Request dispatchers behaving abnormally

[Abdul Kalam]

hi,
consider this,
1. I have a serv1 which has its get method implementede and has some parameters obtained from a previous servlet now i use requestdispatchers forward or include method to call another servlet serv2
getServletContext().getRequestDispatcher("servname?aa=12").forward(request,response)
Now when i obtain the request parameters in the get method of the called servlet i get the entire parameters including aa.
2.Now if i try the same in post method then till serv1 i get all the parameters but in the called servlet when i try to retrieve the parameters it does not return anything.
The strange thing is when i print the content length and query string then the querystring consists of all the parameters while the content length contains some length value.
Why is this happening
bye