cisco 300-410 Implementing Cisco Enterprise Advanced Routing and Services (ENARSI) Exam Dumps and Practice Test Questions Set 2 Q16-30
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Question 16:
A network engineer is configuring OSPF on a multi-area network. After configuration, some routers in area 1 do not see routes from area 2, even though the ABR has advertised them. What is the most likely cause of the missing routes?
A) The routers in area 1 are configured as stub areas.
B) The ABR has failed to advertise Type-1 LSAs.
C) The OSPF cost on the backbone links is too high.
D) The routers in area 1 have mismatched OSPF process IDs.
Answer: A)
Explanation:
In OSPF, area types dictate which LSAs are allowed into an area and which routes are visible. Stub areas are designed to reduce resource usage on edge routers by limiting external LSA flooding. When an area is configured as a stub, it cannot accept Type-5 external LSAs from ASBRs. Instead, the ABR injects a default route to provide external reachability. This design improves scalability by keeping the link-state database smaller and reducing CPU load on routers in the area.
In this scenario, routers in area 1 do not see specific routes from area 2. If area 1 is a stub, any external routes that would normally propagate into it as Type-5 LSAs are suppressed. The routers still see intra-area and inter-area routes but rely on the default route provided by the ABR for external destinations. This behavior is entirely consistent with the missing routes observed and explains why some routes are absent despite the ABR advertising them.
Considering the second possibility, Type-1 LSAs describe the router itself within an area and are essential for neighbor discovery and SPF calculation. If Type-1 LSAs were not advertised, the routers would fail to form adjacencies entirely, and area 1 routers would not even recognize the ABR. Since the problem describes missing external routes rather than adjacency failure, the failure to advertise Type-1 LSAs is unlikely.
Regarding OSPF cost on the backbone links, this metric affects path selection, not LSA propagation. High link costs would influence which path is chosen to reach a destination, but would not prevent routes from appearing in the link-state database or routing table. Therefore, this cannot explain the complete absence of external routes.
Mismatched OSPF process IDs are local to each router and do not affect LSA propagation across area boundaries. OSPF uses process IDs internally to distinguish multiple instances on a router, but mismatches would not prevent Type-5 LSA flooding or cause selective route loss.
The correct explanation is that area 1 is configured as a stub. This prevents Type-5 external LSAs from entering the area and forces routers to rely on the default route from the ABR. The missing routes are not an error but a result of OSPF area-type design, confirming stub behavior as the root cause.
Question 17:
An engineer is deploying BGP in a multi-homed enterprise environment. After enabling BGP route dampening, convergence after flapping prefixes improves, but certain prefixes take significantly longer to appear in the routing table. What is the main reason for this behavior?
A) Route dampening suppresses unstable prefixes for a period of time to stabilize the network.
B) Route dampening redistributes routes more slowly between iBGP peers.
C) Route dampening changes the next-hop attribute, delaying convergence.
D) Route dampening reduces the preference for external routes compared to internal routes.
Answer: A)
Explanation:
BGP route dampening is a mechanism designed to improve network stability by suppressing prefixes that repeatedly flap. Each time a prefix changes state, it accumulates a penalty. Once the penalty exceeds a configurable suppress threshold, the route is withdrawn from the routing table and not advertised to neighbors. The route remains suppressed until its penalty decays below the reuse threshold, at which point it is reintroduced. This mechanism prevents repeated flapping from overwhelming routers with excessive updates.
The delayed appearance of prefixes is expected behavior under route dampening. The network sacrifices immediate convergence for overall stability, ensuring that unstable prefixes do not continually generate updates and cause CPU or memory spikes. Flapping routes are penalized to prevent them from oscillating in and out of the routing table, which could destabilize both iBGP and eBGP neighbors.
Considering alternative explanations, dampening does not redistribute routes more slowly. The update process itself is immediate once the route is no longer suppressed. It simply prevents propagation of unstable prefixes, not redistribution between peers.
Changing the next-hop attribute is unrelated to route dampening. Next-hop adjustments are typically made by route-reflectors, route policies, or the next-hop-self configuration, which affects forwarding paths but not the timing of route installation due to dampening.
