User Sync question across thress sites

Hi,

I have 2 sub sites sync'd to my main master site. If someone is logged in to the main site, can they also be logged in when they go to a sub site, or do they have to login there too.

A use case..I have a support site set-up and say someone is in my main site and we want them to raise a ticket on the support site and we link them through, will they have to login there again to raise the ticket. I t would be great if they were seen as logged in from the main site...and not have to do that twice..

Hope that makes sense...

thanks,

imacg