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Q161. Which OSPF mechanism allows a router to reduce the amount of inter-area routing information by advertising a summary route?

1) LSA filtering
2) Route summarization
3) Passive interface
4) SPF throttling

Correct Answer: 2) Route summarization

Explanation: OSPF route summarization allows multiple contiguous networks to be represented by a single summary prefix. This reduces the number of individual routes that need to be carried and can make a routing domain more scalable. Inter-area summarization is normally configured on an ABR, while external route summarization can be configured on an ASBR. Effective summarization can also reduce the impact of topology changes because changes to individual networks may remain hidden behind the summary. LSA filtering controls the advertisement of LSAs, passive interfaces suppress neighbor formation on an interface, and SPF throttling controls SPF calculations.

Q162. Which OSPF command can be used to manually configure the cost of an interface?

1) ip ospf cost
2) ip ospf priority
3) ospf network cost
4) router ospf cost

Correct Answer: 1) ip ospf cost

Explanation: The ip ospf cost command allows an administrator to manually configure the OSPF cost of an interface. OSPF uses interface costs when calculating the shortest path through the topology. A lower total cost is generally preferred. Manually changing the cost can be useful when an administrator wants to influence path selection without changing the physical interface bandwidth. The command is configured in interface configuration mode. OSPF priority is instead used to influence DR and BDR elections on multiaccess networks. The other options are not valid Cisco IOS commands for directly setting an interface’s OSPF cost.

Q163. In EIGRP, what is the primary role of a feasible successor?

1) It acts as a backup loop-free route
2) It becomes the EIGRP router ID
3) It replaces the EIGRP topology table
4) It advertises only default routes

Correct Answer: 1) It acts as a backup loop-free route

Explanation: An EIGRP feasible successor is a neighboring router that provides a loop-free backup path to a destination. For a route to qualify as a feasible successor, the neighbor must satisfy the EIGRP feasibility condition. When the current successor becomes unavailable, EIGRP can install the feasible successor without necessarily performing a complete recomputation for that destination. This contributes to fast convergence. The feasible successor is not an EIGRP router ID and does not replace the topology table. It also is not restricted to advertising default routes. Feasible successors are particularly valuable in networks requiring rapid recovery from link failures.

Q164. Which EIGRP command displays detailed information from the EIGRP topology table?

1) show ip route eigrp
2) show ip eigrp topology
3) show eigrp interfaces brief
4) show ip protocols bgp

Correct Answer: 2) show ip eigrp topology

Explanation: The show ip eigrp topology command displays information contained in the EIGRP topology table. It can show destination networks, feasible distances, reported distances, successors, and feasible successors. This information is extremely useful when troubleshooting EIGRP route selection and convergence behavior. The routing table contains only routes selected for installation, whereas the topology table provides additional information about paths learned by EIGRP. show ip route eigrp focuses on EIGRP routes installed in the routing table. Therefore, the topology command is the best choice when detailed EIGRP path information is required.

Q165. Which BGP attribute is locally significant to a Cisco router and can be used to select the preferred outbound path?

1) MED
2) Weight
3) Origin
4) AS_PATH

Correct Answer: 2) Weight

Explanation: Cisco BGP Weight is a locally significant attribute used to influence route selection on a single router. A higher weight is preferred. Unlike Local Preference, which can be propagated throughout an autonomous system through iBGP, Weight is not advertised to BGP peers. This makes it useful when an administrator wants one specific router to prefer one BGP path without affecting other routers. MED generally influences inbound path selection by a neighboring AS, while AS_PATH length is another BGP selection factor. The origin attribute indicates how a route was introduced into BGP. Therefore, Weight is the correct answer.

Q166. Which BGP attribute helps prevent routing loops between autonomous systems?

1) AS_PATH
2) Local Preference
3) Weight
4) MED

Correct Answer: 1) AS_PATH

Explanation: The BGP AS_PATH attribute records the autonomous systems through which a route advertisement has passed. When a BGP router receives a route containing its own autonomous system number in the AS_PATH, it can reject the route, preventing the route from being accepted back into the same AS and helping prevent routing loops. AS_PATH length can also influence BGP best-path selection, with shorter paths generally preferred when other higher-priority attributes are equal. Local Preference and Weight are used to influence path selection, while MED can influence how neighboring autonomous systems select an entry path.

Q167. Which command provides a concise overview of BGP neighbors and their session states?

1) show ip bgp summary
2) show ip ospf database
3) show ip eigrp topology
4) show interfaces counters

Correct Answer: 1) show ip bgp summary

Explanation: The show ip bgp summary command is one of the most useful commands for troubleshooting BGP. It provides a concise summary of the local BGP process and configured neighbors. Administrators can see information such as the BGP router ID, local autonomous system, neighbor addresses, remote AS numbers, message counters, and the current state or number of prefixes received from each peer. A BGP session in the Established state indicates that the BGP relationship is operational. The other commands focus on OSPF, EIGRP, or interface statistics and do not provide the same concise BGP neighbor overview.

