Link aggregation with 802.3ad

It may be hard to believe, but 1G bit/sec is just not enough bandwidth for many network connections between LAN switches and from switches to high-demand network servers. Along with the bandwidth-consuming applications at ISPs, application service providers, streaming media providers and the like, traditional network administrators may also be feeling the bandwidth pinch at their server connections.

Relief, in the form of standards-based link aggregation technology for LAN switch ports and high-end server network interface cards (NIC), is on the way. Completed in mid-1999, the IEEE 802.3ad standard defines how two or more Gigabit Ethernet connections can be combined to load share, load balance and provide better resiliency for high-bandwidth network connections.

This is not a new concept. Many vendors have supported proprietary "trunking" schemes for 10/100M bit/sec Ethernet and FDDI for a few years. What's new this time is the standardized implementation of link aggregation. 802.3ad means the end of proprietary techniques that have been used to group together multiple slow-speed ports to get a faster point-to-point logical link.

What is link aggregation?

Cisco's proprietary Inter-Switch Link trunking (ISL) and Adaptec's Duralink port aggregation software are just a few of the popular names for aggregation techniques that have been supported by many switch and NIC manufacturers. The basic problem with these techniques, and the reason we now have 802.3ad, is that they were only designed to work with a single brand of network equipment. This meant major headaches for network administrators who needed the faster "aggregated" links but wanted to use a different brand of networking hardware.

As switch and NIC vendors begin introducing support for 802.3ad in their products, you can expect increased compatibility across different NICs and switches, as well as a few other major benefits.

First, the standard applies to 10M, 100M and 1,000M bit/sec Ethernet. Aggregated links can use a combination of these speeds on a single logical link. This increases the options available when you have one remaining gigabit port and three or four 100M bit/sec ports available between switches. You can add link bandwidth incrementally and affordably. Network traffic is dynamically distributed across ports, so administration of what data actually flows across a given port is taken care of automatically within the aggregated link.

As network bandwidth needs continue to grow, scalability is key. This is obviously a good thing for backbone connections that could benefit from the addition of a few more gigabits of throughput.

Don't overlook link aggregation for servers. High-performance servers are now beginning to support network I/O in the gigabit range.

Not only can servers routinely send and receive that much data, there also are a few machines with CPU cycles left over to do some application work. As servers become more capable annd network throughput requirements become more severe, link aggregation allows for expansion, all the way to 8G bit/sec of full-duplex support.

Added robustness

The other major plus is reliability. With link speeds that can reach as high as 8G bit/sec, it would be a disaster to have the link fail. Mission-critical switch links and server connections need to be as dependable as they are capable. They can't just go down if a single cable is mistakenly unplugged. That's where 802.3ad also offers an interesting side benefit.

The link aggregation standard provides inherent, automatic redundancy on point-to-point links. In other words, should one of the multiple ports used in a link fail, network traffic is dynamically redirected to flow across the remaining good ports in the link. The redirection is fast and triggered when a switch learns that a media access control address has been automatically reassigned from one link port to another in the same link. The switch then sends the data to the new port location, and the network continues to operate with virtually no interruption in service.

Today, only a handful of products support actual 802.3ad, so don't expect too much, too soon. However, in the next few months, many manufacturers will introduce link aggregation support. This is something that many network administrators would, and should, want for their network.

This story, "Link aggregation with 802.3ad" was originally published by Network World.

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