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Q381. Which OSPF command is used to manually configure the Router ID?
1) router-id 1.1.1.1
2) ospf router-id 1.1.1.1
3) set router-id 1.1.1.1
4) ip ospf router-id 1.1.1.1
Correct Answer: 1) router-id 1.1.1.1
Explanation:
The router-id command is used under the OSPF routing process to manually configure the OSPF Router ID. For example, under router ospf 10, an administrator can enter router-id 1.1.1.1. Manually configuring the Router ID provides predictable identification and avoids depending on interface-address selection. If the Router ID is changed after the OSPF process is already running, the OSPF process may need to be restarted or the routing process cleared for the new value to take effect. A stable Router ID is particularly useful when troubleshooting OSPF databases and neighbor relationships because LSAs identify their originating routers using this value. The other commands listed are not valid Cisco IOS commands for configuring an OSPF Router ID.
Q382. Which condition can prevent two OSPF routers from forming an adjacency even when their IP addresses are correctly configured?
1) Different OSPF area IDs
2) Different interface descriptions
3) Different hostname values
4) Different loopback descriptions
Correct Answer: 1) Different OSPF area IDs
Explanation:
OSPF neighbors must belong to the same OSPF area on a shared link in order to form an adjacency. If one router places an interface in Area 0 while the neighboring router places its corresponding interface in Area 1, the OSPF parameters do not match and the adjacency will not successfully form. Area membership is fundamental because it determines the link-state database scope and the role of the interface within the OSPF topology. Interface descriptions and hostname values do not affect OSPF neighbor formation. Loopback descriptions are also irrelevant to adjacency establishment. During troubleshooting, administrators should check the OSPF area, network type, timers, authentication, MTU, and other required parameters when an expected OSPF neighbor remains down.
Q383. Which OSPF feature can help reduce the size of routing information advertised between areas?
1) SPF throttling
2) Route summarization
3) BFD
4) Router priority
Correct Answer: 2) Route summarization
Explanation:
OSPF route summarization allows an administrator to represent multiple more-specific routes with a single summarized route. In a multi-area OSPF design, summarization can be configured on an Area Border Router to reduce the amount of detailed routing information advertised between areas. This can reduce routing-table size, limit unnecessary topology information, and improve scalability. Summarization is particularly useful when multiple networks can be represented by a larger aggregate prefix. SPF throttling serves a different purpose by controlling the timing of SPF calculations. BFD provides fast failure detection, while router priority influences DR and BDR elections on multiaccess networks. Therefore, route summarization is the feature most directly associated with reducing routing information between OSPF areas.
Q384. What is the primary purpose of OSPF LSA filtering?
1) To encrypt OSPF packets
2) To increase interface bandwidth
3) To control which LSAs are propagated in specific parts of the OSPF domain
4) To assign Router IDs automatically
Correct Answer: 3) To control which LSAs are propagated in specific parts of the OSPF domain
Explanation:
OSPF LSA filtering provides control over which link-state information is allowed to propagate through particular areas or interfaces, depending on the filtering mechanism being used. This can help control routing information, reduce unnecessary advertisements, and improve scalability in carefully designed environments. LSA filtering must be implemented cautiously because OSPF relies on consistent link-state information to calculate paths. Incorrect filtering can create incomplete routing information or unexpected reachability problems. LSA filtering does not encrypt OSPF traffic, increase physical bandwidth, or automatically assign Router IDs. Encryption or authentication is handled through OSPF security mechanisms, bandwidth depends on the underlying interface, and Router IDs are selected or configured separately. Therefore, controlling LSA propagation is the correct purpose.
Q385. Which EIGRP component determines whether a reported distance can qualify a route as a feasible successor?
1) Split horizon
2) Administrative distance
3) Feasibility condition
4) Router ID
Correct Answer: 3) Feasibility condition
Explanation:
The EIGRP feasibility condition is used to determine whether a neighboring router can provide a loop-free backup path. For a route to qualify as a feasible successor, the neighbor’s reported distance must be less than the current feasible distance for that destination. This condition provides a loop-free criterion without requiring the router to perform a full recomputation before using the alternate path. If a valid feasible successor exists, EIGRP can quickly switch to it when the current successor becomes unavailable. Split horizon is another loop-prevention mechanism, but it controls route advertisement behavior rather than determining feasible-successor eligibility. Administrative distance ranks routing sources, while Router ID is not the mechanism used for EIGRP feasibility calculations.
