Technical Tip: FortiOS SD-WAN sla-compare-method feature Overview

[sjoshi]

Description

This article describes the use case of sla-compare-method in SDWAN rule when using more than one SLA target.

Scope

FortiGate.

Solution

This SLA comparison method allows for defining an order of preference for SLA Targets.

SLA Targets which are considered more critical can be given a higher order of preference by the administrator.

Suppose there are two SLA targets configured, one for latency and another for packet loss.
So, if the administrator wants to give a higher preference to packet loss than the latency, then the administrator can set the sla-compare-method to order and select the packet loss SLA before latency.

Let’s take an example over here: -

> SDWAN rule: -

 GUI configuration:-

 CLI configuration:-

# config system sdwan

# config service

# edit "1"

# show

config service

    edit 1

        set name "Internet-Rule"

        set mode sla

        set dst "all"

        set src "all"

        config sla

            edit "PacketLoss"

                set id 1

            next

            edit "Latency"

                set id 1

            next

        end

        set priority-members 1 2

    next

end

> SLA configuration: -

Latency SLA:-

Packet Loss SLA:-

> Set sla-compare-method as order.

config system sdwan

config service

edit 1

set sla-compare-method

order     Compare SLA value based on the order of health-check.

number    Compare SLA value based on the number of satisfied health-check. 

For example, if there is more than 5% packet loss over Port3 and there is latency higher than 100ms for port1, since the administrator has configured to give preference to packet loss SLA therefore port1 will be chosen as the preferred route even though it has higher latency.

The table below shows the result of 'set sla-compare-method order' compared to 'set sla-compare-method number'.

SLA Status	Port 1		Port 3		sla-compare-method order	sla-compare-method number
	Latency	Packet loss	Latency	Packet Loss
Optimal	✓	✓	✓	✓	Sla identical	Sla identical
SLA failure 1	✗	✓	✓	✓	>SLA for Loss (preferred) is identical on port1 & port3 >Tie breaker goes to Latency >Port 3 is preferred	>SLA thresholds pass for port1=1 >SLA thresholds pass for port3=2 >PORT3 is preferred
SLA failure 2	✗	✓	✓	✗	>SLA for loss is preferred so in port 3 Loss is failed >Port 1 is preferred	>SLA results are identical since number of SLA pass is identical