OpenBSD Interface Bonding

The following is used on OpenBSD since version 3.9 to combine two physical interfaces (fxp1, fxp2) into a single virtual interface (trunk0). This method allows one to take the feeds from a traditional two-output tap and present a single virtual interface to NSM applications.

# ifconfig fxp1 up
# ifconfig fxp2 up
# ifconfig trunk0 trunkport fxp1 up
# ifconfig trunk0 trunkport fxp2 up
# ifconfig trunk0 trunkproto roundrobin up

To make this configuration permanent between reboots:

echo "up" > /etc/hostname.fxp1
echo "up" > /etc/hostname.fxp2
echo "trunkproto roundrobin trunkport fxp1 trunkport fxp2" > /etc/hostname.trunk0

Remember to replace fxp1 and fxp2 with the network interfaces on your OpenBSD system (e.g., em0, xl0, rl0, etc.).

OpenBSD 3.8 only supported the round robin trunk protocol.

References

trunk(4)
I believe I first heard of this technique here.