Why doesn't CodeIgniter session work on subdomains?

Question

MySQL

There is a project on CodeIgniter 3.1.0. Session variables are not saved when moving from subdomain to subdomain of the site. That is, logged in to novosibirsk.site.ru, go to moskva.site.ru and the session is reset, but when we return to the first subdomain, the user remains logged in there.
Settings:
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 0;
$config['sess_expire_on_close'] = TRUE;
$config['sess_encrypt_cookie'] = TRUE;
$config['sess_use_database'] = TRUE;
$config['sess_table_name'] = 'ci_sessions';
$config['sess_match_ip'] = TRUE;
$config['sess_match_useragent'] = FALSE;
$config['sess_time_to_update' ] = 300000000;
$config['cookie_prefix'] = "mysite";
$config['cookie_domain'] = ".mysite.com";
$config['cookie_path'] = "/";
$config['cookie_secure'] = FALSE;

Answer 1

Solution for version 3.1.0:

$config['sess_driver']     		= 'database';
$config['sess_cookie_name']     = 'sitename';
$config['sess_save_path']      = 'ci_sessions';
$config['sess_match_ip']        = TRUE;
$config['cookie_prefix']	= 'sitename_';
$config['cookie_domain']	= '.sitename.com';
$config['cookie_path']		= '/';
$config['cookie_secure']	= FALSE;
$config['cookie_httponly'] 	= FALSE;

Answer 2

Mb sess_expire_on_close make FALSE ?