Backing up and load balancing access links

Today, we’ll finish up our discussion of high-availability reinforcement options in branch offices. For protecting the last-mile access link (generally the most vulnerable segment of the network), most branch-office router makers offer dial and ISDN backup, which doesn’t give you much bandwidth but is usually affordable. It works well, for example, for protecting low-speed point-of-sale information transfers from retail sites.

However, dial/ISDN might not be sufficient for mission-critical locations or larger locations where the impact of an outage, even for a short time, costs a lot in lost revenue, productivity or customers. In such cases, you might wish to buy a second dedicated WAN link with the same or slightly lower bandwidth, depending on budget.

This alternative requires no manual intervention, and the downtime experienced is the time it takes for the Layer 3 protocols to converge (unless you are running a virtual router protocol in each router, as discussed last time, which enables multiple routers to look like one big router, speeding things up).  Speed of the secondary link is limited only by how much you’ll spend on redundancy.

To decide on the appropriate backup approach, you should try to determine the monthly recurring cost of that second link vs. the cost per minute or hour of downtime to the organization, if that is calculable. Infonetics Research, for example, reports that an enterprise of 1,000 employees or more averages 3.7 hours of WAN outages and 3.4 hours of service degradation per month. Overall, network downtime and service degradation costs companies of this size $32.7 million each year in lost revenue and productivity of networked employees, the researcher says.

If you are doing a one-for-one link backup, check with your router vendor to see if there is support for a load-balancing mechanism so that both links are functioning at all times. If one fails, the other is still in place to back it up. Otherwise, the back-up link is idle most of the time, awaiting a disaster and wasting capacity. Load-balancing across the two circuits will provide a better cost-justification for installing the second circuit.