Technical Tip: SD-WAN Lowest cost (SLA) strategy at a glance

Description

This article describes SD-WAN Lowest cost (SLA).

Scope

FortiGate.

Solution

In the Lowest Cost (SLA) strategy, FortiGate assigns a cost value to each SD-WAN member based on administrative preference or link type. It continuously monitors SLA performance metrics such as latency, jitter, and packet loss to determine if each link meets the defined thresholds. If multiple members satisfy the SLA criteria, the traffic is forwarded in order of member cost, followed by the configuration order.

The following image explains the Lowest Cost (SLA) strategy, the green boxes represent SD-WAN members in SLA, while the red boxes represent SD-WAN members outside of SLA. It describes different scenarios to identify which link will carry the Gmail traffic.  

SD-WAN settings:

config system sdwan
 config members
   edit 1
     set interface "wan1"
     set cost 10
   next
   edit 2
     set interface "wan2"
     set cost 5
   next
 end
 config health-check
   edit "google"
     set server "google.com"
     set members 1 2
     config sla
       edit 1
         set latency-threshold 10
         set jitter-threshold 5
       next
     end
   next
 end
 config service
   edit 1
     set name "gmail"
     set mode sla
     set internet-service enable
     set internet-service-id 65646
     config sla
       edit "google"
         set id 1
       next
     end
     set priority-members 1 2
   next
 end
end

To diagnose the Performance SLA status:

diagnose sys sdwan 
health-check google
Health Check(google):
Seq(1): state(alive), packet-loss(0.000%) latency(14.563), jitter(4.334) sla_map=0x0
Seq(2): state(alive), packet-loss(0.000%) latency(12.633), jitter(6.265) sla_map=0x0

diagnose sys sdwan service 1
Service(1): Address Mode(IPV4) flags=0x0nnTOS(0x0/0x0), Protocol(0: 1->65535), Mode(sla)
Members:
1: Seq_num(2), alive, sla(0x1), cfg_order(1), selectedn2: Seq_num(1), alive, sla(0x1), cfg_order(0), selected

Note: Starting from FortiOS v7.4.4, the diagnose sys sdwan service command is now divided into two separate commands for IPv4 and IPv6.

For IPv4:

diagnose sys sdwan service4

For IPv6:

diagnose sys sdwan service6

Note: Starting FortiGate v7.4, the maximum-bandwidth strategy has been removed and replaced with the load-balance setting under manual mode and the lowest-cost strategies.