Question

A media company runs a large file-transfer workload between two VPCs attached to the same AWS Transit Gateway. On-premises testing showed high throughput using 9001-byte jumbo frames, but after migrating to AWS the application experiences sporadic large-payload failures and reduced throughput. A packet capture on the receiving instance shows that some large packets never arrive, while smaller packets pass normally. The instances themselves are configured with an MTU of 9001. What is the most likely cause and the correct remediation?

Accepted Answer

Transit Gateway supports a maximum MTU of 8500 bytes for traffic between VPC attachments, and packets larger than this are dropped without fragmentation; lower the instance/application MTU to 8500 or less.

Answer

Transit Gateway fragments all packets over 1500 bytes; disable jumbo frames entirely and set the instance MTU to 1500 to restore reliability.

Answer

The VPC route tables are missing a jumbo-frame propagation entry; add a static route with the jumbo flag enabled on both attachments.

Answer

Path MTU Discovery is failing because ICMP is blocked; open ICMP type 3 code 4 in the security groups so 9001-byte frames pass through Transit Gateway unchanged.