T3126 timeout will occur if the BSS is not properly connected to the core. The cell will still transmit details of the cell (in particular the LAC). When the MS camps on a cell with a different LAC it will try to update the HLR with the new LA. This can only occur once the MS has an active channel, and this can only occur if there is a route to the core. Once the core connection is broken, the MS will keep trying to make a connection and keep failing resulting in contineous T3126 timeouts.