A switch's port can either be a simple access port, an up/downlink to a switch or hub, a trunk port for VLAN transport, or a member of a Fast/Giga EtherChannel port group. A UNIX gateway can be connected to another one via a crossover link, can be connected to a switch, can form a VLAN trunk to another trunking-capable neighbor, or can form high-bandwidth multiport EtherChannel connections.
EtherChannels are often constructed with dedicated dual or quad Fast Ethernet NICs and are able to transport and trunk VLANs. At the time of this writing, they offer an alternative to Gigabit Ethernet NICs, especially with low-end 32-bit servers that might have difficulties feeding Gigabit Ethernet. Nevertheless, it is perfectly feasible to use four isolated NICs of good quality. The only requirement is that the other side of the link is EtherChannel-capable as well. As time passes, we will see a similar feature for Gigabit Ethernet gaining momentum. In general, you will come across one of the following EtherChannel or EtherChannel-like implementations:
FreeBSD EtherChannel kernel patch, which supports Cisco Fast EtherChannel (via the Netgraph facility). Two or four ports can be combined into a single aggregate interface.
Linux Ethernet channel bonding.
Cisco Proprietary Fast EtherChannel (featuring PAgP, or Port Aggregation Protocol).
The IEEE 802.3AD link aggregation standard (featuring LACP, or Link Aggregate Control Protocol).
Solaris Ethernet trunking.
Proprietary drivers for dedicated dual and quad interfaces with configuration utilities.
Useful EtherChannel-related links are presented in the "Recommended Reading" section at the end of this chapter.