A switch receives an Ethernet frame that contains source and destination MAC addresses that are...

On Junos-based Ethernet switching platforms used in data centers, Layer 2 forwarding is handled primarily in the forwarding plane. When a switch receives a frame, it first performs source MAC learning. If the source MAC address is not already present in the Ethernet switching table for that VLAN or bridge domain, the switch creates a new entry that maps the source MAC to the ingress interface and the associated VLAN context. This learning step is fundamental to building the MAC table dynamically and enables efficient forwarding for subsequent frames destined back to that source.

Next, the switch attempts to forward the frame based on the destination MAC lookup in the same VLAN or bridge domain. Because the destination MAC is also not in the Ethernet switching table, the frame is treated as an unknown unicast. The default behavior for unknown unicast in a Layer 2 broadcast domain is to flood the frame out all other interfaces that belong to that VLAN or bridge domain, excluding the ingress interface. Flooding ensures the frame has the best chance of reaching the correct destination host. When the destination responds, the switch then learns that MAC address as a source on the return traffic, allowing future traffic to be forwarded as known unicast instead of flooded.

The switch does not drop the frame by default, and it does not forward the frame to the Routing Engine because this is normal Layer 2 bridging behavior, not a control-plane routing decision.