What action does the Junos OS take when an EX Series switch receives an Ethernet frame with a destination MAC address not found in the bridging table?

When an EX Series switch receives an Ethernet frame with a destination MAC address that is not present in its bridging table, the Junos OS takes the action of learning the source MAC address and flooding the frame. This behavior is essential for ensuring that the network can dynamically adapt to new devices and their corresponding MAC addresses.

By learning the source MAC address, the switch can update its bridging table, allowing it to forward future frames directly to that MAC address without flooding. Flooding the frame means that it sends the frame out of all active ports (except the one on which it was received) to reach its intended destination, which is unknown at that moment. This process ensures that the frame can reach other devices on the network that might have the destination MAC address.

Once the destination device responds, the switch learns that device's MAC address and updates its forwarding table, enhancing the efficiency of future data transmissions. This action is part of the fundamental operations of Ethernet switching, wherein the switch learns and adapts to the network topology, fostering efficient communication.