Redirect loop on sub-sites

Can’t tell from your short description. so here’s some things to check:

You can’t have domain mapping for the main site, the site that you use wp-admin from. For the domain mapping to work, WordPress itself has to be on an un-mapped domain. http://www.mysite.com/wp-admin/

For DNS to work on shared hosting and most other configurations (i.e. unless you’re a server expert and want a lot more work!) you can not install WordPress in a subfolder and do domain mapping. You can, however install WordPress in the Root of a Parked Domain or Add-on Domain. http://www.mysite.com/wp-admin/ not http://www.mysite.com/blog1/wp-admin/

Ron Rennick says in one of these support messages, “Domain parking is mainly a cPanel term. Most other web host control panels call it a domain alias. If you are using cPanel then parking the domain is the preferred implementation (over using an addon domain).”

My detailed installation and configuration instructions:
http://lcblog.lernerconsult.com/2012-wordpress-3-multi-site-domain-mapping/

p.s. The Domain Mapping development version works best:
https://wordpress.org/plugins/wordpress-mu-domain-mapping/developers/
click on the Current Version link, currently 0.5.5.1