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.