Two ports, no waiting? The pros and cons of link aggregation

So you've unboxed your shiny new NAS drive, and you've noticed that round the back there's not one but two gigabit Ethernet ports. Great news - that means you can use two connections at once for double the bandwidth, right?

Well, yes and no. You may indeed be able to use two cables to get a faster link between the NAS box and the router, using a technology called 802.3ad Link Aggregation. But there are several potential hitches to be aware of.

The first is that not all routers support 802.3ad. It's a business-grade technology that's unlikely to be built into your basic, ISP-provided router. Even many upmarket domestic models don't include it.

And even assuming you can enable 802.3ad, you still won't see faster file transfers. If you think about it for a moment, it's easy to understand why. Your NAS box might now be capable of sending data to the router at an aggregate speed of 2Gb/sec, but the still router only has the same line to your PC as it had before. It certainly won't be faster than 1Gb/sec, and if you're on Wi-Fi it will be quite a bit slower.

What's more, in our tests we found that 802.3ad incurred an additional network overhead, which actually slowed down file transfers. Top speeds typically dropped from around 110MB/sec to around 80MB/sec.

So does 802.3ad have any point at all? Well, yes it does - if you have two clients wanting to access the NAS box at the same time. In this scenario, 802.3ad lets the router automatically split the traffic up across the two ports, so each computer effectively gets its own dedicated gigabit connection, rather than battling over a single link. In an enterprise environment with dozens of clients, that can provide a big bandwidth boost, especially on high-end NAS units with four or more ports.

But for a home NAS, or a small office, you're probably better off without the overhead - even if that does mean tolerating a bit of contention on the rare occasion when two clients make heavy demands of the NAS at the same time.

Load balancing and failover

Another possible use for that second port is load balancing. This is similar to link aggregation, but instead of assigning each client its own Ethernet port, it aims to distribute the total load evenly across both ports. This doesn't make individual file transfers any faster, since there's still a bottleneck between the router and the client, but, like link aggregation, it can speed things up when multiple clients are accessing the NAS appliance at once.

Until a few years ago, load balancing required specific router support, but many NAS units now support a technique called Adaptive Load Balancing (ALB). This should work with any router, with no configuration required - so if your NAS box offers this feature, you might as well give it a try.

A final option for that second Ethernet port is to hook it up to a second switch or router, so that if your main connection should fail for any reason, the NAS unit will "failover" to the second network and remain accessible.

The idea might again sound more suited to a big corporate office than a domestic setting - after all, not many of us keep a backup network running just in case. But let's say you're travelling, and want to be confident that your NAS will remain accessible while you're on the other side of the world.

With a failover configuration, you could invest in a cheap 3G router, stick in a pay-as-you-go SIM, and set it up as your secondary connection. If your router goes pop - or even if your ISP goes down - your NAS unit should automatically switch to the second connection, so your files remain accessible. It goes without saying, though, that you should thoroughly test such a setup before setting off for the airport...

This article originally appeared in PC Pro issue 276