route 192.168.2.0/24 dev eth-left proto kernel scope link src 192.168.2.2
route 192.168.3.0/24 dev eth-right proto kernel scope link src 192.168.3.2
O atributo src é o problema. Deve ficar assim:
route 192.168.2.0/24 dev eth-left proto kernel scope link
route 192.168.3.0/24 dev eth-right proto kernel scope link