Refer to the exhibit. A site-to-site tunnel between two sites is not coming up. Based on the debugs, what is the cause of this issue?

Question

Options

• AAn authentication failure occurs on the remote peer.
• BA certificate fragmentation issue occurs between both sides.
• CUDP 4500 traffic from the peer does not reach the router.
• DAn authentication failure occurs on the router.

• A
  14% (8)
• B
  7% (4)
• C
  53% (31)
• D
  26% (15)

Explanation

UDP port 4500 is used for IKE NAT Traversal (NAT-T), which encapsulates IKE and ESP traffic when NAT is detected between VPN peers. When the debug output shows IKE Phase 1 or Phase 2 negotiation attempts being sent but receiving no response, it indicates the router is not receiving return UDP 4500 packets from the remote peer. This is a connectivity or firewall/ACL issue - not an authentication or certificate problem - because the negotiation never progresses far enough to fail on credentials. Options A and D (authentication failures) would show explicit HASH or AUTH payload rejection messages. Option B (certificate fragmentation) would appear only in certificate-based PKI setups with specific fragmentation error messages. The absence of any inbound IKE response on UDP 4500 points to a network reachability or filtering problem dropping those packets before they arrive.

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