Emanuele
When applied on an interface access lists are uni-directional. You can apply an access list inbound on the interface and apply an access list outbound on the interface if you want a bi-directional effect.
I am not sure that I understand what you are trying to accomplish. I think that I understand that you do not want LAN A to send to LAN B. I am not clear if you want LAN B to be able to send to LAN A, which it sort of sounds like. The problem with this is how to differentiate something coming from LAN A to LAN B which is a response to something that originated from LAN B versus something originated from LAN A. For TCP connections you can use the established concept in the access list, but there is not a good way to handle UDP, ICMP, etc.
If you do not want either subnet to communicate with the other then I suggest that you write 2 access lists. The first access list would deny traffic with a source in LAN A and a destination in LAN B and would permit other traffic. This access list would be applied outbound on LAN A interface. The second access list would deny traffic with a source in LAN B and a destination in LAN A and would permit other traffic. This access list would be applied outbound on LAN B interface. If you do this I do not see a need for an inbound filter on either interface.
If I have not understood your question correctly please clarify what you are attempting to accomplish.
HTH
Rick
HTH
Rick
