Below are my findings:
In order to recognize routing targets IIS has to be able to process SPPING HTTP method
To test run this code in Powershell:
$url = "http://your-Routing-Target-Server-Name"
$myReq = [System.Net.HttpWebRequest]::Create($url)
$myReq.Method = "SPPING";
$response = $myReq.GetResponse();
$response.StatusCode
If you get the following error message:
Exception calling "GetResponse" with "0" argument(s): "The remote server returned an error: (405) Method Not Allowed."
that means that web front end is not set up to process SPPING HTTP method
To resolve the issue run the following commands on each routing target server:
Import-Module WebAdministration
add-WebConfiguration /system.webserver/handlers "IIS:\" -value @{
name = "SPPINGVerbHandler"
verb = "SPPING"
path = "*"
modules = "ProtocolSupportModule"
requireAccess = "None"
}
This will add a handler for SPPING verb to IIS configuration.
Run the test script again to make sure this works.