Technical Tip: SD-WAN Best Quality (SLA) Customized Profile

[jiahoong112]

Description	This article describes how to configure the Best Quality SLA Customized Profile for FortiGate SD-WAN and what the metrics mean. This is usually used when more than 1 SLA metric is required to determine link switchover. For example, Latency as the Quality Criteria is prioritized. This means that the link with the lower latency will always be selected. In an event where a link failover happens and the recovered link comes back up with high packet loss but lower latency than the backup link, the recovered link will still be selected as it has lower latency. This will cause problems because, despite the recovered link having lower latency, it still has high packet loss. To prevent this, the Packet Loss metric would be the primary deciding factor for link selection, followed by Latency. If Packet Loss is 0, then Latency will be the deciding factor for link selection.
Scope	FortiGate SD-WAN Best Quality Performance SLA - Customized Profile.
Solution	When the Customized Profile metric is used, FortiGate uses a weight-based formula to calculate a value called the link quality index that represents the quality of the member based on its latency, jitter, packet loss, and available bidirectional bandwidth. The lower the link quality index, the higher the member preference. The administrator assigns the weight for each metric. To configure this, go to Network -> SD-WAN -> SD-WAN Rules. Select the Rule to configure. On the Quality criteria dropdown, select Customized Profile. 4 categories will be visible. Latency, Jitter, Packet loss, and Bandwidth weight. The higher the metric weight, the more influence the weight metric has on the link quality index. If a metric is to be ignored, assign it a weight of 0. Link Quality Index = (packet-loss-weight * packet loss) + (latency-weight * latency) + (jitter-weight * jitter) + (bandwidth-weight / bandwidth). In the example given in the Description above, prioritize Packet Loss as the main selection metric, followed by Latency.  If Packet Loss is the same on both links, then Latency will be the deciding factor on which link traffic gets to be forwarded in. In this screenshot, Packet Loss (2) weight is the highest, followed by Latency (1). Jitter and Bandwidth are not considered in this, so their weights are set to 0.  To have the Link Selection use all 4 metrics following the order Packet Loss -> Latency -> Bandwidth -> Jitter, the weight metric for each category would be 4, 3, 2, 1 in this order. Note that if tie-break is set to fib-best-match the selection logic of the outgoing interface no longer relies solely on the quality of the links. For example: On a rule with members 'port1' and 'port2'. If port1 has the best quality based on the Link Quality Index and has the best route to the destination, port1 is selected as the outgoing interface. If port1 has the best quality based on the Link Quality Index, and port2 has the best route to the destination, port2 is selected as the outgoing interface.