GNS3 server is running on a VM (ESXi host). Second interface eth1 connects from the VM to the default vSwitch0. The interface tap0 is a loopback interface on the GNS3 server, which I used to connect into my GNS3 topology. Interface br0 bridges the tap0 interface to interface eth1.
gns3@gns3vm:~$ cat /etc/tap0up.sh
#!/bin/bash
#Set TAP0
tunctl
#Configure TAP1
ifconfig tap0 up
root@gns3vm:/home/gns3# cat /etc/rc.local | grep sudo
sudo sh /etc/tap0up.sh
root@gns3vm:/home/gns3# sudo cat /etc/network/interfaces
# Bridge between tap0 and eth1
auto br0
iface br0 inet manual
bridge_ports tap0 eth1
bridge_stp off