ANS-C01 Question 189
Single answerYou are managing a hybrid cloud environment where an on-premises data center is connected to AWS using a Direct Connect connection. Your on-premises network uses BGP (Border Gateway Protocol) for dynamic routing, and you have configured a Virtual Private Gateway in your VPC. During a maintenance window, you notice that specific routes from your on-premises network are not being propagated to your VPC route tables. What could be the possible cause of this issue?
- A
The BGP session between the Virtual Private Gateway and your on-premises router is down.
- B
The route propagation feature is disabled in the VPC route table associated with the subnet.
- C
The route you are trying to propagate exceeds the AWS limit of 100 prefixes for BGP routes.
- D
Your on-premises router is advertising the routes with a higher metric, making them less preferred.
Show answer and explanation
Correct answer: B
Explanation
In this scenario, the issue is that specific routes are not being propagated to the VPC route table. For dynamic routes received via BGP to be automatically added to the route table, the route propagation feature must be enabled in the VPC route table. If it is disabled, routes will not appear, regardless of the BGP session status or the advertised metrics. Therefore, the most likely cause is that route propagation is disabled in the VPC route table.
- A. Incorrect.
If the BGP session is down, no routes will be propagated at all. However, this question specifies that 'specific routes' are not being propagated, so this is not the root cause.
- B. Correct.
If the route propagation feature is disabled in the VPC route table, dynamic routes received via BGP will not appear in the route table, even if the BGP session is active. This matches the issue described in the question.
- C. Incorrect.
AWS does limit the number of prefixes to 100 for BGP routes, but exceeding this limit would typically result in an error or a complete failure to propagate any routes rather than specific routes not propagating.
- D. Incorrect.
A higher BGP metric (or local preference) would affect route selection but would not prevent the route from being propagated entirely. Therefore, this is not the root cause.