[nsp] cat3750 and ip address on sub-interfaces?

[fredrik-ml at krixor.xy.org]

Hi

While getting acquainted with a Catalyst 3750, I found the following
limitation :

lab-3750-gw#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
lab-3750-gw(config)#interface GigabitEthernet1/0/1
lab-3750-gw(config-if)# no switchport
lab-3750-gw(config-if)# no ip address
lab-3750-gw(config-if)#interface GigabitEthernet1/0/1.123
lab-3750-gw(config-subif)#ip address 192.168.90.1 255.255.255.0

Configuring IP routing on a LAN subinterface is only allowed if that
subinterface is already configured as part of an IEEE 802.10, IEEE
802.1Q, or ISL vLAN.

lab-3750-gw(config-subif)#encapsulation dot1q 123
                           ^
% Invalid input detected at '^' marker.

lab-3750-gw(config-subif)#

I then found the following document 

http://www.cisco.com/en/US/customer/products/hw/switches/ps5023/products_configuration_guide_chapter09186a00801a6535.html

which says 

>Routed Ports
>
>A routed port is a physical port that acts like a port on a router; it
>does not have to be connected to a router. A routed port is not
>associated with a particular VLAN, as is an access port. A routed port
>behaves like a regular router interface, except that it does not
>support VLAN subinterfaces.

Can someone explain where this limitation comes from? I'm hoping it's a
software thing that a later IOS will remedy (currently running
12.1(14)EA1a).

Thanks,

Fredrik