The port 1/1/1 is configured in qinq mode, enabling it to process frames with either a single 802.1Q tag (S-tag) or two tags (an outer S-tag and an inner C-tag). The SAP 1/1/1:100. is configured within VPLS 2. The acts as a wildcard for the inner C-tag. Therefore, any frame arriving on port 1/1/1 with an outer (top) tag of 100, regardless of the inner tag's value, will be matched to this SAP and processed by VPLS 2. Consequently, the source MAC address of such a frame will be learned in the forwarding database (FDB) of VPLS 2.

A. While it is true that untagged frames will be dropped because no SAP is configured to match them, this describes a default behavior for unhandled traffic, not the function of a specifically configured service.

B. This statement is technically correct as a frame with tags 200.300 would match SAP 1/1/1:200.300 in VPLS 3. However, option E describes a broader rule involving a wildcard, a key QinQ feature likely being tested.

C. This is incorrect. The port is explicitly in qinq mode, and SAPs 1/1/1:100. and 1/1/1:200.300 are configured specifically to accept and process frames with two VLAN tags.

D. This statement is technically correct as a single-tagged frame with VLAN ID 500 would match SAP 1/1/1:500 in VPLS 1. Similar to option B, it describes a valid but specific case.

1. Nokia 7x50 SR OS Services Architecture Guide, R19.10.R1 (3HE15086AAADTQZZA 01), Section: "QinQ Access SAPs", Page 288.

This section details the configuration of SAPs on a port in qinq mode. It explicitly states: "The SAP is specified using the format port-id:s-tag.c-tag or port-id:s-tag. for double-

A Virtual Private LAN Service (VPLS) requires Provider Edge (PE) routers to emulate the functions of a traditional Layer 2 Ethernet switch. The three fundamental functions are:

1. MAC Learning (A): The PE router learns the source MAC addresses of frames arriving from both customer-facing ports (SAPs) and network-facing tunnels (SDPs), populating a Forwarding Database (FDB).

2. Frame Replication (B): For frames with an unknown destination MAC address, as well as for broadcast and multicast frames (BUM traffic), the PE router must replicate and flood the frame to all other SAPs and SDPs within that VPLS instance.

3. Frame Forwarding (D): When a frame with a known destination MAC address is received, the PE router performs a lookup in its FDB and forwards the frame directly to the specific SAP or SDP associated with that MAC address.

C. Frame fragmentation: This is a Layer 3 (IP) function. VPLS operates at Layer 2; Ethernet frames are not fragmented. If a frame exceeds the transport MTU, it is dropped.

E. Must discard frames with an unknown destination MAC: This is incorrect. A VPLS PE, like a switch, must flood (replicate) frames with unknown destination MACs to enable MAC learning and communication across the emulated LAN.

F. MSTP: While a VPLS can transparently tunnel customer MSTP BPDUs, the PE router itself does not participate in the customer's Spanning Tree Protocol. The VPLS core uses split-horizon rules on SDPs to prevent loops.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VPLS Service" Chapter, Section: "Learning and Forwarding": "The 7750 SR OS VPLS service is designed to emulate an Ethernet LAN segment over an IP/MPLS core... This emulation requires the 7750 SR OS to perform MAC address learning, flooding of unknown unicast, broadcast, and multicast frames, and forwarding of unicast frames to known MAC addresses." (Confirms A, B, D; refutes E).

2. RFC 4762: Virtual Private LAN Service (VPLS) Using BGP for Auto-Discovery and Signaling, Section 5: "Packet Forwarding": "When a PE router receives an Ethernet frame from a CE device, it must first learn the source MAC address... If the destination MAC address is a broadcast or multicast address, or if the destination MAC address has not yet been learned by the PE, the PE MUST flood the frame... if the destination MAC address has been learned, the PE forwards the frame to the appropriate destination." (Confirms A, B, D; refutes E).

3. Nokia 7750 SR OS Services Guide, Release 21.10.R1, "VPLS Service" Chapter, Section: "Spanning Tree Protocol": "The VPLS service is transparent to the customer’s STP BPDUs... The PE does not participate in the customer’s STP topology." (Refutes F).

