What does it do?
Backup servers provide an extra layer of redundancy to ensure that your site continues running when your primary origin is unavailable. StackPath's CDN loads content from the backup origin if the primary origin is unavailable.
The CDN considers your primary origin unavailable if it receives a 4xx or 5xx response code when fetching content.
When and how should I use it?
Configure a backup server when your site has backup or redundant infrastructure.