Question on FortiGate SD-WAN

3 replies
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I have 2 WAN links provided by 2 different ISPs with load balancing in HA, and as it happened one of them have been down for a couple of days and the other one is working but occasionally going down.

As an emergency solution I plugged in a cellular 5G router to a free port and added the port to the SD-WAN zone.

Would it affect the load balancing between the original links if I raised the cost of the cellular link and given it a lower priority? I don't want to keep the traffic going through it if either of the main links is working fine.

Toshi_Esumi

SuperUser

It’s depending on how your the mode of each SD-WAN rule is set. For example, if it’s set as manual, the interface selection is based on preference order rather than interface cost. The cost is used in Lowest cost (SLA) mode if the interfaces meet required SLA targets.

Toshi

sjoshi

Staff

Hi @xoubie,

Which port you have connected the 5G router, Is that interface on FGT part of sdwan?

What is your requirement.

Traffic should go from the wan link and 5g router connection should be back up?

Regards,

Salon Raj Joshi

Thanks, Salon

osoleimani

Staff

Hi xoubie,

One additional detail is important for your requirement. A higher SD-WAN member cost does not by itself guarantee that the 5G link will remain backup-only.

In FortiOS 8.0, when an SD-WAN rule uses Lowest Cost (SLA) with load balancing enabled, FortiGate load-balances across all members that meet the required SLA; the member cost is not considered for that load-balancing selection.

Therefore, if you want the 5G connection to be backup-only, check whether the applicable SD-WAN rule is using load balancing. The rule design needs to distinguish the two primary links from the backup member rather than relying on a higher cost alone.

FortiOS 8.0 — SD-WAN WAN link selection with load balancing

Hope this helps clarify how the 5G link will be treated in your SD-WAN configuration.

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