Lowest cost (SLA) strategy
SD-WAN rules are used to control how sessions are distributed to SD-WAN members. Rules can be configured in one of five modes:
auto: Interfaces are assigned a priority based on quality.
- Manual (
manual): Interfaces are manually assigned a priority.
- Best Quality (
priority): Interface are assigned a priority based on the link-cost-factor of the interface. See Best quality strategy.
- Lowest Cost (SLA) (
sla): Interfaces are assigned a priority based on selected SLA settings.
- Maximize Bandwidth (SLA) (
load-balance): Traffic is distributed among all available links based on the selected load balancing algorithm. See Maximize bandwidth (SLA) strategy.
When using Lowest Cost (SLA) mode (
sla in the CLI), SD-WAN will choose the lowest cost link that satisfies SLA to forward traffic. The lowest possible cost is
0. If multiple eligible links have the same cost, the Interface preference order will be used to select a link.
In this example, your wan1 and wan2 SD-WAN interfaces connect to two ISPs that both go to the public internet. The cost of wan2 is less than that of wan1. You want to configure Gmail services to use the lowest cost interface, but the link quality must meet a standard of latency: 10ms, and jitter: 5ms.
To configure an SD-WAN rule to use Lowest Cost (SLA):
- On the FortiGate, add wan1 and wan2 as SD-WAN members, then add a policy and static route. See SD-WAN quick start for details.
- Create a new Performance SLA named google that includes an SLA Target with Latency threshold = 10ms and Jitter threshold = 5ms. See Link monitoring example.
- Go to Network > SD-WAN Rules.
- Click Create New. The Priority Rule page opens.
- Enter a name for the rule, such as gmail.
- Configure the following settings:
Lowest Cost (SLA)
wan1 and wan2
Required SLA target
google (created in step 2).
- Click OK to create the rule.
To configure an SD-WAN rule to use SLA:
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
If no SD-WAN zone is specified, members are added to the default virtual-wan-link zone.
To diagnose the Performance SLA status:
FGT # 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 FGT # diagnose sys sdwan service 1 Service(1): Address Mode(IPV4) flags=0x0 TOS(0x0/0x0), Protocol(0: 1->65535), Mode(sla) Members:<<BR>> 1: Seq_num(2), alive, sla(0x1), cfg_order(1), selected 2: Seq_num(1), alive, sla(0x1), cfg_order(0), selected Internet Service: Google.Gmail(65646)
When both wan1 and wan2 meet the SLA requirements, Gmail traffic will only use wan2. If only wan1 meets the SLA requirements, Gmail traffic will only use wan1, even though it has a higher cost. If neither interface meets the requirements, wan2 will be used.
If both interface had the same cost and both met the SLA requirements, the first link configured in
set priority-members would be used.