How Should Network Engineers Increase Payload Size Through a Transit Gateway?

ANS-C01 By islever Editorial Last updated

Short answer

D Instruct the application team to set the maximum transmission unit (MTU) of the system’s network interfaces to 8500 bytes.

Question

A company is running its application servers on Amazon EC2 instances. The EC2 instances run in separate VPCs that are connected by a transit gateway. The EC2 instances launch in a private subnet with a route to the transit gateway for internal and external connectivity. The external connectivity is provided by a VPC with firewall devices that perform an inspection for packets that ingress and egress through an internet gateway. A network engineer needs to help the company’s application team increase the payload size per packet delivery between the EC2 instances. All network connectivity must be through the transit gateway What should the network engineer do to meet these requirements?
  1. Enable jumbo frames on the transit gateway. Instruct the application team to set the maximum transmission unit (MTU) of the system’s network interfaces to 9001 bytes.
  2. Instruct the application team to set the maximum transmission unit (MTU) of the VPC to 8500 bytes.
  3. Instruct the application team to set up enhanced networking on the system by using the enhanced networking adapter. Set the maximum transmission unit (MTU) to 9001 bytes.
  4. Instruct the application team to set the maximum transmission unit (MTU) of the system’s network interfaces to 8500 bytes. Correct Answer

Key Takeaways

This question tests both the default MTU for Amazon EC2 (9001 bytes) and the hard 8500-byte frame size limit of AWS Transit Gateway, where the most common trap is assuming 9001-byte jumbo frames will work end-to-end.

The AWS Certified Advanced Networking Specialty exam tests knowledge of transit gateway MTU limits. For EC2-to-EC2 traffic through a transit gateway, setting the system MTU to 8500 bytes is the correct answer because transit gateways drop packets larger than 8500 bytes.

Common mistake: The most common wrong answer is A: enabling jumbo frames and setting the MTU to 9001 bytes. This ignores the transit gateway's 8500-byte maximum, which causes any larger frame to be silently dropped, so the change would actually break connectivity.

Expert Analysis

Why the Answer Is Correct

Transit Gateway supports an MTU of 8500 bytes on attachments and drops any packet larger than 8500 bytes that arrives at the gateway. Setting the EC2 instance's network interface MTU to 8500 bytes ensures the largest possible payload that the transit gateway can forward, avoiding fragmentation and dropped packets. The question explicitly requires that all connectivity must pass through the transit gateway, so the application path's bottleneck is the transit gateway's limit. Therefore, configuring the system interfaces to 8500 bytes meets the requirement for the maximum payload size.

Why the Other Options Are Wrong

Option A fails because setting the MTU to 9001 bytes exceeds the transit gateway's maximum supported MTU of 8500, and the gateway will drop those frames. Option B is incorrect because the MTU is a property of a network interface or a network path, not a VPC, and the VPC itself does not have a configurable MTU in this context. Option C is also wrong: enhanced networking can improve throughput and latency, but it does not change the transit gateway's 8500-byte frame-size cap, and setting the interface to 9001 would still cause dropped frames. Only D correctly aligns the interface MTU with the transit gateway's maximum MTU.

Community Comment Notes

Community comments overwhelmingly support D, with several citing the official transit gateway quotas documentation. One commenter, referencing likes=3, notes: "Packets with a size larger than 8500 bytes that arrive at the transit gateway are dropped." Another commenter correctly explains that 8500 bytes is the maximum MTU supported by Transit Gateway and that setting the system interfaces to 8500 prevents fragmentation. A few comments, such as likes=1 answer=A, are incorrect because they focus on EC2's jumbo-frame capability without considering the transit gateway's lower limit. The consensus among AWS experts is to match the MTU of the EC2 instances to the maximum supported by the transit gateway, not to the 9001-byte jumbo-frame standard used for VPC-local traffic.

Official Reference

https://docs.aws.amazon.com/vpc/latest/tgw/transit-gateway-quotas.html

This link opens the official vendor documentation in a new tab.

Exam Strategy

When a question involves an AWS managed service in the data path, check the documentation for that service's MTU or maximum frame-size limit. Remember that EC2 instances default to 9001-byte jumbo frames, but Transit Gateway only supports 8500 bytes, so you must set the interface MTU to 8500 when all traffic traverses a transit gateway.

Community Votes

D
66%
A
34%

66% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 8 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 8

  1. Momiac5 👍 3
    D Question: "All network connectivity must be through the transit gateway" Docs: Packets with a size larger than 8500 bytes that arrive at the transit gateway are dropped. https://docs.aws.amazon.com/vpc/latest/tgw/transit-gateway-quotas.html
  2. woorkim 👍 2 Answered D
    The correct answer is D because: 8500 bytes is the maximum MTU supported by Transit Gateway Setting the system interfaces to 8500 ensures compatibility Prevents packet fragmentation Maximizes payload size while maintaining network stability Works with the existing architecture including firewalls
  3. 0acf5ca 👍 1 Answered D
    D - All network connectivity between EC2 instances must be through the transit gateway. Although MTU 9001 is enabled on EC2 by default and MTU 8500 is enabled on TGW by default, any frame more than 8500 bytes would be silently dropped by TGW.
  4. 49ca6f2 👍 1 Answered C
    I think answer is C. TGW supports 8500 MTU by default and enforces MSS Clamping on all traffic. Hence if the EC2 is 9001 with Enhanced networking , MSS clamping on TGW will ensure that EC2 sends only 8500 and does not exceeds the MTU of TGW.
  5. a4002bd 👍 1 Answered D
    https://docs.aws.amazon.com/vpc/latest/tgw/transit-gateway-quotas.html D. Packets with a size larger than 8500 bytes that arrive at the transit gateway are dropped.
Show 3 more comments
  1. ArunRav 👍 1 Answered A
    Enabling Jumbo frames and setting up MTU of the network interfaces to 9001 will help to improve the connectivity.
  2. ashk123456 👍 1 Answered A
    To achieve jumbo frame support, the MTU should be set to 9001 bytes, not 8500 bytes.
  3. 304faa7 👍 1
    Correct Answer is D

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