Troubleshooting Tip: Throughput issue when traffic flows between 2 VLANs

[riteshpv]

Description	This article describes the reasons behind throughput issues in the 1xx series FortiSwitch and provides potential solutions.
Scope	FortiSwitch models F1xxE and FS1xxF.
Solution	Consider 4 PC connected to a Fortiswitch. PC1, PC2 on vlan1 and PC3, PC4 on vlan2. Here, the FortiSwitch model is FS1xxE or FS1xxF. Below are throughput test results done using iPerf. In iPerf between PC1 and PC2, results give around 900 ~ 950Mbps. In iPerf between PC3 and PC4, results give around 900 ~ 950Mbps. In iPerf between PC1 and PC3/PC4, results give less than 13Mbps. In iPerf between PC2 and PC3/PC4, results give less than 13Mbps. The throughput is good when traffic is within the VLAN. When passed between VLANs, the throughput drops. Reason: In this scenario, the FortiSwitch is utilized as the router to route traffic between VLANs. However, this functionality is impacted on FortiSwitch models that support only software-based routing. Software-based routing is supported in specific models like FS1xxE or FS1xxF. It is possible to confirm which FortiSwitch (FSW) models support 'Software-based routing only' by referring to the 'feature matrix' document. Look under Layer3 -> Software-based routing only (IPv4/IPv6) for the relevant information. Solution: Use an external router like FortiGate to route traffic between VLANs. Consider upgrading to a higher-end FortiSwitch model that supports hardware-based routing. Identify these devices by consulting the 'feature matrix' document, specifically under Layer3 -> Hardware-based routing (IPv4/IPv6).