HD
Despite what the book says about "ignoring" unknown aircraft in Class A airspace, it's sometimes very difficult to do so!
The book has changed slightly in recent years. If you believe the aircraft has made an unauthorised penetration of CAS (or have info it has), then you can now take action to avoid it. From MATS Part 1 for airspace classes A-D:
Neither avoiding action nor traffic information shall be passed unless radar derived or other information indicates that an aircraft is lost, has experienced a radio failure, or has made an unauthorised penetration of the airspace.
Pierre
I can see exactly where you are coming from. But to take your argument to its logical conclusion, should pilots no longer react to TCAS RAs
since they have no way of knowing whether the intruder aircraft has verified Mode C or not ?? I think there is a global acceptance in the pilot world that on a very small number of occasions the Mode C might not be what it seems, but for the vast majority it will be correct and so TCAS action, when commanded as an RA based on it, is the wisest thing to follow.