From Rob Hoes:
To isolate network traffic between VMs (e.g. for security reasons) one can use VLANs. The number of possible VLANs on a network, however, is limited, and setting up a VLAN requires configuring the physical switches in the network. GRE tunnels provide a similar, though more flexible solution.
The wiki document at http://wiki.xensource.com/xenwiki/Tunnelling proposes a design that integrates the use of tunnelling in the XenAPI. The design relies on the recent introduction of the Open vSwitch in XCP, and requires an Open vSwitch (OpenFlow) controller (further referred to as “the controller”) to set up and maintain the actual GRE tunnels. The following series of patches implements the XAPI part of this design.
Feedback is appreciated!