Noticed that with PPPoE authentication in ISP connectivity there is a difference in the number of repeated PPPoE connectivity attempts during boot time, i.e. with option force_link '1' there are less attempts than without that option set.
Is upstream packet egress prevented until link_state actually becomes true (or is forced by the aforementioned setting) and/or netfilter (firewall) rules are loaded?
Since there is no mechanism for netifd to discover the DSL line status of an external modem those repeatedly futile PPPoE discovery attempts are likely happening during the time the modem initiates DSL line sync with the ISP and until completed PPPoE discovery will fail.
Getting the DSL line status from an external modem is probably not possible without knowing how to access/extract the information from the modem.
solved the futile PADI discovery attempts; in this case (external xDSL modem) it takes 20 seconds for the first discovery packet being received from the ISP. No more futile discovery attempts since then.