When a single internet line fails, sales, phones and cloud applications can all stop together. Learn how to assess different infrastructure, failover, load balancing and 4G/5G backup options.
Email, accounting software, e-invoicing, payment terminals, IP phones and remote access all depend on the internet in most businesses today. When the connection drops, it is not just that web pages won't load: sales stop, phones go quiet and branches can't reach head office. A redundant connection is a second path, put in place so that the failure of a single line doesn't bring all your work to a halt.
Why isn't one line enough?
Internet outages are not always caused by a fault at the operator's core. A cable cut during roadworks, a problem in the building's distribution box, a failed modem, a power cut or a misconfigured setting all have the same result. Some of these faults are fixed in minutes, while repairs involving physical damage can take days.
What an outage costs your business depends on how reliant you are on the internet. Start by answering one question: if the internet were down for half a day, which work would stop and which could wait? If the answer is "almost everything stops", a redundant connection is not a luxury but part of business continuity.
The second line must really be a second path
The most common mistake with redundancy is having two lines that are different on paper but the same on the ground. Taking lines from two different operators may not be enough on its own: some operators deliver their service over another company's infrastructure. In that case, both lines can enter your building through the same cable, the same box and the same route, and a single excavation can cut both.
So when ordering a line, ask: Which infrastructure is the service delivered over? From which direction, and at which point, does the cable enter the building? Which exchange or distribution point does the connection go to? Where possible, choose two lines based on different technologies; for example, one could be fibre and the other a different cable infrastructure, a radio link or a mobile connection. A different route and a different technology reduce the chance of both lines failing for the same reason.
Backup options
| Option | Strength | What to watch |
|---|---|---|
| Second fixed line over different infrastructure and route | Permanent, high-capacity backup | Confirm that the infrastructure and building entry really are separate |
| 4G/5G mobile backup | Independent of cabling, quick to set up | Coverage, indoor signal, data allowance and no fixed IP address |
| Radio link | Independent of underground cables | Needs line of sight, can be affected by weather |
| Second line from the same operator | Partial protection against equipment failure | Shared infrastructure means it often doesn't help when a cable is cut |
The backup line doesn't need the same capacity as the primary. Look at which work must continue during an outage: if payment terminals, phones and the accounting software keep running, large file transfers can wait a few hours. Sizing the backup to this minimum need avoids unnecessary cost.
Ways to use two lines together
Failover (active-passive)
In this setup, all traffic goes over the primary line, and the second line only comes into play when the primary fails. The firewall or router continuously checks whether the primary line is working and moves traffic to the backup when it detects a problem. It is simple to set up and manage, and suits backups with a data allowance, such as a mobile connection.
How the check is done matters. A setting that only looks at whether the modem link is up may miss a case where the modem is working but the internet is down on the operator's side. A method that regularly checks whether a few reliable destinations on the internet can be reached is more dependable. Also decide whether traffic should return to the primary line automatically once it is back.
Load balancing (active-active)
With load balancing, both lines are used at the same time and users' connections are shared between them. The backup line doesn't sit idle, and you make use of the combined capacity. However, this usually works per connection: a single file download won't reach the combined speed of both lines; different connections are simply spread across different lines.
Some applications check which IP address a connection comes from. Online banking, cloud applications restricted by IP address, or VPN tunnels between branches can drop if the address changes mid-session. Define rules that pin this kind of traffic to a specific line, switching to the other line only when the primary fails.
If you aren't notified when traffic switches to the backup line, the primary line can stay faulty for weeks and you will only notice when the backup fails too. Configure the system to send an email or message alert whenever the line changes.
What to watch with a 4G/5G backup
Because it is independent of cabled infrastructure, a mobile connection makes a good backup, but it has some limits. Measure the signal in advance inside the building, at the spot where the device will sit. Basements and buildings with thick walls may need an external antenna.
Mobile lines usually come with a data allowance. During an outage, heavy traffic such as computer updates, cloud backups and video can use it up quickly. Define rules so that only business-critical traffic goes over the backup line. Mobile connections also often lack a fixed, publicly reachable IP address; if you have a server that is accessed from outside or a branch VPN, plan in advance how these services will work on the backup line.
Don't forget single points of failure
Even with two lines, if both are connected to the same firewall, the same network switch and the same power socket, a failure in any of these will still stop everything. Put the modems, firewall and core switch on an uninterruptible power supply (UPS). If an outage would have a major impact on the business, consider deploying the firewall as a high-availability (HA) pair of two devices.
If you have services accessed from outside, the two lines will have different IP addresses, so your domain name (DNS) records and VPN settings need to include the second address too. Otherwise, even with the internet working, no one outside will be able to reach you.
Contracts and testing
Check the service level agreement (SLA) in your line contracts: response time to faults, target repair time and how to report a fault should all be in writing. Keep the account numbers and contact details needed to log a fault somewhere you can reach during an outage.
A redundant setup only inspires confidence once it has been tested. At regular intervals, outside working hours, unplug the primary line and test the switch to the backup. Check that phones, payment terminals, the VPN and critical applications work on the backup line, then watch the return to the primary line. Repeat the test after every configuration change.
Checklist
- Has it been established which work stops during an outage and how long it can wait?
- Has it been confirmed that the two lines use different infrastructure and enter the building by different routes?
- Is the line check based on reaching destinations on the internet, not just on the modem link?
- Are IP-sensitive applications and VPN tunnels pinned to a specific line?
- For a mobile backup, has the indoor signal been measured and has traffic been restricted to protect the data allowance?
- Are the modem, firewall and core switch on a UPS?
- Is an alert sent when traffic switches to the backup line?
- Are failover and failback tested regularly?