Building the bulletproof Ethernet

Long popular for its resiliency benefits, the technology of link aggregation is finally about to achieve standardhood.

The IEEE has worked out much of the technical details of link aggregation in its 802.3ad specification, and the group expects to finalize the standard in March.

Link aggregation is a technique for combining two or more Ethernet connections into one logical link, or trunk. For example, a user could set up four 100M bit/sec Fast Ethernet connections running in parallel between two switches, but both switches would handle traffic as if there were a single, 400M bit/sec pipe between them.

A user might do this if the connection requires more than 100M bit/sec, but less than the full gigabit per second of Gigabit Ethernet.

"We didn't have enough of a bandwidth need for Gigabit Ethernet," says Chuck Yoke, manager of technology architecture at Janus, a financial services firm. The company uses Cisco's proprietary link aggregation technology, Fast EtherChannel, to run multiple Fast Ethernet connections from its switches to its servers.

Yoke says he didn't want to install Gigabit Ethernet equipment and fiber optic lines to support the technology when the company had plenty of slower-speed equipment and copper cabling available. "We had open Fast Ethernet ports on the Cisco Catalysts and Ethernet patch cables galore," Yoke says.

Another compelling reason to set up a network this way is for its built-in resiliency. If one of the parallel connections fails, the others can continue to run. In less than a second, a switch can detect that a link has gone down and reassign packet flows to the surviving links (see graphic).

Link aggregation between different vendors' equipment is possible today. So why is a standard necessary?

The switches can only interoperate at a very basic level, explains Cam Cullen, senior technical marketing manager at 3Com. With the standard, the switches would be able to perform higher-level functions, such as catching configuration errors.

Another advantage of having a standard is that third-party network management tools, such as Hewlett-Packard's OpenView, would be able to see aggregated links, Cullen says. This would make troubleshooting easier.

The IEEE 802.3ad specification uses Link Aggregation Control Protocol to verify the link configurations and to send packets to each of the physical links in the trunk. The protocol also provides the mechanism for adding and subtracting links from the trunk.

The upcoming standard works for all speeds of Ethernet, so users could even establish multigigabit links composed of Gigabit Ethernet connections.

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