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, or if the request times out.
Response Code Exclusion
If you have some 4xx or 5xx response codes that you don't want to load from the backup (for example 403 Permission Denied response codes), you can add those codes to your Exclude Codes list. Codes can be added individually or with wildcards.
When and how should I use it?
Configure a backup server when your site has backup or redundant infrastructure.