What happens when a packet enters a Junos device with no matching destination entry in the forwarding table?

When a packet enters a Junos device and there is no matching destination entry in the forwarding table, the correct behavior is for the Packet Forwarding Engine (PFE) to respond to the source with a destination unreachable notification message. This response indicates to the source that the packet could not be delivered to the intended destination because there is no corresponding entry in the forwarding table to guide the packet to its destination.

This mechanism is crucial for network troubleshooting and proper communication in IP networks. By informing the sender of the issue, it allows for corrective actions, such as adjusting routing configurations or verifying the network path. The communication of a destination unreachable message helps maintain the integrity and reliability of data transmission across the network.