To use multiple physical links as a single large link, you need a way to distribute traffic between the links. OpenBSD supports five different ways to distribute frames between trunk members, though not all will work in all environments. For a complete list see trunk(4), but the protocols I recommend for real-world use are Link Aggregation Control Protocol (LACP), roundrobin, and failover. LACP is the industry standard for link aggregation. The physical interfaces are bonded into a single virtual interface with roughly the same bandwidth as the sum of the individual interfaces. LACP is very fault-tolerant, and just about every high-end managed switch should support it. If your switch supports LACP, use it, but you must configure LACP on the switch ports before this kind of trunk will pass traffic.

In the roundrobin method, OpenBSD sends frames across the trunk’s active connections using a roundrobin scheduler. The trunk accepts incoming packets on any port, and a roundrobin scheduler rotates between the trunk connections, with error and edge handling added on top. Roundrobin trunks don’t need any special switch configuration; they just need two ports in the same VLAN.

In the case of failover, OpenBSD sends and receives all traffic over the first port in the trunk, and if that port fails, it switches to another active port. The failover method doesn’t give you any additional bandwidth, but requires absolutely no support from the switch, and it even works on old-fashioned hubs.

As an example, let’s configure ports em0 and em1 into failover trunk trunk0. The underlying ports have never been configured before, so begin by activating these interfaces without any configuration.

# ifconfig em0 up
# ifconfig em1 up

Now create the failover trunk with ifconfig(8) and add these ports to it to make the trunk0 interface usable.

# ifconfig trunk0 trunkproto failover
# ifconfig trunk0 trunkport em0
# ifconfig trunk0 trunkport em1

You could do this all in one long ifconfig command, but I find simpler, shorter commands easier to understand when learning.

Assign the interface an IP address just as you would a physical interface, and add a default gateway to your system.

# ifconfig trunk0 192.0.2.8 netmask 255.255.255.0
# route add default 192.0.2.1

You should now have a failover trunk attached to your local network. To configure another trunk protocol, just specify the desired trunk protocol when you create the trunk. You’ll find a complete list of trunk protocols in trunk(4).

Configure your trunk in /etc/hostname.if. For example, suppose you need to edit hostname.em0, hostname.em1, and hostname.trunk0. Both of the em files contain only a single word:

up

This activates the interfaces, but does no configuration.

hostname.trunk0 is more complicated.

trunkproto failover
trunkport em0
trunkport em1
192.0.2.8 netmask 255.255.255.0

You can put all of these entries in a single line, just as you can configure the trunk with a single ifconfig command, but again, I find multiple lines easier to read and understand.

Your trunk should now start at boot.

Note that trunks do not necessarily need to consist of interfaces that use the same type of physical medium. If you’re feeling adventurous, you could try to replicate what some OpenBSD developers and users have been known to do: Trunk together a wired and a wireless network interface, and have all your connections survive (graceful failover, remember?) when you yank the plug out of your Ethernet port, or if you plug yourself back in and take your access point down for maintenance.