4A0-108 Practice Questions

The Bootstrap Router (BSR) protocol is a dynamic mechanism used in Protocol Independent Multicast - Sparse Mode (PIM-SM) for distributing Rendezvous Point (RP) information. Multiple routers can be configured as candidate-BSRs (C-BSRs), but they undergo an election process. Each C-BSR advertises its priority, and the candidate with the highest priority is elected as the single BSR for the entire PIM domain. If priorities are equal, the highest IP address serves as a tie-breaker. This election ensures that there is only one active BSR at any time, providing a single, consistent source of RP-set information to all routers in the domain.

A. A router can be configured as a static RP for specific multicast groups while also being configured as a BSR candidate for dynamic RP discovery for other groups.

B. A single router can be configured to function as both a candidate-RP (C-RP) and a candidate-BSR (C-BSR) simultaneously. This is a common network design choice.

C. A PIM domain is designed to support multiple active RPs. The BSR mechanism distributes a set of available RPs, and routers map different multicast groups to different RPs.

1. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Multicast Routing Protocols Guide, Release 21.10.R1

For Correct Answer (D): In the "PIM" chapter, section "Bootstrap Router (BSR)", it states: "Each C-BSR collects BSMs and elects a single BSR for the PIM domain using an election algorithm. The C-BSR with the highest priority is elected as the BSR. If all C-BSRs have the same priority, the C-BSR with the highest IP address is elected as the BSR." (This confirms the election of a single BSR).

For Incorrect Answer (C): In the "PIM" chapter, section "Rendezvous Point (RP)", it describes the RP-Set: "The elected BSR builds a set of C-RPs, called the RP-Set, and includes the RP-Set in the BSMs that it periodically originates." (This confirms the existence of a set of multiple RPs).

For Incorrect Answer (B): The configuration examples within the "PIM" chapter demonstrate that the bootstrap-candidate and rp-candidate commands can be configured under the same configure>router>pim context, indicating a single router can perform both roles.

2. RFC 5059: Bootstrap Router (BSR) Mechanism for PIM

For Correct Answer (D): Section 3.1, "BSR Election," details the process where all BSR candidates in a domain elect a single BSR. It specifies, "The BSR election procedure is simple: a candidate BSR includes its own priority and address in the Bootstrap messages it originates. A C-BSR which hears about a BSR with a higher priority value... ceases to send Bootstrap messages... This ensures that eventually only one BSR is left active in the domain."

A router, by its fundamental design, operates at Layer 3 and serves as a boundary for Layer 2 broadcast domains. When a Nokia 7750 SR, acting in its routing capacity, receives a packet with a Layer 3 broadcast destination address (e.g., 255.255.255.255 or a directed broadcast), its default behavior is to not forward the packet to any other interface. This is a core principle of IP routing that prevents broadcast storms from propagating across an entire network. The router will process the packet locally if it is the intended recipient (for example, for a routing protocol), but it will not route it onward. Therefore, from a forwarding perspective, the packet is dropped.

B. Forwarding the packet is incorrect as this would violate the router's role as a broadcast domain boundary, potentially leading to network-wide broadcast storms.

C. This describes the behavior of a Layer 2 switch within a single broadcast domain (VLAN), not the default Layer 3 function of a router.

D. Flooding a packet back out the port it was received on is incorrect behavior for any standard networking device and would create network loops.

1. Nokia 7750 SR OS Router Configuration Guide, Release 22.10.R1: In the "IP Helper Addressing" section, the documentation explains the need for helper addresses. It states, "When a router receives a broadcast packet, the packet is not forwarded." This confirms that the default action is to not forward (i.e., drop) broadcasts. The existence of the IP helper feature is to selectively override this default behavior for specific UDP broadcasts like DHCP. (Reference Section: "IP Helper Addressing").

2. Stallings, W. (2016). Data and Computer Communications (10th ed.). Pearson. Chapter 18, "Internet Protocol," explains that routers interconnect networks and that a key function is to contain broadcasts. The text clarifies that routers do not forward limited or directed broadcast datagrams by default, effectively dropping them at the network boundary. (Reference Section: Chapter 18.2, "IP Addresses"). This is a standard textbook used in university computer science and engineering curricula.

Draft Rosen, also known as PIM/GRE MVPN, is a traditional architecture for providing multicast services over an MPLS VPN backbone. A core characteristic of this model is the use of Generic Routing Encapsulation (GRE) to tunnel customer multicast data packets across the service provider's core network. The provider core itself uses PIM to build Multicast Distribution Trees (MDTs) that carry this GRE-encapsulated traffic between PE routers. Customer VRFs also run their own instances of PIM to manage multicast state within the VPN. MPLS encapsulation for multicast data traffic is a feature of Next-Generation MVPN (NG-MVPN) architectures (defined in RFC 6513/6514), not the original Draft Rosen model.

