When is MAC limiting performed on a frame from an unknown source MAC address?

The process of MAC limiting is utilized to control the number of MAC addresses that can be learned on a specific port, preventing potential network issues caused by unauthorized devices or network loops. The correct point at which MAC limiting is performed is during the action taken by the ingress Packet Forwarding Engine (PFE) when it processes a frame from an unknown source MAC address.

This process begins when the ingress PFE receives a frame from a device with an unknown MAC address. At this stage, MAC limiting is enforced as it looks up the MAC address in its existing table. If the MAC address is unknown, and the limit set for that port has been reached, it will restrict further learning, effectively stopping the addition of that MAC address to the bridge table.

This means that the action involving MAC limiting is directly associated with the ingress PFE's operational tasks, specifically how it manages incoming frames based on MAC addresses. Understanding this process is crucial because it ensures that only a defined number of legitimate MAC addresses can be learned, contributing to the stability and security of the network.