What could cause a virtual machine to be unreachable after migration between two ESXi hosts using vSphere vMotion?

The situation described in the question involves a virtual machine becoming unreachable after a migration between two ESXi hosts using vSphere vMotion. The correct cause for this condition is an incorrect VLAN tag on the destination portgroup.

When a virtual machine is migrated between ESXi hosts, it remains logically connected to the same network configuration. If the destination portgroup is improperly configured with the wrong VLAN tag, the virtual machine will not be able to communicate with other devices on the network due to mismatched network settings. VLAN tagging is crucial for ensuring that traffic is routed correctly across different segments of a network. If the tag is different from what the virtual machine expects or needs for access, it will be unable to establish network connectivity, thereby becoming unreachable.

Mismatched MTU settings could also affect network performance or connectivity, but the direct impact of unreachable status is primarily tied to VLAN configuration. Regarding RDMs (Raw Device Mappings) and the usage of different datastores, while these can affect storage accessibility or performance, they do not directly relate to the machine's immediate reachability on the network post-migration.