Citrix NetScaler virtual machine Configured in HA pair stopped working after upgrading ESX 4.1 to ESXi 5.1 (2323236)

Customer reported that Couple of Virtual Machines running the Citrix NetScaler VPX application configured in HA pair has stopped working all of a sudden and has asked me if there was something that has been changed at the ESX Host Networking recently.
The reason for this question was that as soon as he enables HA in those virtual Machines the HA is crashed and they were no longer pinging to each other. They have tried all the possibilities at their front and to their bad luck none has worked. This is without knowing that the ESX environment was upgraded to ESXi 5.1 and asked me if I can help in this regard.
Since I have migrated the whole environment from ESX 4.1 to ESXi 5.1 Update 3 (2323236), I quickly google’d the issue (Netscaler VPX + ESXi 5.1) and found the following solution which has helped fixing them in the matter of 5-10 minutes.
To work around this issue use one of these options: 
  • Install NetScaler VPX appliance on a previously known good VMware version and build.
  • Add the line hw.em.txd=512 in the loader.conf.local file.
 To add the line hw.em.txd=512 in the loader.conf.local file:
1. Log in to the Citrix NetScaler virtual machine appliance as root.

2. Locate the loader.conf.local file on the NetScaler virtual machine appliance by running this command:

find / -name loader.conf.local

Note: The loader.conf.local file may not exist. If so, create this file under directory /flash/boot.

3. Add this line in the loader.conf.local file.

hw.em.txd=512

4. Save the changes.
5. Restart the NetScaler virtual machine appliance.
Source(KB Article):

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