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Question 221

Which multicast protocol is commonly used by hosts to signal their interest in joining an IPv4 multicast group?

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

Correct Answer: 2

Explanation

Internet Group Management Protocol (IGMP) is used by IPv4 hosts to communicate multicast group membership information to local multicast routers. A host can send an IGMP membership report when it wants to receive traffic for a particular multicast group. The multicast router then uses this information when determining where multicast traffic should be forwarded. PIM performs multicast routing between routers, while MSDP exchanges information about multicast sources between domains or Rendezvous Points. BGP is primarily a unicast routing protocol. Therefore, IGMP is the correct protocol for IPv4 multicast group membership signaling.

Question 222

Which PIM mode normally uses a Rendezvous Point for initial multicast distribution?

1. PIM Sparse Mode
2. PIM Dense Mode
3. PIM Passive Mode
4. PIM Loopback Mode

Correct Answer: 1

Explanation

PIM Sparse Mode (PIM-SM) uses a Rendezvous Point (RP) as part of its initial multicast distribution model. Receivers join multicast trees through the RP before traffic may transition to a source-specific shortest-path tree depending on the implementation and configuration. The RP provides a common point for receivers and sources to initially discover each other. PIM Dense Mode follows a different flood-and-prune model and does not rely on an RP for normal operation. Passive and Loopback Mode are not standard PIM multicast-routing modes. Therefore, PIM Sparse Mode is correct.

Question 223

What is the primary role of a Rendezvous Point in PIM Sparse Mode?

1. Assign IP addresses
2. Provide a meeting point for multicast sources and receivers
3. Distribute MPLS labels
4. Perform BGP route reflection

Correct Answer: 2

Explanation

A Rendezvous Point (RP) acts as a common meeting point between multicast sources and receivers in PIM Sparse Mode. Multicast receivers can join the shared tree toward the RP, while sources can register their multicast traffic with the RP. The RP therefore helps establish initial multicast connectivity before routers may move toward a source-specific tree. The RP does not assign IP addresses, distribute MPLS labels, or perform BGP route reflection. Those functions belong to other network mechanisms. Therefore, providing a meeting point for multicast sources and receivers is the correct answer.

Question 224

Which multicast architecture provides multiple redundant Rendezvous Points that share the same logical RP address?

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

Correct Answer: 2

Explanation

Anycast-RP allows multiple routers to operate as redundant Rendezvous Points while using the same RP address. Multicast sources and receivers can therefore reach the nearest available RP according to the underlying routing topology. This improves RP resiliency and can distribute multicast control-plane responsibilities across multiple locations. PIM-SSM does not use an RP for normal source-specific multicast operation. IGMP Snooping operates at Layer 2, while MSDP is commonly used with multiple RPs to exchange source information but is not itself the mechanism that creates the Anycast-RP architecture. Therefore, Anycast-RP is correct.

Question 225

Which protocol can exchange multicast source information between PIM domains or Rendezvous Points?

1. MSDP
2. LDP
3. LACP
4. NTP

Correct Answer: 1

Explanation

Multicast Source Discovery Protocol (MSDP) can exchange information about active multicast sources between PIM domains or between Rendezvous Points in appropriate multicast architectures. This allows receivers in one domain to learn about sources located in another domain and supports interdomain multicast designs. LDP distributes MPLS labels, LACP negotiates link aggregation, and NTP synchronizes system clocks. MSDP is therefore specifically associated with multicast source discovery across administrative or PIM boundaries. The protocol works with multicast routing mechanisms rather than replacing PIM itself.

Question 226

Which PIM mode is designed for multicast applications where receivers explicitly identify the desired source?

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

Correct Answer: 3

Explanation

PIM Source-Specific Multicast (PIM-SSM) is designed for multicast applications where receivers specify both the multicast source and group. Instead of relying on a shared Rendezvous Point, the receiver joins a source-specific tree. This simplifies the multicast control plane for applications where the source is known in advance. PIM-DM uses a dense-mode flood-and-prune approach, while traditional PIM-SM uses an RP-based model. PIM-BIDIR is designed for bidirectional multicast traffic patterns. Therefore, PIM-SSM is the correct answer.

Question 227

Which multicast routing concept prevents a router from forwarding a multicast packet if the packet arrives on an interface that is not considered the correct reverse path toward the source?

