Question: What action does the network take if it receives a ROUTING AREA UPDATE REQUEST message containing the P-TMSI type IE and does not follow the use of the most significant bit of the <LAC> to distinguish the node type?
Option 1: The network uses the P-TMSI type IE to determine whether the mobile identity is a native P-TMSI or a mapped P-TMSI.
Option 2: The network ignores the P-TMSI type IE and proceeds with the routing area update procedure.
Option 3: The network sends an error message to the MS indicating that it does not support the use of P-TMSI type IE.
Option 4: The network initiates a location update procedure instead of GMM common procedures.
Explanation: According to the specification, if the network receives a ROUTING AREA UPDATE REQUEST message containing the P-TMSI type IE and does not follow the use of the most significant bit of the <LAC> to distinguish the node type, it shall use the P-TMSI type IE to determine whether the mobile identity is a native P-TMSI or a mapped P-TMSI. This allows the network to correctly identify the MS and initiate GMM common procedures if necessary.