cross-AZ data transfer costs at the TGW layer

Question

Dear Team - i was going through below post 
**https://repost.aws/questions/QUpnNmoU-LTOmQ_3jr18OEKg**

As per Gen-AI response,

Transit Gateway does maintain zonal affinity by default. Traffic entering TGW in AZ-A will exit to the same AZ in the destination VPC when possible. This means that traffic from your ALB in AZ-A will be routed to the NLB's ENI in AZ-A, assuming there's a TGW attachment in that AZ. **This behavior helps minimize cross-AZ data transfer costs at the TGW layer.**

i wanted to double check on last highlighted sentence.  Is the above behavior really help tp minimize cross-az data transfer charge at TGW layer ? if traffic entering AZ-A and going to AZ-B, will there be cross-AZ charge at TGW layer ?

Ref # https://aws.amazon.com/about-aws/whats-new/2022/04/aws-data-transfer-price-reduction-privatelink-transit-gateway-client-vpn-services/

Thanks Team.

Leo K · Accepted Answer

@JD, yes, your interpretation is correct and the wording by the AI bot is misleading. There are no cross-AZ data transfer charges for any traffic between a TGW attachment in one AZ and a resource in a different AZ, or a resource in one AZ sending traffic through the TGW to a resource in a different AZ. The appliance mode doesn't affect costs.

The traffic costs are covered by the USD 0.02/GB data processing fee that the TGW charges for all traffic sent to it in any way other than by another TGW through a TGW peering attachment. In the latter case, the data processing fee will have been charged already by the peered TGW (or a third TGW possibly chained behind multiple TGW peering attachments).

Taz · Answer

Transit Gateway maintains zonal affinity to keep packets within the same Availability Zone whenever both attachments have ENIs in that zone. When traffic enters the TGW through an attachment in AZ‑A and exits to another attachment also in AZ‑A, it stays local and avoids inter‑AZ data transfer inside the TGW infrastructure. However, if the destination attachment does not have an ENI in AZ‑A, or if routing sends the flow to an attachment hosted only in another zone (for example AZ‑B), the packet will cross AZ boundaries inside the TGW, and AWS charges standard inter‑AZ data transfer rates in addition to the per‑GB TGW data processing fee.
—Taz

JD · Answer

Above GenAI Answer states **" When Appliance Mode is enabled on a Transit Gateway attachment, AZ affinity is no longer maintained, and traffic is allowed to cross AZs. However, as announced in April 2022, you will not incur any inter-AZ data transfer charges in this specific scenario (when using Appliance Mode with Transit Gateway)."**

However, as per 2022 announcement

**AWS Transit Gateway: All data transfer charges across Availability Zones (AZs) over private IP address within the same AWS Region via Transit Gateway are now free of charge. **

as per above, irrespective of appliance mode, weather it is enabled or not, there is no cross-az charges at TGW layer....

someone pl correct my understanding

re:Post Agent · Answer

Yes, the statement is correct. Transit Gateway's default behavior of maintaining Availability Zone (AZ) affinity does help minimize cross-AZ data transfer charges.

By default, Transit Gateway preserves AZ affinity, meaning that traffic entering the Transit Gateway in one AZ will exit in the same AZ when possible. This design helps reduce cross-AZ data transfer costs because traffic doesn't need to traverse between different AZs unnecessarily.

When traffic stays within the same AZ throughout its journey (from source to Transit Gateway to destination), you avoid the standard inter-AZ data transfer charges that would otherwise apply when traffic crosses AZ boundaries.

It's worth noting that this default behavior can be modified with the "Appliance Mode" setting. When Appliance Mode is enabled on a Transit Gateway attachment, AZ affinity is no longer maintained, and traffic is allowed to cross AZs. However, as announced in April 2022, you will not incur any inter-AZ data transfer charges in this specific scenario (when using Appliance Mode with Transit Gateway).

This is particularly relevant for architectures using security appliances or inspection services where maintaining flow symmetry is important, such as when using Gateway Load Balancer for traffic inspection.  
**Sources**  
[GWLB + Cross AZ Charges | AWS re:Post](https://repost.aws/questions/QUcorq4fpiQk-DaC2dGX-nkA/gwlb-cross-az-charges)  
[egress inspection   Firewall appliance | AWS re:Post](https://repost.aws/questions/QUP7CURzNMTTSp4IAArVWGcQ/egress-inspection-firewall-appliance)  
[Hyperplane TGW latency | AWS re:Post](https://repost.aws/questions/QUomd1wdNYTPOMq8QUpSQcnQ/hyperplane-tgw-latency)

cross-AZ data transfer costs at the TGW layer

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