I haven't ever used the KSZ8873 but I've used other Micrel parts in the past. I notice that this is a Managed switch, which usually means it doesn't switch any packets between ports until switching is enabled by software. I also noted in the errata sheet that the default is "don't switch" in I2C master mode.
I would suggest looking at register values over the management interface to make sure switching has indeed been enabled. Also check rate policing to make sure it's allowing ports to transmit.
Hope this helps.