Google Professional Cloud Network Engineer Question 746
Select 3Google Cloud PlatformYou have configured a Cloud Router in Google Cloud to establish a BGP session with an on-premises router over a VPN connection. However, the BGP session is not being established, and the Cloud Router logs show 'BGP NOTIFICATION Message: Hold Timer Expired'. What actions should you take to resolve this issue?
- A
Verify that the on-premises router's BGP hold timer matches the Cloud Router's hold timer.
- B
Check if the BGP session is using the correct peer IP addresses on both the Cloud Router and on-premises router.
- C
Ensure that the Cloud Router has the correct ASN configured to match the on-premises router's ASN.
- D
Verify that the firewall rules allow TCP traffic on port 179 between the Cloud Router and the on-premises router.
- E
Increase the MTU size on the VPN tunnel to improve packet delivery.
Show answer and explanation
Correct answers: A, B, D
Explanation
The 'Hold Timer Expired' error in BGP indicates that the BGP session between peers is not being maintained due to missing keepalive messages. Key causes include mismatched BGP hold timers, incorrect peer IP addresses, or firewall rules blocking TCP traffic on port 179. Verifying and addressing these configurations ensures the BGP session can be successfully established. However, configuring the ASN or adjusting the MTU size are not relevant to resolving this specific issue.
- A. Correct.
The 'Hold Timer Expired' error can occur if the BGP hold timers between peers do not match. It's important to verify and align the hold timers on both the Cloud Router and the on-premises router.
- B. Correct.
If the peer IP addresses are misconfigured, the BGP session cannot be established, leading to connection issues such as the hold timer expiring.
- C. Incorrect.
The Cloud Router does not require the ASN to match the on-premises router's ASN because BGP allows peering between different ASNs. This is not related to the issue.
- D. Correct.
Firewall rules must allow TCP traffic on port 179, which is the port used by BGP. If this traffic is blocked, the BGP session will fail to establish.
- E. Incorrect.
MTU size is not related to the 'Hold Timer Expired' error and does not directly impact BGP session establishment. Adjusting the MTU size in this scenario is unlikely to resolve the issue.