dhcp with syslog as an option

I've got a DHCP server doing IPv4 and IPv6 DHCP, I've been asked that any device automatically use a syslog server as part of one of the DHCP options. I've been passing:

option log-servers x.x.x.x; 

In the config, but so far I've not managed to log anything, am I doing something wrong? It's impossible to test as well (logger writes to syslog). Telnet works so I know the IP/port is up (TCP and UDP), not really sure what's going wrong - perhaps TCPdump isn't out of the question.

I should probably mention this is for DOCSIS modems (although it doesn't present work for anything).

Thanks guys