Drush within Multisites I am using drush on ubuntu 14.04. I have more than 20 site on my localhost /var/www with virtualHost setting. I have a question that whenever i go to /var/www/site1 and run drush dl module_name. It download the module in sites/all/modules directory in site1 folder. Now i cd to /var/www/site2 and run drush dl module_name. It will again download the module in site2 module directory.

In every site i have to do drush dl for same module. Is there any method so that i don't need to do drush dl every time. I just do drush dl module, it will check from all other sites for particular module, if not found then will download from internet. I already doing cp and scp for same module. But i want to do with drush. 
I am very greedy to save my internet bandwidth.

Thanks in Advance.
you can use:

drush dl modulename --cache


From drush help dl:

Download Drupal core or projects from drupal.org (Drupal core, modules, themes or profiles) and other sources. It will automatically figure out which project version you want based on its recommended release,
or you may specify a particular version.

...

Examples:

 drush dl webform --cache                  Download webform. Fetch and populate the download cache as needed.

Options:
 --cache                                   Cache release XML and tarballs or git clones. Git clones use git's --reference option.                                                                         
   --cache-duration-releasexml             Expire duration (in seconds) for release XML. Defaults to 86400 (24 hours).
...