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According to "Packet flow in Netfilter and General Networking" drawing in Wikipedia Netfilter article:

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..one of the first hops in packet flow right after the traffic control and queuing is bridge check. What exactly happens in bridge check? When does frame continue in Link Layer and when does it continue in Network Layer?

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When you add a network interface to a bridge (e.g. brctl addif br0 eth0), all received packets are diverted to the bridge. If you try to set an IP address on the interface, it will be accepted, but you won't be able to use it to receive any packets - this is a common pitfall. In the source code, the bridge sets an rx_handler on the interface, and consumes every packet.

This is what is meant by the bridge check in this diagram. The choice is simply based on whether the network interface belongs to a bridge.

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