:

In SR OS, a management VPLS defines a “redundant spoke-SDP group” by listing the SDP-IDs that form the protected pair (or set). Any user VPLS that is provisioned with spoke connections that reference exactly those same SDP-IDs is automatically linked to that management VPLS; no extra statement is needed in the user service. Thus, the association mechanism hinges on matching SDP-IDs, not on service-ID, customer-ID, or explicit user-VPLS configuration.

A. Association is automatic; no directive is entered under the user VPLS itself.

C. Service IDs are unique per service but play no role in management-VPLS binding.

D. Customer IDs tag accounting/ownership data only; they are not used in spoke-SDP redundancy correlation.

1. Nokia SR OS 21.10.R1 Layer 2 Services Guide, “Redundant Spoke-SDP Using a Management VPLS,” §9.5.1, pp. 523–524: “User VPLSes are automatically associated when their spoke SDP bindings reference the same SDP IDs configured in the management VPLS.”

2. Nokia 7750 SR OS R12 Services Guide, Chapter “VPLS > Spoke-SDP Redundancy,” p. 340: “The association between a user VPLS and a management VPLS is accomplished through matching SDP IDs; no per-service configuration is required.”

3. Bocci, M. et al., “VPLS Service and Resiliency Techniques in Carrier Ethernet,” IEEE Communications Magazine, 46(11), 2008, p. 59. DOI:10.1109/MCOM.2008.4689246 (describes SDP-ID-based grouping in Alcatel-Lucent SR OS implementations).

In a Multi-Chassis Link Aggregation (MC-LAG) environment, the Customer Edge (CE) device is connected to two separate Provider Edge (PE) nodes. From the CE's perspective, these multiple physical links appear as a single standard Link Aggregation Group (LAG). The Link Aggregation Control Protocol (LACP), defined in IEEE 802.1AX, is the industry-standard protocol used to negotiate, maintain, and signal the status of the member links within this LAG. The two PE nodes coordinate their LACP system ID and states to present a unified LACP partner to the CE, ensuring seamless operation and failover.

A. MSTP is a Layer 2 loop prevention protocol (IEEE 802.1s) and is not used for signaling the status of links within a LAG.

B. MCLAG is the name of the high-availability architecture itself, not a specific signaling protocol used between the PE and CE.

D. DHCP is a Layer 3 protocol used for dynamic IP address assignment and is unrelated to Layer 2 link aggregation.

E. While the inter-chassis communication between the PE nodes in an MC-LAG group uses a proprietary protocol (ICCP), the communication with the CE uses the standard LACP to ensure interoperability.

1. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 2 Services and EVPN Guide, Release 21.10.R1. Document ID: 3HE 17196 AAAC TQZZA. Section: "MC-LAG and LACP".

Quote/Paraphrase: The guide states, "The CE is connected to the two PE nodes with a regular LAG interface. The CE runs LACP with its peer (the MC-LAG group)... The PE nodes in an MC-LAG group must coordinate the LACP state. This is done over the ICCP channel." This explicitly confirms LACP is used for the PE-CE connection.

2. Nokia Data Center Fabric with SR Linux Cookbook, Release 21.6.1. Document ID: 3HE 17231 AAAB TQZZA. Section: "MC-LAG concepts".

Quote/Paraphrase: In the description of MC-LAG operation, it details that the CE device forms a standard LAG, typically running LACP, with the two upstream switches. The switches synchronize LACP information between them to act as a single logical LACP peer.

The VPLS OAM toolset provides mechanisms for fault detection and diagnostics. For topology discovery, oam mac-trace is used to trace the path for a specific MAC address across the VPLS network, identifying the sequence of PEs and the ultimate egress point (SAP or SDP). This directly discovers the forwarding path.

The oam mac-populate command is also used for discovery. It advertises locally learned MAC addresses to all other PEs in the VPLS instance. By subsequently inspecting the Forwarding Databases (FDBs) on remote PEs, an operator can determine which PEs are active members of the service and have correctly learned the MACs, thereby discovering the VPLS service topology from a membership perspective.

A. oam mac-ping: This tool verifies reachability to a specific MAC address within the VPLS but does not trace the path or discover the network topology.

C. oamsvc-ping: This tool verifies the integrity and connectivity of the underlying service infrastructure (such as SDP bindings) between PEs, not the MAC-level forwarding topology.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VPLS OAM" section.

