According to the vSphere 4.1 Release Notes, vSphere 4.1 and 4.0 should gracefully co-exist in a Linked Mode configuration. This, however, does not seem to fully be the case. We ran into an issue where we could not mount ISOs within the 4.0 vCenter’s virtual machines; it would simply hang at “Connecting.” A glance at the client logs (C:\Documents and Settings\<Username>\Local Settings\Application Data\VMware\vpx\viclient-*.log) shows the following:

[        :Failed t:P:16] 2010-12-17 02:20:07.811  System.Net.WebException: The remote server returned an error: (501) Not Implemented.
Server stack trace:
at System.Net.HttpWebRequest.GetResponse()
Exception rethrown at [0]:
…
at System.Net.HttpWebRequest.GetResponse()
at VirtualInfrastructure.Utils.WebDownload.GetResponse()

[        :Failed t:P:16] 2010-12-17 02:20:07.811  System.Net.WebException: The remote server returned an error: (501) Not Implemented.Server stack trace:    at System.Net.HttpWebRequest.GetResponse()Exception rethrown at [0]: …   at System.Net.HttpWebRequest.GetResponse()   at VirtualInfrastructure.Utils.WebDownload.GetResponse()

From speaking with VMware support, they believe this is an issue with the client not gracefully communicating to the older vCenter. Basically, it is trying to send functions to the older version that only exist in 4.1. When you have Linked Mode with different versions of vSphere, it prompts (actually, it requires) you to download both vSphere clients, so one would presume this would be for issues like this… unfortunately, that doesn’t seem to happen.

There are a few ways to resolve this:

  1. Unlink the vCenter instances.
  2. Upgrade the 4.0 version to 4.1.
  3. Accept that ISO mounting within vCenter will not work on the 4.0 vCenter, and manually connect to the VM through the host (thus bypassing vCenter) to mount the ISO. Avoiding vCenter “resolves” the issue.

If none of the above work for you, perhaps it is worth hounding VMware Support to resolve this, since it is listed as supported in documentation (Note: Support tried to claim this configuration was not supported during our case, but acknowledged it was after being presented the vSphere 4.1 Release Notes). Their final response was that having 4.1 and 4.0 in tandem was only meant to be a “temporary migration” setup. It wasn’t really worth fighting the good fight here since we are upgrading the 4.0 instance to 4.1 soon anyway.