Consider the following sequence:

ifconfig eth0 1.1.1.1 netmask 255.255.0.0
ifconfig eth0:1 1.1.1.2 netmask 255.255.255.0

Route table now shows two routes. That's fine by me.

ifconfig eth0 1.1.1.1 netmask 255.255.255.0

Now the route table shows only one route. That's also fine

ifconfig eth0:1 down

Route deleted, and so is eth0's IP address.

Bug or feature ?