What could cause a virtual machine to no longer be pinged after migrating between two ESXi hosts using vSphere vMotion?

The reason a virtual machine may no longer be reachable via ping after being migrated between two ESXi hosts using vSphere vMotion is typically due to an incorrect VLAN tag in the destination portgroup. In a virtualized environment, VLAN tagging is crucial for ensuring that network traffic is appropriately segregated. Each portgroup is associated with a specific VLAN ID, and if the migration to a new host involves a portgroup that is configured for a different VLAN than what the VM was connected to on the source host, the VM will not be able to communicate with the network as expected. This means it won't respond to pings since it will be trying to communicate on a VLAN that doesn't match the network configuration it was originally a part of.

In contrast, an MTU mismatch can affect traffic, but this issue tends to manifest in larger packets rather than simply losing ping responses. RDMs being attached may have implications for storage and not necessarily for network connectivity. Using a different datastore should not cause ping issues either, as datastores generally do not affect the VM's network configuration directly. Therefore, an incorrect VLAN tag is the most plausible cause of the connectivity problem in this scenario.