In networking, high availability (HA) is essential for businesses striving to maintain uninterrupted operations. By ensuring redundancy and failover, HA setups aim to provide resilience against failures. However, lurking within many of these deployments is a silent disruptor: split brain.

This article explains what split brain is, how it can devastate high availability setups—particularly in router and firewall configurations—and why traditional failover methods, like those of MikroTik, can exacerbate the issue. Finally, we’ll explore how Fusion’s SD-WAN architecture delivers an elegant solution to split brain, ensuring continuous and reliable connectivity.

What Is High Availability & Split Brain?

High availability is a system design principle that ensures critical services remain operational during hardware, software, or connectivity failures. Typically, this is achieved by deploying redundant systems, such as firewalls, routers, or links, configured for failover.

However, redundancy isn’t enough to guarantee reliability. When communication breaks down between redundant systems, a dangerous phenomenon called split brain can occur.

What Is Split Brain?

Split brain happens when redundant systems lose communication with each other but continue to function independently. Without coordination, these systems can:

Make Conflicting Decisions: Two systems may assume they’re active, causing conflicting routing or firewall rule enforcement.
Disrupt Traffic Flows: Active-active misalignment may lead to dropped or misrouted packets.
Compromise Security: Conflicting firewall states can leave critical vulnerabilities exposed.

In HA setups, split brain is often triggered by link flaps, congestion, or last-mile outages, creating chaos in what should have been a seamless failover system.

The Pitfall of Using Traditional Failover Methods Like MikroTik

Many businesses attempt to implement last-mile link redundancy using traditional failover solutions, such as those offered by MikroTik routers. While MikroTik’s failover abilities are robust for basic redundancy, they are prone to split brain in high availability setups. Here’s why:

1. No Central Arbitration

Failover in MikroTik and similar systems relies on local logic at each site. If one link fails, the router shifts traffic to the backup link. However, if communication between sites is disrupted, both ends may misinterpret the situation, resulting in:

Both routers marking different links as active, leading to split brain.
Inconsistent routing decisions that disrupt site-to-site communication.

2. Misdiagnosing Failures

With link redundancy at the last mile, a router may mistakenly assume an upstream issue is local. This results in:

Repeated failovers back and forth (flapping), which destabilises the network.
Outages that are incorrectly attributed to local issues instead of upstream problems.

3. Site Outages Due to Split Brain

For example, a MikroTik setup with two redundant links may experience a failure on the primary link. If the backup link activates without proper coordination, the router at the other site may also initiate its backup, causing conflicting routes. The result? A site-wide outage despite having redundant links—the very problem redundancy is supposed to prevent.

Why Traditional Firewalls & Routers Fall Short

Even in high-end setups with firewalls and routers configured for HA, most solutions fail to address split brain effectively. These configurations lack:

Central Arbitration: No neutral entity to resolve conflicts.
Proactive Synchronisation: Inconsistent states during failover lead to race conditions.
Advanced Diagnostics: Limited visibility into root causes like link flaps or upstream issues.

Without these safeguards, traditional HA setups are reactive at best, leaving businesses vulnerable to downtime during critical moments.

Fusion SD-WAN | The Ultimate Solution to Split Brain

Fusion’s SD-WAN architecture provides a smarter, more reliable approach to high availability, addressing the shortcomings of traditional setups. By integrating central arbitration and intelligent failover mechanisms, Fusion eliminates the risks of split brain entirely.

1. Hub-and-Spoke Architecture

Fusion’s SD-WAN is built on a hub-and-spoke model, where:

Aggregators (Aggs) act as central hubs for connectivity and routing.
Edges serve as spokes, connecting sites to the central hub.

This architecture inherently prevents peer-to-peer communication issues that are common in traditional setups.

2. Central Arbitration via Management Servers

The secret to Fusion’s reliability lies in its management servers, which serve as the arbitrators in the network. When a failure occurs, the management servers:

Resolve Conflicts: Ensure only one link or device assumes the active role.
Synchronise Routing States: Maintain consistent configurations across the network.
Intelligently Redirect Traffic: Automatically reroute traffic to available paths without causing flapping or split brain.

3. Resilient Last-Mile Connectivity

Fusion SD-WAN’s failover mechanism is designed to handle even the most complex last-mile scenarios:

If one link fails, traffic is seamlessly shifted to the backup link.
Failover decisions are coordinated centrally, ensuring no conflicting states arise.
Unlike traditional setups, Fusion provides visibility into fibre continuity, signal levels, and upstream diagnostics, enabling faster troubleshooting.

4. Continuity and Reliability

Even in extreme cases of link flaps or upstream issues, Fusion’s SD-WAN ensures that businesses remain online. The hub-and-spoke design, combined with arbitration, guarantees that traffic flows smoothly, avoiding the chaos of split brain.

Why Fusion SD-WAN Matters

In today’s always-on world, downtime isn’t just an inconvenience—it’s a business killer. Traditional failover methods, like those of MikroTik, fall short in high availability environments, exposing businesses to split brain and the resulting outages.

Fusion SD-WAN offers a future-proof solution that ensures uninterrupted connectivity, no matter what challenges arise. With its unique architecture, central arbitration, and advanced diagnostics, Fusion eliminates split brain and provides businesses with the reliable, high-performance network they need to succeed.

Key Takeaways

Split brain is a silent disruptor that undermines high availability by causing conflicting states in redundant systems.
Traditional failover methods, such as MikroTik, lack the safeguards needed to prevent split brain, leading to site-wide outages.
Fusion SD-WAN’s hub-and-spoke architecture, management servers, and intelligent failover eliminate split brain and ensure seamless connectivity.
By adopting Fusion SD-WAN, businesses gain unparalleled reliability, visibility, and control, transforming their networks into resilient, high-performance environments.

For businesses ready to leave split brain and downtime behind, Fusion’s SD-WAN is the ultimate solution.

🚜The Silent Killer of High Availability | How Split Brain Can Cripple Your Network & Why Fusion's SD-WAN Is the Ultimate Solution🎯