Similarly, dampening does not inherently adjust route preference. Preference (local preference) determines which route is preferred when multiple paths exist, but the suppression of a prefix is independent of preference. The route remains suppressed until its penalty decays, regardless of its relative preference.
The primary reason prefixes take longer to appear in the routing table is the suppression mechanism built into route dampening. This delay ensures that unstable prefixes do not continuously impact network stability and that only prefixes deemed sufficiently stable are readvertised, confirming that the behavior described is an intentional feature of the protocol.
Question 18:
In a DMVPN Phase 3 deployment, some spoke routers consistently send traffic through the hub even when direct spoke-to-spoke tunnels are available. What is the most likely cause of this suboptimal routing?
A) NHRP redirect messages are missing from the hub.
B) EIGRP variance is misconfigured on the hub.
C) GRE keepalives are disabled on the spoke tunnels.
D) MTU mismatches between the hub and spokes prevent tunnel formation.
Answer: A)
Explanation:
In DMVPN Phase 3, the hub router is responsible for informing spokes that direct tunnels between spokes can be formed. This is achieved via NHRP redirect messages. When a spoke sends traffic to another spoke via the hub, the hub can reply with a redirect message indicating that a direct path exists. The originating spoke can then initiate a direct tunnel to the remote spoke.
If these redirect messages are not generated or lost due to configuration errors, spokes remain unaware that a direct tunnel is possible. Consequently, all inter-spoke traffic continues to traverse the hub, resulting in suboptimal routing. This behavior aligns exactly with the observed symptom: spokes rely on the hub even though the network architecture allows for direct communication.
Misconfigured EIGRP variance affects load balancing across multiple unequal-cost paths. It does not impact the formation of DMVPN tunnels or NHRP redirect functionality. Therefore, variance misconfiguration cannot explain why traffic is unnecessarily sent through the hub.
Disabling GRE keepalives may delay detection of tunnel failures, but does not prevent tunnel formation. The absence of keepalives affects the operational status of tunnels, not the knowledge of available direct paths between spokes.
MTU mismatches can lead to packet fragmentation or blackholing, but do not prevent the control-plane signaling required to form direct tunnels. The presence of a functioning DMVPN hub and GRE tunnels indicates that MTU mismatches are unlikely to be the cause of the routing behavior described.
The suboptimal routing is most logically caused by missing NHRP redirect messages. Without these messages, spokes cannot learn about direct paths, and all traffic continues to flow through the hub. This explains why spokes fail to take advantage of the Phase 3 architecture designed to optimize routing efficiency.
Question 19:
A network is using BGP to connect multiple data centers. After a link failure, certain routes learned from one peer take minutes to reappear in the routing table. Which configuration or feature is responsible for delaying the installation of these routes?
A) BGP route dampening
B) BGP multipath load balancing
C) BGP next-hop-self
D) BGP synchronization
Answer: A)
Explanation:
Route dampening in BGP is intended to prevent unstable prefixes from continually flapping in the network. Each time a prefix changes state (up or down), a penalty is added to that route. Once the accumulated penalty exceeds the suppress threshold, the route is temporarily removed from the routing table and withheld from updates. The route is reintroduced only after its penalty decays below the reuse threshold. This mechanism stabilizes the network at the cost of delayed convergence.
The delayed installation described in this scenario is characteristic of route dampening. Flapping prefixes are suppressed, which explains why routes do not immediately reappear after the link is restored. The mechanism ensures that repeated instability does not overwhelm the control plane or propagate unnecessary updates to other peers.
Multipath load balancing only affects the selection of paths in the forwarding table, allowing traffic to be distributed across multiple equal or unequal-cost paths. It does not suppress prefixes or delay their propagation.
Next-hop-self modifies the next-hop attribute in BGP advertisements to ensure reachability from downstream peers. It does not influence route suppression or convergence time.
BGP synchronization requires routes to exist in the IGP before being advertised to BGP peers. While it slows route propagation in specific legacy scenarios, it is not typically used in modern networks and does not account for suppression of flapping prefixes across multiple data centers.
The delayed reappearance of routes is best explained by BGP route dampening, which suppresses unstable prefixes to stabilize the network, confirming it as the primary cause.
