Contributed by todd on from the redundant-redundancy dept.
First released in OpenBSD 3.8, trunk(4) is used to aggregate multiple network interfaces as one virtual trunk interface. In English, this means you take two network cards, plug them both into the same (or a different switch) and you continue functioning even if you unplug one. As released in 3.8, the trunk(4) interface implemented a simple roundrobin protocol.
New in -current, there is a failover protocol. This permits specifying one of the network cards as the master, and a secondary one to be used if the primary one is observed to be unplugged. One ingenious use of this new feature is for users who bridge their wireless network and wired networks. One may use a trunk interface to assign the IP address of a laptop. The trunk interface then in turn uses the ethernet interface of the laptop as master, and the wireless interface as the failover port. Unplugging the laptop to go sit in the breeze on the porch permits no connections to be reset, while returning to plugin the wired network permits a download from a local server to greatly increase in speed.
Servers will likely want round-robin which also permits to aggregate bandwidth. This means faster than 100mbit speeds with multiple 100mbit ethernet cards, for example.
With a `super redundant setup' of failover trunking, one can setup a system with 6 nics in each of two machines and several switches that result in a super resilient setup in which no one piece of equipment (including a switch) failing will permit loss of traffic.
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