Iptables forward rules: br0 or ppp0

[user2k10]

I have an RT-AC68U and it is connected to my fibre modem via the wan port.

I want to add some rules to the FORWARD chain to block outgoing and incoming packets from the Internet.

I am confused about when to use br0 and when to use ppp0.

To block outgoing packets to the internet do I specify -i br0 -o ppp0 or do I only specify -o ppp0?

To block incoming packets from the internet do I specify -i ppp0 -o br0 or do I only specify -i ppp0?

TIA

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