A cooperative's engineer needs to filter traffic arriving from subscribers into a POP router's aggregation interface, while leaving traffic the router sends back out to those same subscribers unaffected. Under family inet on that interface, which statement should reference the firewall filter?

Junos always requires a firewall filter to be attached to an interface's family with an explicit direction — input or output — there is no directionless attachment; each direction is its own statement under family inet. Input applies the filter to packets arriving on that interface, which is exactly what covers traffic coming from subscribers into the aggregation port, so that is the correct choice here. Output would instead govern traffic the router transmits out that same interface toward subscribers, which the engineer specifically wants to leave unaffected, so it does not fit the requirement. The idea of a single directionless filter statement misunderstands Junos syntax — direction is mandatory at the point of attachment, and covering both directions requires configuring input and output separately, each naming a filter. Referencing the filter from routing-options tied to a protocol confuses firewall filters, which act on interfaces and packets, with routing policy, which is applied to routing protocols and shapes the routing table rather than interface traffic. Getting the direction right matters operationally: an engineer who accidentally applies the restriction as output instead of input would find subscriber-originated traffic unfiltered while unrelated router-to-subscriber traffic gets blocked. To confirm the binding is correct, run 'show configuration interfaces <name> unit <n> family inet' and verify the filter name appears under the input line, not output.