Q386. Which EIGRP command displays detailed information about successor and feasible successor paths?
1) show ip eigrp topology
2) show ip eigrp traffic
3) show ip eigrp interfaces
4) show ip eigrp neighbors
Correct Answer: 1) show ip eigrp topology
Explanation:
The show ip eigrp topology command displays information from the EIGRP topology table, including destinations, successor routes, feasible successors when available, feasible distance, reported distance, and other path information. This makes the command particularly useful when troubleshooting EIGRP route selection and determining why an alternate path is or is not eligible as a feasible successor. The show ip eigrp neighbors command focuses on neighbor relationships and provides information such as neighbor addresses and uptime. show ip eigrp interfaces provides information about interfaces participating in EIGRP, while show ip eigrp traffic displays EIGRP packet statistics. Therefore, the topology command is the best choice for examining successor and feasible successor information.
Q387. Which EIGRP feature can prevent a router from advertising certain routes through a specific interface?
1) Route reflector
2) Distribute list
3) BGP community
4) MPLS label
Correct Answer: 2) Distribute list
Explanation:
An EIGRP distribute list can filter routing updates so that selected routes are permitted or denied through a specified interface or routing process. It can work with mechanisms such as access lists or prefix lists to identify routes that should be advertised or accepted. This provides administrators with control over routing information without necessarily removing the routes from the local routing table. For example, a distribute list can prevent selected prefixes from being advertised to a particular neighbor while allowing other routes to continue being exchanged. A BGP route reflector is a BGP scalability mechanism, communities are BGP policy attributes, and MPLS labels are used for label switching. Therefore, the distribute list is the appropriate EIGRP filtering mechanism.
Q388. Which BGP attribute is normally preferred when it has the highest value and is used to select an outbound path on Cisco routers?
1) MED
2) Local Preference
3) AS_PATH length
4) Origin code
Correct Answer: 2) Local Preference
Explanation:
BGP Local Preference is an important attribute for controlling outbound traffic within an autonomous system. When comparing otherwise valid paths, Cisco BGP prefers the route with the highest Local Preference. The attribute is distributed among iBGP peers so that multiple internal routers can make a consistent decision about the preferred exit point. For example, an organization with two Internet connections can assign a higher Local Preference to routes learned through its preferred provider. AS_PATH is generally preferred when shorter, while MED is generally preferred when lower and under the applicable comparison conditions. Origin codes also participate in the BGP decision process but are not preferred based on a higher numerical value. Therefore, the highest Local Preference is preferred.
Q389. Which BGP attribute is generally preferred when it has the lowest value, assuming the paths reach the appropriate stage of the best-path selection process?
1) Local Preference
2) Weight
3) MED
4) AS_PATH
Correct Answer: 3) MED
Explanation:
The Multi-Exit Discriminator, or MED, is generally compared as a lower-is-better attribute in BGP path selection. MED can be used by an autonomous system to suggest which entry point another autonomous system should prefer when multiple connections exist between the same autonomous systems. For example, a company could advertise a lower MED on one link to encourage inbound traffic through that connection. MED behavior can depend on the specific BGP topology and Cisco configuration, including whether routes originate from the same neighboring AS. Local Preference and Weight use higher-is-better logic, while AS_PATH generally favors a shorter path. Therefore, among the listed attributes, MED is the one generally preferred when its value is lower.
Q390. Which BGP mechanism is designed to prevent an unstable route from repeatedly entering and leaving the routing table?
1) Route dampening
2) Route reflection
3) Local Preference
4) Peer group
Correct Answer: 1) Route dampening
Explanation:
BGP route dampening is designed to reduce the impact of route instability, commonly called route flapping. A route that repeatedly becomes unavailable and then available again can generate many BGP updates and consume processing resources across the network. Route dampening assigns penalties to unstable routes. When a route exceeds configured thresholds, it can be suppressed temporarily. As the route remains stable, the penalty decays, eventually allowing the route to be considered again. Route reflection solves the iBGP full-mesh scaling problem, Local Preference influences outbound path selection, and peer groups simplify configuration for multiple BGP neighbors. Therefore, route dampening is the BGP mechanism specifically designed to mitigate repeated route instability.
