This read-only Counter64 counts the number of session packets received from the peer by this PPPoE session while its own operational status was down. A non-zero and growing value here indicates the subscriber's CPE continued sending traffic even though the AC-side session had gone operationally down, pointing to a state mismatch between the two ends. Seeing this counter climb on a session repeatedly falling into oper-down status suggests the CPE isn't detecting the down state and needs its own keepalive tuning.