Handling HCX OSAM migrations of VMs that have loopback adapter

Products

• VMware HCX

Issue/Introduction

• A loopback adapter is a virtual network interface that enables a computer to run and test network applications and services internally. It simulates a network connection without the need for a physical network card or cable, allowing for self-communication within the OS. The default MAC address assigned to loopback adapters is 02:00:4C:4F:4F:50.
  
  If HCX migrates multiple VMs to the same destination portgroup in the cloud, each configured with a loopback adapter that has the same MAC address, it can cause a significant impact on the network. This scenario may result in a complete network outage for the portgroup since HCX treats loopback adapters as standard network interface cards.
  
  The situation can escalate into a "broadcast storm," where continuous flooding of the network with unnecessary traffic saturates it, consuming all available bandwidth. Overwhelming resources can severely degrade performance, destabilize the network, and ultimately render the network segment unusable.

• In the Windows OS, the loopback adapter may appear like the following:
  
  
  
• In the Migration Group wizard, you see two vNICs assigned to the VM:
  
  
  
  

Environment

VMware HCX

Cause

Loopback adapters facilitate internal testing of network applications and services by creating a virtual interface within the operating system, often assigned the MAC address 02:00:4C:4F:4F:50 in Windows Machines. As part of its intended migration behavior, HCX Manager recognizes and processes these virtual adapters as standard network interfaces. Therefore, it is crucial for customers to exercise caution and pay close attention to their deployment, as introducing multiple loopback adapters with identical MAC addresses into the same portgroup can trigger a broadcast storm, leading to severe network disruption or outage.

Resolution

As a recommendation, please proceed with the following:

• If feasible, remove the loopback adapter in the source virtual machine before the migration
• In HCX Manager UI > Migration > New Mobility Group > Select the destination portgroup for the loopback adapter to "None":
  
  

Additional Information

The following article detail issue encountered due to usage of loopback network adapter : HCX OSAM migration switchover failed for Windows VM with error - Failed to perform fix-up operation on Migrated VM. Second Stage fixup failed