Technical Tip: Recommended TTL settings in FortiADC Global Load Balance

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Description	This article describes the FortiADC FQDN DNS Host Zone TTL Settings in Global Load Balance.
Scope	FortiADC.
Solution	One of the key features of FortiADC Global Load Balance (GLB) is to dynamically respond to a DNS query request of an FQDN hostname according to the configured virtual servers’ health. Any delays or caches in the FQDN DNS resolution could lead to extended outages or instability. It is recommended to shorten the TTL of the FortiADC GLB FQDN host DNS resolution. In the example, an FQDN Host has been pre-configured and associated with a Virtual Server Pool. By default, the zone records generated from the FQDN Host creation shall have a TTL of 86400 seconds, which equals 24 hours or 1 day. The zone records or the FQDN host records would have the TTL of '-1', which means it will use the zone level TTL by default. A DNS resolution test on the FQDN host returned with a TTL of 86400. In the case of one of the virtual servers failure, the recursive DNS servers could retain the DNS result of the failed virtual server until the next TTL expires which results in users’ downtime experience. To avoid extended outages or intermittent caused by the TTL, it is recommended to reduce the TTL to a lower value such as 300 (5 minutes). The DNS zone TTL can also be set to zero '0' second, which every FQDN DNS query will always refer to the FortiADC GLB DNS server, but it may increase the load of the FortiADC DNS server due to frequent queries of the FQDN hosts. Related document: Configuring hosts. Configuring DNS zones.