Hi @Assaf L ,
you can't add availability zones support to an existing application gateway.
If you previously deployed Azure Application Gateway Standard v2 or Azure Application Gateway Standard v2 + WAF v2 without zonal support, you must redeploy these services to enable zone redundancy.
There are 2 options how to migrate: https://learn.microsoft.com/en-us/azure/availability-zones/migrate-app-gateway-v2
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Regards
Andreas Baumgarten