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How does OVS GRE tunnel port get network package?

I'm debugging a network issue and like to know the magic behind OVS GRE tunnel.

Let's say a GRE tunnel between host A and B, on host A I have:

Bridge br-tun
        Port "gre-0a0a0114"
            Interface "gre-0a0a0114"
                type: gre
                options: {df_default="true", in_key=flow, local_ip="10.10.1.21", out_key=flow, remote_ip="10.10.1.20"}

Host A has NIC eth1 using IP 10.10.1.21.

and on Host B I have:

Bridge br-tun
Port "gre-0a0a0115"
    Interface "gre-0a0a0115"
        type: gre
        options: {df_default="true", in_key=flow, local_ip="10.10.1.20", out_key=flow, remote_ip="10.10.1.21"}

Host B has NIC eth1 using IP 10.10.1.20.

When a package sent out from host A to B, will it first reach eth1 on Host B? Or will it reach Port "gre-0a0a0115" on br-tun directly? If the package reaches eth1 first then how does port "gre-0a0a0115" get the traffic?

I'm asking this because I found somehow the traffic reaches eth1 on host B but nothing on GRE tunnel port "gre-0a0a0115" in my environment.

Thanks in advance!