Question 20:
In an MPLS TE network, RSVP-TE tunnels fail to establish even though every link reports sufficient bandwidth. What is the most probable cause?
A) Link attribute constraints, such as administrative groups, prevent the computed explicit path.
B) RSVP authentication mismatch prevents reservation installation.
C) RSVP soft-state refresh timers are too long.
D) The IGP metric of the path is incorrectly configured.
Answer: A)
Explanation:
Traffic Engineering (TE) in MPLS allows explicit path computation considering bandwidth, constraints, and link attributes. Administrative groups (TE colors) tag links to control which tunnels may traverse them. Even if links have sufficient bandwidth, CSPF may reject a path if the requested constraints cannot be satisfied. This is the most common reason tunnels fail when resources appear available: the TE path is feasible in terms of bandwidth but infeasible due to constraints.
RSVP authentication mismatches typically cause reservation failures, but would generate error messages; the problem here is that the path computation itself fails, which is independent of authentication.
Soft-state refresh timers control the maintenance of RSVP reservations. Timers too long may cause state removal during network instability, but do not prevent initial path computation.
Incorrect IGP metrics could influence the choice of unconstrained paths, but do not prevent RSVP-TE from computing a feasible constrained path if link attributes permit it.
The explicit path constraints, specifically administrative group mismatches, are the primary reason for TE path computation failure, explaining the symptoms observed in the MPLS TE network.
Question 21:
A network engineer configures OSPF sham links between two VRFs over an MPLS network to extend the OSPF backbone. After configuration, the sham link is up, but the OSPF routers on the remote site still prefer a non-backbone path through other routers. What is the most likely reason the sham link is not being preferred?
A) The sham link’s cost is higher than the alternative path.
B) OSPF authentication is mismatched on the sham link.
C) The MPLS labels are not correctly assigned.
D) The remote site routers have mismatched router IDs.
Answer: A)
Explanation:
OSPF sham links are used in MPLS VPN environments to simulate a direct backbone connection between two non-contiguous areas, effectively extending Area 0 across VPN boundaries. Sham links are treated as logical point-to-point links and allow the OSPF backbone to see the remote site as part of Area 0. However, the path selection behavior of OSPF depends primarily on the cost associated with each path. If the shamlink is higher than an alternative path, OSPF’s SPF algorithm will prefer the lower-cost path even though the sham link exists and is fully operational. In this scenario, the sham link is up, indicating that Layer 3 connectivity and OSPF adjacency are established. Despite this, the routers on the remote site prefer a non-backbone path because OSPF calculates the path with the lowest cumulative cost and chooses that route for forwarding.
Sham link cost is explicitly configurable and represents a logical metric for the virtual link. If not carefully assigned, the sham link may inherit default metrics or retain a cost that exceeds alternative paths through other routers in the VPN, causing it to be ignored for traffic forwarding. To resolve this, engineers should ensure that the sham link cost is set lower than any competing path to enforce route preference.
Considering other possible causes, OSPF authentication mismatches would prevent the sham link from forming at all. An adjacency would fail, LSAs would not be exchanged, and the sham link would be in the down state. Since the sham link is reported as up, authentication mismatches can be ruled out.
MPLS label assignment is essential for forwarding traffic across the MPLS network. Incorrect labels could cause packet delivery failures, but in this case, the sham link is operational from the OSPF perspective. Labeling problems would manifest as blackhole traffic or unreachable LSAs, not simply as preference alternative paths.
Mismatched router IDs affect OSPF DR/BDR election and tie-breaking in SPF calculations but do not override the fundamental cost comparison used to select a path. The SPF algorithm considers router IDs only in tie situations, not in determining the lowest-cost path. Therefore, mismatched IDs would not cause the sham link to be bypassed in favor of another path with a lower cumulative cost.
The most likely explanation is the cost of the sham link relative to alternative paths. OSPF inherently prioritizes the path with the lowest cost, and a higher-cost sham link—even when fully operational—will not be chosen. This highlights the importance of carefully configuring the sham link cost in MPLS VPN designs to ensure the logical backbone path is preferred and traffic forwarding aligns with intended network design principles. By adjusting the sham link cost to a value lower than competing routes, engineers can enforce OSPF route preference across the VPN and ensure proper backbone extension.
