Often times you want the convenience of seemlessly switching over from your wired connection to wireless connection and vice versa. This is where the trunk interface comes into picture.
What is trunk
trunk is a pseudo interface that allows aggregation of multiple network interfaces as one virtual trunk interface(man 4 trunk). Though trunk supports multiple protocols our focus here is on setting it up in failover mode.
In the failover mode, the first interface added will be a master interface and any subsequent interfaces added will be failover interfaces. In this example, I will set up my ethernet interface em0 as master and wireless interface urtwn0 as a failover interface by adding them to trunk0. What this means is whenever I disconnect my ethernet cable from the laptop, it seamlessly switches to wireless.
You will not see an ip address assigned to member interfaces themselves but ip is added to the trunk0 interface.
How to set up trunk
OpenBSD man page for trunk(4) has an example to set it up using command line options. This is good if you just need once or you just want to test it. But you will have to run the commands after every reboot. If you like to make this permanent you can follow below steps.
The below commands assume that your wired interface is em0 and wireless interface is urtwn0.
Bring up the wired interface
# echo up > /etc/hostname.em0
Bring up the wireless interface. You need to replace mynwid with your SSID and replace mypasswd with your wifi password.
# echo nwid mynwid > /etc/hostname.urtwn0
# echo wpakey mypasswd >> /etc/hostname.urtwn0
# echo up >> /etc/hostname.urtwn0
Now we will set up the trunk in a failover mode and add these two interfaces.
# cat > /etc/hostname.trunk0 <<EOF
trunkproto failover
trunkport em0
trunkport urtwn0
dhcp
rtsol
EOF
The dhcp in file indicates to get the ip address dynamically and rtsol will get the ipv6 address.