The Network Extension Service extends a Layer 2 network from the on-premises vSphere data center to vCloud Air.

The Layer 2 Concentrator virtual service appliances make extension possible. The Layer 2 Concentrators have two interfaces:

Internal trunk interface: Handles virtual machine traffic on-premises for the extended networks using a translational bridge mapping to a corresponding stretched network in vCloud Air.

Uplink interface: Hybrid Cloud Manager uses this interface to send encapsulated overlay traffic to and from vCloud Air. Application data travels through this interface.

As shown in Comparing Layer 2 Concentrators, the two concentrators are based on different technology. The newer high-throughput L2C is a much larger appliance, as described in Virtual Machine Resource Requirements.

Comparing Layer 2 Concentrators


Standard L2C



TCP/SSL, Internet, or Direct Connect

IP SEC, Direct Connect

IP Address

Can use default non-routable IP in a fleet deployment, or a routable IP address

Requires routed management IP address in all deployments.

NAT rules



Proximity routing



Jumbo frame support



Max connections per vLAN



Layer 2 Appliance Position in the Single-Path Deployment

The configuration wizard's default strategy is to define the Cloud Gateway, then place the Layer 2 Concentrator "behind" the Cloud Gateway. The WAN optimizer bootstraps from the Cloud Gateway so it uses the gateway’s address by default, as shown in Figure 1.

If the appliance is a standard L2C, this "single-path" strategy routes all traffic through the Cloud Gateway and uses only one IP address. Internally, the Layer 2 Concentrator is assigned a non-routable IP address, so the IP address assigned to the Cloud Gateway is the only one consumed.

If you are using an HT L2C, you must assign a routable IP address, therefore two IP addresses are consumed.

Layer 2 Appliance Position for an Alternate Path

If you have a single-path deployment you can add multiple "standalone" appliances, where an L2C with a unique IP address forms a separate edge. Standalone appliances can be used to route vLANs through the cloud gateway, to extend a network to vCloud Air, or to carry communication between virtual machines.

If you use an HT L2C to extend a network, each vLAN can have only one HT L2C connection. A Direct Connect private line is a typical.

The standard L2C permits multiple connections, for scaling out a deployment. See Network Extension.