1. Split horizon
2. Reverse Path Forwarding
3. Route reflection
4. Label swapping

Correct Answer: 2

Explanation

Reverse Path Forwarding (RPF) is a fundamental multicast forwarding mechanism. When a multicast packet arrives, the router checks its unicast routing information to determine the expected interface toward the source. If the packet arrives on the expected RPF interface, it can be considered eligible for forwarding; otherwise, it may be discarded. This prevents many multicast forwarding loops and helps ensure that traffic follows a valid path from the source. Split horizon, route reflection, and MPLS label swapping serve different purposes. Therefore, Reverse Path Forwarding is correct.

Question 228

Which technology can carry multicast VPN traffic across an MPLS provider network?

1. MVPN
2. VRRP
3. LACP
4. SSH

Correct Answer: 1

Explanation

Multicast VPN (MVPN) extends multicast services across an MPLS provider network. It allows customer multicast traffic to be transported between geographically separated sites while maintaining separation between different VPNs. MVPN architectures can use different control-plane and provider-tunnel mechanisms depending on the implementation, including BGP-based signaling and multicast transport technologies. VRRP provides first-hop gateway redundancy, LACP provides link aggregation, and SSH provides secure management access. Therefore, MVPN is the correct technology for transporting multicast VPN traffic across an MPLS service provider infrastructure.

Question 229

Which MPLS technology can establish point-to-multipoint traffic-engineered tunnels using RSVP signaling?

1. RSVP-TE P2MP
2. LDP
3. BFD
4. EVPN

Correct Answer: 1

Explanation

RSVP-TE can support point-to-multipoint (P2MP) traffic-engineered LSPs. A P2MP LSP has one root or ingress point and multiple destination leaves, allowing replicated traffic to follow engineered paths through the MPLS network. This can be useful for multicast-like transport requirements and specific service provider applications. LDP primarily provides label distribution following IGP-derived paths, BFD provides rapid failure detection, and EVPN provides Ethernet VPN services. Therefore, RSVP-TE P2MP is the correct technology for establishing point-to-multipoint traffic-engineered LSPs.

Question 230

Which MPLS forwarding action replaces an incoming label with another label before forwarding the packet?

1. Push
2. Pop
3. Swap
4. Punt

Correct Answer: 3

Explanation

MPLS label swapping occurs when an LSR receives a packet with an incoming label and replaces that label with an outgoing label before forwarding the packet. The outgoing label and interface are determined from the LFIB. Push adds an MPLS label to a packet, while pop removes a label. These operations allow MPLS packets to traverse a Label Switched Path through the provider network. Punt generally refers to sending traffic to a CPU rather than performing normal hardware forwarding. Therefore, swap is the correct MPLS forwarding action.

Question 231

Which MPLS forwarding operation adds a new label to the top of an existing label stack?

1. Pop
2. Swap
3. Push
4. Penultimate hop popping

Correct Answer: 3

Explanation

The MPLS push operation adds a label to the packet’s label stack. This is commonly performed when traffic enters an MPLS domain or when an additional service or transport label needs to be imposed. MPLS VPN packets can carry multiple labels, with the outer label typically representing transport forwarding and an inner label identifying the VPN or service context. Pop removes a label, while swap replaces one label with another. Penultimate Hop Popping specifically removes the outer label before the egress PE under applicable conditions. Therefore, push is correct.

Question 232

Which MPLS label-stack field indicates whether a label is the bottom label in the stack?

1. EXP
2. TTL
3. BoS
4. Label

Correct Answer: 3

Explanation

The Bottom of Stack (BoS) bit identifies whether an MPLS label is the final label in the label stack. A value indicating the bottom of the stack allows the receiving device to determine that no additional MPLS labels follow the current label. MPLS packets can contain multiple labels, particularly in VPN and traffic-engineering environments. The Traffic Class field is used for QoS-related information, TTL helps limit packet lifetime, and the Label field contains the actual label value. Therefore, BoS is the correct field for identifying the bottom label.

Question 233

Which mechanism can allow an MPLS VPN PE router to determine the customer VRF associated with an incoming VPN label?

1. VPN/service label
2. OSPF cost
3. BGP MED
4. IGMP group

Correct Answer: 1

Explanation

In an MPLS Layer 3 VPN, the VPN or service label identifies the VPN forwarding context at the egress PE. After the transport label has been removed or processed, the PE can use the VPN label to determine the appropriate VRF and customer route associated with the packet. This enables multiple customers to use overlapping address spaces while maintaining traffic separation. OSPF cost determines IGP path selection, BGP MED influences certain BGP path-selection decisions, and an IGMP group identifies multicast membership. Therefore, the VPN/service label is correct.

Question 234

Which MPLS mechanism is typically used by the penultimate router to remove the outer transport label before traffic reaches the egress PE?