Question 22:
A network engineer configures BGP multipath on an enterprise edge router to allow traffic to multiple upstream providers. After configuration, traffic still uses a single path even though multiple BGP paths are in the table. What is the most likely reason BGP multipath is not used?
A) The paths are not equal in AS path, origin, or MED attributes.
B) The BGP router is running iBGP instead of eBGP.
C) The BGP next-hop is unreachable for one of the paths.
D) BGP update suppression is enabled.
Answer: A)
Explanation:
BGP multipath allows traffic to be forwarded along multiple equal-cost paths to the same destination, increasing bandwidth utilization and providing redundancy. However, multipath is only used when the BGP paths are considered equivalent according to the BGP best-path selection rules. These rules include multiple attributes such as AS path, origin type, MED, local preference, and next-hop reachability. If any attribute differs between the candidate paths, BGP considers them unequal and will select a single best path for forwarding. In this scenario, multiple paths are present in the BGP table, but traffic continues to use a single path because the paths differ in one or more critical attributes. For example, if the AS path length differs or the MED is not identical, BGP will not consider the paths equivalent and will forward traffic only on the selected best path.
The role of iBGP versus eBGP does not inherently prevent multipath. Both iBGP and eBGP can support multipath, although certain attributes must be carefully considered. For iBGP, the next-hop attribute must be reachable within the AS, and path equality still requires attribute consistency. Simply running iBGP does not prevent multipath; the critical factor is attribute equivalence.
Next-hop reachability is important because if one of the multiple paths has an unreachable next hop, traffic cannot be forwarded along that path. However, in this scenario, the paths are installed in the BGP table, which typically indicates the next hop is reachable at the time of installation. The problem is not path availability but rather that BGP considers the paths unequal due to differing attributes.
BGP update suppression affects how frequently updates are sent to peers, but does not prevent the local router from selecting multiple paths for forwarding. Suppression mechanisms aim to reduce control-plane overhead but do not influence the forwarding-plane path selection for already learned routes.
In summary, the most likely reason multipath is not being utilized is that the candidate BGP paths are not equal in key attributes such as AS path, origin type, or MED. BGP requires strict equivalence to install multiple paths into the forwarding table. By ensuring that these attributes match across the paths intended for multipath usage, the network engineer can achieve load sharing and redundancy as intended. This emphasizes the importance of careful BGP attribute configuration when implementing multipath in multi-provider environments.
Question 23:
A network engineer deploys IPsec site-to-site VPNs using VTIs and OSPF. Although the VPN tunnels come up, routes from the remote site do not appear in the OSPF database. What is the most likely reason?
A) The OSPF network type on the VTI interface is incompatible with the peer.
B) The IPsec encryption algorithm is incompatible with OSPF.
C) The remote router has a lower OSPF router ID.
D) The dead-interval timers are mismatched.
Answer: A)
Explanation:
VTIs (Virtual Tunnel Interfaces) allow OSPF to operate over IPsec site-to-site VPNs as if they were point-to-point interfaces. For OSPF to successfully exchange LSAs over a VTI, both endpoints must use compatible network types. The network type determines how OSPF forms adjacencies, exchanges LSAs, and elects a DR/BDR if necessary. For example, a mismatch between broadcast and point-to-point types may allow hello packets to be exchanged, but the adjacency may not progress to the full state required for LSDB synchronization. In this scenario, OSPF hello packets are seen on the tunnel, confirming that basic connectivity exists. However, the OSPF database does not populate, indicating that the adjacency is not fully operational at the LSDB level. This behavior is consistent with incompatible network type configuration on the VTI interface, which prevents full database exchange.
IPsec encryption algorithms such as AES or 3DES do not affect OSPF protocol operation directly. As long as the tunnel comes up and traffic is successfully encrypted and decrypted, OSPF packets can traverse the VPN. The presence of hello packets confirms that encryption is functioning. Therefore, the encryption type does not explain the missing routes.
