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Question 121

Which QoS mechanism controls the rate at which traffic is transmitted by buffering excess packets?

1. Shaping
2. Classification
3. Policing
4. Marking

Correct Answer: 1

Explanation

Traffic shaping controls the rate at which packets are transmitted by temporarily buffering packets that exceed the configured rate. Instead of immediately discarding excess traffic, shaping can hold packets in a queue and release them according to the configured traffic profile. This makes shaping particularly useful when traffic needs to conform to a service provider’s committed rate or a downstream interface’s capacity. Policing also controls traffic rates but typically handles excess traffic by dropping or remarking packets rather than buffering them for later transmission. Classification identifies traffic, while marking changes packet QoS values. Therefore, shaping is correct.

Question 122

Which QoS mechanism typically drops or remarks packets when traffic exceeds a configured rate?

1. Queuing
2. Shaping
3. Policing
4. Classification

Correct Answer: 3

Explanation

QoS policing monitors traffic against a configured rate and takes an action when traffic exceeds the allowed threshold. Depending on the policy, excess packets may be dropped or have their markings changed. Unlike shaping, policing does not normally buffer excess packets for later transmission. Classification determines which traffic belongs to a particular class, while queuing determines how packets are stored and scheduled during congestion. Policing is commonly used at service boundaries to enforce contracted bandwidth or traffic profiles. Therefore, policing is the correct mechanism when excess traffic needs to be dropped or remarked.

Question 123

Which QoS technique provides strict priority treatment for delay-sensitive traffic such as voice?

1. WRED
2. LLQ
3. CBWFQ
4. Traffic shaping

Correct Answer: 2

Explanation

Low Latency Queuing (LLQ) provides a strict priority queue for traffic that is sensitive to delay and jitter, such as voice or certain real-time applications. During congestion, packets in the priority queue are serviced before packets in other queues, helping reduce latency. LLQ is commonly implemented as an extension of Class-Based Weighted Fair Queuing (CBWFQ). WRED is designed for congestion avoidance, while traffic shaping controls the transmission rate. Therefore, LLQ is the appropriate QoS mechanism for strict priority treatment of delay-sensitive traffic.

Question 124

Which congestion-avoidance mechanism can randomly drop packets before a queue becomes completely full?

1. LLQ
2. Policing
3. WRED
4. Classification

Correct Answer: 3

Explanation

Weighted Random Early Detection (WRED) is a congestion-avoidance mechanism that can begin dropping packets before a queue reaches full capacity. By monitoring queue depth and packet characteristics, WRED can reduce the likelihood of severe congestion and tail-drop behavior. It is often associated with differentiated treatment of traffic based on packet markings. LLQ provides priority scheduling, policing enforces traffic rates, and classification identifies traffic classes. WRED is therefore useful for managing congestion proactively rather than waiting until the queue is completely full.

Question 125

Which QoS function changes a packet’s DSCP value to communicate its desired treatment through the network?

1. Marking
2. Shaping
3. Queuing
4. Policing

Correct Answer: 1

Explanation

QoS marking modifies packet classification fields so that downstream devices can recognize the desired forwarding treatment. For IP traffic, DSCP is commonly used to indicate the packet’s Differentiated Services treatment. Once marked, packets can be placed into appropriate queues or receive specific forwarding behavior at subsequent network devices. Shaping controls transmission rates, queuing determines packet scheduling, and policing enforces traffic-rate limits. Marking is therefore the correct function when the goal is to change a packet’s DSCP value for downstream QoS processing.

Question 126

Which MPLS header field is commonly used to carry QoS-related traffic-class information?

1. Label
2. Bottom-of-Stack
3. Traffic Class
4. TTL

Correct Answer: 3

Explanation

The MPLS Traffic Class (TC) field is used for functions such as Quality of Service and Explicit Congestion Notification. It is commonly used to preserve or communicate traffic-class information across an MPLS network. The Label field identifies the forwarding equivalence class or forwarding context, the Bottom-of-Stack bit indicates whether a label is the last label in the stack, and TTL helps prevent packets from circulating indefinitely. Therefore, the MPLS Traffic Class field is the appropriate field for QoS-related classification and treatment.

Question 127

Which multicast protocol is used by hosts to signal their IPv4 multicast group membership to local routers?

1. PIM
2. IGMP
3. MSDP
4. BGP

Correct Answer: 2

Explanation

Internet Group Management Protocol (IGMP) allows IPv4 hosts to communicate multicast group membership information to their directly connected multicast routers. Hosts can use IGMP messages to join or leave multicast groups, while routers use the membership information to determine whether multicast traffic should be forwarded onto a particular interface. Protocol Independent Multicast (PIM) handles multicast routing between routers, while MSDP can exchange source information between multicast domains. BGP is primarily used for unicast and other routing applications. Therefore, IGMP is the correct protocol for host-to-router multicast membership signaling.

