To be clear, suggest can you use traffic generator such like IXIA connect one your BCM5389 board to test trunk? Use three ports giga link as following configuration figure for example.
One port S4 from traffic generator IXIA to inject traffic to BCM5389 board port Port4 and forward to one of trunk ports (Port0 and Port1) for Port0 example.
The Port0 and Port1 also connect to traffic generator port S0 and S1 respectively.
And you may observe the IXIA S0 will receive the traffic, then unplug the cable between IXIA port S0 and BCM5389 Port0 and it will be failover to Port1. And IXIA port S1 receive the traffic.BCM5389 trunk registers write:- Reg(0x32: 0x01)=0x08- Reg(0x32: 0x90)=0x01
IXIA BCM5389 device1
S4 ---------> Port4----------
S0 <-------- Port0 <--------
S1 <-------- Port1