Symptoms

During the Image Build process, while the system files are copied from the client’s local hard drive to the virtual disk, the process takes an extended period of time to complete. Opening the virtual disks Status icon in the lower right corner of the System Tray shows a large number of retries (in the thousands). In some instances, the Image Build process may not complete at all.

In the newer versions of certain NICs, hardware vendors have introduced a feature called "Large Send Offload," which is enabled by default. If network switches are unable to support this new feature, Ardence packets may be dropped by the switch, causing the Image Build process to take a long time to complete or not complete at all. To disable this feature, open the Advanced Properties tab of the Network Interface Card (NIC) configuration and disable Large Send Offload.

Resolution

Large Send Offload, also known as Segmentation Offload, allows the host to present an entire buffers worth of data to the NIC even if it is larger than the current Maximum Transmission Unit (MTU) size. The NIC then manages dividing and assembling the data into TCP segments for transmission over the network.

Caution! Using the Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Citrix cannot guarantee that problems resulting from the incorrect use of the Registry Editor can be solved. Use the Registry Editor at your own risk. Back up the registry before editing it.

For some NICs there are no setting in the NIC Properties and they require a registry key change to disable it. If the option is not there add the following entry to the registry:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\BNNS\Parameters\
DWORD = EnableOffload
Value “0”

Note: On some NICs this is listed as TCP/IP Offloading.

More Information

Use of the Large Send Offload feature may result in jumbo packets that are not supported by all network switches. In this scenario, it would be necessary to disable this feature within the Advanced properties of the NIC to allow the Image Build process to run normally.

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