Sophos Weight Calculation Tool
Optimize your Sophos Firewall Weighted Round Robin (WRR) configuration for multi-WAN setups.
Load Balancing Weight Calculator
(Link 1 : Link 2)
Configuration Table
| Interface | Speed (Mbps) | Sophos Weight (Integer) | Traffic Share |
|---|
Traffic Distribution Visual
*Chart represents the proportional traffic distribution based on calculated weights.
What is Sophos Weight Calculation?
Sophos weight calculation is a critical process for network administrators configuring Weighted Round Robin (WRR) load balancing on Sophos XG and XGS firewalls. When an organization utilizes multiple Internet Service Provider (ISP) links—often referred to as a Multi-WAN setup—the firewall must decide how to distribute outgoing traffic across these connections.
Unlike simple failover, which only uses a backup link when the primary fails, weighted load balancing actively uses all available bandwidth. The "weight" is a numerical value assigned to each gateway that dictates the proportion of traffic it should handle relative to others. Correctly calculating these weights ensures that a 100 Mbps fiber line handles twice as much traffic as a 50 Mbps DSL line, preventing saturation of slower links while maximizing the return on investment (ROI) for faster connections.
Sophos Weight Formula and Explanation
The core principle of the sophos weight calculation is establishing a ratio of bandwidth capacities. The firewall does not inherently know the speed of your ISP links; the administrator must define this relationship using integer weights.
The formula to determine the weight ($W$) for a specific link ($i$) is:
Weight_i = (Bandwidth_i / GCD_of_all_Bandwidths)
Where GCD is the Greatest Common Divisor. Alternatively, for simpler configuration, you can use normalized percentages or direct integer mapping (e.g., 10 for 10Mbps, 100 for 100Mbps), provided the values do not exceed the firewall's maximum allowed input (typically 100).
Variable Definitions
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Bandwidth | Throughput capacity of the link | Mbps / Gbps | 10 – 10,000 |
| Weight | Configuration value for load balancing | Integer | 1 – 100 |
| Traffic Share | Percentage of total sessions assigned | Percentage (%) | 0 – 100% |
Practical Examples
Example 1: Asymmetric Connections
Scenario: A corporate office has a primary Fiber line at 200 Mbps and a backup Cable line at 50 Mbps.
- Input: Link 1 = 200, Link 2 = 50
- Calculation: The ratio is 200:50, which simplifies to 4:1.
- Result: Assign Weight 4 to the Fiber gateway and Weight 1 to the Cable gateway.
- Outcome: For every 5 connections, 4 go through Fiber, 1 goes through Cable.
Example 2: Equal Load Balancing
Scenario: A branch office uses two identical 100 Mbps lines for redundancy.
- Input: Link 1 = 100, Link 2 = 100
- Calculation: The ratio is 1:1.
- Result: Assign Weight 1 to both gateways.
- Outcome: Traffic is split exactly 50/50, doubling the effective throughput to 200 Mbps total.
How to Use This Sophos Weight Calculator
- Identify Bandwidth: Check your ISP service level agreements (SLAs) to determine the upstream/downstream speeds of all your WAN links.
- Enter Values: Input the speeds into the "Link 1" and "Link 2" fields above. Use the optional field for a third line if applicable.
- Review Ratio: Look at the "Recommended Weight Ratio". This is the simplified integer format.
- Configure Firewall: Log in to your Sophos Admin Console, navigate to Network > WAN Link Manager, and edit each gateway. Enter the calculated integers into the "Weight" field.
- Analyze Distribution: Use the chart to visualize how traffic will be split. If the "Secondary Link Load" is too high for a metered connection, manually reduce its weight.
Key Factors That Affect Sophos Weight Results
While the raw bandwidth calculation is the starting point, several financial and technical factors should influence your final configuration:
- Latency and Jitter: A high-bandwidth satellite link might have high latency. You may want to lower its weight to prevent VoIP traffic from suffering, even if the raw speed is high.
- Data Caps & Overage Costs: If your secondary link has a data cap (e.g., LTE backup), mathematically it might support a weight of 5, but financially you should set it to 1 or configure it as "Active-Backup" to avoid overage fees.
- Upload vs. Download Asymmetry: Most business connections are synchronous (SD-WAN, Fiber), but some are asynchronous (Coax). Ensure you calculate weights based on the bottleneck speed (usually upload) if you host services.
- Session Persistence: Sophos applies weights to sessions, not individual packets. Heavy file downloads can skew the actual bandwidth usage compared to the calculated weight.
- ISP Stability: If a link frequently drops packets, assign it a lower weight than its bandwidth suggests to minimize user impact during instability.
- Link Overhead: PPPoE or VPN encapsulation adds overhead. A 100 Mbps tunnel is effectively ~85-90 Mbps. Use the effective throughput for accurate weighting.
Frequently Asked Questions (FAQ)
Does this calculator work for Sophos UTM 9?
Yes, the mathematical principle of Weighted Round Robin applies to Sophos UTM 9, Sophos XG, and Sophos XGS firewall series. The interface labels may vary slightly (e.g., "Weight" vs "Traffic Ratio").
Can I use decimals for weights?
No. Sophos firewalls typically require integer values for gateway weights. This tool rounds and simplifies ratios to provide valid integer outputs.
What happens if I set weights to 1 and 1 for uneven links?
If you have a 100Mbps and a 10Mbps link both set to weight 1, traffic is split 50/50. The 10Mbps link will become saturated and drop packets, while the 100Mbps link remains 90% idle. This is inefficient and causes poor user experience.
Should I include my backup LTE line in the calculation?
Only if you intend to use it in an "Active-Active" configuration. If it is purely for failover ("Active-Passive"), you do not need to assign a load balancing weight relative to the primary line.
How does this affect VoIP traffic?
Weighted balancing can cause VoIP issues if packets take different paths. It is best practice to use SD-WAN policy routes to pin VoIP traffic to a specific high-quality link, regardless of the general load balancing weights.
What is the maximum weight I can enter?
On most Sophos XG/XGS firmware versions, the weight value is an integer between 1 and 100.
Does weight calculation account for real-time congestion?
No. WRR is a static configuration. It does not dynamically adjust based on real-time packet loss or latency unless you configure specific health check rules to mark a gateway as "down".
Why is my calculated ratio different from my speed test?
Speed tests show momentary capacity. Weights should be based on the committed information rate (CIR) or the guaranteed bandwidth from your ISP contract for stability.
Related Tools and Resources
- Network Throughput Calculator – Estimate real-world data transfer speeds.
- Bandwidth ROI Estimator – Calculate the financial return of upgrading your ISP links.
- Sophos Licensing Cost Guide – Understand the costs associated with XGS hardware.
- Subnet Mask Cheat Sheet – Quick reference for IP addressing and CIDR notation.
- High Availability Uptime Calculator – Measure the reliability of your redundant firewall setup.
- Data Transfer Time Tool – How long will your backups take on weighted links?