Ring Topology Ethernet Backplane - 3 Port 1GbE Switch
To create  loop topology you will need the appropriate PHY port and chip to support it assuming you can afford the interface with high volume.

E.g. http://www.broadcom.com/products/Physical-Layer/Gigabit-Ethernet-PHYs/BCM5421xE-Family

Consider stub bus topology with a differential 100 Ohm impedance backplane such as Flexbus only does 10MHz while Canbus is only 1MHz.  if you want low cost this is it.

Considering the complexity of 1GHz signal, loop topology will have reliability issues if any repeater is bad and any T-bus is limited due to reflections of the stub lengths about 50MHz per metre of bus, this will not be easy unless you have successfully done 100MHz backplanes.  

For 1 GHz rates,  the PHY supplies the clock and the peripheral must synchronize to it.