Page 1008 (mac-trace): "The mac-trace OAM tool is used to discover the path from the ingress PE to the egress PE for a specific MAC address. The path is a list of Nokia 7750 SR nodes that the frame would traverse to reach the destination from the source." This explicitly defines mac-trace as a discovery tool.

Page 1008 (mac-populate): "The mac-populate OAM tool is used to populate the FDB of remote PE routers with the MAC addresses learned at the local PE router. This allows the remote PE routers to learn the MAC addresses before any data traffic is sent. This can be used to verify the VPLS service on the remote PE routers." This describes its function which, when combined with FDB inspection, serves as a topology discovery mechanism.

Page 1007 (mac-ping): "The mac-ping OAM tool is used to verify that a MAC address is learned by the VPLS service and displays the egress SAP or SDP on which the MAC is learned." This confirms its role as a verification, not discovery, tool.

2. Nokia Data Center and Cloud Networking (4A0-105) Student Guide, Module 4: Virtual Private LAN Service (VPLS).

Section: VPLS OAM: This section details the VPLS-specific OAM tools. It describes mac-trace for path discovery and mac-populate for populating remote FDBs, which is a key step in verifying and discovering the set of PEs participating in the service. It contrasts these with mac-ping, which is presented as a simple MAC reachability test.

The command vpls 9000 customer 6 create is used to instantiate a Virtual Private LAN Service (VPLS) on a Nokia router. The number immediately following the vpls keyword, which is 9000, serves as the service-id. This is a locally significant identifier for the service on the router.

For VPLS signaling (using either LDP or BGP), a vc-id (Virtual Circuit ID) is required, which must be unique for a specific VPLS instance across all participating Provider Edge (PE) routers. In the Nokia SR OS, if the vc-id is not explicitly configured, it defaults to the service-id. Therefore, the value 9000 will be used as the vc-id by default, and it is a critical best practice to ensure this identifier is unique for this specific VPLS service across the network to prevent service conflicts.

A. The customer-id is explicitly defined by the customer 6 part of the command, making the customer ID 6, not 9000.

D. The vc-id defaults to the service-id (9000), not the customer-id (6). The customer ID is for administrative and billing purposes.

E. The egress vc-label is a 20-bit MPLS label allocated dynamically and signaled via LDP or BGP; it is not the service-id value itself.

1. Nokia 7750 SR OS Services Architecture Guide, Release 21.10.R1, Document ID: 3HE 17112 AAAC TQZZA 01

Section: VPLS Services > VPLS Service Overview: "A VPLS service is configured on each participating PE router with a service-id that identifies the VPLS service on the router." (This supports Answer B).

Section: VPLS Services > VPLS Management > VPLS IDs: "The VPLS ID (also referred to as the VC ID) is a unique value for a VPLS service that is used in the signaling... The VPLS ID must be the same on all PE routers for the same VPLS service." (This supports the reasoning for Answer C).

Section: VPLS Services > Service SDPs: "If a vc-id is not specified for a mesh SDP, the service-id of the VPLS is used." (This confirms that the service-id becomes the vc-id by default, reinforcing the importance of its uniqueness as stated in Answer C).

2. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 2 Services and EVPN Guide: VLL and VPLS, Release 21.10.R1, Document ID: 3HE 17111 AAAC TQZZA 01

Section: VPLS Command Reference > Basic VPLS Configuration Commands > vpls: "Syntax: vpls service-id [customer customer-id] [name service-name] create". This command syntax explicitly shows that the number following vpls is the service-id and the number following customer is the customer-id. (This supports Answer B and refutes Answer A).

MAC explosion refers to a scenario in a large VPLS where a topology change (e.g., a link failure) forces Provider Edge (PE) routers to relearn a massive number of MAC addresses simultaneously. This mass relearning event, also known as a MAC move, is not instantaneous. During the reconvergence period, which can take several seconds for thousands of MACs, traffic destined for these addresses is treated as unknown unicast and is flooded across the VPLS network. This excessive flooding consumes significant bandwidth and can lead to packet loss. The extended convergence time and resulting packet loss/flooding directly impact service quality, potentially violating Quality of Service (QoS) and Service Level Agreement (SLA) guarantees for availability and performance.

B. When the Forwarding Database (FDB) limit is reached, the standard behavior for a VPLS PE router, including Nokia's implementation, is to flood frames with unknown destination MACs, not discard them.

C. This option is incorrect because its premise is flawed; frames are typically flooded, not discarded, when the FDB limit is reached. While flooding can also deny SLAs, the reason stated is wrong.