Question 128

Which PIM mode uses a Rendezvous Point for initial multicast source discovery and shared-tree operation?

1. PIM-SSM
2. PIM-DM
3. PIM-SM
4. IGMP

Correct Answer: 3

Explanation

PIM Sparse Mode (PIM-SM) uses a Rendezvous Point (RP) as part of its shared-tree operation. Receivers initially join a multicast distribution tree associated with the RP, and source information can subsequently be learned and optimized depending on the network configuration. PIM-SSM uses source-specific multicast and does not require an RP for source discovery. IGMP manages host membership rather than routing between multicast routers, while PIM-DM uses a different dense-mode forwarding model. Therefore, PIM-SM is the correct answer.

Question 129

Which multicast model allows receivers to explicitly identify both the desired source and multicast group?

1. Anycast-RP
2. PIM-SM
3. SSM
4. MSDP

Correct Answer: 3

Explanation

Source-Specific Multicast (SSM) allows a receiver to specify both the multicast source and multicast group it wants to receive. This creates an explicit channel represented conceptually by a source and group pair. SSM can simplify multicast operation because it does not require a traditional Rendezvous Point for source discovery. PIM-SM commonly uses an RP during initial tree construction, Anycast-RP provides RP redundancy, and MSDP exchanges source information between multicast domains. Therefore, SSM is the correct multicast model for explicit source-and-group selection.

Question 130

Which multicast protocol is commonly used between PIM domains to exchange information about active multicast sources?

1. MSDP
2. IGMP
3. LDP
4. RSVP

Correct Answer: 1

Explanation

Multicast Source Discovery Protocol (MSDP) can be used to exchange information about active multicast sources between different PIM-SM domains. It allows one multicast domain to learn about sources located in another domain so that receivers can potentially obtain the associated multicast traffic. IGMP operates between hosts and local multicast routers, while LDP and RSVP are associated with MPLS functions. MSDP is therefore the appropriate protocol when source information needs to be exchanged between multicast routing domains.

Question 131

Which multicast mechanism provides redundancy by allowing multiple Rendezvous Points to share the same RP address?

1. PIM-SSM
2. Anycast-RP
3. IGMP Snooping
4. PIM-DM

Correct Answer: 2

Explanation

Anycast-RP provides redundancy for multicast Rendezvous Points by allowing multiple routers to use the same RP address. Multicast clients and routers can therefore reach the nearest RP according to the routing topology. This design can improve resilience because the failure of one RP does not necessarily eliminate RP availability. PIM-SSM uses source-specific multicast and does not require an RP, while IGMP Snooping is a Layer 2 switch feature used to control multicast forwarding. PIM-DM uses dense-mode behavior. Therefore, Anycast-RP is correct.

Question 132

Which protocol is primarily responsible for distributing MPLS labels for IP prefixes in a traditional MPLS network?

1. LACP
2. RSVP
3. LDP
4. IGMP

Correct Answer: 3

Explanation

Label Distribution Protocol (LDP) is commonly used to distribute MPLS label bindings for routes learned through an underlying IGP. Routers establish LDP sessions with neighbors and exchange information that allows labeled paths to be built through the MPLS network. RSVP-TE can also establish traffic-engineered MPLS paths but uses resource reservation and explicit signaling. LACP provides Ethernet link aggregation, while IGMP handles IPv4 multicast group membership. Therefore, LDP is the correct protocol for traditional MPLS label distribution.

Question 133

Which MPLS VPN label is primarily associated with identifying the customer VPN at the egress PE router?

1. Transport label
2. VPN/service label
3. Ethernet VLAN tag
4. DSCP value

Correct Answer: 2

Explanation

In an MPLS Layer 3 VPN, the VPN or service label identifies the appropriate VPN context at the egress PE router. A typical packet can carry an outer transport label that guides the packet across the provider core and an inner VPN label that identifies the customer service or VRF context at the egress. The transport label is removed or changed as the packet traverses the MPLS network. VLAN tags and DSCP values serve different functions. Therefore, the VPN/service label is the correct answer.

Question 134

Which MPLS VPN architecture uses separate customer routing tables on PE routers?