Designing Ethernet networks with redundancy involves creating multiple physical paths between switches to prevent single points of failure. However, these physical loops are problematic for standard Ethernet (Layer 2) forwarding, which lacks an inherent loop-avoidance mechanism like the IP TTL field. Unmanaged loops cause broadcast storms, MAC address table instability, and duplication of frames, which can quickly render a network unusable. Therefore, a fundamental design requirement is to implement a protocol, such as the Spanning Tree Protocol (STP) or its modern variants (RSTP, MSTP), to logically prune the redundant paths, creating a single, loop-free forwarding topology while maintaining the physical paths for failover.

A. TTL (Time to Live) is a mechanism used in Layer 3 (IP) packets to prevent routing loops, not in Layer 2 Ethernet frames.

B. In a looped environment, the MAC address table becomes unstable (flapping), but the learning rate itself is not doubled.

C. Redundancy does not double the number of devices or MAC addresses; thus, the required FDB (MAC table) size does not double.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1: In the "Spanning Tree Protocol" section, it states, "The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your network. Loops are deadly to a network." This directly confirms the need for a protocol to manage loops created by redundancy. (Reference found in sections describing Layer 2 services like VPLS and Epipe).

2. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. In Chapter 6, Section 6.4.3 "Link-Layer Switches," the text explains: "To deal with the problem of loops in redundant topologies, switches use the Spanning Tree Protocol (STP)... The protocol's goal is to create a spanning tree from the graph of switches, resulting in a loop-free topology." This is standard university courseware material.

3. IEEE Std 802.1D-2004, IEEE Standard for Local and metropolitan area networks—Media Access Control (MAC) Bridges. Clause 1, "Overview," states that the standard specifies an algorithm for bridges to "provide for the automatic reconfiguration of the active topology... to provide loop-free data paths." This standard is the foundational source for Spanning Tree Protocol.

The statement is correct. The primary objective of the Spanning Tree Protocol (STP) is to prevent Layer 2 forwarding loops in a network with redundant links by creating a single, loop-free logical topology. It accomplishes this by electing a Root Bridge and ensuring every other switch has only one active, lowest-cost path towards that Root Bridge. All other redundant paths are logically blocked.

Consequently, the path taken between any two non-root switches is not necessarily the most direct or lowest-latency physical path. Instead, traffic may have to traverse "up" the spanning tree towards a common ancestor (or all the way to the Root Bridge) and then "down" to the destination switch. A more direct physical link between the two switches might exist but will be in a blocking state to maintain the loop-free topology. Therefore, STP guarantees a valid, loop-free path, but this path is often suboptimal compared to the shortest physical path available.

The "False" option is incorrect because the potential for suboptimal pathing is a fundamental and well-documented characteristic of STP's design. The protocol's algorithm prioritizes loop prevention over path optimization between all pairs of nodes. Newer protocols like Shortest Path Bridging (SPB) were developed specifically to address this and other STP limitations.

1. Nokia Official Documentation: Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 2 Services and EVPN Guide, Release 22.10.R1, Section: "Spanning Tree Protocol (STP)". The documentation describes how STP, RSTP, and MSTP operate by creating a single active path to a root, which inherently means other direct paths are blocked. The guide states, "The Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on bridges and switches... STP creates a single spanning tree for the entire bridged network, which results in a single active path between any two nodes." This confirms the creation of a single path, which is not necessarily the most optimal one if a more direct link is blocked.

2. Academic Publication (Original Protocol Design): Perlman, R. (1985). "An algorithm for distributed computation of a spanning tree in an extended LAN." ACM SIGCOMM Computer Communication Review, 15(4), 44–53. https://doi.org/10.1145/318951.319004. While the paper details the algorithm for creating a loop-free tree, subsequent analysis and networking pedagogy built upon this work universally acknowledge that the resulting tree does not guarantee shortest paths between all pairs of nodes, only between each node and the root.

3. University Courseware: Kurose, J. F., & Ross, K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. In Chapter 6, "The Link Layer: Links, Access Networks, and LANs," the text explains the operation of spanning tree. It clarifies that the protocol forces traffic to follow the branches of the tree, even if a "shortcut" link (which is blocked by STP) exists, thus leading to suboptimal routing between some pairs of switches. This textbook is standard curriculum in numerous university computer science and engineering programs.

