ANS-C01 MTU (Maximum Transmission Unit) Practice Question
A company is using AWS Database Migration Service (DMS) to replicate data from an on-premises Oracle database to an Amazon RDS for Oracle instance. The replication is failing intermittently with connection timeouts. The network connectivity uses a Direct Connect private VIF. What should the network team investigate first?
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 MTU settings on the on-premises router and the AWS Direct Connect interface.
Correct answer: Option A is correct because MTU mismatch between the on-premises router and AWS Direct Connect interface can cause packet fragmentation or drops, leading to intermittent connection timeouts. The default MTU for Direct Connect is 1500 bytes, but if the on-premises router uses a larger MTU (e.g., 9001 for jumbo frames), packets may be dropped or fragmented, causing timeouts. Option B is incorrect because routing misconfiguration would cause persistent failure, not intermittent. Option C is incorrect because a VPN is not used as a primary path when Direct Connect is available. Option D is incorrect because security groups apply to traffic within the VPC, not to on-premises traffic over Direct Connect.
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 MTU settings on the on-premises router and the AWS Direct Connect interface.
Why this is correct
MTU mismatch can cause intermittent connectivity issues, especially with DMS.
- ✗
The route table of the VPC to ensure it has a route to the on-premises CIDR via the Direct Connect virtual interface.
Why it's wrong here
If routing were missing, it would fail consistently, not intermittently.
- ✗
The VPN connection status as a backup path.
Why it's wrong here
Only Direct Connect is used, no VPN.
- ✗
The security group of the RDS instance to ensure it allows traffic from the on-premises IP range.
Why it's wrong here
Security groups are for VPC resources, but the issue is intermittent, not consistent blocking.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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