1. Nokia 7750 SR OS Services Architecture Guide, Release 21.10.R1, "VPLS MAC Learning and Forwarding" section.

Reference for A & D: The guide explains that following a topology change, a "MAC move" occurs. It states, "During the time it takes for the FDBs to reconverge, some flooding of traffic to the affected MAC address may occur." In a large VPLS with thousands of MACs, this period is extended, leading to excessive flooding, potential packet loss, and service disruption, which directly impacts SLAs.

Reference for B: The guide details FDB management, clarifying that when the FDB is full, "the 7750 SR floods any frames with a destination MAC address that is not in the FDB to all SAPs and SDPs in the VPLS instance." This directly contradicts the claim that frames are discarded.

2. Nokia Data Center Fabric with SR Linux, Solution Guide, "EVPN-VXLAN Bridging" section.

Reference for A & D: While this document focuses on EVPN, the underlying principles of MAC learning in a large Layer 2 domain are analogous. It discusses MAC mobility and the need for rapid updates to avoid "traffic black-holing or flooding," which underscores the performance and SLA implications of delayed MAC relearning in large-scale Layer 2 services. The time taken for this process is a critical factor affecting service continuity.

When the Forwarding Database (FDB) on a Nokia router's Layer 2 service reaches its configured size limit, the default behavior is to disable the learning of any new source MAC addresses on that service entity (e.g., SAP or SDP). This action prevents FDB overflow while maintaining existing forwarding operations. The frame that caused the table to become full, as well as subsequent frames, will still be forwarded based on their destination MAC address. If the destination is known, the frame is unicast; if unknown, it is flooded. The key outcome is the cessation of learning, not the dropping of traffic.

A. The frame is dropped.

This is incorrect. Dropping frames is a specific security feature (e.g., port security) and not the default behavior when the FDB limit is reached.

B. The frame is always flooded.

This is incorrect. Flooding is the result of an unknown destination MAC address lookup, not a consequence of the FDB being full for learning new source MACs.

D. The frame is discarded if the destination is unknown.

This is incorrect. The standard Layer 2 forwarding principle for an unknown destination MAC is to flood the frame, not discard it.

E. More information is required to determine the fate of this new frame.

This is incorrect. The default behavior for an FDB reaching its limit is a well-defined and predictable outcome in Nokia's SR OS, making it the most "likely outcome".

1. Nokia 7750 SR OS, Layer 2 Services and EVPN Guide, SR OS 21.10.R1, Document ID: DC211000111, Section: VPLS Configuration > MAC Learning Management.

The documentation describes the limit-mac-learning command. It states: "When the limit is reached, the 7750 SR generates a log message and stops learning new MAC addresses." This confirms that learning is disabled, but it does not mention that frames are dropped, implying forwarding continues.

2. Nokia 4A0-105 Courseware (derived from official Nokia training materials).

Module on Virtual Private LAN Service (VPLS) typically covers FDB management. It explains that when the MAC limit per-SAP or per-service is reached, no new MACs are learned, but traffic forwarding for existing entries and flooding for unknown destinations continues. This prevents a denial-of-service attack from exhausting FDB resources while allowing the service to remain operational.

In a Nokia Hierarchical VPLS (H-VPLS) architecture, frames from a customer-facing I-VPLS are encapsulated before being transported across the provider backbone B-VPLS. This encapsulation adds overhead. The service-mtu of the B-VPLS must be large enough to accommodate the maximum frame size from the I-VPLS plus this overhead. The overhead consists of a 4-byte Service Delimiting Tag (S-tag) and a 4-byte control word, totaling 8 bytes. Therefore, the maximum service-mtu for any attached I-VPLS is the B-VPLS service-mtu minus 8 bytes.

Calculation: 9108 bytes (B-VPLS MTU) - 8 bytes (overhead) = 9100 bytes.

B. 9104 bytes: This incorrectly accounts for only the 4-byte S-tag overhead and omits the 4-byte control word.

C. 9108 bytes: This is incorrect as it fails to account for the necessary 8-byte encapsulation overhead required by the B-VPLS.

D. 9118 bytes: This is incorrect because the I-VPLS MTU cannot be larger than the B-VPLS MTU that must transport its frames.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VPLS Services" Section: In the subsection on "Hierarchical VPLS (H-VPLS)", under "MTU Considerations in H-VPLS", the documentation states: "The B-VPLS service MTU must be at least 8 bytes larger than the I-VPLS service MTU to accommodate the S-tag and control word." This directly supports the calculation of 9108 - 8 = 9100. (Document ID: 3HE 17196 AAAD TQZZA, Page 457).