The OSPF router ID primarily affects tie-breaking during SPF calculation and DR/BDR election. A lower router ID does not prevent OSPF from forming adjacencies or exchanging LSAs over a point-to-point interface. Hence, it is unlikely that the router ID is the root cause of the problem.
Dead-interval timers control the frequency of hello timeout detection. While mismatched timers can prevent adjacency formation if differences are significant, in this case, hello packets are being observed, indicating that the dead intervals are compatible enough for neighbor discovery. The lack of LSDB entries points more toward network type incompatibility than timer mismatch.
The fundamental cause is the network type mismatch on the VTI interfaces. OSPF requires compatible network types to form full adjacencies and exchange LSAs. Even though the tunnel is operational and hello packets are exchanged, incompatible types prevent LSDB synchronization, resulting in the absence of remote site routes in the OSPF database. Correcting the network type to ensure compatibility will allow OSPF to fully exchange routing information across the IPsec VTI.
Question 24:
An engineer is troubleshooting BGP VPNv4 routes in a multi-tenant MPLS network. A PE router shows certain prefixes in the MP-BGP table but they do not appear in the customer VRF routing table. What is the most likely cause?
A) The route target import configuration on the VRF does not match the advertised route targets.
B) MPLS labels were incorrectly assigned by the P router.
C) The route distinguisher conflicts with another VRF.
D) The CE router is advertising prefixes with a private AS number.
Answer: A)
Explanation:
In MPLS L3 VPNs, each customer VRF relies on route-target (RT) extended communities to determine which prefixes from the MP-BGP table should be imported into the local VRF routing table. The route distinguisher (RD) is used to make VPNv4 prefixes globally unique across the provider network, but the actual mapping of prefixes into VRFs is determined by the RT. When a PE router receives MP-BGP routes, it examines the route-targets attached to each prefix and compares them to the VRF’s import RT list. Only if there is a match will the prefix be installed in the VRF routing table.
If prefixes appear in the MP-BGP table but are missing from the VRF, it almost always indicates that the route-target import list on the VRF does not include the route-targets of the advertised prefixes. This is a common operational mistake when adding new customer VPNs or configuring multi-tenant connectivity. The MP-BGP table reflects the presence of routes in the provider core, but the VRF sees no mapping due to the missing RT import configuration.
Considering alternative explanations, MPLS label assignment affects data-plane forwarding rather than control-plane VRF installation. If labels were misconfigured on the P routers, traffic forwarding might fail, but the prefixes would still appear in the VRF routing table once imported. Therefore, MPLS label misassignment cannot account for the missing routes.
A route distinguisher conflict might cause duplicate prefixes across VRFs, but RDs do not prevent prefixes from being installed in a VRF if the import RT matches. Conflicts usually generate warnings or may cause unintended overlap but do not explain why MP-BGP routes are visible yet not imported.
Similarly, advertising prefixes with a private AS number does not prevent VRF import. The MP-BGP table will still carry the prefix, and the RT matching mechanism governs VRF installation. Private AS usage may trigger ASN rewrite or specific policy considerations for eBGP peers, but it is unrelated to VRF mapping based on RTs.
Thus, the missing VRF routes are almost certainly caused by route-target import misconfiguration. The MP-BGP table contains the routes, but without a matching RT on the VRF, the PE router will not install them in the VRF routing table. Correctly configuring the VRF import RT to include the advertised route-targets resolves this issue, ensuring proper customer route visibility and reachability.
Question 25:
A network engineer observes that MPLS Traffic Engineering (TE) RSVP tunnels fail to establish even though each link shows sufficient bandwidth. What is the most likely cause?
A) Link attribute constraints, such as TE colors, prevent the computed path from being feasible.
B) RSVP authentication between the head-end and intermediate routers is mismatched.
C) RSVP soft-state refresh intervals are too long.
D) The IGP metric of the desired path is incorrectly configured.
Answer: A)
Explanation:
MPLS Traffic Engineering (TE) uses constrained shortest-path first (CSPF) algorithms to calculate tunnel paths that satisfy constraints such as bandwidth, administrative groups (TE colors), explicit exclusions, and node or link priorities. Even if individual links report sufficient bandwidth, the path may still be considered infeasible if it violates one of the configured constraints. The most common scenario is that an explicit path object or tunnel policy specifies TE colors or link attributes that do not match the candidate links. CSPF evaluates all constraints together; if any link on the computed path does not meet the policy requirements, the path is rejected, and the tunnel cannot be established.