Q168. Which MPLS VPN component is used to distinguish identical customer IPv4 prefixes?

1) Route Target
2) Route Distinguisher
3) VLAN priority
4) BGP Weight

Correct Answer: 2) Route Distinguisher

Explanation: A Route Distinguisher (RD) makes IPv4 customer routes unique within the MPLS VPN provider’s VPNv4 routing environment. Different customers can use overlapping address spaces, such as 192.168.1.0/24, while remaining logically separate. The RD is combined with the IPv4 prefix to create a unique VPNv4 route. This allows the provider’s BGP infrastructure to maintain separate routes even when customers use identical addressing. A Route Target has a different purpose: it controls route import and export between VRFs. VLAN priority and BGP Weight do not provide the required VPN route uniqueness.

Q169. Which MPLS VPN component controls the import and export of VPN routes between VRFs?

1) Route Target
2) Route Distinguisher
3) MPLS EXP bit only
4) OSPF router ID

Correct Answer: 1) Route Target

Explanation: Route Targets are extended BGP community values used to control which VPN routes are imported into or exported from VRFs. When a PE router exports a route, it attaches one or more Route Targets. Other VRFs can be configured to import routes matching those Route Targets. This provides flexible control over which customer sites can communicate. The Route Distinguisher provides uniqueness for overlapping prefixes but does not determine VPN membership. MPLS traffic-class information and OSPF router IDs serve different purposes. Therefore, Route Targets are the correct mechanism for controlling MPLS VPN route import and export.

Q170. Which DMVPN component allows a spoke to dynamically learn the NBMA address associated with another spoke?

1) BGP
2) NHRP
3) OSPF
4) SNMP

Correct Answer: 2) NHRP

Explanation: Next Hop Resolution Protocol (NHRP) is a fundamental component of DMVPN. It allows routers to dynamically map protocol addresses, such as tunnel IP addresses, to NBMA addresses used by the underlying transport network. In a DMVPN environment, NHRP helps spokes discover the information needed to establish direct spoke-to-spoke tunnels when supported by the DMVPN design and phase. This reduces the need for every spoke to send all traffic through the hub. BGP and OSPF can be used as routing protocols over DMVPN, but they do not perform the NHRP address-resolution function.

Q171. Which DMVPN phase supports dynamic spoke-to-spoke tunnels using NHRP?

1) Phase 1 only
2) Phase 2
3) Phase 0 only
4) Phase 4

Correct Answer: 2) Phase 2

Explanation: DMVPN Phase 2 supports dynamic spoke-to-spoke communication using NHRP. In this design, a spoke can learn the NBMA address of another spoke and establish a direct tunnel rather than sending all traffic through the hub. The routing design must also support appropriate next-hop information so that the spoke can use the direct path. DMVPN Phase 1 generally uses hub-and-spoke forwarding, while Phase 3 introduces additional mechanisms that improve scalability and allow more flexible spoke-to-spoke communication. Understanding the differences between DMVPN phases is important when designing and troubleshooting dynamic multipoint VPN networks.

Q172. Which command can be used to display NHRP mappings on a Cisco router?

1) show dmvpn
2) show ip nhrp
3) show ip bgp neighbors
4) show crypto ikev2 sa

Correct Answer: 2) show ip nhrp

Explanation: The show ip nhrp command displays NHRP information, including mappings between protocol addresses and NBMA addresses. It can help administrators determine whether NHRP registrations and resolutions are functioning correctly in DMVPN environments. When troubleshooting spoke-to-spoke connectivity, checking NHRP mappings can reveal whether a spoke has learned the required information about another tunnel endpoint. The show dmvpn command provides a broader DMVPN overview, while BGP and IKEv2 commands focus on routing and VPN security functions. Therefore, show ip nhrp is the appropriate command when specifically examining NHRP mappings.

Q173. Which protocol is responsible for negotiating IPsec security parameters before protected traffic is exchanged?

1) IKE
2) ARP
3) FTP
4) STP

Correct Answer: 1) IKE

Explanation: Internet Key Exchange (IKE) is used to negotiate security parameters and establish the security associations needed for IPsec. During the negotiation process, peers authenticate each other and agree on cryptographic algorithms and other security parameters. IKEv2 is a modern version commonly used in enterprise VPN deployments. Once the necessary security associations are established, protocols such as ESP can protect data traffic. ARP performs IPv4-to-MAC address resolution, FTP transfers files, and STP prevents Layer 2 loops. Therefore, IKE is responsible for negotiating the security parameters required before protected IPsec traffic can be exchanged.

Q174. Which IPsec protocol provides confidentiality by encrypting the protected payload?

