Question: What happens when an MME receives a PDN DISCONNECT REQUEST message with an EPS bearer identity that does not belong to the default EPS bearer context of an established PDN connection?
Option 1: The MME sends a PDN DISCONNECT ACCEPT message.
Option 2: The MME discards the message and takes no further action.
Option 3: The MME sends a PDN DISCONNECT REJECT message with ESM cause #42 "bearer resource allocation failure".
Option 4: The MME sends a PDN DISCONNECT REJECT message with ESM cause #43 "invalid EPS bearer identity".
Explanation: When the linked EPS bearer identity in the PDN DISCONNECT REQUEST message does not belong to the default EPS bearer context of an established PDN connection, the MME replies with a PDN DISCONNECT REJECT message citing ESM cause #43 "invalid EPS bearer identity". This ensures that the network maintains accurate bearer contexts and prevents potential misconfigurations or security breaches.