1. L3VPN
2. L2VPN
3. IPsec only
4. GRE only

Correct Answer: 1

Explanation

MPLS Layer 3 VPNs use Virtual Routing and Forwarding (VRF) instances on PE routers to maintain separate routing information for different customers. Each VRF can contain customer-specific routes and policies, allowing overlapping customer address spaces to coexist. MP-BGP is commonly used to exchange VPN routes between PE routers, while MPLS labels provide data-plane transport. Layer 2 VPNs instead provide Layer 2 connectivity such as point-to-point or multipoint services. Therefore, L3VPN is the architecture associated with separate customer routing tables on PE routers.

Question 135

Which BGP community attribute is commonly used to indicate that a route should not be advertised outside the local autonomous system?

1. NO_EXPORT
2. LOCAL_PREF
3. NEXT_HOP
4. ORIGIN

Correct Answer: 1

Explanation

The well-known BGP community NO_EXPORT indicates that a route should not be advertised outside the receiving autonomous system. This community can be used to control the propagation of routes between autonomous systems while still allowing internal BGP distribution. LOCAL_PREF is a path-selection attribute used within an AS, NEXT_HOP identifies the next-hop address, and ORIGIN indicates how a route entered BGP. Community-based policies provide flexible control over route propagation and treatment. Therefore, NO_EXPORT is the correct community attribute for restricting advertisement outside the local AS.

Question 136

Which BGP community is commonly used to indicate that a route should not be advertised to any BGP peer?

1. NO_EXPORT
2. NO_ADVERTISE
3. NOPEER
4. INTERNET

Correct Answer: 2

Explanation

The NO_ADVERTISE well-known BGP community instructs a BGP speaker not to advertise the associated route to any BGP peer. This provides stronger propagation control than NO_EXPORT, which allows advertisement within the local autonomous system but prevents advertisement beyond it. NOPEER has a more specific purpose related to peer-provider relationships, while INTERNET indicates that the route can be advertised to the Internet. Therefore, NO_ADVERTISE is the correct community when the route should not be advertised to BGP peers.

Question 137

Which BGP security mechanism can validate whether an advertised prefix is authorized to originate from a particular autonomous system?

1. RPKI
2. LACP
3. STP
4. NTP

Correct Answer: 1

Explanation

Resource Public Key Infrastructure (RPKI) can be used to validate BGP route-origin information through Route Origin Authorizations (ROAs). A ROA specifies which autonomous system is authorized to originate a particular IP prefix and can also define the maximum prefix length. Routers can use RPKI validation results to classify routes as valid, invalid, or not found and apply routing policies accordingly. LACP manages link aggregation, STP prevents Layer 2 loops, and NTP synchronizes time. Therefore, RPKI is the appropriate technology for BGP route-origin validation.

Question 138

Which mechanism can protect a router’s control plane by limiting the rate of unwanted traffic directed toward CPU-bound protocols?

1. NAT
2. CoPP
3. LDP
4. VRRP

Correct Answer: 2

Explanation

Control Plane Policing (CoPP) protects the router’s control plane by classifying and rate-limiting traffic destined for the device CPU. Routing protocols, management protocols, and other control-plane services can consume CPU resources when exposed to excessive or malicious traffic. CoPP policies can permit required traffic while limiting or dropping unwanted traffic according to configured rates. NAT performs address translation, LDP distributes MPLS labels, and VRRP provides gateway redundancy. Therefore, CoPP is the correct mechanism for protecting the control plane from excessive traffic.

Question 139

Which technology allows a router to verify that an IPv4 packet arrived through the interface that would normally be used to reach its source address?

1. RPKI
2. uRPF
3. CoPP
4. SNMP

Correct Answer: 2

Explanation

Unicast Reverse Path Forwarding (uRPF) helps protect networks against source-address spoofing by checking whether the source address of an incoming packet is reachable through the expected interface according to the routing information. Depending on the configured uRPF mode, the router can perform strict or more flexible source validation. This makes uRPF useful for filtering traffic with potentially spoofed source addresses. RPKI validates BGP route origins, CoPP protects the control plane, and SNMP provides network management functions. Therefore, uRPF is the correct technology for source-address validation.

Question 140

Which high-availability feature allows a routing device to continue forwarding traffic while the control plane restarts during certain failures?

1. Route Dampening
2. Graceful Restart
3. Route Reflector
4. NAT

Correct Answer: 2

Explanation

Graceful Restart (GR) allows routing protocols and forwarding devices to maintain forwarding behavior during certain control-plane restart events. Neighboring devices can temporarily preserve routing information while the restarting router reestablishes its control-plane sessions. This can reduce traffic disruption during planned or unplanned control-plane restarts. The exact behavior depends on protocol support and configuration. Route Dampening addresses unstable routes, Route Reflectors improve BGP scalability, and NAT performs address translation. Therefore, Graceful Restart is the correct high-availability mechanism in this scenario.