Q391. Which BGP feature reduces the need for a full mesh of iBGP sessions?
1) MED
2) Route reflector
3) Weight
4) AS-path prepending
Correct Answer: 2) Route reflector
Explanation:
A BGP route reflector reduces the requirement for a full mesh of internal BGP sessions. In traditional iBGP, routes learned from one iBGP peer are not normally advertised to another iBGP peer, which requires every iBGP router to establish sessions with every other iBGP router. As the number of routers increases, this becomes difficult to manage. A route reflector provides a scalable alternative by allowing selected clients to exchange routes through a route reflector. The reflector can advertise routes between its clients according to BGP rules. MED influences path selection, Weight is a Cisco-specific local attribute, and AS-path prepending modifies path attractiveness. Therefore, route reflection is the correct scalability mechanism.
Q392. In an MPLS Layer 3 VPN, what is the primary purpose of a Route Distinguisher?
1) Encrypt VPN traffic
2) Create globally unique VPNv4 route identities
3) Assign customer IP addresses
4) Select the OSPF DR
Correct Answer: 2) Create globally unique VPNv4 route identities
Explanation:
A Route Distinguisher, or RD, is used in MPLS Layer 3 VPNs to make overlapping customer IPv4 prefixes unique within the provider’s VPNv4 routing environment. Different customers can use the same private prefix, such as 10.1.1.0/24, and the RD creates a unique VPNv4 representation of each route. This allows the provider network to maintain separate routing information for customers that use overlapping address spaces. The RD itself does not determine which customer routes are imported into a particular VRF; that function is associated with Route Targets. An RD also does not provide encryption, assign customer addresses, or participate in OSPF DR selection. Therefore, creating unique VPNv4 route identities is the primary purpose of an RD.
Q393. Which MPLS device normally forwards labeled packets through the provider core without maintaining customer-specific VRF tables?
1) CE router
2) PE router
3) P router
4) Customer firewall
Correct Answer: 3) P router
Explanation:
A Provider, or P, router operates inside the MPLS provider core and normally does not maintain customer-specific VRF routing information. Its primary responsibility is to forward MPLS-labeled packets through the provider backbone using the labels associated with the forwarding path. PE routers sit at the edge of the provider network and maintain customer VRFs, while CE routers connect customer networks to the provider edge. Keeping customer-specific information out of the P routers allows the provider core to scale more efficiently because core routers do not need individual routing information for every customer VPN. The P router therefore focuses on label switching and transport across the provider core rather than customer-specific VPN routing decisions.
Q394. Which DMVPN component provides the central service that maintains NHRP mappings for spoke routers?
1) NHRP NHS
2) DHCP server
3) BGP route reflector
4) MPLS P router
Correct Answer: 1) NHRP NHS
Explanation:
The Next Hop Server, or NHS, is a key component of DMVPN and NHRP operation. In a traditional DMVPN design, the hub commonly functions as the NHRP NHS. Spokes register their tunnel and NBMA information with the NHS, allowing the hub to maintain mappings between tunnel addresses and underlying NBMA addresses. These mappings can then be used to establish more efficient spoke-to-spoke communication depending on the DMVPN phase and routing design. A DHCP server provides host configuration, a BGP route reflector serves a different purpose in BGP, and an MPLS P router operates within an MPLS provider core. Therefore, the NHRP NHS is responsible for maintaining the relevant NHRP mapping information.
Q395. Which IPsec protocol provides confidentiality through encryption and can also provide integrity and authentication?
1) AH
2) ESP
3) GRE
4) NHRP
Correct Answer: 2) ESP
Explanation:
Encapsulating Security Payload, or ESP, is the IPsec protocol commonly used to provide confidentiality through encryption. ESP can also provide data integrity, authentication, and replay protection depending on the selected algorithms and configuration. This makes ESP suitable for protecting data transported across untrusted networks. Authentication Header, or AH, provides integrity and authentication but does not provide encryption, so it does not offer confidentiality. GRE is a tunneling protocol and does not inherently encrypt traffic, while NHRP is used primarily for address resolution in technologies such as DMVPN. Because ESP can encrypt the protected payload and provide additional security services, it is the correct answer for IPsec confidentiality.
Q396. Which QoS technique reserves a strict priority queue for delay-sensitive traffic such as voice?