A. PIM in the VRF is a fundamental component, used to learn about customer multicast sources and receivers and to build customer-specific multicast trees.

B. PIM is required in the service provider's core to establish the default and data MDTs that transport the encapsulated customer multicast traffic between PE routers.

C. GRE is the specified encapsulation method in Draft Rosen for tunneling customer multicast packets across the provider's IP/MPLS core network.

1. Nokia 7750 SR OS Layer 3 Services Guide, Release 22.10.R1, Section: "MVPN Overview". The document states, "Draft Rosen Layer 3 MVPNs use PIM to create and signal the provider tunnels, and GRE to encapsulate the multicast traffic." This explicitly confirms the use of PIM in the core and GRE encapsulation. It also describes the requirement for PIM within the VRF instance.

2. RFC 6513: Multicast in MPLS/BGP IP VPNs, Section 1: "Introduction". This document, which defines Next-Generation MVPN, contrasts the new framework with the older method. It states that the prior art (Draft Rosen) "requires the use of PIM in the SP network" and "uses GRE encapsulation for the C-multicast data packets". It then introduces the use of MPLS P-tunnels (such as P2MP LSPs) as a key feature of the new framework, distinguishing it from Draft Rosen.

3. RFC 4364: BGP/MPLS IP Virtual Private Networks (VPNs), Section 11: "Multicast". This RFC describes the foundational principles of Draft Rosen MVPN. It details the creation of a "Default MDT" for each VPN in the provider core using PIM and the encapsulation of customer multicast packets (C-packets) inside provider packets (P-packets) for transport across these tunnels.

The command show router 100 pim neighbor queries for PIM neighbors within the context of VPRN service 100. The provided configuration shows PIM enabled on the customer-facing interface "toCE1" within this VPRN. This establishes a PIM adjacency with the connected CE router, accounting for one neighbor.

The scenario implies a multi-PE network for a functional MVPN service. The use of PIM within the VPRN context indicates a Draft-Rosen (PIM/GRE) MVPN model. In this model, PEs establish GRE tunnels (Multicast Tunnels) among themselves and run PIM over these tunnels to exchange multicast routing information. Assuming the simplest functional multi-PE topology (two PEs), PE1 will form a second PIM adjacency with the remote PE over its GRE tunnel. Therefore, PE1 has two PIM neighbors in VPRN 100: the local CE and the remote PE.

A. This is incorrect because it only accounts for the PIM neighbor on the customer-facing (SAP) interface and ignores the required inter-PE neighbor for MVPN functionality.

C. This is incorrect as there is no information to assume a topology with three PEs. The most reasonable assumption for a functional MVPN is the minimum of two PEs.

D. This is incorrect because the configuration explicitly enables PIM within VPRN 100 on the "toCE1" interface, and the command is specific to that VPRN instance.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VPRN services" chapter, section "Multicast in VPRN Services": This document describes the Draft-Rosen MVPN model. It states, "PEs that are configured in a VPRN’s M-VPRN will automatically set up a full mesh of GRE tunnels (or MTs) between them... PIM is used as the PE-PE routing protocol to exchange C-multicast routing information." This confirms that PEs form PIM adjacencies with other PEs over tunnels within the VPRN context.

2. Nokia 7750 SR OS Routing Protocols Guide, Release 21.10.R1, "Protocol Independent Multicast" chapter, section "PIM Neighbor Adjacency": This section explains that PIM routers become neighbors by exchanging Hello messages on PIM-enabled interfaces. In an MVPN, this applies to both the physical customer-facing interfaces (SAPs) and the logical multicast tunnel interfaces connecting PEs. The show router pim neighbor command displays these adjacencies.

The Protocol Independent Multicast - Sparse Mode (PIM-SM) Bootstrap Router (BSR) election mechanism is preemptive. According to RFC 5059, when a router in a PIM domain receives a Bootstrap message from a candidate BSR, it compares the priority in the message to that of its currently known BSR. If the priority of the new candidate is higher, the router immediately and unconditionally accepts the new candidate as the active BSR for the domain. This ensures that the router with the highest configured priority is always selected as the BSR as soon as its advertisements are received.

A. The BSR election is preemptive; a higher priority candidate does not need to wait for the existing BSR to fail.

B. There is no "bootstrap message wait interval" for preemption; the switch to a higher-priority BSR is immediate.

D. The preemptive behavior based on priority is a fundamental part of the PIM BSR standard and is not a configurable parameter.

