Conditional cross-zone loadbalancing

Question

Hello,
with the latest enhancements on TGW appliance mode (https://aws.amazon.com/de/blogs/networking-and-content-delivery/performance-and-metrics-enhancements-for-aws-transit-gateway-and-aws-cloud-wan/) a TGW will now keep the local AZ if src and dst are in the same AZ and the inspection VPC also has an attachment there (i.e. it will not add an AZ-hop in that case).

Inside my Inspection VPC I still might have a potential (unnecessary) AZ-Hop because I use cross-zone LB on the GWLB (because I have two FWs in total but only one per AZ).

I could imagine a new feature to have cross-zone LB enabled but only under the condition that the local-AZ does not have a healthy target in the TG. This way we would have HA across AZs but for normal cases use the shortest path and avoid latency and cross-AZ data transfer cost.

Is such a feature realistic, has anyone though about this?

re:Post Agent · Answer

While AWS hasn't officially announced a feature for conditional cross-zone load balancing as you've described, your idea is an interesting one that aligns with AWS's general approach to optimizing performance and reducing costs.

Currently, cross-zone load balancing for Gateway Load Balancers (GWLB) distributes traffic across all registered and healthy targets, regardless of which Availability Zones (AZs) these targets are in. This can indeed lead to unnecessary AZ hops and associated inter-AZ data transfer charges in some scenarios.

Your proposed feature of enabling cross-zone load balancing only when the local AZ doesn't have a healthy target in the target group is an intriguing concept. It would potentially offer the best of both worlds - maintaining high availability across AZs while minimizing latency and cross-AZ data transfer costs under normal conditions.

This approach would be particularly beneficial in setups like yours, where you have two firewalls in total but only one per AZ. It could help ensure that traffic stays within the same AZ when possible, leveraging the recent enhancements to Transit Gateway (TGW) appliance mode that you mentioned.

While such a feature isn't currently available, it's not unrealistic to think AWS might consider implementing something similar in the future. AWS regularly introduces new features and optimizations based on customer needs and feedback. The idea aligns well with AWS's focus on performance optimization and cost-efficiency.

In the meantime, you might want to carefully weigh the trade-offs between enabling cross-zone load balancing (for better fault tolerance) and disabling it (for potentially lower latency and costs) based on your specific use case and requirements. You could also consider submitting a feature request to AWS support or through their feedback channels to bring attention to this idea.

Remember that while this feature isn't currently available, AWS continues to evolve its services. It's always worth keeping an eye on AWS announcements and documentation for potential future enhancements in this area.  
**Sources**  
[Using cross-zone load balancing with zonal shift | Networking & Content Delivery](https://aws.amazon.com/blogs/networking-and-content-delivery/using-cross-zone-load-balancing-with-zonal-shift/)  
[GWLB + Cross AZ Charges | AWS re:Post](https://repost.aws/questions/QUcorq4fpiQk-DaC2dGX-nkA/gwlb-cross-az-charges)

Conditional cross-zone loadbalancing

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