I am trying to configure App Engine Flexible to talk to Container Engine.

It appears that App Engine Flexible is just a container engine that is deployed with some automatic features. And by default, it uses network "default"

For container engine, it is also deployed with in network "default".

And I followed the instruction from https://cloud.google.com/container-engine/docs/internal-load-balancing to configure an internal load balancing ip for the kubernete cluster.

I then checked my appengine instance IP through SSH. The IPs are obviously in the same subnet but I am still unable to access it. However, using external IP is totally fine.

According to: https://cloud.google.com/compute/docs/vpc/

A VPC network is a virtual version of the traditional physical networks that exist within and between physical data centers. A VPC network provides connectivity for your Compute Engine virtual machine (VM) instances, Container Engine containers, App Engine Flex services, and other network-related resources.

So I assume this should work? Any suggestion is greatly appreciated. Thanks in advance!