RSVP authentication mismatch is a different issue. While it can prevent successful reservation establishment or tear down a tunnel after path computation, it does not cause CSPF to declare a path infeasible during the computation phase. Authentication errors generate explicit log messages, which are different from path-computation failure due to constraint violations.
Soft-state refresh timers are related to maintaining RSVP reservations once they have been established. If refresh intervals are too long, RSVP state could expire prematurely, causing an active tunnel to fail. However, the failure described here occurs during initial tunnel establishment, not after the tunnel is operational, so soft-state refresh timing is unlikely to be the root cause.
IGP metric misconfiguration affects unconstrained SPF calculations but does not influence TE path computation in the presence of explicit constraints. CSPF bases its decisions on both bandwidth availability and link attributes, not solely on IGP metrics. A path may be chosen with a higher metric but still satisfy TE constraints, while a path with the lowest IGP metric may be rejected if it violates administrative attributes.
Therefore, the most probable explanation is that the TE path computation fails because one or more links on the desired path do not match the configured link attributes such as TE colors. This prevents the tunnel from being established despite available bandwidth. Engineers can resolve this by verifying administrative group configuration on all links, ensuring that the tunnel’s explicit path or affinity constraints are compatible with the available links, and rerunning CSPF. This behavior demonstrates the importance of considering both bandwidth and link attributes in TE design and troubleshooting.
Question 26:
A network engineer is deploying DMVPN Phase 3 across multiple branches. Some spoke routers always send traffic to the hub even though a direct spoke-to-spoke tunnel should exist. What is the most likely reason?
A) NHRP redirect messages are not being sent by the hub.
B) GRE keepalives are disabled on the spoke tunnels.
C) MTU mismatches between hub and spokes prevent tunnel formation.
D) EIGRP variance is misconfigured on the hub.
Answer: A)
Explanation:
DMVPN Phase 3 uses NHRP (Next Hop Resolution Protocol) to enable direct spoke-to-spoke communication without sending all traffic through the hub. When a spoke sends traffic to another spoke for the first time, the hub intercepts it and replies with an NHRP redirect message, indicating the optimal direct path. The originating spoke then establishes a dynamic GRE tunnel to the remote spoke.
If NHRP redirect messages are missing or blocked, spokes remain unaware of the possibility of a direct tunnel. All traffic continues to flow through the hub, resulting in suboptimal routing and increased latency. This behavior matches the scenario described, where spokes always send traffic via the hub despite Phase 3 capabilities.
Disabling GRE keepalives may prevent rapid detection of tunnel failures but does not prevent NHRP redirection. The tunnel would still form once the redirect message is received.
MTU mismatches can cause packet fragmentation or loss, but they do not prevent NHRP messaging or the establishment of the logical GRE tunnels needed for direct spoke-to-spoke communication.
Misconfigured EIGRP variance affects load balancing across unequal-cost paths but has no impact on DMVPN tunnel formation or NHRP redirect functionality.
The root cause is therefore the absence of NHRP redirect messages from the hub. Correctly configuring and ensuring the hub sends redirects enables Phase 3 spokes to form direct tunnels, achieving optimal routing and reducing unnecessary hub traffic.
Question 27:
A network engineer observes that after enabling BGP route dampening, certain prefixes take significantly longer to reappear in the routing table even though the link is stable. What is the most likely reason?
A) Route dampening suppresses unstable prefixes to improve network stability.
B) Route dampening slows redistribution between iBGP and eBGP peers.
C) BGP multipath is disabled, forcing a single path selection.
D) The next-hop attribute is not reachable.
Answer: A)
Explanation:
BGP route dampening is designed to enhance network stability by suppressing prefixes that repeatedly flap. Each time a prefix changes its state, it accumulates a penalty. When the penalty exceeds a configured suppress threshold, the prefix is temporarily withdrawn from the BGP routing table and withheld from advertisements to neighbors. The prefix remains suppressed until its penalty decays below the reuse threshold, at which point it is reintroduced into the table. This suppression prevents unstable prefixes from overwhelming routers with continuous updates, which can lead to CPU spikes and routing instability.
