Hi,
your linux box is connected to an ethernet switch where vlan tagging is configured (trunc) and you want to configure your network manually by ifconfig?
Load the module for tagged vlans 802.1q.
root@debdev ~# modprobe 8021q
Add the vlan to the interface. In this example VLAN ID 11 at Interface eth0
root@debdev ~# vconfig add eth0 11 Added VLAN with VID == 11 to IF -:eth0:-
Configure IP Address
root@debdev ~# ifconfig eth0.11 192.168.254.11 netmask 255.255.255.0 up
And add a (default) route if necessary
root@debdev ~# route add default gw 192.168.254.1