Unable to assign public IP from different resource group to load balancer

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Is this a request for help?: request

Is this a BUG REPORT or FEATURE REQUEST? (choose one): BUG REPORT

Orchestrator and version (e.g. Kubernetes, DC/OS, Swarm) Kubernetes v1.7.9

What happened: I reserved a static public ip in one resource group rg1, let's say 1.2.3.4 and wanted to use assign this to the load balancer of my k8s cluster which is running inside another resource group rg2. I deployed the according k8s service service1 with the following specs:

  type: LoadBalancer
  loadBalancerIP: 1.2.3.4

When applying this spec, the EXTERNAL-IP field stays in <pending> and when calling describe on that service, I see the following message:

  Warning  CreatingLoadBalancerFailed  1m (x8 over 36m)  service-controller  Error creating load balancer (will retry): Failed to create load balancer for service default/service1: user supplied IP Address 1.2.3.4 was not found

When moving the ip into rg2, it will eventually be assigned to the load balancer:

  Normal   CreatingLoadBalancer        34m (x9 over 1h)  service-controller  Creating load balancer
  Normal   CreatedLoadBalancer         32m               service-controller  Created load balancer

What you expected to happen: I would like to be able to assign an ip from one resource group to the load balancer in another resource group.

How to reproduce it (as minimally and precisely as possible): Create static public IP in different resource group and try to assign it to one of your services.

Anything else we need to know: The cluster was created using acs-engine v0.9.4 and is deployed inside a custom VNET.