1) AH
2) ESP
3) ICMP
4) GRE

Correct Answer: 2) ESP

Explanation: Encapsulating Security Payload (ESP) provides confidentiality by encrypting protected IP traffic. ESP can also provide integrity, authentication, and anti-replay protection depending on the configured security algorithms and mode. It is widely used in site-to-site and remote-access IPsec VPNs. Authentication Header (AH) can provide integrity and authentication but does not provide encryption. GRE is a tunneling protocol and does not inherently encrypt traffic. ICMP is used for control and diagnostic messages. Therefore, ESP is the appropriate IPsec protocol when confidentiality through encryption is required.

Q175. Which mechanism can provide rapid detection of a forwarding-path failure for routing protocols?

1) BFD
2) DNS
3) TFTP
4) CDP

Correct Answer: 1) BFD

Explanation: Bidirectional Forwarding Detection (BFD) provides rapid detection of failures between neighboring network devices. Routing protocols can use BFD information to react more quickly than they might using their normal neighbor-dead or hold timers. This can improve convergence, especially in environments where fast recovery is important. BFD is independent of the specific routing protocol and can be integrated with protocols such as OSPF, EIGRP, and BGP. DNS resolves names, TFTP provides simple file transfer, and CDP discovers directly connected Cisco devices. Therefore, BFD is the appropriate mechanism for rapid forwarding-path failure detection.

Q176. Which Cisco feature can limit the number of BGP prefixes accepted from a neighbor?

1) Prefix limit
2) PortFast
3) Root guard
4) DHCP snooping

Correct Answer: 1) Prefix limit

Explanation: BGP prefix limits can be configured to control the maximum number of prefixes accepted from a BGP neighbor. This provides a useful protection mechanism because an unexpected routing-table increase can consume memory and processing resources or indicate a routing problem. If the configured limit is exceeded, the router can take a configured action, such as warning or shutting down the session depending on the implementation and configuration. PortFast and root guard are Spanning Tree features, while DHCP snooping protects against rogue DHCP behavior. Therefore, BGP prefix limiting is the appropriate mechanism for controlling received route counts.

Q177. Which routing technique allows a static route to be used only when the primary route is unavailable?

1) Route redistribution
2) Floating static route
3) Equal-cost multipath
4) Policy-based routing

Correct Answer: 2) Floating static route

Explanation: A floating static route provides a backup path by assigning the static route an administrative distance higher than the primary route. As long as the preferred route remains in the routing table, the floating static route is not selected. If the preferred route disappears, the floating static route can become active. This is a simple and effective method for providing backup connectivity. Equal-cost multipath can use multiple equivalent paths simultaneously, while policy-based routing makes forwarding decisions based on configured policies rather than the normal routing-table process. Route redistribution moves routes between routing protocols rather than simply providing a backup static path.

Q178. Which feature allows a router to make forwarding decisions based on criteria other than the destination routing table?

1) Policy-Based Routing
2) STP
3) DHCP relay
4) NTP

Correct Answer: 1) Policy-Based Routing

Explanation: Policy-Based Routing (PBR) allows administrators to influence packet forwarding based on policies that can evaluate characteristics such as source address, destination address, protocol, or other matching criteria. Instead of relying exclusively on the normal destination-based routing table, PBR can direct selected traffic toward a specific next hop or interface. This can be useful for traffic engineering, security policies, and specialized routing requirements. STP operates at Layer 2 to prevent loops, DHCP relay forwards DHCP requests across Layer 3 boundaries, and NTP synchronizes clocks. Therefore, PBR is the technology that provides policy-driven forwarding decisions.

Q179. Which command can be used to verify policy-based routing configuration and statistics on a Cisco router?

1) show ip policy
2) show ip ospf database
3) show ip bgp summary
4) show spanning-tree

Correct Answer: 1) show ip policy

Explanation: The show ip policy command can be used to verify policy-based routing policies applied to interfaces. It helps administrators determine whether a route map is associated with an interface for PBR. Additional commands, such as show route-map, can provide information about the route-map configuration and matching statistics. When troubleshooting PBR, administrators should verify that the policy is applied to the correct incoming interface and that packets match the configured criteria. The other commands focus on OSPF, BGP, or Spanning Tree information and do not directly verify interface-level PBR application.

Q180. Which mechanism can be used to detect and prevent duplicate IPv4 address assignments through DHCP snooping information?

1) Dynamic ARP Inspection
2) BGP MED
3) OSPF summarization
4) MPLS LDP

Correct Answer: 1) Dynamic ARP Inspection

Explanation: Dynamic ARP Inspection (DAI) uses information from the DHCP snooping binding database to validate ARP packets on untrusted switch ports. It can help protect against ARP spoofing and certain address-related attacks by checking whether the claimed IP-to-MAC binding matches trusted DHCP information. Although DAI is not itself a DHCP server or a general duplicate-address detection mechanism, it leverages trusted bindings to validate Layer 2 address relationships. BGP MED, OSPF summarization, and MPLS LDP serve entirely different purposes. Therefore, DAI is the appropriate security mechanism among the listed choices.