Linux kernel shows a lot of usefull data in proc filesystem. Good example is
/proc/net/dev file - among others, you can see how many data are transfered
through the interface.
If you have a router/firewall you can easily monitor how much bandwidth your
whole network uses. Let's assume your WAN interface is eth0.

Create the RRD archive:
rrdtool create eth0.rrd -s 30\
        DS:in:COUNTER:90:U:U\
        DS:out:COUNTER:90:U:U\
        RRA:AVERAGE:0.99:2:60\
        RRA:AVERAGE:0.99:120:72\
        RRA:MAX:0.99:120:72\
        RRA:AVERAGE:0.99:20160:53\
        RRA:MAX:0.99:20160:53

Put into rrdcollect.conf:
file:///proc/net/dev
" eth0:%d %*d %*d %*d %*d %*d %*d %*d %d %*s" eth0.rrd:in,out