All physical ports in the same aggregate must appear on a single switch, or even on a single ASIC on this switch, as is the case on an HP Procurve. An ASIC (Application Specific Integrated Circuit) represents a port grouping here.
However, for stackable switches, traffic may be balanced on separate switches.
For example, on the Cisco 3750 switch range, the technology is called Cross-Stack EtherChannel and setting it up requires two switches in a VSS (Virtual Switching System) configuration, allowing both physical switches to run as a logical switch.
Another example is the proprietary protocol DMLT (Distributed Multi-Link Trunking) developed by Avaya.
A highly resilient infrastructure can therefore be implemented, but with significant capital investment on layer 2 hardware (switches), and in terms of administration time. Examples of fault-tolerant infrastructures will be given further on in this document.