A fault isolation method, computer system, and apparatus, which are capable of monitoring a state of a second endpoint device in the extended domain, and setting a device state record according to the state of the second endpoint device; after an access request between the second endpoint device and the primary domain is received, querying the device state record according to identifier information that is of the second endpoint device and in the access request, and determining the state of the second endpoint device; and if the state of the second endpoint device is a fault state, discarding the access request to prevent communication between the faulty second endpoint device and the primary domain and prevent spreading a fault to the primary domain, thereby ensuring system reliability.