I have a root domain website www.example.com and multiple subdomains - sub1.example.com, sub2.example.com, sub3.example.com.
All these subdomains have sitemap index file of their own, sub1.example.com/sitemap_index.xml, sub2.example.com/sitemap_index.xml, etc.. and it is being indexed properly.
Is it required for me to list all the sitemap index files in robots.txt of the root domain (www.example.com/robots.txt)? Or is it not mandatory?
Please explain how sitemap listing works across subdomains. Thank you.