What can cause an IKEv2 tunnel to go down after it was initially brought up...

The correct answer is A .

The study guide explains the IKEv2 exchange order very clearly:

“The initial exchanges are: IKE_SA_INIT and IKE_AUTH.”

“Create_Child_SA exchange: Creates a new child SA or rekeys an existing child SA.”

It also states:

“After successful IKE_SA_INIT and IKE_AUTH exchanges, the CHILD_SA exchange takes place. In this exchange, the peers negotiate the CHILD_SA and the traffic selectors — traffic selector responder (TSr) and traffic selector initiator (TSi).”

That is why A is correct: if the tunnel was initially brought up successfully , then the initial exchanges already succeeded. A later problem during CREATE_CHILD_SA , especially with traffic selectors/phase 2 selectors , can cause the tunnel to fail during rekey or child-SA renegotiation.

Why the other options are wrong:

B is wrong because proposal mismatch for the IKE SA is handled during IKE_SA_INIT , not after the tunnel is already up. The study guide says IKE_SA_INIT negotiates the security settings to protect the IKE traffic

C is wrong because a pre-shared key mismatch is part of authentication and would prevent successful initial establishment during IKE_AUTH . The study guide shows that after IKE_AUTH, “authentication succeeded” and “established IKE SA” when it works

D is wrong because a Diffie-Hellman mismatch belongs to IKE_SA_INIT , which happens before the tunnel comes up. The study guide also states: “By IKEv2 design, no Diffie-Hellman public key is exchanged during an IKE_AUTH exchange.”

So the verified answer is: A .