Is it possible to share forms authentication *only* across sub-sub domains?

Question

I know I can share forms authentication across 2 subdomains sub1.domain.com, sub2.domain.com by using the following Web.config setting:

<authentication mode="Forms">
    <forms domain="domain.com" />
</authentication>

However, I have 2 "sub-sub" domains (not sure if there's a better term for that) a.sub.domain.com, and b.sub.domain.com. Now I want to share the forms authentication cookies only on sub.domain.com, and not my entire domain.com. Is it possible to do this?

I tried the obvious solution below but it doesn't work, and always redirects back to the login page:

<authentication mode="Forms">
    <forms domain="sub.domain.com" />
</authentication>

Is it possible to share the cookies only across domains that end with sub.domain.com?

Answer 1

So it turned out that my old cookies in my browser were somehow conflicting with the new setting.

After clearing all domain.com cookies, the solution I was trying now works, in case anyone faces a similar situation.

Also note I believe it's preferable to prepend a . to the domain:

<authentication mode="Forms">
    <forms domain=".sub.domain.com" />
</authentication>