IIS Application Request Routing(Monitoring and management) Powershell

I am working with iis arr powershell... In my iis i have one web farm and in that i have two servers "server1" & "server2"......

My task is

1. Make server1 unavailable gracefully
2. restart server1 iis
3. make available server1...
4. Make server2 unavailable gracefully
5. restart server2 iis
6. make available server2......

but the problem is after making server1 "unavailable gracefully" and after restarting the server1 iis the "Availability" and "Health Status"(in monitoring and management) is going to "unknown" state then now iam unable to make server1 available its giving exception like

Exception is :

Exception calling "Execute" with "0" argument(s): "The request is not supported. (Exception from HRESULT: 0x80070032)"
At line:19 char:28
+     $methodInstance.Execute <<<< ()
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : DotNetMethodException

can any body know the solution how to make available(if is in unknown state) using powershell

I had the same problem and the culprit seems to be the "unknown" Availability and Health Status.

This is due to not having configured a correct health check for your site in the "Health Test" section, a url that can be tested to detect if the site is online. Once configured the status will change from "unknown" to "Healthy" and you will be able to do a rolling update with zero downtime.

Health Test