To stretch a VLAN means to expand its ability to host servers and other devices within the existing infrastructure. Using a VLAN to unite your networks makes it easier for other devices to connect to the network since they all share the same domain.
This concept goes well in hand with Data Center Interconnect (DCI), a networking industry term referring to the bridging of layer 2 domains between geographically distinct sites. Examples of DCI protocol is Cisco’s Overlay Transport Virtualization (OTV) and HP’s Ethernet Virtual Interconnect (EVA).
When do you stretch your network?
V2V Data Center Migrations and long distance workload mobility requirements.
Risks and critics
It might appear counter intuitive to stretch the connection between two sites, which by definition are meant to be isolated, to improve data resilience and avoid complete loss of service ( a network failure in DC1 will propagate to DC2, e.g. bridging loops).
Trying to satisfy DC InterConnectivity can increase complexity, reduce performance (e.g. running a LAN latency based app on a WAN-like network) and availability (what if the connectivity between the sites goes down?), and increase the overall cost of your network.
Shall I use this approach?
Stretching VLan is quite a controversial topic, however knowing your risks ahead and planning to extend your VLAN for a reduced time frame should get you where you want safely.