Abstract: In complex networks, data frames may be routed through different systems having different frame or addressing requirements. When the frames are transferred or bridged between such systems, known changes may have to be made in the frame contents to accommodate these requirements. To maintain the integrity of error checking provided by Cyclical Redundancy Checking (CRC) techniques, a system receiving a frame uses an improved Frame Check Sequence value modification technique which modifies a Frame Check Sequence (FCS) field value only as a function of the known or planned changes to be made in the frame. If unplanned changes (that is, errors) are introduced at the receiving system, application of standard CRC error checking techniques at the next system to receive the frame will indicate those errors.