Symfony 2.1:

In parameters.yml I have
sub_dir: /mnc

In config.yml:

sonata:
resource: '@SonataAdminBundle/Resources/config/routing/sonata_admin.xml'
prefix: %sub_dir%/secured/admin
_sonata_admin:
resource: .
type: sonata_admin
prefix: /mnc/secured/admin

Up to here everything works fine, but if I use

sonata:
resource: '@SonataAdminBundle/Resources/config/routing/sonata_admin.xml'
prefix: %sub_dir%/secured/admin
_sonata_admin:
resource: .
type: sonata_admin
prefix: %sub_dir%/secured/admin

I get a route not found error.
I checked that in the latter case the line

if (0 === strpos($pathinfo, '/%sub_dir%/secured/admin')) {

is generated in the cache file appdevUrlMatcher.php, which should in fact be

if (0 === strpos($pathinfo, '/mnc/secured/admin')) {.

Regards