What is Live Migration?
Live migration refers to the process of moving a virtual machine running in a server virtualization environment to another physical server without stopping the operating system or applications. During live migration, the memory image of the target virtual machine is transferred in its entirety to the destination physical server. While a very brief network disruption may occur during the migration, users (clients) of the virtual machine can continue using it without noticing the migration.
For live migration to succeed, the destination physical server must be connected to the same Layer 2 (L2) network as the original physical server. If the destination server is connected to a different L2 network, the subnet address changes, which in turn changes the virtual machine's IP address, preventing seamless communication continuity from the client side. In other words, live migration cannot be performed across routers or Layer 3 (L3) switches.
Technologies like VXLAN (Virtual eXtensible Local Area Network) and NVGRE (Network Virtualization using Generic Routing Encapsulation) address this issue. These are network overlay techniques that virtually create an L2 network over an L3 network, enabling live migration across routers and Layer 3 switches.