In this scenario, prefixes take longer to appear even though the link is stable. This behavior is entirely consistent with route dampening. The mechanism does not immediately readmit previously flapping prefixes; it enforces a wait period until the route’s penalty has sufficiently decayed. This ensures that only prefixes considered stable are propagated, improving overall network stability. Even stable links can experience delayed route readmission if the prefixes were previously flapping.
Considering other possibilities, redistribution between iBGP and eBGP peers is unaffected by route dampening. While redistribution processes determine which routes are advertised across domains, dampening affects route visibility based on flapping behavior, not the speed of redistribution. Therefore, the delayed appearance is unrelated to redistribution speed.
BGP multipath allows multiple equal-cost paths to be installed in the forwarding table. Disabling multipath would prevent load-sharing but would not suppress prefixes or delay route installation. Multipath considerations only affect the forwarding plane, whereas route dampening affects the control plane by withholding prefixes from the table.
Next-hop reachability is another factor in route selection. A route with an unreachable next-hop cannot be installed in the routing table. However, in this scenario, the prefixes are delayed even though the link is stable and reachable, indicating that next-hop reachability is not the cause. The delay is due to intentional suppression rather than connectivity issues.
The correct explanation is that route dampening suppresses previously unstable prefixes to prevent repeated flapping. Even when a link becomes stable, the prefix will not immediately reappear until the penalty decays below the reuse threshold. This mechanism ensures stability but introduces a controlled delay in convergence, which matches the behavior described. Network engineers must carefully configure suppress and reuse thresholds to balance stability with convergence time, ensuring that essential prefixes do not remain suppressed unnecessarily.
Question 28:
In a DMVPN Phase 3 network, some spoke routers always send traffic to the hub even though direct spoke-to-spoke tunnels exist. What is the most likely cause?
A) NHRP redirect messages are missing from the hub.
B) GRE keepalives are disabled on the tunnels.
C) MTU mismatches prevent direct tunnels from forming.
D) EIGRP variance is misconfigured.
Answer: A)
Explanation:
DMVPN Phase 3 introduces the capability for spokes to communicate directly without routing traffic through the hub, reducing latency and hub load. When a spoke initiates traffic to another spoke, the hub intercepts the first packet and replies with an NHRP redirect message, informing the originating spoke that a direct tunnel can be established. The spoke then forms a dynamic GRE tunnel to the remote spoke for subsequent traffic.
If NHRP redirect messages are not sent or blocked by the hub, spokes remain unaware that direct tunnels are possible. Consequently, all inter-spoke traffic continues to traverse the hub, causing suboptimal routing and higher latency. This matches the scenario described, where traffic consistently flows through the hub despite Phase 3 features being enabled.
Disabling GRE keepalives affects the detection of tunnel failures but does not prevent NHRP messages or the formation of direct tunnels. Tunnels may still be established dynamically if NHRP redirects are received.
MTU mismatches can lead to packet fragmentation or blackholing, but they do not interfere with NHRP messaging or the establishment of GRE tunnels. MTU issues would typically result in packet loss or communication errors rather than a consistent hub-centric routing behavior.
EIGRP variance affects load balancing across multiple unequal-cost paths within a routed network. While variance settings can influence traffic distribution, they do not control the establishment of DMVPN tunnels or the reception of NHRP redirects. Misconfiguration of variance cannot explain why direct tunnels are ignored.
The root cause is therefore the absence of NHRP redirect messages from the hub. Ensuring that the hub is configured to send redirects allows spokes to learn about optimal paths and establish direct tunnels. Once NHRP messaging is operational, Phase 3 DMVPN can achieve efficient spoke-to-spoke routing, reducing reliance on the hub and improving network performance.
Question 29:
A network engineer is troubleshooting OSPF over IPsec VTIs. Tunnels are up and OSPF hellos are seen, but remote routes are missing in the OSPF database. What is the most likely cause?
A) The OSPF network type on the VTI interface is incompatible with the peer.
B) The IPsec encryption algorithm is incompatible with OSPF.