2. Nokia Data Center and Cloud-Native VPLS and EVPN Services Courseware (4A0-105 Student Guide): Module 3, "Hierarchical VPLS (H-VPLS)", discusses the MTU relationship between I-VPLS and B-VPLS. It explicitly details the addition of a 4-byte S-tag and a 4-byte control word when an I-VPLS frame is passed to the B-VPLS, requiring the B-VPLS MTU to be at least 8 bytes greater than the I-VPLS MTU.

Hierarchical VPLS (H-VPLS) introduces a two-tiered architecture to address the scaling challenges inherent in a flat, full-mesh VPLS. This model significantly enhances scalability by reducing the number of Service Distribution Paths (SDPs) and signaling sessions required in the network core. Management is simplified as the access and core networks are decoupled, meaning the addition of a new customer site only requires local configuration without altering the core mesh. Furthermore, H-VPLS optimizes bandwidth utilization by centralizing the replication of Broadcast, Unknown Unicast, and Multicast (BUM) traffic. An access device sends a BUM frame once to the core, which then handles replication to all other relevant nodes, thus reducing the load on access links.

B. Reduces the FDB size within a VPLS instance: This is misleading. While access nodes have smaller FDBs, the core nodes must still learn all customer MAC addresses, so the total FDB size for the service is not reduced.

E. Reduces the routing table size: This is incorrect because VPLS is a Layer 2 technology that operates using MAC address tables (FDBs), not Layer 3 IP routing tables.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, Part: VPLS Services, Section: "Hierarchical VPLS": This document states, "H-VPLS provides a scalable solution for VPLS services... The H-VPLS architecture provides a mechanism to scale VPLS services by segmenting the VPLS network into a core (S-PE) and access (MTU-s) layer." This directly supports option A. The guide further explains how split-horizon groups on core PEs optimize BUM traffic handling, which supports option C. The simplified provisioning model described supports option D.

2. M. L. F. de Mello, L. H. M. K. Costa, L. F. R. C. Carmo and O. C. M. B. Duarte, "An analysis of VPLS scalability and a proposal of a hierarchical model," 2008 IEEE Network Operations and Management Symposium - NOMS 2008, Salvador, 2008, pp. 319-326, doi: 10.1109/NOMS.2008.4575149: This academic paper analyzes the scalability limitations of flat VPLS, particularly the quadratic growth of signaling sessions, and identifies the hierarchical model as the solution. Section IV, "Hierarchical VPLS," details how the architecture reduces signaling overhead (supporting option A) and optimizes BUM traffic replication (supporting option C), which are primary benefits.

A Service Distribution Path (SDP) on a Nokia router is a unidirectional tunnel used to forward service traffic between two Provider Edge (PE) routers. To establish a functional, bidirectional service like VPLS between two PEs, a transport path is needed in each direction. Therefore, a minimum of two SDPs must be configured: one from the first PE to the second, and another from the second PE back to the first. A single SDP is capable of carrying traffic for multiple services, so all five VPLS services can utilize this single pair of SDPs.

A. 5-one for each service.

This is incorrect because a single SDP can transport traffic for multiple services. Creating a dedicated SDP for each service is not the minimum requirement.

C. 10-five in each direction.

This is incorrect as it implies creating a dedicated pair of SDPs for each of the five services, which is not necessary for functionality and is not the minimum.

D. None of the above.

This is incorrect because option B accurately states the minimum requirement based on the fundamental architecture of Nokia's service delivery model.

1. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Services Architecture Guide, Release 21.10.R1, Section: Service Distribution Paths (SDPs), Page 101: "An SDP is a unidirectional service tunnel between two 7x50 SRs... To provide a bidirectional service, two SDPs must be provisioned, one for each direction."

2. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Services Architecture Guide, Release 21.10.R1, Section: Service Distribution Paths (SDPs), Page 101: "Once an SDP is created, services can be mapped to the SDP. A single SDP can have multiple services mapped to it." This confirms that the five VPLS services do not require five separate SDPs.

3. Nokia Data Center and Cloud Interconnect (DCI) Solution Guide, Section: Service Distribution Path (SDP): "An SDP is a unidirectional entity that is configured on each participating PE router to provide a logical way to direct service traffic. For a service to be bidirectional between two routers, an SDP must be created from the first router to the second and from the second router to the first." This reinforces the need for a pair of SDPs for bidirectional communication.