jsp will lost its session while subdomain-rewrite applied

my system is tomcat combinded apache.

I want to do SEO by apache mod rewrite so as to hard pages likewise, and devided my company's applications into several modules/subdomain( hosting in one jsp server), so as to promote each individually, such as bussines, forum, blog, news, etc.

after everything has been closely to be done, I found one unexpect result that the jsp session not only related to the application/context directory but also the hostname, so that while the audience login with one subdomain, but change to another domain(or subdomain), his session will be lost. Unless I use redirect,however, it probably will disable the SEO(sounds like to browse by hardpage link) efforts.

can I get the tomcat/jsp to regconize the sessions only by context/dir?

thanks
Frederick Z