The disable-learning command, when configured at the VPLS service level on a Nokia Service Router, only prevents the learning of new source MAC addresses. It does not affect the aging process for MAC entries that are already present in the Forwarding Database (FDB). These existing, previously learned MAC entries will continue to be subject to the configured MAC aging timer and will be removed from the FDB if they are not refreshed by subsequent traffic before the timer expires.

The statement is false because it incorrectly assumes that disabling the learning function also disables the aging function. In Nokia's SR OS, MAC learning and MAC aging are distinct processes. The disable-learning command specifically targets the population of the FDB with new entries, while the mac-aging timer independently manages the lifecycle of all dynamic entries within the FDB.

1. Nokia 7750 SR OS Services Guide, Release 21.10.R1, Document Part Number: 3HE 17680 AAAD TQZZA, Section: VPLS Configuration Commands.

In the description for the disable-learning command, the documentation clarifies its function is to stop the learning of new MAC addresses. It does not state that it stops the aging of existing entries. The mac-aging command is described separately as the mechanism that controls the removal of inactive dynamic MAC addresses. This separation of functions confirms that disabling one does not disable the other.

2. Nokia 4A0-105 Courseware: Nokia Virtual Private LAN Services, Student Guide, Module 3: VPLS Configuration and Implementation.

This module details the VPLS forwarding plane. It explains that the FDB is populated through MAC learning. It also describes the aging mechanism as a separate, continuous process that removes stale entries to maintain FDB health. The disable-learning feature is presented as a way to create a static FDB or a hub-and-spoke topology, which only affects the ingress learning process, not the maintenance (aging) of existing entries.

In the Rapid Spanning Tree Protocol (RSTP), as defined by IEEE 802.1w, a Backup Port is a port that provides a redundant, non-forwarding path to a network segment for which another port on the same switch is already the Designated Port. This scenario typically occurs when a switch has two or more ports connected to the same shared collision domain (for example, connected to a hub). The port with the lower port ID (higher priority) on the switch becomes the Designated Port for that segment, and the other port connected to the same segment transitions to the Backup Port role. The Backup Port receives BPDUs from its own switch's Designated Port.

The statement is a correct definition of the Backup Port role in RSTP. Therefore, the "False" option is incorrect. A common point of confusion is the difference between a Backup Port and an Alternate Port. An Alternate Port provides a redundant path toward the Root Bridge and receives BPDUs from a Designated Port on a different switch, whereas a Backup Port provides redundancy for a segment and receives BPDUs from a Designated Port on the same switch.

1. Nokia 7750 SR OS Services Configuration Guide, Release 21.10.R1: In the section describing Spanning Tree, the port roles are defined. The guide clarifies the Backup Port role in the context of RSTP and MSTP. "A backup port provides a redundant path to the segment to which it is connected. A backup port exists when a bridge has two or more connections to the same segment (for example, through a hub)." This inherently means the Designated Port for that segment is on the same bridge. (Reference: Section on "RSTP" and "Port States and Roles").

2. IEEE Std 802.1Q-2018, "Bridges and Bridged Networks": This standard incorporates RSTP. In Clause 13.5, "Port Roles," the Backup Port role is formally defined. It states: "A port’s role is Backup Port if it receives more useful Spanning Tree information from a Designated Port on the same Bridge." This explicitly confirms that the Backup Port and the corresponding Designated Port are on the same switch (Bridge). (Reference: Clause 13.5, page 478).

3. "Spanning Tree Protocol: A deep dive into the 802.1D standard" - University of California, Berkeley, EECS Department Course Material: Course notes for advanced networking often detail RSTP port roles. Lecture materials explain that "A backup port is a port blocked by receiving BPDUs from a designated port on the same switch. This happens when two ports of a switch are connected to the same segment." (Reference: CS168/268 Course Notes on Spanning Tree Protocol).

In Nokia-SR OS terminology a “MAC explosion” (also called MAC flooding) is the situation in which a bridge learns a very large number of different source MAC addresses in a short time, eventually exhausting the forwarding database. It is characterised by thousands of new, unique MAC addresses being added—​not by the same addresses being repeatedly withdrawn and relearned.

The behaviour in which the same MAC address is continually withdrawn from one port/service and relearned on another is called “MAC move” or “MAC flapping,” not MAC explosion.

