Originally Posted by TransWorldOne

The last segment is YY, which means it can be on any carrier. It doesn't need to be on AA or an AA codeshare. So not having service on AA CUN-GDL shouldn't be an issue.

Of course, if there is conflicting language somewhere else stating all travel on AA-only, the YY wildcard may be trumped.

If you really want this fare ticketed, call a good travel agent to see if it is possible.