1. Label Push
2. PHP
3. Route Reflection
4. ECMP

Correct Answer: 2

Explanation

Penultimate Hop Popping (PHP) allows the router immediately before the egress PE to remove the outer MPLS transport label. This reduces label-processing work at the egress router and can simplify the final forwarding operation. PHP is commonly implemented using the Implicit Null label. The egress PE can then process the remaining packet or service label as required. Label Push adds labels, Route Reflection is a BGP scalability mechanism, and ECMP provides multiple equal-cost forwarding paths. Therefore, PHP is the correct mechanism.

Question 235

Which routing feature allows traffic to be distributed across multiple equal-cost next hops?

1. ECMP
2. MED
3. Route Dampening
4. CoPP

Correct Answer: 1

Explanation

Equal-Cost Multipath (ECMP) allows a router to use multiple next hops when they have equivalent routing costs according to the relevant routing protocol and forwarding rules. ECMP can improve link utilization and provide redundancy by distributing traffic across multiple paths. The exact load-balancing behavior depends on the platform and hashing method, and traffic is often distributed per flow rather than per packet. MED is a BGP path attribute, Route Dampening addresses route instability, and CoPP protects the control plane. Therefore, ECMP is the correct feature.

Question 236

Which BGP capability allows a router to advertise more than one path for the same destination prefix to a neighbor?

1. Add-Path
2. Graceful Restart
3. Route Dampening
4. Next-Hop-Self

Correct Answer: 1

Explanation

BGP Add-Path allows a BGP speaker to advertise multiple paths for the same destination prefix instead of advertising only its selected best path. This can improve path diversity and reduce situations where a downstream router has knowledge of only one available route. Add-Path can be useful for traffic engineering, redundancy, and faster convergence in appropriate designs. Graceful Restart addresses control-plane restart behavior, Route Dampening suppresses unstable routes, and Next-Hop-Self changes the next-hop attribute. Therefore, Add-Path is the correct capability.

Question 237

Which BGP feature allows a router to request that its neighbor resend routing information without tearing down the BGP session?

1. Route Refresh
2. BGP Restart
3. AS Override
4. MED

Correct Answer: 1

Explanation

BGP Route Refresh allows a router to request updated routing information from a neighbor without resetting the established BGP session. This is particularly useful after changing inbound routing policy because the router can request the neighbor to re-advertise routes and then apply the new policy. Without Route Refresh, operators historically used approaches such as soft reconfiguration or session resets, depending on the implementation. AS Override modifies customer AS information in certain VPN scenarios, while MED influences path selection. Therefore, Route Refresh is correct.

Question 238

Which BGP mechanism can help an operator apply different routing policies based on a tag attached to routes?

1. BGP Community
2. MPLS TTL
3. OSPF Router ID
4. IS-IS DIS

Correct Answer: 1

Explanation

BGP Communities allow routes to carry tags that can be matched by routing policies. Operators can use these tags to classify routes and apply actions such as changing local preference, controlling advertisements, or influencing route acceptance. Standard communities, extended communities, and large communities provide different formats and use cases. MPLS TTL controls packet lifetime, the OSPF Router ID identifies an OSPF router, and the IS-IS DIS represents a designated router-like role on broadcast networks. Therefore, BGP Community is the correct mechanism for policy-based route tagging.

Question 239

Which QoS mechanism limits the rate of traffic by dropping or remarking packets that exceed a configured rate?

1. Shaping
2. Policing
3. LLQ
4. ECMP

Correct Answer: 2

Explanation

QoS policing monitors traffic against a configured rate and can take actions when traffic exceeds the allowed rate. Depending on the configuration, excess packets may be dropped or remarked. Policing is useful when traffic must be constrained to a defined service rate, particularly at service boundaries. Traffic shaping also controls transmission rate but generally buffers excess traffic rather than immediately dropping it, allowing bursts to be smoothed. LLQ provides strict priority scheduling, while ECMP distributes traffic across equal-cost paths. Therefore, policing is the correct mechanism for rate enforcement through dropping or remarking.

Question 240

Which QoS mechanism buffers excess packets and transmits them later to smooth the traffic rate?

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

Correct Answer: 2

Explanation

Traffic shaping controls the transmission rate by buffering excess packets and sending them later according to the configured shaping rate. This smooths bursts and can help match traffic to a downstream service rate. Unlike policing, which may drop or remark traffic that exceeds the configured rate, shaping generally attempts to retain packets by placing them into a queue. WRED is a congestion-avoidance mechanism that can selectively drop packets, while marking changes packet classification information. Therefore, shaping is the correct QoS mechanism for buffering and smoothing excess traffic.