1. RFC 5059: Bootstrap Router (BSR) Mechanism for PIM, Section 3.1, "BSR Election", Page 6. The document states: "A router that receives a Bootstrap message first checks the BSR Priority field. If the Priority of the new BSR is higher than the Priority of the existing BSR, the new BSR is unconditionally accepted."

2. Nokia 7750 SR OS R21.10.R1 Multicast Routing Protocols Guide, "PIM > Bootstrap Router (BSR)", Page 128. The guide describes the election process: "The candidate BSR with the highest priority value is elected as the BSR. If the priority values are the same, the candidate BSR with the highest IP address is elected as the BSR." This implies standard, preemptive RFC-compliant behavior.

A PIM register message is a control-plane message that is unicast from the First-Hop Router (FHR), Router A, to the Rendezvous Point (RP), Router D. When an intermediate router, such as Router B, receives this packet, it processes it as a standard unicast datagram. Router B will perform a lookup for the destination IP address (the RP's address) in its unicast routing table and forward the packet towards the next hop according to the best path determined by the IGP. The Reverse Path Forwarding (RPF) check is performed on multicast data packets, not on unicast PIM register messages in transit.

A. The RPF check is performed on multicast data packets to prevent loops, not on unicast PIM register messages being forwarded by an intermediate router.

C. This describes the forwarding of a native multicast packet on the shared tree (,G), which is incorrect as the message is a unicast PIM register packet.

D. This describes the forwarding of a native multicast packet on the source-specific tree (S,G), which is incorrect as the message is a unicast PIM register packet.

1. Nokia 7750 SR OS 21.10.R1 Multicast Routing Protocols Guide (December 2021), Page 100, "PIM-SM Operation". The guide states: "When a DR receives a multicast packet from a directly attached source, it encapsulates the packet in a PIM register message and sends the register message to the RP." This confirms the packet is sent to the RP, and standard unicast forwarding rules apply for transit routers.

2. RFC 7761: Protocol Independent Multicast - Sparse Mode (PIM-SM), Section 4.4.1, "Registering a Source". It specifies: "The DR encapsulates the data packet in a PIM Register message and sends it via unicast to the RP for that group." This establishes that the register message is a unicast packet, and therefore, intermediate routers like Router B will forward it based on their unicast routing tables.

3. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Chapter 4, Section 4.8.2 "IP Multicasting". University-level textbooks on networking describe the PIM-SM registration process where the FHR unicasts an encapsulated packet to the RP, which is then forwarded by intermediate routers using standard unicast routing logic.

In Next-Generation Multicast VPN (NG-MVPN), an MDT Join TLV is carried within a BGP C-multicast route advertisement. It is sent by a Provider Edge (PE) router that has a downstream receiver for a specific customer source and group (C-S, C-G). This message signals to the source-PE that it should forward this multicast stream.

To receive this traffic, the interested PE (the one that sent the MDT Join TLV) must join the specific multicast distribution tree in the provider core, known as a Selective-PMSI (S-PMSI) or Data-MDT. This is a source-specific tree. Therefore, the PE sends a PIM (S, G) Join into the provider core. The PIM Join is a standard IP control plane packet and is not encapsulated in GRE; GRE is a data plane encapsulation used for the actual multicast traffic.

A. PIM Join messages are control plane packets and are not encapsulated in GRE. GRE is a data plane encapsulation for the multicast traffic itself.

C. A (, G) PIM Join is used to build the shared tree for the Default-MDT, not the source-specific tree (Data-MDT) triggered by an MDT Join TLV.

D. A (, G) PIM Join is incorrect because the MDT Join TLV is for a specific source, necessitating a source-specific (S, G) join in the provider core.

1. RFC 6513: Multicast in MPLS/BGP IP VPNs, Section 9.1, "S-PMSI Instantiated by PIM". This section states: "A PE that receives and accepts the S-PMSI advertisement, and that needs to receive the C-traffic carried over the specified P-tunnel, acts as a PIM router that needs to join the P-multicast tree. It sends a PIM Join message upstream." This confirms a PIM Join is sent. The context of S-PMSI implies a source-specific tree, hence an (S,G) join.

2. RFC 6514: BGP Encodings and Procedures for Multicast in MPLS/BGP IP VPNs, Section 5.1, "Source Tree Join C-multicast route". This section details the route that carries the MDT Join TLV. The purpose is to signal interest in a specific (S,G) channel, which leads to the creation of a source-specific P-tunnel that the interested PE must join.

3. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Triple Play Service Delivery Architecture Guide, Release 20.10.R1, Section: "NG-MVPN Data MDT". The documentation explains: "When an egress PE receives the C-multicast route with the data-MDT address, it will initiate a PIM (S,G) join toward the ingress PE, where S is the ingress PE and G is the data-MDT address." This explicitly confirms an (S,G) PIM join is sent by the interested (egress) PE.

