ANS-C01 Network Design Practice Question

Exhibit

Refer to the exhibit.

```
Tunnel1:
  State: UP
  Last Status Change: 2024-03-15 10:23:45 UTC
  Details: Tunnel is in UP state with BGP established.
Tunnel2:
  State: DOWN
  Last Status Change: 2024-03-15 10:25:12 UTC
  Details: Tunnel is in DOWN state due to phase 2 negotiation failure.
```

A network engineer has configured an AWS Site-to-Site VPN connection between a VPC and an on-premises network. The engineer checks the VPN status and sees the output above. What is the MOST likely cause of Tunnel2 being down?

⚠ Common exam trap

AWS often tests the distinction between IKE phase 1 (pre-shared key, authentication) and IPsec phase 2 (encryption/integrity algorithms); candidates mistakenly attribute a single tunnel failure to a pre-shared key mismatch, but AWS assigns unique keys per tunnel, so a key error would affect only that tunnel—making algorithm mismatch the more subtle and likely cause when only one tunnel is down.

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The IPsec encryption or integrity algorithms do not match between the AWS VPN endpoint and the on-premises device.

Tunnel2 is down while Tunnel1 is up, indicating a per-tunnel configuration mismatch. The most common cause is a mismatch in IPsec security parameters (encryption, integrity, or DH group) between the AWS VPN endpoint and the on-premises device for that specific tunnel. AWS uses separate pre-shared keys and IPsec settings per tunnel, so a mismatch in algorithms would affect only the misconfigured tunnel.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
The BGP ASN on the on-premises device is misconfigured.
Why it's wrong here
BGP issues would not cause Phase 2 failure.
✗
The route tables in the VPC do not have a route to the on-premises network.
Why it's wrong here
Routing does not affect tunnel status.
✗
The IKE pre-shared key used for Tunnel2 is incorrect.
Why it's wrong here
Incorrect PSK would cause Phase 1 failure, not Phase 2.
✓
The IPsec encryption or integrity algorithms do not match between the AWS VPN endpoint and the on-premises device.
Why this is correct
Mismatched IPsec parameters cause Phase 2 negotiation failure.

Quick reference

VPN Protocol Comparison

Protocol	Port	Encryption	Authentication	Use Case
IKEv2 / IPsec	UDP 500 / 4500	AES-256	Certificates / PSK	Site-to-site & remote access
SSL / TLS VPN	TCP 443	TLS 1.3	Certificates / MFA	Clientless remote access
L2TP / IPsec	UDP 1701	AES (IPsec)	PSK / Certificates	Legacy remote access
WireGuard	UDP 51820	ChaCha20	Public keys	Modern high-performance VPN
PPTP	TCP 1723	MPPE (weak)	MS-CHAPv2	Legacy — avoid in production

PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.

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Last reviewed: Jun 30, 2026

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