For an AWS VPC site cluster of 3 nodes (1 CE in each AZ), when defining an origin pool with the following details:
Type: DNS Name of Origin Server on given Sites
DNS Name: <AWS NLB FQDN, e.g. my-nlb-123456789012.us-west-2.elb.amazonaws.com>
SIte: <AWS VPC site>
The CE resolves the NLB FQDN to multiple IP addresses, 1 for each AZ in which the NLB is present. E.g.
$ dig my-nlb-123456789012.us-west-2.elb.amazonaws.com +short
10.0.1.100 <-AZ A
10.0.2.50 <- AZ B
10.0.3.56 <- AZ C
Traffic is then load balanced from CE to each of the NLB IPs, which can lead to inter-AZ traffic flow that comes with a cost (Inter-AZ data transfer is free for ALB, not NLB). E.g. CE in AZ-A can send traffic to NLB IP in AZ-B or AZ-C
I have a customer who wants to reduce this inter-AZ data transfer cost by having the CE prefer NLB IPs in the same AZ. AWS provides a mechanism to select NLB in certain AZs by prefixing the NLB domain with the AZ name, e.g. us-west-2a.my-nlb-123456789012.us-west-2.elb.amazonaws.com
Example implementation could be:
User provide an NLB fqdn my-nlb-123456789012.us-west-2.elb.amazonaws.com
User toggle "prefer local AZ"
Multiple endpoints are created for the NLB FQDN - 1 for each AZ, e.g.
each CE (hopefully being aware of the AZ it's in), will assign higher weight or preference the endpoint in the same AZ