[libvirt-users] creating a network on existing bridge

Hello, Simply I want to create a network on an existing bridge. My XML: <network> <name>nn1</name> <bridge name="br-nn1" /> </network> when i want to start network I got "File exist" error. error: Failed to start network nn1 error: Unable to create bridge br-nn1: File exist I could define on older releases but now I can't. Ubuntu 15.04 / libvirt version: 1.2.12 virsh version: Compiled against library: libvirt 1.2.12 Using library: libvirt 1.2.12 Using API: QEMU 1.2.12 Running hypervisor: QEMU 2.2.0 Is there any way to activate network? Regards.