linux box作为trunk透明。也就是说trunk可以透过linux box

VLAN2---------------|           |
VLAN3---------------| switch|--------| linux box|--------|switch|--------VLAN3
                                                                                               |___VLAN2

//建vlan虚拟接口
vconfig add eth0 2
vconfig add eth0 3
vconfig add eth1 2
vconfig add eth1 3

//启用端口
ifconfig eth0.2 up
ifconfig eth0.3 up
ifconfig eth1.2 up
ifconfig eth1.3 up

//建桥
brctl addbr bridge2
brctl addbr bridge3

//加入端口
brctl addif bridge2 eth0.2
brctl addif bridge2 eth1.2
brctl addif bridge3 eth0.3
brctl addif bridge3 eth1.3