LTE: When and why MME decides to assign new GUTI?

Question

When a UE attaches to a network for the first time, it uses its IMSI to request access to the network & obtains a GUTI in Attach Accept message allocated by the MME, during TAU procedure MME may assign new GUTI .
When and why MME decides to assign new GUTI?

Answer 1

Usually when tracking area update request receives at MME that is different from the current serving MME, GUTI reallocation procedure executes in the network. Sometime UE provided GUTI is invalid in the MME that's why a new GUTI is provided by the MME.