To summarise all answers I'h got to my question:
- there is RFC recommendations about implementation for this filtering;
- CISCO have such filtering in their new forwarding scheme;
- there is (already) experimental IOS images supported such filtering;
- for now, this filtering is not possible (in CISCO) at the access
servers, but at the core routers only.
This means something like _CISCO know about this problem and work around
it, but there is technical problems in implementing this for the low-end
routers_ - this is as I understood all answers (not only from CISCO).
Thanks to all who have answered.
Aleksei.