ANS-C01 Network Implementation Practice Question
A company is implementing a multicast application in AWS. The application requires that multicast traffic be forwarded between Amazon EC2 instances in different VPCs. The company has set up a multicast domain using AWS Transit Gateway Connect with multicast support. The multicast group is using the IP address 239.0.1.10. The network engineer has confirmed that the EC2 instances are registered as multicast members and that the Transit Gateway multicast domain is configured correctly. However, receivers in VPC B are not receiving multicast traffic from senders in VPC A. What is the MOST likely cause of this issue?
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 security group for the sender EC2 instances does not allow outbound UDP traffic to the multicast group address.
The issue is that the sender EC2 instances' security group must allow outbound UDP traffic to the multicast group address 239.0.1.10. Even though the Transit Gateway multicast domain is correctly configured, security groups act as a virtual firewall at the instance level and can block multicast traffic if the appropriate outbound rule is not present. Option C is correct because without this rule, the traffic is blocked at the source. Option A is incorrect because multicast groups are not tied to specific subnets; they are associated with the transit gateway. Option B is incorrect because TTL is not a limiting factor in this scenario; AWS Transit Gateway multicast handles traffic within the domain regardless of TTL. Option D is incorrect because IGMP join messages are not required; multicast group members are statically registered in the Transit Gateway multicast domain.
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 multicast group is not associated with the correct subnet in VPC A.
Why it's wrong here
Multicast groups are not subnet-specific.
- ✗
The multicast traffic has a TTL value of 1, which prevents it from leaving the source subnet.
Why it's wrong here
TTL is not an issue within AWS Transit Gateway multicast.
- ✓
The security group for the sender EC2 instances does not allow outbound UDP traffic to the multicast group address.
Why this is correct
Security groups control outbound traffic; if outbound rule missing, traffic is dropped.
- ✗
The receiver instances are not sending IGMP join messages to the multicast group.
Why it's wrong here
AWS Transit Gateway multicast does not use IGMP; membership is static.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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