C) The OSPF router ID on the remote device is lower.
D) Dead-interval timers are mismatched.
Answer: A)
Explanation:
Virtual Tunnel Interfaces (VTIs) allow OSPF to operate over IPsec VPNs as if they were point-to-point interfaces. For OSPF to exchange LSAs successfully over a VTI, both endpoints must have compatible network types. Network type determines adjacency behavior, LSA exchange mechanisms, and DR/BDR election where applicable.
If network types are incompatible—for example, one interface configured as broadcast and the other as point-to-point—OSPF hellos may still be exchanged, but the adjacency may not progress to the full state required for LSDB synchronization. This prevents LSAs from being exchanged, leaving the remote routes missing in the database. This scenario exactly matches the problem described: hellos are visible, but the database remains incomplete.
The IPsec encryption algorithm (AES, 3DES, etc.) does not affect OSPF operation. As long as the tunnel is operational and packets can traverse it, OSPF packets are delivered. The observation of hello packets confirms that encryption is functioning correctly.
A lower router ID affects tie-breaking and DR/BDR election in broadcast networks but does not prevent LSDB synchronization. The router will still form full adjacencies and exchange LSAs as long as network types and timers are compatible.
Dead-interval timers, if mismatched, can prevent adjacency formation entirely. However, since hellos are observed consistently, the timers are compatible enough to allow neighbor detection. The problem of missing LSAs is better explained by network type incompatibility rather than timer mismatch.
The fundamental issue is therefore incompatible network types on the VTI interfaces. Correctly aligning network types ensures that OSPF adjacency reaches the full state and LSAs are exchanged, populating the OSPF database with remote routes. Engineers must verify and configure matching network types on both ends of the VTI to ensure proper OSPF operation over IPsec.
Question 30:
In a multi-area OSPF deployment, a router in area 1 does not see routes from area 2, even though the ABR advertises them. What is the most likely reason?
A) Area 1 is configured as a stub.
B) Type-1 LSAs are not advertised by the ABR.
C) The backbone link cost is too high.
D) OSPF process IDs are mismatched.
Answer: A)
Explanation:
OSPF area types control which LSAs are allowed into the area and which routes are visible. A stub area is designed to reduce routing overhead on edge routers by limiting external LSA flooding. In a stub area, Type-5 LSAs (external routes) from ASBRs are not allowed. Instead, the ABR injects a default route to provide reachability to external destinations.
In this scenario, the routers in area 1 do not see routes from area 2, which is consistent with stub area behavior. OSPF continues to propagate intra-area and inter-area routes, but external routes are suppressed to reduce the LSDB size and CPU load. The missing routes are expected because of the stub configuration.
Type-1 LSAs describe router links within the area. Failure to advertise Type-1 LSAs would prevent adjacency formation and area participation entirely, not selectively suppress external routes. Since area 1 routers form adjacencies and see other intra-area routes, Type-1 LSA issues can be ruled out.
In a network using link-state routing protocols, such as OSPF, the cost assigned to a backbone link influences the path selection process but does not affect the propagation of link-state advertisements (LSAs) within an area. Each router calculates the shortest path to every destination based on the cumulative cost of traversing links, meaning that a higher-cost link is less likely to be chosen as part of the preferred route because the algorithm favors paths with lower total cost. However, this does not prevent LSAs from being distributed across the network. LSAs carry information about network topology, link states, and metrics, and they are flooded throughout an area to ensure all routers maintain a consistent and complete map of the network. Even if a link has a high cost, its presence and attributes are still advertised, allowing routers to make informed decisions about alternative routes and failover scenarios. Therefore, while cost can influence which paths are selected for forwarding traffic, it does not cause the routes themselves to disappear from the routing table. This separation ensures that network visibility and redundancy are maintained, allowing routers to adapt dynamically to topology changes while preferring more efficient paths for data delivery.
Mismatched OSPF process IDs are local to each router and do not affect LSA propagation across areas. They only affect the identification of OSPF instances on a single router.
Therefore, the missing routes are caused by the stub configuration, which intentionally suppresses Type-5 LSAs and forces the use of a default route from the ABR. This aligns exactly with OSPF design principles for stub areas.