In IGMP version 3, a receiver uses an EXCLUDE mode report to specify sources from which it does not want to receive traffic. The report essentially states, "send me traffic for this group from all sources, except for the ones in this list." When the EXCLUDE list is empty, the receiver is excluding zero sources. This logically means the receiver is willing to accept traffic from any and all sources for that multicast group. This is the standard method for an IGMPv3-capable host to signal a join for an Any-Source Multicast (ASM) group, which is functionally equivalent to a join in IGMPv1 or IGMPv2.

A. The report is a complete state declaration. An empty list is a valid and final state, not an indication that more information will follow in a subsequent message.

C. This report is an explicit request to receive multicast traffic for a group. A host that does not wish to join a group would simply not send a report.

D. Leaving a group in IGMPv3 is typically signaled by sending a report that changes the filter mode to INCLUDE with an empty source list (TOIN {}).

1. IETF RFC 3376, "Internet Group Management Protocol, Version 3", Section 5.2, Paragraph 2: "A MODEISEXCLUDE record with an empty source list indicates that the interface has a filter mode of EXCLUDE for that group, with an empty exclusion list. That is, it is requesting traffic from all sources."

2. Nokia 7750 SR OS Multicast Routing Protocols Guide, Release 21.10.R1, "IGMP", Section: IGMPv3: This guide details the protocol operations, explaining that an EXCLUDE report with a null source set (EXCLUDE {}) is the mechanism for joining an ASM group, indicating a desire to receive from any source.

3. Cain, B., et al. "Internet Group Management Protocol, Version 3 (IGMPv3) and Multicast Listener Discovery, Version 2 (MLDv2) for Source-Specific Multicast (SSM) Address Assignments." IEEE Network, vol. 17, no. 1, 2003, pp. 10-15. DOI: 10.1109/MNET.2003.1174174: This academic paper discusses IGMPv3 mechanics, clarifying that the EXCLUDE {} state is the functional equivalent of an IGMPv2 join, requesting traffic from all potential sources.

For an I-PMSI that uses RSVP-TE P2MP LSPs, the ingress PE (root) issues one RSVP Path message for every direct child (next-hop) at the first branching point of the tree. In the topology shown, PE1’s explicit-route contains two distinct next-hops (toward P1 and P2). Consequently, PE1 must originate two separate Path messages, one per branch, which are then forwarded and replicated further downstream by the P-routers. Hence, exactly two Path messages leave PE1.

A. One Path message would be used only if there were a single next-hop branch, which is not the case.

C. Three Path messages would require three direct branches from PE1, not shown in the diagram.

D. The count relates to the number of first-hop branches, not the total number of receivers deeper in the tree.

1. RFC 4875 “Extensions to RSVP-TE for Point-to-Multipoint TE LSPs”, §9, pp. 20-22: root replicates a Path message per branch leaving the root.

2. Nokia 7750 SR OS MPLS Guide, Release 21.5.R1, §7.7.1 “P2MP LSP Path Message Replication”, pp. 400-403.

3. Nokia 7750 SR OS Layer 3 Multicast VPN Guide, Release 21.5.R1, §11.3 “I-PMSI over RSVP-TE P2MP”, p. 556.

The multicast address 224.0.0.1 is the reserved "All Systems" address on the local subnet. According to the Internet Group Management Protocol (IGMP) standards (RFC 1112, RFC 2236, and RFC 3376), all multicast-capable hosts are permanent members of this group on every network interface. This membership is implicit and is explicitly excluded from the reporting mechanism. The protocol standards state that a Membership Report message is never sent for the all-systems-multicast address (224.0.0.1).

B, C, D, E: These options suggest a periodic report, which is incorrect as reports for the 224.0.0.1 group are never sent.

F: While hosts send reports for other groups in response to queries, the 224.0.0.1 group is a specific exception and is never reported.

1. IETF RFC 2236, "Internet Group Management Protocol, Version 2": Section 2, "Protocol Description," states, "The all-hosts group (224.0.0.1) is a special group that all multicast-capable hosts must join on every network interface, but membership in this group is never reported."

2. IETF RFC 3376, "Internet Group Management Protocol, Version 3": Section 4.2.12, "Sending Membership Reports," explicitly states, "A Membership Report message is never sent for the all-systems-multicast address (224.0.0.1)."

3. Nokia 7750 SR OS R21.10.R1 Multicast Routing Protocols Guide: In the "IGMP" chapter, the "IGMP Overview" section describes protocol behavior consistent with IETF standards, noting that well-known addresses like 224.0.0.1 have special handling and are not subject to normal join/report mechanisms. (Document ID: 3HE 17279 AAAB TQZZA, Edition 01).