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.