Load-balanced BTnet: two lines working together

How load-balanced BTnet works

Load balancing is one of the three resilience options in BT's service schedule for BTnet. BT configures a secondary access line for dual running with your primary access line. Instead of waiting on standby, as in failover, both lines are in use.

1. Normal running. Both BTnet access lines are live and carry traffic, as set up in BT's configuration.
2. One line fails. A fault takes one access line out of service.
3. Traffic moves to the other line. Traffic can flow over the remaining line, subject to there being enough capacity on it.
4. The fault is fixed. The failed line is repaired through BT's normal fault process and returns to dual running.

As with the other options, BT's terms ask you to connect the equipment provided for the secondary line to the same segment of your LAN as the equipment for the primary line.

How traffic is shared between the two lines in normal running depends on the configuration, the routing protocol and the speeds you choose. It is confirmed in the design for your site rather than assumed. Do not plan on the two lines behaving as a single faster connection for every application; plan on two lines, each able to carry traffic.

The capacity question

BT's description carries an important condition: if one line fails, traffic can flow over the other subject to sufficient capacity being available on it. In practice that means the real test of a load-balanced design is the day one line is down.

Suppose a site needs the capacity of both lines on a busy afternoon. If one fails, the other cannot carry everything, and something has to give: calls break up, uploads stall or cloud apps slow. There are a few ways to avoid that:

• Size each line to carry the essentials alone. Work out what must keep running, then make sure either line can carry it. Our bandwidth guide helps you list and size what uses the connection.
• Prioritise on your own network. Quality of service on your router or firewall can keep calls and key apps ahead of bulk traffic such as backups and updates while you are running on one line.
• Leave room to grow. Bandwidth can change within the bearer through Bandwidth Flex, but a bigger change may need a larger bearer. See bearers and port speeds.

If you find that matching both lines is more than you need, a lower-speed back-up line may be the better fit.

Load balancing or failover?

Neither is better in every case. Load balancing suits sites that like the idea of using what they pay for every day and are disciplined about capacity. Failover suits sites that want a clear primary and a clean standby. Both depend on the same physical questions: separate routes, separate entry points and resilient power.

Who load balancing suits

Load-balanced BTnet tends to suit:

• Busy sites with steady, heavy use, where spreading traffic over two lines is useful in normal running
• Sites with a mix of traffic, such as calls and cloud apps alongside large uploads, CCTV or backups
• Head offices and hubs that carry traffic for other sites or many remote users
• Organisations with in-house network skills, or an IT provider, comfortable with routing and quality of service on two lines

If your team runs its own edge equipment on BTnet wires only, plan how that equipment is made resilient too. Two lines into one firewall still leaves one device that can fail.

An illustrative example

The figures that matter are specific to your site, so here is the method rather than numbers. Take a busy office that runs a cloud phone system, Microsoft 365, a video meeting room, nightly backups to the cloud and CCTV that uploads to a cloud service.

In this example, each line would be sized to carry the first three rows on its own, with headroom, while the last rows are allowed to pause. In normal running the site benefits from both lines; during a fault, it keeps working on either. Our bandwidth guide explains how to estimate each row for your own site, and we can review your figures with you before you order.

What load balancing does not cover

Two lines do not protect against a power cut in the comms room, a failure of your own router or firewall, a fault on your office network or an outage at a cloud service. Those need their own plans: resilient power, resilient edge equipment, a sound LAN, and a clear idea of what you will do if a key cloud service is unavailable.

Planning, cost and the SLA

Before you commit, we work through these points with you:

• What each line must carry on its own, and the bandwidth that implies
• Route separation and entry points, confirmed at the site survey
• Power for the lines and equipment, which BT's terms make your responsibility where BT's equipment is on site
• IP addressing, VPNs and remote access, and how they behave when one line is down; see static IP addresses on BTnet
• How you will test that the site keeps working on either line

Under BT's terms, where resilience uses dual access bearers, service introduction charges apply per bearer, including the secondary line, as do cancellation charges if the order is cancelled before the service goes live. BT applies availability credits at the impacted site if a qualifying incident hits a resilient service; see the BTnet SLA. We do not publish prices: the factors are on BT leased line cost.

To size a load-balanced design for your site, contact us and ask us for a quote.