Why the Other Option Is Wrong:

• True – Choosing True would equate MAC explosion with the continual withdrawal and relearning of the same MAC addresses. That behaviour is explicitly defined by Nokia documentation as “MAC move/flap,” not MAC explosion; therefore the statement is incorrect.

1. Nokia (Alcatel-Lucent) SR OS Layer 2 Services and Bridging Guide, Release R12.0, Section 7.6 “MAC Flooding (MAC Explosion)”, pp. 342-343: “MAC flooding occurs when a large number of frames with unique source MAC addresses are received … creating an explosion of new MAC entries.”

2. Nokia SR OS Layer 2 Services and Bridging Guide, Release R12.0, Section 7.7 “MAC Move Protection”, p. 350: “MAC move (also called MAC flapping) is the repeated withdrawal and relearning of the same MAC address on different ports/services.”

3. Aalto University, CS-E4180 “Advanced Topics in Networking,” Lecture notes on Layer-2 security, slide 18: distinguishes ‘MAC flooding/explosion’ from ‘MAC flapping’ (single MAC moving between ports).

These sources collectively confirm that MAC explosion involves many new MAC addresses being learned, whereas withdrawal and relearning of the same MAC address describes MAC flapping.

The statement is false. By default, a Nokia MC-LAG is configured to operate in a non-revertive mode. This means that when a failed link or node recovers, traffic will not automatically switch back (revert) to it. The system maintains the traffic on the currently active link to prevent potential instability or traffic disruption that could be caused by link flapping. Revertive behavior must be explicitly enabled by configuring the revert-time parameter within the mc-lag configuration context.

The "True" option is incorrect because it describes revertive behavior, which is not the default for Nokia's MC-LAG implementation. The default non-revertive behavior is designed to prioritize service stability over returning to a specific primary path.

1. Nokia 7450 ESS, 7750 SR, 7950 XRS, and VSR LAG and MC-LAG Guide, SR OS Release 21.10.R1, Document Part Number: 3HE 17313 AAAD TQZZA, Section: MC-LAG Configuration Commands.

Reference Detail: In the description of the revert-time command under the config>lag>mc-lag context, the documentation states: "This command enables revertive operation for an MC-LAG instance... By default, MC-LAG operates in non-revertive mode." This explicitly confirms that the default behavior is non-revertive, and an administrator must configure a revert timer to enable the behavior described in the question.

The statement is false because it incorrectly describes the relationship that defines an Alternate Port. An Alternate Port in Rapid Spanning Tree Protocol (RSTP) provides a redundant, non-forwarding path toward the Root Bridge. It is a backup for the switch's Root Port. An Alternate Port is established when a port receives superior Bridge Protocol Data Units (BPDUs) from a Designated Port on a different (neighboring) switch.

The scenario described in the question—where a port provides redundancy for a Designated Port on the same switch (for example, when two ports on a switch are connected to the same hub or collision domain)—defines a Backup Port, not an Alternate Port.

The statement is not True. While it is possible for a switch to have both an Alternate Port and a Designated Port simultaneously (on different network segments), this is not the defining characteristic of an Alternate Port. The question implies a definitional relationship that is specific to the Backup Port role. The core distinction is that an Alternate Port is a backup for a Root Port, whereas a Backup Port is a backup for a Designated Port.

1. IEEE Std 802.1D-2004, "IEEE Standard for Local and metropolitan area networks—Media Access Control (MAC) Bridges":

Section 17.4, Port Roles: This section formally defines the RSTP port roles. It specifies: "An Alternate Port...receives more useful Designated Bridge information from another Bridge than the Root Port...". It also defines a Backup Port: "A Backup Port...receives more useful Designated Bridge information from the same Bridge it is on...". This directly contradicts the question's statement for an Alternate Port.

2. Nokia 7750 SR OS Services Architecture Guide, Release 21.10.R1:

Chapter: Spanning Tree Protocol (STP), Section: RSTP: The documentation defines the port roles consistent with the IEEE standard. It describes an Alternate port as providing "an alternate path to the root bridge from the root port." It defines a Backup port as providing "a redundant path to a segment where another bridge port already connects... A backup port exists when a bridge has two or more connections to the same segment... The backup port is the port that is not the designated port." This confirms the question is describing a Backup Port's relationship, not an Alternate Port's.

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