1) WRED
2) Policing
3) LLQ
4) FIFO
Correct Answer: 3) LLQ
Explanation:
Low Latency Queuing, or LLQ, adds a strict priority queue to a class-based QoS configuration. It is commonly used for delay-sensitive traffic such as voice because packets placed into the priority queue receive preferential treatment during congestion. The goal is to reduce latency and jitter for real-time applications while still allowing other traffic classes to receive their configured bandwidth. LLQ is commonly implemented as an extension of Class-Based Weighted Fair Queuing. WRED focuses on congestion avoidance through early packet drops, policing controls traffic rates and can drop or remark packets, and FIFO processes packets according to their arrival order without providing a dedicated priority queue. Therefore, LLQ is the correct mechanism.
Q397. What is the primary difference between QoS policing and shaping?
1) Policing buffers excess traffic while shaping always drops it
2) Shaping can buffer excess traffic, while policing can drop or remark it
3) Both mechanisms always encrypt excess traffic
4) Shaping is used only for routing protocols
Correct Answer: 2) Shaping can buffer excess traffic, while policing can drop or remark it
Explanation:
QoS policing and shaping both control traffic rates, but they handle traffic exceeding the configured rate differently. Policing generally operates by measuring traffic against a rate and taking an immediate action when traffic exceeds the configured threshold. Depending on configuration, excess traffic can be dropped or remarked. Traffic shaping, on the other hand, can buffer excess packets and transmit them later when bandwidth becomes available, smoothing the traffic rate over time. This makes shaping useful when a device needs to conform traffic to a provider’s contracted rate without immediately discarding bursts. Neither mechanism inherently encrypts traffic, and shaping is not limited to routing protocols. Therefore, buffering versus immediate treatment of excess traffic is the key distinction.
Q398. Which IPv6 mechanism allows a host to automatically configure an address based on information provided by a router?
1) SLAAC
2) NAT
3) ARP
4) RARP
Correct Answer: 1) SLAAC
Explanation:
Stateless Address Autoconfiguration, or SLAAC, allows IPv6 hosts to automatically configure their addresses using information learned from Router Advertisement messages. The router provides an IPv6 prefix and other network information, and the host can generate its interface address without requiring a traditional DHCP server for the address itself. SLAAC simplifies IPv6 deployment and is a fundamental part of IPv6 address configuration. IPv6 does not use ARP; it uses Neighbor Discovery Protocol for functions such as address resolution and neighbor discovery. NAT is not required for standard IPv6 address configuration, and RARP is an obsolete IPv4-era mechanism. Therefore, SLAAC is the correct mechanism for automatic IPv6 address configuration using router-provided information.
Q399. Which command can be used to verify the Cisco router’s CEF forwarding entries for IPv4 destinations?
1) show ip cef
2) show ip ospf database
3) show ip eigrp neighbors
4) show crypto isakmp sa
Correct Answer: 1) show ip cef
Explanation:
The show ip cef command displays information from the Cisco Express Forwarding table. CEF uses a Forwarding Information Base and adjacency information to make efficient packet-forwarding decisions. When troubleshooting forwarding problems, the command can help an administrator determine how a destination prefix is represented in the CEF table and which next hop or interface is associated with it. The OSPF database command displays link-state information, EIGRP neighbors shows EIGRP neighbor relationships, and the ISAKMP command displays information about IKE security associations. These commands are useful in their respective areas but do not directly show the CEF forwarding entries. Therefore, show ip cef is the correct troubleshooting command.
Q400. Which command is most useful for verifying the operational status, IP address, and line protocol state of router interfaces?
1) show ip bgp
2) show interfaces
3) show ip nhrp
4) show mpls ldp neighbor
Correct Answer: 2) show interfaces
Explanation:
The show interfaces command provides detailed information about router interfaces, including interface status, line protocol status, IP-related information where applicable, traffic counters, errors, packet statistics, bandwidth, duplex settings, and other operational details. This makes it one of the most important commands for diagnosing physical and data-link problems. Administrators can use it to determine whether an interface is administratively down, physically down, or operational. show ip bgp focuses on BGP routing information, show ip nhrp provides NHRP information, and show mpls ldp neighbor verifies LDP relationships. When troubleshooting an interface that appears unreachable or unstable, show interfaces provides the detailed operational information needed to investigate the problem.